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MAERSK TRAINING CENTRE Drilling Section

Copyright © Maersk Training Centre a/s. All rights reserved. No part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written permission of Maersk Training Centre a/s.

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Table of content: 01

Pressure in the earth crust 01.01 Sedimentation 02.01 Compression 03.01 Pressure 04.01 Pressure in fluids 05.01 Pressure gradient 06.01 Abnormal/subnormal pressure

Page 007

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Pressure balance in the well bore 01.02 Pressure balance 02.02 Overbalance and underbalance 03.02 Lost circulation 04.02 Rate of penetration versus overbalance 05.02 Drilling break 06.02 Necessary overbalance 07.02 Trip margin 08.02 Riser margin 09.02 Relationship 10.02 Equivalent drilling fluid density

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Dynamic pressure regime when circulating 01.03 Circulation of drilling fluid 02.03 Dynamic pressure in the well bore

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Consideration with a closed in well 01.04 Closed in well 02.04 U-tube

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Properties of gasses and gas laws Page 036 01.05 Drilling with underbalance 02.05 Properties of gas and gas laws 03.05 Expansion of gas 04.05 Formation strength 05.05 Leak-off test 06.05 Maximum allowable annular surface pressure

06

Drilling fluid volume and capacities Page 044 01.06 Calculating drilling fluid volume – capacities 02.06 Drilling fluid volume and capacities from tables 03.06 Surface to bit strokes & bit to surface strokes 04.06 Use of barite to increase drilling fluid volume 05.06 Volume increase due to barite addition

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Wellbore kicks 01.07 02.07 03.07 04.07 05.07 06.07

Page 053 Kick occurrences Warning signals Warning signals while drilling Warning signals while tripping or making connection Procedure for shutting in the well Pressure after shut in

08

Circulating a kick out of the well bore Page 069 01.08 General points 02.08 Circulating out an influx using Driller’s Method 03.08 Wait and Weight Method or Engineer’s Method 04.08 The Concurrent Method 05.08 Advantages and disadvantages of the three methods 06.08 Pressure control schemes

09

Calculations of density and pressure gradient of an influx 01.09 General points 02.09 Example

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Lost circulation 01.10 02.10 03.10 04.10

Page 097 General Causes of lost circulation Well control with partly lost circulation Well control with total lost circulation

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Volumetric wellcontrol and other 01.11 General 02.11 Volumetric Method – Specification required 03.11 Volumetric Method – Handling 04.11 Lubrication Technique 05.11 Volumetric Method – Example 06.11 Low Choke Method – Dynamic Kill 07.11 Bullheading

12

Kick with bit off bottom Page 113 01.12 Introduction 02.12 Stripping 03.12 Closing Procedures 04.12 Rig layout for combined stripping and volumetric method 05.12 Procedure 06.12 Snubbing


Page 102

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Gas cut drilling fluid 01.13 General 02.13 Causes of gas cut drilling fluid 03.13 Gas kicks in Oil Based Mud 04.13 Influx volume

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Deviated and Horizontal well control 01.14 Introduction 02.14 Complications 03.14 Horizontal well control example 04.14 Wait and Weight Method 05.14 Driller’s Method 06.14 Horizontal well kill method

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Pulling Pipe 01.15 02.15 03.15 04.15 05.15

Page 138


Introduction Pumping slug Inadequate hole fill Hole not taking correct amount of fluid Hole not giving correct amount of fluid

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Abbreviations: A Atm BHA BHP BOP Cap DC DP EDC EFD EOB FCP FOSV Ft G Gal GMD GMR GPM HCR HPHT H2S ICP ID KMW KOP Lb Lbs/ft LOT MAASP MD MW MWF OBM OD OH OMW P PA Pc PDP Pf Ph PL PPG M:\IWCF Surface\3\1\Section 5.doc

Area Atmosphere Bottom hole assembly Bottom hole pressure Blow out preventer Capacity Drill collar Drill pipe Equivalent circulating density Equivalent formation density End of build Final circulating pressure Full opening safety valve Feet Pressure gradient psi/ft Gallons Gas migration distance Gas migration rate Gallons per minute High closing ratio High Pressure/High Temperature Hydrogen sulfide Initial circulating pressure Inside diameter Kill mud weight Kick off point Pounds Pounds per feet Leak off test Maximum allowable annular surface pressure Measured depth Mud weight Final mud weight Oil base mud Outside diameter Open hole Original mud weight Pressure Pressure annulus Pressure dynamic Pressure drill pipe Pressure formation (pore pressure) Pressure hydrostatic Pressure loss Pound per gallon © MTC

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Part per million Pound per inch² Pressure while drilling Rate of penetration Rotation per minute Reduced rate circulating pressure Safety factor Shut in casing pressure Shut in drill pipe pressure Strokes per minute Sacks Temperature True vertical depth Volume Water base mud Weight on bit

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PRESSURE IN THE EARTHS CRUST

01.01 Sedimentation: The theory behind the pressure being present in the different depths in the earth rock formations are based on the historic development during millions of years where settling of particles has taken place in the ocean. Large and small rock particles are transported by rivers and streams, ice and wind and deposited on the seabed offshore. In the sea several different chemical substances are present which also separates from the water and sink to the seabed. Amongst others carbonates, sulphates and chlorides are known to be dissolved in the seawater. Small organisms which live in the sea has a life cyclus and when they die their solid remains also sink to the seabed. When this process continues during millions of years the layers of settling will obtain a considerable thickness on the sea floor. 02.01 Compression: The rock particles and solid matter will eventually become more and more compacted as they bear more and more weight from the overlaying deposits. As this process continues the water that is found between the rock particles will usually escape. However there will usually be small cavities left between the particles, which contain the remaining water. These cavities or void spaces make the rock formations more or less porous. A porous formation can contain fluids, gas or hydrocarbons. As compression and compaction continue during time, combined with thermal and chemical processes the unconsolidated particles will eventually become rock formations within the earth crust. These sedimentary rock formations are generally porous, and the pores are filled with a fluid or gas.

SHALE Porous/

impermeable Porous/

SANDSTONE permeable Tight and

SALT without pores Fig 01


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If communication between the cavities or pores in the formation is present this allows the fluid to flow away and escape. Under certain conditions the formation fluid can become trapped. If a porous fluid-bearing formation becomes covered with an impermeable layer of rock such as a clay stone, the fluid becomes trapped. 03.01 Pressure Before describing the conditions in which the formation fluids are found at different depths in the rock formations the terms mass, density, force, energy and pressure will be considered. Mass Mass is defined as the term for a quantity of matter. The unit of measurement that is used is the pound. Density Density is an expression giving the mass of gas, fluid or solid matter in relationship to its volume, E.I. mass per unit volume. Other means to express density is the term relative density. By relative density is understood, the mass of a particular volume of substance divided by the mass of an equal volume of fresh water. Due to the definition of the relative density it remains dimensionless. In this lecture mass in pounds, and volume in gallons is used, therefore the density is given in pounds per gallon (ppg). Force When a mass hangs by a string, a force will keep the string in tension. The product of gravitational acceleration and the mass causes the force itself.

Mass Power

Fig 02


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This force can be measured by a dynamometer, Fig. 03. This instrument consists of a spring. One end is fixed and the other end shows on a scale how much gravity force is exerted.

Scale Pointer

Fig 03

Force is expressed in the unit pounds-force, which is defined as follows. One pound-force is the force, which will influence a body with a one pound mass when subjected to a gravitational acceleration of 9.80665 m/s2. The gravitational acceleration of 9.80665 m/s2 is present at latitude 45° North on the earth's globe. Gravitational acceleration differs in various parts of the globe. This means that one pound-force is not an equal value everywhere on the globe. As an example the gravitational acceleration at the North Pole is equal to 9.831 m/s2, which gives a force influence on a mass of one pound according to the following G = 1x

9,831 = 1.0025 [ pounds ] 9,80665

At the equator the gravitational acceleration = 9,781 m/s2 The force influence on one pound mass becomes G = 1x

9,781 = 0,9974 [ pounds ] 9,80665

In practice this variation in gravitational acceleration is ignored and a one pound mass is considered to exert a one pound-force influence.


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Pressure Pressure is defined in physics as force per area unit. Force Area unit

Pressure =

The total force, which acts on a plane, is divided by the area of the plane. The result is called pressure. The unit for force is pounds-force and the unit for area is square inch. Therefore the unit for pressure will be: Pressure =

Pounds [pounds per square inch ] Square inch M = 1 pound G = 1 pound ( 45° latitude North ) g = 9,80665 m/s2 A = 1 inch2

M G

Pressure (P) = P x

A

G 1 = = 1 A 1

Fig 04

04.01 Pressure in fluids Considering a vertical cylindrical volume of static fresh water with a cross-sectional area of one inch2 and height of 10 ft, the pressure at the bottom of this cylinder can be calculated The fluids total volume is 1 in2 x 10 x 12 = 120 in3 10 ft

The density of fresh water is 8.34 ppg

2

1 inch

8.34 pounds per gallon =

8.34 x 7,48 pounds / inch3 1728

Fig 05 M:\IWCF Surface\3\1\Section 5.doc

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The mass of the fluid column will be M = 8.34

1 pounds gallon ft 3 x 7,48 x x 120 inch3 = 4,33 pounds 3 3 gallons 1728 inch ft

The pressure at the base of the fluid column is caused by gravitational acceleration that acts on the fluid column divided by the fluid columns' cross sectional area. Ph =

4.33 pound = 4.33 psi 1 inch 2

It is important to realise that the pressure at the bottom of a static fluid column is only depending on the vertical height of the column and the density of the fluid. 05.01 Pressure gradient Considering a porous and permeable rock formation in which the pores are filled with fresh water (density 8.34 ppg). It is now possible to calculate the pressure at 5000 feet depth Ph =

4,33 x 5000 = 2165 psi 10

It is also possible to calculate the pressure increase that every foot of depth will represent. Pressure increase per ft =

2165 = 0.433 psi pr ft 5000

This quantity which represents pressure increase in psi/ft is named Pressure Gradient 8G. When the pressure gradient for a fluid or gas is known it is easy to calculate the pressure at any given depth. From the shown example of freshwater (8.34 ppg) and pressure gradient (0.433 psi/ft) it is possible to calculate the pressure gradient for a fluid or for a gas with a density of 1 ppg. Pressure gradient for 1 ppg =

0,433 = 0,052 psi / ft 8,34

With this new figure it is now possible to calculate the pressure gradient for any fluid or gas. Pressure gradient = 0.052 x density in ppg Example: Calculate the pressure gradient for a fluid with the density 10.4 ppg. M:\IWCF Surface\3\1\Section 5.doc

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0.052 x 10.4 = 0.541 psi/ft

Calculate the pressure exerted from this fluid at a depth of 4000 ft Answer:

0.541 x 4000 = 2164 psi

Fig 06 shows different pressure gradients and illustrates how pressure increases with depth-

DEPTH 0

1

Gas grad. 0.07 psi/ft 2

Oil grad. 0.30 psi/ft 3

Fresh W. grad 0.433 psi/ft 4

Salt W. grad 0.465 psi/ft 5

10 ppg grad. 0.52 psi/ft

2500 6

15 ppg grad. 0.7785 psi/ft 7

5000

1000

2000

3000

4000

21 ppg grad. 1.091 psi/ft

5000

PRESSURE

Fig 06


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06.01 Abnormal / Subnormal Pressure So far it has been assumed that there is a direct proportional relation between formation pressure and fluid density and true vertical depth from the surface. That means that the formation fluid pressure is only affected by the fluid density and from the true vertical depth. The influence of the overlying rock formations has so far not been considered. The reason is that in case of a permeable and porous formation system every single rock particle rests upon or leans up against other particles just below and to the side of it. Therefore the rock structure supports its own weight, and regardless of depth does not affect the formation fluid pressure. Artesian Well NORMAL FORMATION PRESSURE AT THE WELL UNTILL BELOW THE CAP ROCK LAKE

POROUS SANDSTONE BELOW CAP ROCK

When talking about artesian wells, we are normally talking about water wells where we have a porous sandstone witch has communication to higher laying areas creating abnormal pressure below a cap rock.

HYDROSTATIC PRESSURE FROM FORMATION WATER COLUMN

Fig 07

Under compaction Let us consider that at a particular period in a rock formations' development it was not possible for the formation fluids to escape since an impermeable formation type placed on top prevents this from happening. Therefore the rock particles can not be compacted and consolidated sufficiently to carry the weight of the overlying rock. Since the fluid trapped in between the particles could not escape the fluid will be exposed to compressing forces. These forces result in an increased formation fluid pressure, which is abnormal at the given depth. It can be realised that the trapped formation fluid has to carry the weight of the overlaying formation, along with the formation rock in which it is trapped. In a situation such as this the formation pressure will be greatly different from a calculated normal pressure/depth forecast. Example: A formation at 5000 ft depth contains formation fluid. The formation fluid has communication to the surface through porous and permeable formation rock. See fig. 08 M:\IWCF Surface\3\1\Section 5.doc

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Formation pressure at 5000 ft will be the fluid column pressure Density for formation fluid = 8.95 ppg Pressure gradient for formation fluid = 8.95 x O.052 = 0.465 psi/ft Pf (Pressure of Formation) = 5000 x O.465 = 2325 psi

5000 ft

5000 ft

2325 psi

Impermeable zone 5147 psi Fig 08

If it is considered that this formation fluid was trapped in an earlier period in the sedimentary process and therefore could not escape the later compaction process, it is possible that the fluid may be exposed to the weight of the overlying rock mass. Assuming formation fluid is 10% and an equivalent formation density of 21 ppg this results in the following formation pressure P f = ( 0.1 x 5000 x 8.95 x 0.052) + (0.9 x 5000 x 21 x 0.052)

Pf = 5146.7 psi This formation fluid is over-pressured or abnormal. Over-pressured formations are often encountered with thick salt sediments and salt domes. Salt does not have the same structure as normal rock formations. Salt is termed a "plastic" formation, which means that it is not self-supporting, it can move and deform under pressure, and (this is not necessarily a rapid process). When pressure is applied to a salt formation it behaves more as fluids rather than as solid matter. The relative strength of salt is very low compared to other rock types. Because of the salt's qualities the weight from the overlying formation including the weight of the salt layers themselves will be transferred to the formation below the salt. The pressure in the salt and in the formation below it will often have a pressure gradient of 1 psi/ft instead of the normal pressure gradient for formation fluid, which is 0.465 psi/ft.


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Abnormal pressures can also occur when an encapsulated and normal pressured formation for the particular depth at a later stage in history with movements or surface erosion is brought closer to the surface. The particular formation in question can be found deeper or shallower in relation to its original position. If it is the case that the formation pressure cannot adjust to its new depth it will hold its original pressure. Example: If a sandstone formation at 4000 ft depth is considered it will have a normal pressure of 1860 psi. On account of geological processes the area of sandstone becomes isolated by impermeable rock. Through earth movements the formation moves to a shallower depth of 2500 ft. In this situation the sandstone will retain it's original 1860 psi pore pressure but he surrounding formation has a pore pressure of 1160 psi. Such an isolated zone is called a high-pressure zone or abnormal pressured zone. It may as well be the case that the isolated sandstone by earth movements was brought down to 5000 ft depth. The normal pressure for 5000 ft would be 2325 psi and the isolated sandstone area with its 1860 psi would become a low-pressure or subnormal-pressured zone.

2500 ft 4000 ft

1160 psi

1860 psi

1860 psi

5000 ft

1860 psi 2325 psi

1860 psi Fig 09

Abnormal pressured formations can also develop because of differences in the contained formation fluid and gas densities. Figure 10 shows an anticline. An anticline is the geological term for an area of formations which, due to earth movements has been pushed upwards to take a shape like a dome. In the figure the anticline consists of porous sandstone which contains gas. A layer of impermeable shale that prevents the gas from escaping caps the sandstone. The formation surrounding the anticline has a pore content of salt water and a base depth of 5000 ft. The formation pressure is considered to be normal. Formation pressure of the salt water bearing rock at 5000 ft will therefore be: M:\IWCF Surface\3\1\Section 5.doc

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P f = 5000 x 0.465 = 2325 psi

3000 ft Anticline

1395 psi 2125 psi

Porous with water

Sandstone with gas

5000 ft

Tight Shale

2325 psi Fig 10

If the sandstone in the anticline contained salt water instead of gas, the formation pressure at the very top of the anticline would be exactly the same as the formation just above. Example: Pf = 3000 x 0.465 = 1395 psi The sandstone however is containing gas, which has a pressure gradient of 0.1 psi/ft. This results in the pressure at top of the anticline to be substantially higher than the calculated 1395 psi for a salt-water formation. The reason is that the hydrostatic pressure of gas within the anticline is much lower than the corresponding hydrostatic pressure of salt water on the outside. Pressure from the 2000 ft high gas column will be: Ph = 2000 x 0.1 = 200 psi Therefore the formation pressure at the very top of the anticline below the cap rock will be: Pf = 2325 - 200 = 2125 psi Formation structures of this type give a real problem if the formations above and/or below will not withstand the 12.45 ppg hydrostatic pressure from the drilling fluid that is required to


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balance the zone at 2000 ft. It may be necessary to set several casing strings in order to isolate the pressure. High-permeability limestone formations have small formation strength gradients, and lost circulation may be the result when the bottom well pressure exceeds formation pressure by as little as 200 psi. This value may be less than the dynamic pressure drop in the annulus or less than a safe trip margin. Such conditions can be risky if insufficient information is available. Transition zones and under compacted shale Wherever massive shale formations are found the risk for transition zones and high pressure is present. This is caused by thick impermeable shale restricting the disposal of formation fluid. Due to new sediments are settled on the seabed increasing weight load is exerted on the shale from the formation above. The water, gas or oil trapped within the shale cannot escape. The result is the development of abnormal pore pressures. The terminology under compacted shales is used to indicate these circumstances. A seal of harder rock often caps the top of the abnormal pressured shale. Just after the cap is penetrated the Rate of Penetration (ROP) increases. The reason is that the shale is easier to drill since the differential pressure between drilling fluid hydrostatic pressure and the formation pressure decreases. A reduction in overbalance results in a faster drilling rate. When the Driller maintains his drilling parameters constant t.i. constant rotary speed, constant weight on bit and constant pump rate, the Rate of Penetration should be constant as well, unless changes in the drilled formation takes place. The indication of changes in the formation can therefore be observed by the Driller by means of changes in Rate of Penetration. To confirm whether the well is still in balance, the Driller must stop and observe if the well is static. The terminology for this operation is "flow checking the well".

UNCONSOLIDATED SHALE-DENSITY INCREASES WITH DEPTH - WATER ESCAPES

SAND WITH COMMUNICATION TO SURFACE

SHALE-DENSITY DECREASES WITH DEPTH-WATER ENCLOSED

ENCLOSED SAND LENS WITH FORMATION FLUID

Fig 11

Whenever thick shales are encountered it is important to be careful and expect abnormal pressure in the formation. Shale related abnormal pressures can occur at any depth from surface to very deep and is the most common reason for abnormal formation pressure. Because the formation fluid in under compacted shale is unable to escape, a typical trend will indicate that the cuttings density decrease with depth. The density decrease with depth can indicate that abnormal pressure is encountered.


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Surcharged formations by underground blowouts A different reason for abnormal formation pressures are the result of previous blowouts underground. Shallower sands can become charged as the result of an uncontrolled underground blow out from an adjacent well or from a bad cement job. Even the well has successfully been closed in on surface the pressure from the deeper zone can communicate to the shallower sand reservoir. When the next well is drilled the abnormal pressure is encountered at the much shallower depth. See Fig 12

UNDERGROUND BLOWOUT

FAULT ZONE

Pf Pf

Fig 12

Fig 13

Surcharged formations by natural causes Shallow formations may also be surcharged by natural causes. This can be the result of a fault in the formations. A fault gives a means of communication between deeper formations with high pressure and shallower formations. The higher pressure escapes into the shallower formation where an abnormal pressure will be the result. See Fig 13.


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PRESSURE BALANCE IN THE WELL BORE

01.02 Pressure balance During drilling of a well the formation pressure must always be counter balanced by an equal amount of pressure exerted from within the well. This is achieved by using a drilling fluid having a sufficient density. Drilling fluid which is a combination of different fluids and chemicals has several important functions in the drilling process but a main function is the ability to give pressure balance in the well. The density of the fluid can be adjusted by adding high density material or by diluting by water. It is in this way that balance and control of the formation pressure can be achieved. 02.02 Overbalance and underbalance Underbalance is the term used when at a particular depth the formation pressure exceeds the hydrostatic pressure exerted by the drilling fluid column. In this situation there is a risk that fluid from the formation will intrude into the wellbore and begin to displace the drilling fluid. On surface the drilling fluid returns rate will increase and later the active drilling fluid pits will show a gain of fluid. If this sequence of events takes place in a well a kick is said to have occurred. The rate of influx is dependent on the degree of underbalance and on the formation's permeability. To drill a well underbalanced is dangerous in most parts of the world and is therefore usually not practised in oil well drilling. However in certain areas where sufficient data are available it is practised anyway mainly because drilling can take place with a high penetration rate. 03.02 Lost circulation Overbalance in the well is present when the drilling fluid hydrostatic pressure exerts a higher pressure than the formation pressure. In this situation formation fluids cannot intrude into the wellbore. The reverse can occur whereby drilling fluid will seep into the formation, and lost circulation may be the result. This is not a desirable situation. If drilling fluid seeps into the formation the formations' permeability becomes reduced. When the overbalance becomes too large the formation will break allowing a large amount of the drilling fluid to flow into the formation. This situation is called lost circulation. When lost circulation has been the result a dangerous situation is created. The drilling fluid level drops and hydrostatic pressure is lost. When hydrostatic pressure is lost the formation pressure higher up becomes underbalanced which can result in a blow out. 04.02 Rate Of Penetration versus overbalance The difference between the hydrostatic pressure exerted by the drilling fluid at the bottom of the wellbore and the formation pressure is called the differential pressure. When the


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hydrostatic pressure exerted by the drilling fluid is higher than the formation pressure the bottom hole pressure is in overbalance. The relationship between differential pressure and Rate of Penetration shows that Rate of Penetration increases when the differential pressure decreases. Penetration is given in feet per minute and differential pressure in psi. Ft/min

15

Rate of Penetration

12

9

6 4 3 psi 1000

2000

P = Differential Pressure

3000

Fig 14

The graph is interesting in several ways. Assume drilling with a differential pressure of 2430 psi in a particular formation with constant drilling parameters E.I. : - Constant Weight on Bit - Constant drilling fluid density - Constant rotary RPM and - Constant pump rate it can be seen that the penetration rate is 4 ft per minute. M:\IWCF Surface\3\1\Section 5.doc

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Without changing any other parameters imagine that the formation pressure increases by 980 psi. This results in a new differential pressure of 1450 psi and a corresponding increased penetration rate to 6 ft per minute. It is realised that when the differential pressure decreases the penetration rate will increase. 05.02 Drilling break An increase in Rate of Penetration (ROP) with constant drilling parameters is called a drilling break. It should be known that a drilling break is an early warning of a kick. If the Driller reacts on the observation by making a flow check the well may still be overbalanced with the pumps stopped. Even that an increase in Rate of Penetration may be caused by other factors than a change in differential pressure, the Driller should always play safe and perform a flow check in order to confirm that the well is in balance. A natural reaction must also be to inform the supervisors of any drilling breaks. 06.02 Necessary overbalance By means of the graph it is seen that to obtain the highest possible penetration rate the degree of overbalance has to be as small as possible. In practice a sufficient overbalance must be used to avoid kicks from taking place. 07.02 Trip margin A situation that can bring the well in underbalance is when the drill string is pulled upwards during a connection and when tripping the string out of the well. The lower part of the drill string acts as a piston that results in reducing the pressure below the string when pulling upwards. When the pressure in the wellbore is reduced the formation fluids can enter the well underneath the bit. To what extent this occurs is dependent on: - How quickly the drill string is pulled upwards - The dimension of the wellbore - Dimensions of the drill string - The rheological characteristics of the drilling fluid - Other factors like degree of balling of the Bottom Hole Assembly etc. To prevent formation fluids from being swabbed into the wellbore caused by any of these reasons in combination a necessary overbalance is used. This small degree of overbalance is called a trip margin. M:\IWCF Surface\3\1\Section 5.doc

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Formation Strength

Fluid Density

Formation Pressure

Surge Pressure

Swab Pressure

Fig 15

Fig. 15 shows the conditions when drilling in normal pressure conditions. The tolerance area (given by the area between the formation strength pressure and the formation pressure) is relatively large. When the drilling fluid density is adjusted to be in the centre of the area, there is only a small risk for swabbing in connection with a trip. There is also allowance for a relatively large surge pressure in excess of the hydrostatic pressure without risk for exceeding the formation strength. Surge pressure in the well is the result of lowering the drill string too quickly. The piston effect results in increasing the pressure below the drill string. Fig 16 and 17 shows different measurements taken with a PWD tool under “normal” tripping conditions. SWAB PRESSURE Pulling Speed (mins/stand

SURGE PRESSURE

Start EMW (G)

End EMW (G)

4

0.965

0.956

140

5

0.964

0.956

7

0.962

8

0.962

Fig 16


Pressure Drop psi

Running Speed (mins/stand

Pump Rate 0 gpm

Pump Rate 180 gpm

Pump Rate 250 gpm

1

295 psi

651 psi

837 psi

124

2

124 psi

434 psi

636 psi

0.958

62

3

93 psi

356 psi

527 psi

0.960

31

4

Fig 17

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08.02 Riser margin When drilling takes place from floating rigs (semi-submersible and drill ship), there can be several hundred feet of distance between the rig and the sea floor. The marine riser connects the rig to the sea floor amongst other to allow returns to be taken to the rig. The drilling fluid that is contained in the marine riser is contributing to balancing the formation pressure in the well. If a marine riser by accident or on purpose is disconnected from the wellhead at the seabed the bottom hole pressure will be reduced. The reason is that the drilling fluid in the marine riser from the well head to the bell nipple is removed and replaced by a shorter column of seawater. An important factor is that the seawater has a lower density than the drilling fluid. To prevent that the reduction in hydrostatic pressure leads to a kick and a blowout a preparation must be made that will ensure that a sufficient overbalance in the well, even with the marine riser disconnected. This overbalance is called a riser margin. It is realised that there are many precautions to take into consideration, when deciding the drilling fluid density to be used in a particular situation.


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09.02 Relationship between hydrostatic pressure, drilling fluid density and true vertical depth Example: Well depth TVD Drilling fluid density

6000 10.5

ft ppg

What is the hydrostatic bottom hole pressure? Answer:

Ph = 10.5 x 0.052 x 6000 =

3276 psi

It is required to increase the hydrostatic bottom hole pressure by 500 psi. Which new drilling fluid density shall be used? Answer:

Ph = 3276 + 500 =

The new drilling fluid density will therefore be: 3776 MW = --------------------- = 6000 x 0.052

3776 psi

12.1 ppg

The increase in drilling fluid density will be: ∆MW = 12.1 - 10.5 =

1.6 ppg

With the new drilling fluid density drill to 9000 ft TVD and calculate the bottom hole pressure at this depth? Answer:

Ph = 12.1 x 0.052 x 9000 =

5663 psi

What is the pressure gradient of this drilling fluid column? Answer:

G drilling fluid = 12.1 x 0.052 =

0.629 psi per foot

This can also be calculated a different way: 5665 Gmud = ------------------- = 9000

0.629 psi per foot

All results comes from utilising the formula: Ph = TVDft x Drilling Fluid Densityppg x 0.052 0.052 is a constant, which represents the pressure gradient in psi per foot for a fluid density equal to 1 ppg. M:\IWCF Surface\3\1\Section 5.doc

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Pressure Gradient G Drilling Fluid = Drilling Fluid Density ppg x 0.052 psi/ft


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10.02 Equivalent drilling fluid Density. Considering a well with a true vertical depth of 6000 ft full of drilling fluid that has a density of 11 ppg. The well is closed-in at the surface with the Blow Out Preventer ( BOP ) and drilling fluid is pumped slowly into the wellbore. Pressure at the top of the well will now increase to 900 psi. See Fig 18 900 psi

Find what the bottom hole pressure in the well will be? It is seen that the pressure now consists of two components. - The hydrostatic pressure from the drilling fluid (which changes with depth) MW 11 ppg

- The static pressure at the surface (which gives a constant extra pressure at all depths in the well).

6000 ft 900 psi

900 psi

Fig 18

Hydrostatic pressure Closed-in pressure Bottom hole pressure

11 x 0.052 x 6000

= = =

3430 psi 900 psi 4330 psi

Which drilling fluid density must be used if the above bottom hole pressure shall be maintained by using only hydrostatic pressure? MW =

4330 = 13.9 [ppg] 6000 x 0.052

The calculated drilling fluid density is called the equivalent drilling fluid density.


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This means that the original 11.0 ppg drilling fluid must be replaced by a drilling fluid which has a density of 13.9 ppg if the same bottom hole pressure shall be present without extra pressure being applied at the top of the well. Pressure in all depths in the well will be different in the two examples. This can be confirmed by simple calculation. What is the pressure at 3000 ft in the two examples? Example with closed-in pressure on surface: 1. P h = 11 x 0.052x 3000 = 1716 psi Applied Static Pressure = 900 psi Total Pressure = 2616 psi

Example without closed-in pressure on surface: 2. P h = 13.9 x 0.052 x 3000 = 2168 psi

It must be realised that pressures throughout the well will be lower, if a particular bottom hole pressure is achieved only by drilling fluid density, rather than using a lower drilling fluid density combined with a static pressure applied at the surface.


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Dynamic pressure regime when circulating

01.03 Circulation of Drilling Fluid Whilst drilling the drilling fluid is continuously circulated to clean out the rock fragments (cuttings) from underneath the bit whilst removing them up to the surface where they are separated from the drilling fluid by the mud cleaning equipment. To establish the circulation in the system it is required to have a dynamic fluid differential pressure between certain areas in the system. This pressure difference represents a certain energy that is used to overcome the resistance against fluid movement, resistance that is existing in the system. This resistance against fluid flow or friction as it is generally called in a hydraulic system is largely dependent upon: - The fluids' rheology (viscosity, density etc.) - The fluids' velocity - Type of flow regime ( laminar or turbulent) If a fluid is pumped through an enclosed pipe system with a constant velocity the resistance in the system depends on the flow area. Where the fluid flow meets diameter reductions, a local increase in velocity is the result and therefore a greater friction. Conversely where the flow meets a larger diameter the velocity will decrease and the friction will consequently also decrease. Recorded Pressure (psi) 1400 1320

80

40

1280

1220

60

1170

50

370

800

0

800

Pressure loss (psi) Fig 19

Fig. 19. shows a circulating fluid system where the initial pressure at the pump is 1400 psi and the final pressure is 0 psi at the flow line. It is seen that the 1400 psi represents the energy required to overcome the friction that is present against the flow of the fluid in the system. Large obstructions to flow give large pressure losses. By means of pressure gauges placed in the system the pressure losses in the different parts of the system can be monitored. M:\IWCF Surface\3\1\Section 5.doc

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Applying these considerations to the circulation of drilling fluid the Fig. 20. shows a pipe system in which the drilling fluid pump ( mud pump ) shall pump drilling fluid through. This simplified pipe system consists of drill pipe, drill collars, bit nozzles and annulus. The drilling fluid enters the top of the drill string with a pressure of 2200 psi. On the way down through the string some of this pressure is lost depending on - The dimensions of the drill pipe (Internal diameter) - The characteristics of the drilling fluid.

DRILL COLLARS ANNULUS

DRILL PIPE

NATIONAL

PSI

BIT P1 P3 P2

P4

P5

Fig 20

P1 P2 P3 P4 P5

= = = = =

Pressure as drilling fluids enters the drill pipe Pressure as drilling fluid enters the drill collars Pressure as drilling fluid enters the bit nozzles Pressure as drilling fluids enters annulus Pressure as drilling fluid enters the flow line

(2200 psi) (1900 psi) (1700 psi) (130 psi) ( 0 psi)

The largest pressure loss in the well system takes place when fluid flows through the bit nozzles that have a relatively small flow-through area. On the way towards the surface through the annulus, the pressure loss will be the lowest in the system, because the friction is not at all large on account of the large cross-sectional area of the annulus. The pressure figures used in Fig. 20. are based on average calculations for a simple rotary assembly, and they show that 94% of the total pressure loss occurs in the drill string and bit nozzles.


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The figures show that to circulate the drilling fluid from the bottom of the well up to the surface it is only necessary to use approximately 6% of the total pump pressure. This dynamic pressure will be transmitted to the bottom hole pressure. When the pump is running and circulation takes place there will be a higher bottom hole pressure than when the pump is stopped. With the pumps stopped only hydrostatic pressure is present in the well to balance the formation pressure. 02.03 Dynamic pressure in the wellbore ( Circulating Pressure) Dynamic Pressure ( PC ) is dependent on three factors: - Components in the flow system (Flow area, length of drill string, nozzles size etc) - The fluid characteristics ( Rheology ) - The flow rate (SPM, liner size, pump efficiency etc) Change in drilling fluid characteristics ( such as viscosity and gel-strength ) can change the friction against flow in a system. A fluid's flow resistance is largely depending on the drilling fluid density. In well control calculations it is accepted that dynamic pressure loss is proportionally depending on drilling fluid density. PC 2 = PC 1 x

PC1 PC2

= =

MW 2 [psi] MW 1

Circulation pressure when drilling fluid density is MW1 Circulation pressure when drilling fluid density is MW2

The expression for the relationship between circulation pressure and drilling fluid density has proved to be realistic in most practical cases. See fig. 21. PSI High fluid dens ity Low fluid dens ity

PC2 PC1 Fig 21


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Example: At 100 SPM the pump pressure is 1000 psi with a drilling fluid density of 10 ppg. What would the pump pressure be at 100 SPM if the drilling fluid density was increased to 12 ppg?? New pump pressure: P C 2 = 1000 x

12 = 1200 [psi] 10

To calculate the new pump pressure it is required to know the original pump pressure, which is read just after the pump ( standpipe pressure ). The third factor that affects the circulation pressure is the speed of the flow of drilling fluid. This velocity of flow is directly related on the pump speed ( SPM = strokes per minute). The relationship between pump speed and dynamic pressure can be expressed as: æ SPM 2 ö PC 2 = PC 1 x ç ÷ è SPM 1 ø

1.86

Where SPM is the number of strokes per minute in the two cases. Example: Circulation pressure is 1200 psi with 40 SPM. What will the circulation pressure be if the pump speed was increased to 80 SPM? Answer: æ 80 ö P C 2 = 1200 x ç ÷ è 40 ø

1.86

= 4356 psi

It is realised that if the pump speed is increased to twice its original value the dynamic pressure will be increased almost fourfold. The graph in Fig. 22 illustrates this fact. The power 1.86 is an experience figure, which is obtained from experiments. However in well control calculations it is generally accepted to use the power 2 in stead of 1.86.


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For well control calculations use the formula below: æ SPM 2 ö PC 2 = PC 1 x ç ÷ è SPM 1 ø

2

Pc

4000 3000 2000 1000

SPM 10

20

30

40

50

60

70

80

90

Fig 22

Fig. 23. shows circulation pressures and pressure losses between the drill string and annulus with three different pump rates.

Pc

4000

80 spm

3000 60 spm 2000 1000

40 spm

Fig 23


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CONSIDERATIONS WITH A CLOSED-IN WELL

01.04 Closed in well Fig. 24 illustrates a wellbore with pressure gauges. The drill string consists of pipe connected to each other, right down to the bottom of the well. Through the bit nozzles, the string is in communication with the annulus. In principle this can represent two pipes, one inside the other but there is only communication at the bottom of the well. On top of the well the BOP equipment is installed. This equipment makes it possible to contain and close off the annulus and its contents. Under the BOP a pressure gauge is installed which measures the surface annulus/casing pressure. On the top of the drill pipe after the pumps another gauge which measures drill pipe pressure is installed. The two gauges are necessary to get an indication of down hole conditions. PDP NATIONAL

PDP

PA

PA BOP

DRILLSTRING

ANNULUS

DRILL STRING

ANNULAR

A DRILLCOLLAR

Fig 24

Fig 25

02.04 U-tube A simplified and equal system can be represented by two tubes standing upright side-by-side and connected at the bottom. The example is called a U-tube. See. Fig. 25. The pressure in the same horizontal levels in the connected system is always the same if fluid density is the same, when no circulation is taken place and no pressures are closed in on the top on any of the two legs. It is seen that the hydrostatic pressure at the bottom of such a U-tube system, irrespective of which leg of the U-tube column is considered will be equal. This is easily confirmed by a simple calculation: M:\IWCF Surface\3\1\Section 5.doc

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Example: True vertical depth = Drilling fluid Density =

10000 10

ft ppg

Both drill pipe and annulus are open at the surface and the U-tube is in balance. Bottom hole pressure Ph at the point A can be found either by the drill pipe or by the annulus when drilling fluid density is uniform : P h = Drilling Fluid Density ppg x 0.052 x True Vertical Depth ft [psi] P h = 10 x 0.052 x 10000 = 5200 [psi]

If the BOP is closed on the annulus and the drilling fluid in the annulus is replaced with saltwater ( 8.34 ppg ) the following can be calculated: The internal contents of the string ( drill pipe and drill collar ) has not changed so PH at A is still 5200, but the hydrostatic pressure in the annulus is only (Fig 26): P ha = 8.34 x 0.052 x 10000 = 4337 psi P a = 5200 - 4340 = 860 psi PA

PA

9 ppg

10000 ft

DRILL STRING

ANNULUS

10000 ft

8.34 ppg

DRILL STRING

10 ppg

PDP

ANNULUS

PDP

8.34 ppg

A


A

Fig 27 © MTC

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Example: Considering the same well with the same bottom hole pressure, but now with 9 ppg drilling fluid in the drill string and upper 7000 ft of annulus, while there remains saltwater in the lower part of the annulus (Fig 27). When PSIDP = Pressure ( Shut in drill pipe ) When PSIA = Pressure ( Shut in annulus ) P SIDP = 5200 - ( 9 x 0.052 x 10000 ) = 520 [psi] P SIA = 5200 - ( 7000 x 9 x 0.052 + 3000 x 8.34 x 0.052 ) = 623 [psi]

The example represents a typical kick situation, where insufficient drilling fluid density has resulted in a saltwater influx into the annulus. The influx has replaced a quantity of drilling fluid. Notice that the drill pipe bottom hole pressure consists of two parts, first the PSIDP value and secondly the hydrostatic pressure of the drilling fluid. The annulus bottom hole pressure consists of three parts. The PSIA value, the hydrostatic pressure of drilling fluid and the hydrostatic pressure of the saltwater.


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PROPERTIES OF GASSES AND GAS LAWS

01.05 Drilling with Underbalance. If drilling takes placed being underbalanced the risk of taking a kick is always present. The influx resulting from a kick can be water, oil or gas. When dealing with gas the drill crew must be aware that gas behaves differently than fluid. 02.05 Properties of gas and Gas Laws A given mass of gas can be compressed or expanded, and as the volume changes the pressure will do the same. Boyles Law states that: P1 x V1 = P2 x V2 or Pressure x Volume = Constant → See Fig 28 Fig 28 PRESSURE 15000 14000 13000 12000 11000 10000 9000 8000 7000 6000 5000 4000 3000 2000 1000

5 M:\IWCF Surface\3\1\Section 5.doc

10

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25

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45

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65

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VOLUME © MTC

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This means that when a given volume V1 with an absolute pressure P1 is changed in pressure or volume we get a new pressure P2 with a new volume V2. Example: V1 = 5 gal V2 = 3 gal Calculate

P1 = 170 psi P2 P1 x V 1 = 170 x 5 = 283 [psi] P1 x V 1 = P 2 x V 2 _ P 2 = 3 V2

It is important to know that gas expands if pressure is reduced. Boyle’s Law is only true when the temperature is constant. If the temperature changes the formula given below is used where → T = temperature P1 x V 1 P x V2 = 2 T1 T2

It must be noted that the temperature to use is an absolute temperature which is given in Kelvin degrees, (ºK ) for the Centigrade system. If the Fahrenheit system is used the absolute temperature must be given in Rankin (ºR ) degrees. ºK is obtained by addition of 273º to the temperature given in Centigrade ºC. T ° K = t °C + 273

ºR is obtained by addition of 460º to the temperature given in Fahrenheit ºF. T ° R = t ° F + 460

Example: V1 = 12 gal V2 = 12 gal

P1 = 90 psi T2 = 80ºC P2 =

T1 = 20ºC

P 1 x V 1 x T 2 90 x 12 x (273 + 80) = = 108 [psi] (273 + 20) x 12 T1 xV 2

Since V1 = V2 pressure increases only through temperature increase.


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Example: The formula, which relates to the properties of gasses, is here used in a practical example. The well has a depth of 10000 ft and there is a swabbed gas bubble on bottom. The drilling fluid density is 12.5. The well is open and in balance. Consequently no closed-in pressure at the surface. The pressure in the gas is therefore equal to the hydrostatic pressure at 10000 ft → Ph. Hydrostatic pressure Ph is 6500 psi. If the BOP is closed and the gas is allowed to rise upwards ( migrate ), the gas volume will not change and in accordance with the gas law the pressure will also remain unchanged. Assuming the temperature is constant the gas would retain its original volume and pressure all the way to the surface. 0 psi

6500 psi

3250 psi

PA

PA

PA

6500 psi 12.5 ppg 12.5 ppg 5000 ft 12.5 ppg 10000 ft

6500 psi

5000 ft

10000 ft

12.5 ppg

6500 psi 6500 psi

9750 psi

13000 psi

Fig 29

Considering that the gas has migrated halfway up the wellbore it will still have a pressure of 6500 psi. The pressure at surface ( annulus ) at this stage would be: (See Fig 29.) P SIA = 6500 - 12.5 x 0.052 x 5000 = 3250 [psi]

Bottom hole pressure: P bottom = 6500 + 12.5 x 0.052 x 5000 = 9750 [psi] M:\IWCF Surface\3\1\Section 5.doc

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When the gas is allowed to rise all the way to the surface without expanding, the pressure at the surface would be 6500 psi. Bottom hole pressure would be: P bottom = 6500 + 12.5 x 0.052 x 10000 = 13000 [psi]

This extreme pressure throughout the wellbore cannot be controlled, and it is not reasonable to assume that the situation would develop all the way as described. The weakest point in the wellbore is normally believed to be at the casing shoe level. When the pressure increases above the strength at the weakest point the formation at that point will fracture. The risk for an underground blow out is high. A gas kick can never be allowed to migrate up through the annulus without expanding. A skilled drill crew must take proper and timely action to avoid the dangerous situation that is likely to occur. In the given example the temperature influence neither the changed height due to annulus geometry was taken into account since these factors only have a small influence in practice. 03.05 Expansion of Gas

Although some kicks are predominantly salt water or oil, at least some gas is usually present. Because salt water and oil do not expand as pressure decreases, they are not as troublesome as gas. It is important for the persons who control blowouts to understand the behaviour of gas in a well. The gas volume change as a result of pressure change is predictable, and this allows calculation under illustrative conditions of changes in bottom well pressure as gas rises through the drilling fluid. When the pressure of a given mass of gas is doubled, the volume is halved. When the pressure is halved the volume is doubled. This relationship between pressure and volume results in the greatest expansion of the gas in the upper part of the well. See Fig 28. Although gas-cut drilling fluid is one of the early indicators of abnormal pressure, minor gascutting results in only a small reduction in the hydrostatic head. In a gas-cut column of drilling fluid, the pressure increases rapidly with depth, so that the volume of gas scattered through the well bore is very small, and reduces the overall drilling fluid density in the well very little. A slug of gas in the bottom of a well is potentially dangerous, because it will expand greatly when it rises or is pumped up. Under low pressure near the surface, it will displace a large amount of drilling fluid from the well and consequently greatly reduce bottom hole pressure giving risk for a blowout. The safe handling of a gas kick requires knowledge about the principle of gas expansion and consequently lowering the pressure in the gas bubble as it is circulated up through the annulus in order to maintain the correct and constant bottom hole pressure. The theoretical knowledge requires practice as well as knowledge about well control equipment. When the gas in a well control situation is circulated to the surface and expanding, more drilling fluid must be allowed to flow out of the annulus than is pumped into the drill pipe. Thus, the pit level will increase. M:\IWCF Surface\3\1\Section 5.doc

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The expected drilling fluid volume increase should be known prior to circulating out the kick. This detail is discussed in the kick control section. To control a correct and constant bottom hole pressure, the surface pressures are used as a parameter for control. This is done by means of the choke in connection with a stroke counter for the mud pumps, and simple recognised procedures. 04.05 Formation strength

From the previous examples it is realised that pressure throughout the wellbore increases when gas rises up the annulus in a closed-in well. Gas must be circulated out of a well under control. One of the most important limitations that should be known is the maximum pressure the formation ( or weak point ) can withstand before it fractures and allows the drilling fluid to flow into the formation. If the formation strength is exceeded in a kick situation there is a high risk for an underground blow-out and perhaps complete loss of control of the well. The formation strength is recorded by means of a leak-off test. 05.05 Leak-off test

A leak-off test can be carried out in various ways. The aim is to find the surface annulus pressure value for when the drilling fluid begins to seep into the formation, without at the same time to cause an actual fracture of the formation. The less drilling fluid volume that is pumped into the formation, the less damage there is caused to the formation. After the test the formation should easily heal again as a result of the drilling fluid's wall building effect. A leak-off test is carried out just after casing has been set and cemented. A leak-off test may be conducted as follows: Between 10 and 30 feet is drilled below the casing shoe to expose virgin hole. The well is circulated to obtain a representative and accurately known drilling fluid density in the well. The well is closed-in and drilling fluid is pumped into the well at a very slow rate. The cement pump is generally used since they have a smaller displacement and thus are easier to control and are fitted with very accurate low pressure gauges. Accurately measured volumes are pumped into the well, one barrel in this example, until an increase in casing pressure is registered. At this point pumping is stopped for about one minute, until the surface annulus pressure has stabilised. When no pressure decrease is observed the pressure is plotted on a graph paper. The pumping is resumed and the same volume is again pumped. The pumps are stopped and the new pressure is plotted after it has stabilised. This procedure is repeated until it is observed that the pressure increase per volume portion is no longer proportional. This is easy seen on the plotted graph at the point where the straight line begins to bend. The pressure on the graph where this happens is the annulus surface leak-off pressure. See Fig. 30.


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PRESSURE

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PRESSURE ANNULUS

DEPTH

1100 1000 900 800 700

* *

600 * *

400

*

300 100

S H O E

*

*

500

200

* * *

* V O L * U M 1 2 3 4 5 6 7 8 9 101112131415 E *

*

Ph

Max Pshoe

P R E S S U R E

Fig 30

The leak-off test should be interrupted at this point. If the pumping is continued the pressure will decrease as a result of an increasing amount of drilling fluid which is injected into the formation. Furthermore the formation strength will be reduced. It has been proven that a leak-off test performed too far has damaged the formation. In that case a second leak-off test will indicate a lower formation strength. Fig. 30 shows the results from a leak-off test carried out after casing has been cemented at 3000 ft. Drilling fluid density was 9.6 ppg. The leak-off pressure is seen to be 720 psi. The combined pressure the casing shoe is exposed to is the hydrostatic pressure of the drilling fluid and the surface leak off pressure and this combined pressure becomes the maximum allowable shoe pressure at any given time. Calculate the maximum allowable pressure at the casing shoe: Answer: P shoe = 9.6 x 0.052 x 3000 + 720 = 2218 [psi]

When we know the maximum allowable shoe pressure, we are able to calculate the equivalent drilling fluid density or maximum allowable drilling fluid density


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Well Control Training Manual Equivalent drilling fluid density =

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2218 = 14.22 [ppg] 3000 x 0.052

06.05 Maximum Allowable Annular Surface Pressure ( MAASP ) MAASP means the highest surface pressure that can be allowed at the top of the casing in excess of hydrostatic pressure that is likely to causes losses at the shoe formation if exceeded. There are three factors that decide the Initial MAASP.

- The maximum pressure that the surface equipment can handle - The maximum pressure the casing can handle - The maximum pressure that the formation at the casing shoe ( or weak point ) can handle. In most cases it is the formation strength at the casing shoe that is the deciding factor, and Initial MAASP is then given from the leak-off test which has previously been described. As the maximum allowable shoe pressure remains constant the hydrostatic pressure inside the casing is the determine factor for the MAASP at any given time-See Fig 31 MAASP = Maximum Allowable Shoe Pressure – Pressure Hydrostatic Inside Casing

Ph

MAASP DOWN

DOWN

= MAASP UP

Ph

=

UP

Fig 31

As illustrated in Fig 31 the MAASP will increase if pressure hydrostatic inside the casing decrease for whatever reason and visa versa.


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One important issue when circulating out a kick is to monitor the Initial MAASP value. If the Initial MAASP is approached before the kick is circulated into the casing the responsible rig management must take safe action. It may be impossible to avoid exceeding the Initial MAASP, but the competent and responsible management may decide to evacuate the rig for non-essential personnel until the situation has proven to be safe. Once the influx is inside the casing the initial value can be exceeded. We will look on how MAASP behave during circulating out a kick later in chapter 08.


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MAERSK TRAINING CENTRE A/S DRILLING SECTION

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44

DRILLING FLUID VOLUME AND CAPACITIES

In routine operations as well as during well control operations it is necessary to know the total drilling fluid volume and the volume for the individual sections in the circulating system. How much volume does the drill string contain and what is the volume in the different parts of the annulus? These questions can easily be answered if the dimensions of the different components in the drill string and annulus are known. There are two ways to find the different capacities and volumes: - By calculating the volumes - By reading tables 01.06 Calculating drilling fluid Volume - Capacities

The internal capacity of drill pipe and drill collars is calculated based on formulas for cylinders. For a cylinder with a diameter d (inches) and a height of 1 foot the volume will be:

D V=Axh p x d2x h [ ft 3 /ft] V= 4 x 144

1 ft

1 ft2 = 144 in2 1 ft3 = 0.1781 bbl Then: V =

2 p x d 2 x 0.1781 d = bbl / ft 4 x 144 1029.4

Fig 32

Calculations of annular capacities are basically calculations of a hollow cylinder, or the difference between two cylinders, - one inside the other. For a hollow cylinder with an outside diameter OD in and inside diameter ID in and a height of 1 ft the following formula can be derived M:\IWCF Surface\3\1\Section 5.doc

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2 2 2 2 OD ID OD - ID = [ bbl / ft ] 1029.4 1029.4 1029.4

The total inside or outside capacity for a certain length of pipe can be worked out by multiplying the capacity in bbl/ft by the length in ft. The result is the capacity in bbl.

1 ft

Example:

OD

Wellbore inside diameter = casing id Vertical depth

= 9-7/8 in = 5.000 ft

Drill pipe Drill collars

= 4.600 ft = 400 ft

5"OD & 4-1/4"ID 7"OD & 2-13/16”ID

Fig 33

Internal capacities drill pipe: 2

(4 1 / 4) V drill pipe = = 0,01754 bbl / ft 1029,4

Total Volume of drill pipe = 0.01754 x 4600 = 80.68 bbl Internal capacities drill collars: 2

(2 13 / 16) V drill collar = = 0,00768 bbl / ft 1029,4

Total Volume drill collars = 0.00768 x 400 = 3.07 bbl Annulus Capacities: V drill pipe =

(9 7 / 8 )2 - 52 = 0,0704 bbl / ft 1029,4

Total Volume between casing and drill pipe = 0.0704 x 4600 = 323.84 bbl V drill collar =

(9 7 / 8 )2 - 7 2 = 0,04713 bbl / ft 1029,4

Total Volume between casing and drill collars = 0.04713 x 400 = 18.85 bbl


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02.06 Drilling fluid Volume and Capacities from Tables

It is common practice to use tables that give capacities in bbl/ft or litre/meter for different sizes of pipe and casing. These tables are made taking into consideration the physical outline of the pipes ( tool-joint etc. ). Tables of this kind can be found in different Data Handbooks or in the Drilling Data Handbook ( DDH ) sixth edition 1991. Section D “Capacities and Annular Volumes” and section G “Pumping and Pressure Losses”. All the tables in DDH is in SI-units, but at the bottom of each table a conversion factor is found in order to convert to oil-field units. Fig 34

Fig. 34 shows an example of a well and drill string. Internal capacity of drill pipe: ( table D7 )

10000 ft of 5" Drill-pipe , 19.5 lbs/ft, Grade G-105. Reading in table: 9.05 l/m,(9.05 x 0.00192 = bbl/ft)

Drill Pipe 5” - 19.5 lbs/ft 10000 ft

9.05 x 0.00192bbl/ft x 10000ft = 173.76 bbl Internal capacity of drill collars: ( table D8 )

500 ft of 7"OD x 2 13/16"ID Drill collar Reading in table: 4.01 l/m,(4.01 x 0.00192 = bbl/ft) ( 4.01 x 0.00192 )bbl / ft x 500 ft = 3.85 bbl Total internal capacity of drill string: 173.76 + 3.85 = 177.61 bbl Volume between drill pipe and casing: ( table D14 )

7500 ft of Casing 9-5/8”, 47 lbs/ft Reading in table: 24.9 l/m, (24.9 x 0.00192 = bbl/ft)

Casing 9-7/8” - 47 lbs/ft 7500 ft

Open Hole - 8-5/8” 3000 ft Drill Collar 7” x 2-13/16” 500 ft

(24.9 x 0.00192)bbl / ft x 7500 ft = 358.56 bbl Volume between drill pipe and Open-Hole: ( table D12 )

2500 ft of 8-5/8” Open-Hole Reading in table: 24.4 l/m, (24.4 x 0.00192 = bbl/ft) (24.4 x 0.00192)bbb / ft x 2500 ft = 117.12 bbl Volume between drill collars and Open-Hole: (table D11 )

500 ft of 8-5/8” Open-Hole M:\IWCF Surface\3\1\Section 5.doc

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Reading in table: 12.9 l/m, (12.9 x 0.00192 = bbl/ft) (12.9 x 0.00192)bbl / ft x 500 ft = 12.38 bbl Total capacity of annulus:

558.56 + 117.12 + 12.38

= 688.06 bbl

By making the above calculations the exact quantities of drilling fluid contained in the different parts of the well is known. 03.06 Surface-to-Bit Strokes & Bit-to-Surface Strokes

The exact number of strokes required to pump from the surface through the drill string to the bit, is known as surface-to-bit strokes. The number of pump strokes required to pump from the bottom of the well to the surface, is known as bit-to-surface strokes. These values can be calculated when the pump displacement per stroke is known. Pump displacement can be found in the DDH Section G table G6. Given: National pump 12-P-160. w/ 6" liners. The number 12 represents the stroke length in inches. Volumetric efficiency 97 %. From the table is read 16.68 l/stroke with volumetric efficiency of 100 %. At the bottom of the table a conversion factor is found to convert into bbl. 16.68 x 0.264 = 0.1048 bbl / stroke 42

With 97 % efficiency the pump output would be: 0.1048 x 97 = 0.1017 bbl / stroke 100

By using the capacity figures in fig. 34 we can now calculate surface-to-bit strokes as follows: Total inside volume of drillstring = Strokes Mud pump output per stroke

Surface-to-bit strokes Respectively bit-to-surface strokes is now calculated. 177.61 Surface ® bit strokes = = 1746 strokes Bit-to-surface strokes 0.1017


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Total annulus capacity = Strokes Mud pump output per stroke 688.06 Bit ® surface strokes: = 6765 strokes 0.1017

The circulating time required is controlled by the speed of the drilling fluid pump. In case of a pump speed of 30 strokes per minute ( SPM ) the result would be: Surface ® bit time = Bit ® surface time =

1746 = 58.2 minutes 30

6765 = 225.5 minutes 30

Another volume that is often necessary to know is the bit-to-shoe time and the corresponding pump strokes. Considering fig. 34 it can be seen that: Bit ® shoe strokes =

117.12 + 12.38 = 1273 strokes 0.1017

Therefore: Bit ® shoe time (at 30 SPM) =


1273 = 42.4 minutes 30

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04.06 Use of Barite to Increase the Drilling fluid Density

The theoretic and actual quantity of barite used to effect a drilling fluid density increase can be calculated beforehand by using the initial drilling fluid density MW ( ppg ) and the drilling fluid density required MWf ( PPG ) (final drilling fluid density). The units will be number of 100 lb sacks per 100 bbl of drilling fluid. Example: An active drilling fluid system contains 900 bbl of drilling fluid with a weight of 10.5 ppg. We want to increase this density to 13.5 ppg by adding barite. How many sacks will be used?

MWf - MWi 0

100 lb Sacks Theoretical

1

MWf 9 10

100

2 3 4 5 6 7 8 9 10

100 lb Sacks Actual 0

11 200 300 400

12

200

13

300

14

400

15

500

16

600 700 800 900

100

17 18 19

500 600 700 800 900 1000 1200 1400

Fig 35

l.

2.

In the homograph fig 35 a straight line is drawn on the scale from 13.5 ppg MWf (final drilling fluid density) to 3.0 ppg on the scale to get MWf - MWi(final drilling fluid density minus initial drilling fluid density). The scale reads 204 sacks per 100 bbl on the scale for theoretical use of 100 lb sacks. By taking the point on the scale for theoretical use where the first line crosses (i.e. 204 sacks) and by drawing a horizontal line across to the scale for actual use from this


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point, the actual true value is seen to be 224 sacks (100 lb sacks) per 100 bbl of drilling fluid to effect the desired increase. 3.

Therefore 224 x

900 = 2016 sacks 100

2016 sacks in the 900 bbl system is required to increase the drilling fluid weight to the desired level. The theoretical quantity of sacks per 100 bbl of drilling fluid can be calculated by the following expression: S = 1490 x

where S MWf MWi

MW f - MW i 35,5 - MW f

= theoretical number of 100 lb sacks of barite = final drilling fluid weight (ppg) = initial drilling fluid weight ((ppg)

35.5 = the calculated density of barite is considered to be 35.5 ppg. It is always necessary to use more barite than the theoretical quantity because of hydration, variations in barite density and volume increases because of addition of other material. 05.06 Volume increase due to Barite Addition

The volume increase (in barrels per 100 bbl of drilling fluid in the system) can also be calculated by initial drilling fluid weight MWi (ppg.) and final drilling fluid weight MWf (ppg.). Example: An active drilling fluid system contains 900 bbl of drilling fluid with a weight of 10.5 ppg. We want to raise this weight up to 13.5 ppg. by adding barite.

How much volume increase will we have in the system? 1.

In the homograph fig. 36 we draw a straight line between 13.5 ppg. on the scale for MWf and 3.0 ppg. on the scale for MWf - MWi, and we notice where this line crosses the Pivot Line.

2.

Where our first line crosses the Pivot Line we draw a horizontal line across to the scale for volume increase and we can read-off that there will occur a 22.5 bbl per 100 bbl increase in drilling fluid volume.


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MWf - MWi 0

MWf 9

1

10

2

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Volume Increase 0

10

12

20

13

30

5

14

40

6

15

3

2 1

4

E LIN OT PIV

7 8

16 17

9

18

10

19

50 60 70 80 90 100 120 140

Fig 36

3. 22.5 x

900 = 202,5 bbls increase Î volume 100

Therefore total volume = 900 + 202.5 = 1103 bbl after completion of weight increase. The volume increase can be calculated with the help of the following expression: V = 155 x C x

MW f - MW i 35.5 - MW f

where V

= the volume increase (bbl/100 bbl)

C

= factor for extra barite, based on MW


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MWf

= final drilling fluid weight (ppg.)

MWi

= initial drilling fluid weight (ppg.)

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This expression assumes that 1.5 gallons of water are used per sack of barite to replace water lost on account of hydration, and a dry barite volume factor of 14.9 sacks per bbl.


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53

WELLBORE KICKS

01.07 Kick Occurrences

A kick can occur when the formation pressure becomes higher than the hydrostatic pressure that the drilling fluid column is exerting in the wellbore. The influx of fluid or gas into the wellbore further reduces the hydrostatic pressure, which results in increased flow at the surface and therefore further influx from the formation. The influx into the well bore shall therefore be stopped as rapidly as possible by closing-in the well. There are two normal reasons why the formation pressure can exceed the hydrostatic pressure in the wellbore: 1.

Pore pressure or formation pressure increase more rapidly than drilling fluid weight

2.

The drilling fluid weight is sufficient to balance the formation when the well is full of drilling fluid, but when the height of the drilling fluid column is reduced for some reason hydrostatic pressure is reduced.

A kick (influx) can be caused by any of the following: 1.

Insufficient drilling fluid weight

2.

Failure to keep well full of drilling fluid

3.

Swabbing

4.

Lost circulation

5.

Drilling fluid cut by gas or water

6.

Abnormal pressure zones

1.

Insufficient Drilling Fluid Weight

This should seldom be the cause of a kick in development wells where formation pressures are known. At a wildcat well where the pressures of the formation are partly unknown the danger for insufficient drilling fluid weight is much greater. In Normal Pressured Formations the pressure gradient is taken as 0.465 psi/ft. of depth. This is the figure for salt water having a salinity of about 100,000 parts per million (ppm). The drilling fluid density required to balance this pressure would be approx. 9 ppg. It must be remembered that the overbalance increases with depth if the formation pressure gradient remains constant. See Fig 37.


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Mud Column Pressure 10 ppg

Formation Pressure 0.465psi/ft

Over Balance Pressure

5.000 ft

2600 psi

2325 psi

275 psi

10.000 ft

5200 psi

4650 psi

550 psi

15.000 ft

7800 psi

6975 psi

825 psi

Overbalance pressure increases with depth

Pf

0

CHAPTER

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DEPTH

Hole Depth

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Ph

PRESSURE

O/B

Fig 37

2.

Failure to Keep Well Full of Drilling fluid

This particular cause of well kicks is one, which should never happen today, but it still does. When a stand of drill pipe is pulled from the well, the volume of the metal pulled must be replaced with drilling fluid. If this is not done, the level of the drilling fluid in the well will drop. Since the bottom hole hydrostatic pressure is the product of the drilling fluid density multiplied by the height of the column, the bottom hole hydrostatic pressure will reduce if the height of the column is reduced. If this reduction in height is appreciable, the bottom well hydrostatic pressure may be reduced to such an extent that the safety margin may be taken away and the well may kick. Different measurements can indicate if the proper amount of drilling fluid is pumped into the well. One possibility is a pit volume monitoring, but large pits will not show small changes; these can best be seen in trip tanks. This is one or more high tanks with a little cross-section, where a little change in volume is easy to see. It should be near the rig floor and calibrated so the Driller can easily see and compare the volumes pumped into the well versus steel pulled out. Another possibility is that the Driller by help of a stroke counter can check the amount of drilling fluid pumped into the well from the pits. Fig 38

384 ft

500 ft

DEPTH

Example: If, while pulling out of a well at 8000' carrying 200 psi hydrostatic overbalance with 10 ppg. drilling fluid, the drilling fluid level was allowed to drop to 384 ft. below the surface, the well would be just on balance.

Pf

Ph

If the level was allowed to drop to 500 ft. a kick would develop. See Fig 38 384 x 10 X ,052 = 200 psi = balance 500 x 10 x ,052 = 260 psi = 60 psi underbalance M:\IWCF Surface\3\1\Section 5.doc

PRESSURE

U/B © MTC

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55

Swabbing

Several things can cause swabbing: a.

Balled bit

b.

Pulling pipe too fast

c.

Poor drilling fluid properties

d.

Heaving or swelling formations

a.

Balled Bit (or Stabilisers, Reamers, or Drill Collars) - When pipe is pulled it acts somewhat like a piston or swab, more so if a bit or other bottom assembly member is balled up. This pulls all or most of the fluid up the well, directly reducing the hydrostatic head on the formation. If the well is almost at balance, only a few feet swabbed can result in a kick, or potential blowout.

b.

Pulling Pipe too fast - This piston action is enhanced when pipe is pulled too fast. The rig supervisor should be sure that the pipe is pulled slowly of bottom for a reasonable distance. However, the well should be watched closely at all times to be sure it is taken the correct amount of fluid. Fig 39 show recommended pulling speed w/16.9 ppg mud in the wellbore.

c.

Drilling fluid Properties - Swabbing problems are compounded by poor drilling fluid proper-ties, such as high viscosity and gels. Drilling fluid in this condition tends to cling to the drill pipe as it moves up or down the well, causing swabbing coming out and lost circulation going in. Fig 39 300 280 260 240 220

Sec. Pr Stand

200 180 160 140 120 100 80 60 40 20 0

1000


2000

3000

4000 BIT DEPTH

5000

6000

7000

8000

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Heaving or Swelling Formations - Swabbing can result if the formations exposed either heave or swell, effectively reducing the diameter of the well and clearance around the bit or stabilisers. In these regions even a clean bit acts like a balled bit or stabiliser.

Normal good practices to prevent or minimise swabbing are aimed at keeping the drilling fluid in good condition, pulling the pipe at a reasonable speed, and using some type of effective lubricant drilling fluid additive to reduce balling. Additives such as blown asphalt, gelsonite, detergent, and EP additives are effective in many cases. Good hydraulics will often help clean a balled-up bit or bottom well assembly. If the well does swab, in spite of best practices, the pipe should be run back to bottom immediately, the drilling fluid circulated out, and its weight increased before making the trip. Sometimes a short trip is made to see if the well actually swabs when several stands of pipe are pulled. 4.

Loss of Circulation

Following can cause loss of circulation: a. b. c. d.

High drilling fluid weight Going into well too fast Underground blowouts Pressure due to annular circulating friction

a.

High Drilling fluid Weight - If the hydrostatic head of the drilling fluid exceeds the fracture gradient of the weakest exposed formation, circulation is lost and the fluid level in the well drops. This reduces the effective hydrostatic head acting against the formations that did not break down. If the drilling fluid level falls far enough to reduce the bottom hole pressure below the formation pressure, the well will begin flowing. Thus, it is important to avoid losing circulation. Should returns cease, loss of hydrostatic head can be minimised by immediately pumping measured volumes of water into the well. Measuring the volumes will enable the drilling supervisor to calculate what density of drilling fluid the formation will support without fracturing.

b.

Going into well too fast - Loss of circulation can also result from too rapid lowering of the drill string. This is similar to swabbing, only in reverse; the piston action forces the drilling fluid into the weakest formation. This problem is compounded if the string has a float in it and the pipe is large compared to the well. Particular discretion is required when running pipe into a well having exposed weaker formations and heavy drilling fluid to counter high formation pressure. Fig 40 show recommended running speed w/16.9 ppg mud in the wellbore.

c.

Underground Blowouts - Loss of circulation due to any cause can create a condition known as an underground blow out. This results when the hydrostatic head at a permeable, exposed formations drops below the formation pressure. Fluids then produce into the well and flow uncontrolled into the zone that has broken down. The situation can be very difficult to control, and the well is usually lost below the formation


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300 280 260 240 220

Sec. Pr Stand

200 180 160 140 120 100 80 60 40 20 0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 BIT DEPTH

Fig 40

that is broken down. It is therefore important that loss of circulation is avoided. d.

Pressure due to Annular Circulating Friction - Another item to be considered when drilling with a heavy drilling fluid near the fracture gradient of the formation is the pressure added by circulating friction. This can be quite large, particularly in small wells with large drill pipe, stabilisers, or large drill pipe rubbers inside the protective casing. It is sometimes necessary to reduce the pumping rate to lower the circulating pressure.

5.

Cut by Gas or Water.

a.

Drilling fluid cut by Gas: Gas cutting of the drilling fluid need not always indicate that kick has occurred. When a porous gas zone is drilled the gas in the pores of the cuttings will be released as the cuttings approach the surface. This will happen despite the fact that a good 200 psi overbalance is carried in drilling fluid column density. Provided the drilling fluid viscosity and gel strength is low, this is no problem and the drilling fluid can easily be degassed at the surface and go back into circulation at full density. If this gas is not released at the surface and is allowed to continually recycle, problems will crop up. When in doubt stop the pump and observe the well for flow.

b.

Drilling fluid Cut by Water: If drilling fluid density is reduced by the addition of water there must be a corresponding rise, equal to the amount influx, in the drilling fluid pits. Therefore, this should seldom happen. If it does, the pit level and flow rate indicators are not functioning.


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Abnormal Pressure

Usually a formation with such pressures gives enough warning that proper steps can be taken. Once these zones are detected, it is normally possible to drill into them a reasonable distance while raising the drilling fluid weight as necessary to control gas entry. However, when pressure due to drilling fluid weight approaches the fracture gradient of the highest exposed formation, it is good practice to set casing. Failure to do this has been the cause of many underground blowouts and lost or junked wells. 02.07 Warning Signals

It is impossible for a blowout to occur under normal conditions without warning of its development. The wellbore and the drilling fluid system is a closed circulation system, and any influx from the formation into the system will show up in the form of increasing returns from the annulus and an increase of total drilling fluid volume in the surface system (drilling fluid tanks). Often while drilling we can get indications at the surface that we are entering a transition zone, that is to say a zone where pressure increases slowly because of the formations relatively slow change of compaction, but sometimes such formations are difficult to interpret. Normally we will have many clear indications of increasing formation pressure before a kick occurs. The following points show the indications that we can often receive at the surface before or when a kick has occurred: 1.

While drilling: a. Drilling rate increase - drilling breaks b. Gas in return drilling fluid - gas-cut drilling fluid c. Chlorides in return drilling fluid - salt water cut drilling fluid d. Change in the density of cuttings e. Change in the size of cuttings f. Fall in circulation pressure g. Temperature increase in return drilling fluid h. Increase in R.P.M. (rotary speed) i. Increase in flow from the wellbore j. Increase in volume in drilling fluid pits (pit gain)

2.

While tripping or while making connections: a. b. c. d. e. f.


Increase in flow from the wellbore Trip gas Connection gas Well-fill after a trip "Tight" well on connections Wellbore not taking the correct amount of drilling fluid to compensate for pipe taken out. © MTC

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03.07 Warning Signals While Drilling

a.

An increase in penetration rate can often be a sign of drilling into a softer formation. Pump pressure can also change because the drilling fluid is simply cleaning out the cuttings under the bit better than previously. These two examples are not of course signals that a kick is about to occur. We discussed earlier also that penetration rate is also dependent upon differential pressure. An increase in penetration rate can also indicate, therefore, that differential pressure is known to be reduced and there is danger of taking a kick. An increase in penetration rate must always be noted and acted upon. The bit must be picked up off bottom, the drilling fluid pumps must be stopped, and 6 8 10 12 2 4 the well must be checked for flow. A drilling break is a sudden change in penetration rate from a low to a higher value. This sudden change in penetration can vary considerably depending upon the actual formation type. In some cases a "break" can be from between 10 ft. to 50 ft./hour, in others maybe only 5 ft. to 10 ft./hour. In all cases where drilling is conducted in areas that are unknown or where high pressures are expected, after a relatively long period of slow drilling is followed by faster drilling, no more than between 2 to 4 feet should be drilled before the pumps are shut down and the well checked for flow. See Fig 41

12:00 12:15 12:30 12:45 13:00 13:15 13:30 13:45 14:00 14:15

A negative drilling break could also be a warning sign that a cap rock is being penetrated and possible higher pressure is contained in the formation below the cap rock. b.

Fig 41

Background gas increases quite suddenly if the bit penetrates a zone of higher pore pressure. This background gas is not gas that intrudes into the wellbore from the formation but gas which is contained between the wellbore cuttings. If this gas has a high pore pressure it will expand considerably on the way up to the surface and may make up 50% of the drilling fluid volume. Such a situation is not so critical if it is properly treated. This gas is removed from the drilling fluid at the surface with the help of a “Degasser”. If the drilling fluid is not properly degassed before it is pumped back down the well the hydrostatic pressure in the well will be lowered and the chance for a kick to occur is possibly. On the other hand, gas in the return drilling fluid could mean that there actually exists an underbalance in the well and gas is intruding. In such a case this is a real kick situation and the necessary steps to contain it must be made immediately.


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c.

The chloride content in drilling fluid, ( fresh-water based drilling fluid ) will normally increase when high-pressure zones are penetrated. However, this increase is impossible to notice when salt-water based drilling fluid, or drilling fluid with high chloride content, is in use. This indication, therefore is not reliable enough on its own to be of any use to us in indicating high pressure zones.

d.

The density of rock formation will nearly always be reduced if it is associated with a high pressure zone. This is because it will have a greater porosity. This is a good indicator if it is possible to examine different cuttings at the shale shaker, and with the help of S.P.M. and annulus capacity decide from which depth they originate.

e.

The size of the cuttings often suddenly change when a high-pressure zone is penetrated, they can become long and splintery in shape.

f.

Changes in pump pressure are a direct result of changing resistance (friction) in the drilling fluid, if formation fluids or gas penetrate the wellbore and intermingle with the drilling fluid. However, it will only be a small part of the drilling fluid in the annulus that becomes affected in this way. Pump pressure will normally fall if a kick occurs, as part of the drilling fluid in the wellbore becomes lighter through reduction in weight and viscosity. If the kick is a gas kick it is possible that the gas forces its way up the annulus of its own accord and pushes the drilling fluid ahead of it. This will cause large pressure fluctuations. STROKES

STROKES

110 PRESSURE

165.5

112 PRESSURE

162.5 Fig 42

A reduction in pump pressure can in some cases give an increase in pump speed. This occurs because of the decreased load on the pump. However, some rigs have drilling fluid pumps that are self-regulating, which is to say that regardless of what loads are imposed on the pumps they will automatically use less or more energy and maintain a constant pre-determined speed. See Fig 42 We should also be aware that pressure reductions can occur through reasons other than kick situations, such as washouts in the drill string etc.


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g.

Flow line (return drilling fluid) temperature often increases when a high pressure zone is penetrated, but although this has been observed in many places throughout the world it is not a particularly trustworthy indication on its own. Drilling fluid temperature can often increase when caustic soda and barite are added, well geometry can also cause temperature increases (higher drilling fluid velocities). A clear and uniform temperature increase that could possibly indicate a formation with a high pressure is best seen when shown graphically in detail.

h.

Rotary table speed (R.P.M.) often increases when a high pressure zone is penetrated. This is because the formation is breaking up easier and, therefore, offering less resistance to the bit.

i.

Increasing flow at the flow-line is immediately the first signal that a kick is occurring. This indication is called a “positive kick indicator” and require no flow check, but an immediately shut in of the well to minimise the size of the influx. See Fig 43

Fig 43

j.

An increase in pit volume will always occur when fluid or gas enters the wellbore, because a proportional amount of drilling fluid is displaced out of the well and into the drilling fluid pits. Any unexplained pit gain is a sure sign of a kick and is also called a “positive kick indicator” where the necessary precaution and steps must be carried out immediately.


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04.07 Warning Signals While Tripping or Making Connections

a.

If drilling fluid returns are observed at the flow-line when the pumps are not running this is a certain sign that formation fluids are flowing into the wellbore. Therefore, the annulus becomes underbalanced and drilling fluid is displaced into the drilling fluid pits. See Fig 43

Pf

PRESSURE

DEPTH

DEPTH

Ph

Pf

Ph+Pl

Pl annulus

Ph

PRESSURE

Fig 43

b.

Trip gas, which is gas that permeates into the wellbore during a trip, will normally increase when a high-pressure zone is penetrated, and drilling fluid weight is not increased to counter balance this. This trip gas is measured by a gas detector at the flow-line that continuously monitors the drilling fluid and will be seen as a peek on the chart during first circulation bottoms up after a trip. Trip gas alone is not a reliable kick indicator.

c.

Connection gas is the name given to the gas which penetrates into the wellbore when circulation is stopped and a new length of pipe added to the drill-string. This connection gas will always increase as a rule when a high pressure zone is penetrated. Connection gas is also monitored by the gas censor at the flow line and is normally not a problem as long the gas is removed from the drilling fluid and not recirculated. To avoid reducing pressure hydrostatic in the annulus more that one slug of connection gas should not be circulated out at any given time.

d.

Well-fill after a trip accompanied by an increase in trip gas can indicate high pressure, but can also be caused by other factors such as poor drilling fluid qualities, swelling formations and incorrect well-filling procedures so therefore it is unreliable in itself as an indicator.

e.

"Tight" well, which is when the formation closes back in on the drill string, can occur when connections are made and can indicate high pressure. This condition can also warn us that there is a danger of the drill string becoming stuck ( sometimes permanently ).


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When the drill string is pulled out of the well the volume of steel which it comprises must be replaced by equivalent volume of drilling fluid, this is achieved by pumping measured amounts of drilling fluid into the well as the drill string is removed. The well shall at all times be kept full of drilling fluid. The amount of drilling fluid needed to fill the well on a trip must be calculated before-hand and the amount used on the trip must be identical, if the amount becomes dissimilar the reasons must immediately be found. This quantity is usually checked every 5 stands. If the well is taking too small an amount of drilling fluid, formation fluids are intruding into the wellbore, and if the well is taking too much drilling fluid, drilling fluid is flowing into the formation, both situations are highly dangerous and must be controlled.

05.07 Procedure for Shutting in the Well

If we are drilling ahead and for any reason we have cause to think that a kick may be developing, the well must immediately be checked for flow. If there is no flow and everything is in order we go back to drilling. If the well is flowing, we shut the well in either using a soft shut-in or a hard shut-in procedure. Soft Shut-in Procedure:

1.

Pick-up from bottom and position drill string, shut down mud pumps and rotation. Flow check. Well flowing. 2.

Open hydraulic side outlet choke valve. 3.

Close BOP (Ram or Annular preventer). 4.

Close adjustable hydraulic choke. 5.

Record SIDPP – SICP – Pit Gain.

Hard Shut-in Procedure:

1.

Pick-up from bottom and position drill string, shut down mud pumps and rotation. Flow check. Well flowing. 2.

Close BOP (Ram or Annular preventer). 3.

Open hydraulic side outlet choke valve. 4.

Record SIDPP – SICP – Pit Gain.

If it is a positive kick indication that is observed keep in mind that no flow check is carried out, but the well is shut in instantly. Remembering what has been said about MAASP, we must observe the casing pressure as it begins to rise and ensure that it does not exceed the pre-determined MAASP value.


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Now that the well is shut-in, the pressure in bottom of the well will soon come into balance with the formation pressure. The different between the two existing methods to close the well in is that the Hard Shut-in Procedure reduces the amount of influx into the wellbore with resulting lesser annulus pressure and surface pressure when circulating out the kick. The purposes of the shut-in procedure are to:

1.

Stop the influx into the wellbore.

2.

Provide a safe rig environment.

3.

Start kill procedures.

The purpose of raising the bit from bottom of the well are to get:

a.

Less chance to get stuck.

b.

Easier to get free if stuck ( you can go up or down ).

c.

The Kelly cock is above rotary table. (if Kelly is used)

d.

Ram type preventer can be used when it is secured that there is not a tool joint opposite this.

e.

The Kelly can be removed.

f.

After killing drill pipe wire line tools can be run in.


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06.07 Pressure after Shut In

Fig. 44 and 45 show the situation just after the well is shut in. On account of the influx in the bottom of the well there can now be read pressure on the standpipe (PSIDPP) and pressure on the casing (PSICP). The cause for the kick is an increase in the formation pressure PF . PSIDPP NATIONAL

PSICP

PSICP PSIDPP

BOP

Gradient of mud

PSIDPP

PSICP

+

+

PHDP

PHA

D E P T H

PH

PH

Gradient of influx Annulus

Drill Pipe

PF

Pf

PRESSURE

PF

Fig 45 Fig 44

This new pressure will be the sum of the hydrostatic pressure from the column of drilling fluid in the drill pipe PHDP and the pressure on standpipe ( PSIDPP ). The new pressure of the formation will also be the sum of the hydrostatic pressure from the column of drilling fluid-gas in annulus ( PHA ) and the pressure on the casing ( PSICP ). PHDP + PSIDPP = PF = PHA + PSICP

The pressure reading on the standpipe PSIDPP alone will be determined by the pressure of the formation ( PF ). The pressure reading on the casing ( PSICP ) will be determined by both the pressure of the formation and the amount of gas, which is flowed into the wellbore. Gas has a pressure gradient of @ 0,17 psi/ft. Drilling fluid with a weight of 10 ppg has a pressure gradient of 0,52 psi/ft. From this is showed the more gas flowed into the wellbore, the lower hydrostatic pressure PHA from the column of drilling fluid-gas, as PF = PHA + PSICP. Fig 45 shows this connection from 3 different quantities of influx.


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PSIDPP and PSICP are gauges connected to the bottom of the well via the drilling fluid. They can be used to calculate the kill drilling fluid density for a kick or to see how much pressure a formation can stand before losing circulation, such as with a leak off test.

For well control the PSIDPP is used to calculate the kill drilling fluid density required for killing a well with a certain influx. The drill pipe is full of clean and homogeneous drilling fluid newly treated from the pits. To know how much to increase the drilling fluid density to kill a well, it is necessary to know how much the original drilling fluid density to begin with is so the PSIDPP is used. The annulus or casing has cuttings and gas or salt water in it, so it is much harder to determine an accurate drilling fluid weight increase from it. The calculation of kill drilling fluid weight is made as follows: Kill MW =

P sidpp + MW 1 TVD x 0,052

Where: Kill MW =

the drilling fluid density required balancing the pressure in the formation.

PSIDPP =

the read back pressure on the standpipe after the well is shut in and the pressure stabilised.

TVD

the true vertical depth of the well.

=

0,052 =

a constant which tells how much the hydrostatic pressure will be changed for every feet fluid column at a fluid with a density equal to 1 PPG.

MW1

Original drilling fluid density while drilling.

=

Fig. 46 shows how to figure the drilling fluid density increase from PSIDPP out from a Chart: PSICP can together with PSIDPP be used to calculate the pressure gradient (density) for the influx by using the following formulae:

SICP —SIDPP (psi) Gradient of influx (psi/ft) = MW x 0.052 — -------------------------------Height Influx(TVD) ft Kick Size Height of Influx along hole = -----------------------------Annular Volume


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Increase required to balance a kick (lb/gal) SIDPP SIDPP x 19.2 Lb/galincrease = -------------------- = -----------------Depth x 0.052 Depth SIDPP 300 400 500 600 700 800

100

200

900

1000

1000

1.9

3.8

5.8

7.7

9.6

11.5

13.5

15.7

17.3

19.2

2000

1.0

1.9

2.9

3.8

4.8

5.8

6.7

7.7

8.6

9.6

3000

0.6

1.3

1.9

2.6

3.2

3.8

4.5

5.1

5.8

6.4

4000

0.5

1.0

1.4

1.9

2.4

2.9

3.4

3.8

4.3

4.8

5000

0.4

0.8

1.2

1.5

1.9

2.3

2.7

3.1

3.5

3.8

6000

0.3

0.6

1.0

1.3

1.6

1.9

2.3

2.6

2.9

3.2

7000

0.3

0.6

0.8

1.1

1.4

1.7

1.9

2.2

2.5

2.8

8000 D E 9000 P 10000 T 11000 H 12000

0.2

0.5

0.7

1.0

1.2

1.4

1.7

1.9

2.2

2.4

0.2

0.4

0.6

0.9

1.1

1.3

1.5

1.7

1.9

2.1

0.2

0.4

0.6

0.8

1.0

1.2

1.3

1.5

1.7

1.9

0.2

0.4

0.5

0.7

0.9

1.1

1.2

1.4

1.6

1.8

0.2

0.3

0.5

0.6

0.8

1.0

1.1

1.3

1.4

1.6

13000

0.1

0.3

0.4

0.6

0.7

0.9

1.0

1.2

1.3

1.5

14000

0.1

0.3

0.4

0.6

0.7

0.8

1.0

1.1

1.2

1.4

15000

0.1

0.3

0.4

0.5

0.6

0.8

0.9

1.1

1.2

1.3

16000

0.1

0.2

0.4

0.5

0.6

0.7

0.8

1.0

1.1

1.2

Fig 46

Rising Pressures after Shut-in:

Often the drill pipe and casing pressures do not stop, but continue to rise. This could be due to: 1. 2.

Low permeability Percolation of gas up through the drilling fluid

Low Permeability: If the permeability of the formation in the kick zone is low, then the influx will come slowly. There will go some time before the influx can create a pressure in the top of the drill string and the annulus respectively, which added to the hydrostatic pressure can balance the pressure in the formation. You must therefore wait until the pressures have stabilised before the accurate PSIDPP and PSICP can be read. Percolation: If the slowly rising pressure is due to gas percolating up the well the pressure does not represent reservoir pressure, but is due to the low density of the gas. On account of this, the gas will raise up through the drilling fluid without expansion. If this situation is handled properly it will cause no major problems. How the situation must be handled is mentioned in section ????? in relation to the volumetric method. Low or no Pressures on Standpipe (PSIDPP) M:\IWCF Surface\3\1\Section 5.doc

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Low pressure: If there is no PSIDPP after the well is Shut-in following could be the reason:

1. 2. 3. 4.

The gauges are broken, malfunctioning or shut off. There is a float in the drill pipe. The well is in balance. The drill string is plugged.

If there is no float install in the drill string, check the gauges on the standpipe manifold to see that this is not the problem. Change gauges as required after isolation to obtain SIDPP. Float in the drill pipe:

If there is a float in the drill string the SIDPP may be zero. Some floats have a 3/16" hole drilled through the float witch will allow the pressure to build up slowly on the drill pipe site. If the float valve provides a complete shut off there are several ways to check for the true shut in pressure. 1.

Pump as slowly as possible (3 to 5 SPM) until the casing pressure starts to rise. Then stop pumping. The pressure after the pump stops should be PSIDPP.

2.

Slowly bring the pump up to kill rate holding casing pressure constant. The circulating drill pipe pressure is identical to the ICP (initial circulating pressure). The PSIDPP can now be calculated using the formula below: PSIDPP =


Circulating drill pipe pressure – Pre-recorded kill rate pressure.

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69

CIRCULATING A KICK OUT OF THE WELLBORE

01.08 General points

It can be seen by the calculations in chapter 02 of this book, that the bottom hole pressure in a well shut in will be balanced by the hydrostatic pressure (both annulus and drill string) and pressure at the surface (casing and standpipe pressure). As long as this bottom hole pressure is held constant no more formation fluid/gas can intrude into the wellbore. If the bottom hole pressure is allowed to fall below the formation pressure, a fresh influx will enter the wellbore and we will have to deal with a second kick. If the bottom hole pressure is increased too much there is a possibility to break down the formation resulting in losses and further complications to the well control problem. To get the influx out of the well, drilling fluid is pumped down the drill string. This displaces the influx higher and higher up the annulus until it reaches the surface where it is vented out of the wellbore via the choke. This can be achieved by holding the bottom hole pressure constant during circulation (i.e. the bottom hole pressure that was registered when the well was shut in). How can it be known at the surface that bottom well pressure is being held constant, under circulation?

We can deduce that if there is no change in the height of the drilling fluid column or drilling fluid properties and, furthermore, no change in the pressure at the surface acting on the drilling fluid column there will not be any change in bottom hole pressure. Therefore we have the possibility of observing changes in the bottom hole pressure by way of the gauges installed respectively on the standpipe and casing. If the bit is at the bottom of the well it is normal practice to use the standpipe (drill string) pressure as a bottom hole pressure indicator. If the drilling fluid weight in the drill string remains constant, a constant standpipe pressure will indicate a constant bottom hole pressure. Well Killing Methods:

There are several methods recognised within the industry to control formation pressure while circulate out a kick and the primary object regardless of method is to keep constant bottom hole pressure. Driller’s Method. Wait and Weight Method. Concurrent Method.


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02.08 Circulating out an Influx using the Driller's Method

This method is also called the "Constant Drill-Pipe Pressure Method" and consists of two steps. First step is to circulate the kick out of the wellbore without changing the drilling fluid density. Second step is to displace the original drilling fluid out of the well bore with heavier drilling fluid (Kill Mud) which will exert enough hydrostatic pressure to balance the formation pressure. After taken an influx and shutting in the well pressure will build up on the standpipe and casing gauges because of the hydrostatic underbalance (drilling fluid weight too low to balance formation pressure). This pressure is known as "Shut In Drill pipe Pressure" PSIDPP. As long as the drill string contains drilling fluid of the original weight, PSIDPP will always exist as an extra pressure registered at the surface required to balance formation pressure. When the influx is circulated out, the pump will have to overcome the PSIDPP + the friction losses in the circulating system at the desire pump rate. The friction losses in the circulating system has been determined previously when checking the RRCP ( Reduced Rate Circulating Pressure) and the only changes to the bottom hole pressure balance is the increase due to friction losses in the annulus which is considered to be very small and therefore not taking into consideration when circulating out the influx. Where and when is the value for RRCP found?

RRCP is the friction loss in the system at a decided pump speed (reduced pump rate). These pressures are noted at several different reduced pump rates, for example at 20, 30, and 40 SPM and the pressure is recorded on the remote choke panel and noted on the pre-recorded kill sheet. The RRCP is normally recorded at the beginning of each shift. Other factors could require the RRCP to be recorded more frequently such as: Change in drilling fluid properties. Change in drill string configuration. Very fast drilling. The pump pressure is called Initial Circulation Pressure ICP and is registered on the standpipe gauge on the remote choke panel. To reach the ICP while keeping constant bottom hole pressure the pumps are slowly brought up to desired reduced rate while keeping casing pressure constant by manipulating the choke. At the desire pump rate the drill pipe pressure is identical to the ICP. ICP = RRCP + SIDPP


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What happens to the casing pressure as an influx is circulated out of the wellbore?

If the influx is fluid (water or oil), the casing pressure will remain constant until the influx begins to vent at the choke. If the influx is gas, the gas must be allowed to expand as it rises up through the annulus, to ensure that bottom hole pressure does not increase. In this situation more drilling fluid is displaced out of the wellbore through the choke than pumped into the well. Therefore the drilling fluid pit level will increase. This also means that when the height of the drilling fluid column in the annulus is reduced and the column of gas increased loss of hydrostatic pressure takes place, but increasing casing pressure compensates for this loss. The expansion of gas depends of the drilling fluid properties and type. See Fig 47. FLUID INFLUX GAS IN WATER BASE DRILLING FLUID GAS IN OIL BASE DRILLING FLUID

1500 1400 1300 1200 CASING PRESSURE

1100 1000 900 800 700 600 500 400 300 200

SICP

SIDPP

100 0

1000

2000

3000 4000 5000 STROKES - BIT TO SURFACE

6000

7000

8000

Fig 47

To ensure that bottom well pressure does not change as a kick is being circulated out standpipe pressure must remain constant all the time. When the influx is circulated out the pump can be stopped and the well closed. Standpipe pressure and casing pressure will now be the same value (PSIDPP) due to that both the drill pipe and the annulus is filled with a homogeneous column of original drilling fluid and no further influx has taken place. The next step consists of replacing the original drilling fluid with a heavier drilling fluid (KMW) that will create sufficient hydrostatic pressure to balance the formation pressure. M:\IWCF Surface\3\1\Section 5.doc

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The drill string is filled gradually with heavy drilling fluid and therefore its hydrostatic pressure will change. The drill pipe pressure must be allowed to decrease and can for that reason not be held constant, but as long as the heavy drilling fluid is confined inside the drill string there will be no change of drilling fluid in the annulus and therefore no change in pressure either. So casing pressure is held constant until the heavy drilling fluid has reached the bit. The new standpipe pressure observed at this stage is the final circulating pressure (FCP) and held constant until the annulus is full of heavy drilling fluid. If the new drilling fluid is the correct weight the well should now be "killed" (dead) and standpipe and casing pressure should be zero when pumps are stopped. KMW FCP = RRCP x -----------------OMW Drillers Method 1st Circulation: Well Kick Data: Hole size Hole depth TVD/MD Casing (9 5/8 in) TVD/MD Drill pipe 5 in capacity Heavy Wall pipe 5 in Capacity Drill collars 6¼ in Capacity Drilling fluid density Capacity open hole x collars Capacity open hole x drill pipe/HWDP Capacity casing x drill pipe Fracture fluid density at the casing shoe SIDPP SICP Mud pumps displacement Slow Circulating Rate Pressure at 30 SPM Pit gain

8½ 11536 9875 0.01741 600 0.00874 880 0.00492 14.0 0.03221 0.04470 0.04891 16.9 530 700 0.1019 650 10.0

in ft ft bbl/ft ft bbl/ft ft bbl/ft ppg bbl/ft bbl/ft bbl/ft ppg psi psi bbl/strk. psi bbl

With the following date given the kill sheet can be filled out and the necessary information be required to kill the well: Internal strokes from surface to bit: Total annulus from bit to surface: Open hole from bit to shoe: Kill fluid density: Initial circulation pressure Final circulation pressure: Initial MAASP with drilling fluid density: New MAASP with kill fluid density: Influx gradient Height of influx around DC Height of influx around DP


1812 5360 620 14.9 1180 692 1489 1027 0.178 310 204

strokes strokes strokes ppg psi psi psi psi psi/ft ft ft

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PDP

Shoe

PA

ANNULUS

OMW OMW

ANNULUS

CBI/NLN

08

DRILL STRING

PA

DRILL STRING

PDP

REVISION

REVIEWED BY

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AUTHORISED BY

OMW

Shoe GAS

BHP

BHP

GAS

Fig 48

Fig 49

Situation Fig 48 shows the start of circulation. The standpipe pressure is equal to PSIDPP plus RRCP(Reduced Rate Circulating Pressure). The pressure at the casing head Pa is equal to PSICP .

While keeping constant casing pressure the pumps are slowly brought up to slow circulating rate, in this case 30 SPM. When the pumps are running at 30 SPM and pressures have stabilized the ICP pressure on the drill pipe gauge is keep constant. ICP = 530 psi + 650 psi = 1180 psi Shoe pressure = 7189 psi + 700 psi = 7889 psi MAASP = 8678 psi - 7189 psi = 1489 psi

SIDPP + RRCP Phshoe + SICP Max Shoe Pressure - Phshoe

Situation Fig 49 shows the gas circulated a way up the annulus.

Drill Pipe Pressure is kept constant while gas is being pumped up through the open hole section and the top of the gas bubble reach the shoe. The gas is expanding allowing the pressure inside bubble to decrease. Casing pressure increasing due to the expanding gas is displacing drilling fluid. CSG P = BHP - (Phmud + Phgas) 775 psi Shoe pressure is increasing with same value as the casing pressure. Shoe P = Phshoe + Csg P 7964 psi MAASP remains constant due to no change in Ph inside the casing. MAASP = Max Shoe Pressure - Phshoe 1489 psi M:\IWCF Surface\3\1\Section 5.doc

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PDP

PA

Shoe

BHP

Fig 50

ANNULUS

GAS

DRILL STRING

OMW OMW

ANNULUS

DRILL STRING

OMW

GAS

OMW

Shoe

BHP

Fig 51

Situation Fig 50 shows the gas circulated inside the casing.

Drill Pipe Pressure is kept constant while gas is being pumped from the open hole section until all the gas is inside the casing so the open hole section is displaced to original drilling fluid. The gas is expanding allowing the pressure inside bubble to decrease. Casing pressure increasing due to the expanding gas is displacing drilling fluid. CSG P = BHP - (Phmud + Phgas) 785 psi Shoe pressure is constantly decreasing from gas reach the shoe until all gas is inside casing. Shoe P = BHP - Phopen hole 7718 psi MAASP start increasing from the first gas enters the casing due to change in Ph inside the casing. MAASP = Max Shoe Pressure - Phshoe 1685 psi Situation Fig 51 shows the gas at the choke.

Drill Pipe Pressure is kept constant while gas is being pumped up inside the casing and the top of the gas bubble reach the choke. The gas is expanding allowing the pressure inside bubble to decrease. Casing pressure increasing due to the expanding gas is displacing drilling fluid and when top of the gas bubble reach the choke casing pressure is increased to max. CSG P = BHP - (Phmud + Phgas) 1580 psi Shoe pressure remains constant from the moment all gas is inside the casing. Shoe P = BHP - Phopen hole 7718 psi M:\IWCF Surface\3\1\Section 5.doc

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MAASP increasing with the same value as the Csg P due to change in Ph inside the casing and will reach max value when gas at choke. MAASP = Max Shoe Pressure - Phshoe 2480 psi

PA

ANNULUS

DRILL STRING

OMW

PDP

OMW

Shoe

BHP

Fig 52

Situation Fig 52 shows that all the gas is now circulated out.

Drill Pipe Pressure is kept constant while gas is being pumped out of the well through the choke. Casing pressure decreasing while drilling fluid is displacing gas in the well bore and will reach SIDPP when all the gas is out of the well. CSG P = BHP - Phmud 530 psi Shoe pressure remains constant while the gas is displaced from the well bore due to no change in Phopen hole. Shoe P = BHP - Phopen hole 7718 psi MAASP decreasing with same value as the Casing Pressure and will reach initial MAASP when the annulus is displaced to original drilling fluid. MAASP = Max Shoe Pressure - Phshoe 1489 psi


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Drillers Method 2nd Circulation:

Shoe

BHP

Fig 53

PA

ANNULUS

OMW KMW

ANNULUS

PDP

DRILL STRING

PA

DRILL STRING

OMW

PDP

OMW

Shoe

BHP

Fig 54

Situation Fig 53 shows the kill fluid is being pumped to the rig floor.

Kill mud is being mixed to 14.9 ppg and 2nd circulation is started. Casing pressure is kept constant while kill fluid fills the drill string. CSG P = BHP - Phmud 530 psi Drillpipe pressure decreasing while kill fluid fills the drill string. DP P = RRCP + (BHP - Phmud) Shoe pressure remains constant while kill mud fills the drill string due to no change in Phopen hole. Shoe P = BHP - Phopen hole 7718 psi MAASP remains constant while kill mud fills the drill string. MAASP = Max Shoe Pressure - Phshoe 1489 psi Situation Fig 54 shows that kill fluid has reached the bit.

Kill fluid reach the bit. Casing pressure is kept constant while kill fluid fills the drill string. CSG P = BHP - Phmud 530 psi Drillpipe pressure decreasing while kill fluid fills the drill string and when kill fluid reach the bit pressure is FCP or RRCP w/14.9 ppg mud. DP P = RRCP w/14.9 ppg 692 psi


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Shoe pressure remains constant while kill mud fills the drill string due to no change in Phopen hole. Shoe P = BHP - Phopen hole 7718 psi MAASP remains constant while kill mud fills the drill string. MAASP = Max Shoe Pressure - Phshoe 1489 psi

Shoe

ANNULUS

KMW

ANNULUS

OMW

KMW

Shoe

BHP

BHP

Fig 55

PA

DRILL STRING

PDP

PA

DRILL STRING

KMW

PDP

Fig 56

Situation Fig 55 shows that kill fluid is on its way up the annulus.

Kill fluid at shoe. Drillpipe pressure is kept constant while kill fluid displaces original mud in annulus. DP P = RRCP w/14.9 ppg 692 psi Casing pressure decreasing as kill fluid moves up the annulus to the shoe. CSG P = BHP - Phmud 469 psi Shoe pressure decreasing as kill fluid moves up the annulus to the shoe with same value as the decrease in Csg P. Shoe P = BHP - Phopen hole 7657 psi MAASP remains constant while kill fluid moves up the annulus to the shoe. MAASP = Max Shoe Pressure - Phshoe 1489 psi Situation Fig 56 shows that the kill fluid has reach the choke.

Kill fluid at the choke. Drillpipe pressure is kept constant while kill fluid displaces original mud in annulus. DP P = RRCP w/14.9 ppg 692 psi M:\IWCF Surface\3\1\Section 5.doc

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Casing pressure decreasing as kill fluid moves up the annulus and will reach 0 psi when kill fluid reach the choke. CSG P = BHP - Phmud 0 psi Shoe pressure remains constant while kill fluid is displacing original mud inside the casing due to no change in Ph open hole. Shoe P = BHP - Phopen hole 7657 psi MAASP decreasing as kill fluid is displacing original mud inside the casing with same value as the drop in casing pressure. MAASP = Max Shoe Pressure - Phshoe 1020 psi CASING PRESSURE

DRILL PIPE PRESSURE 1500 1400 1300 1200 1100 1000 PRESSURE

900 800 700 600 500 400 300 200 100 0

2000

4000

6000

8000

10000

12000

14000

16000

STROKES

Fig 57

Fig 57 shows the pressure relationships between drill pipe and casing. Drill Pipe Pressure 1st circulation: Drill Pipe Pressure constant while displacing annulus to original drilling fluid and removing gas from well bore. Drill Pipe Pressure 2nd circulation: Drill Pipe Pressure decreasing to FCP will kill fluid fills the drill string and then constant while kill fluid displaces original mud in annulus. Casing Pressure 1st circulation: Casing Pressure constantly increasing until gas reach choke. Casing pressure decreasing to SIDPP while gas is displaced from the well bore.


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Casing Pressure 2nd circulation: Casing Pressure constant while kill fluid fills the drill string and then decreasing while kill fluid displace original mud in annulus. Shoe Pressure 1st circulation: Increase with SICP when taking influx. Increase while gas is moving up in the open hole section. Decrease while gas enters the casing. Constant after open hole has been displaced to drilling fluid. Shoe Pressure 2nd circulation: Shoe pressure constant while kill fluid fills the drill string. Shoe pressure decreasing while kill fluid displaces original mud in open hole. Shoe pressure constant while kill fluid displaces original mud inside casing. MAASP Pressure 1st circulation: Constant while gas is moving up open hole section. Increase quickly while gas is entering casing. Increase slowly with same value as Csg P while gas moves up inside casing. Decreasing to initial value while gas is displaced from the well bore. MAASP Pressure 2nd circulation: MAASP constant while kill fluid fills the drill string. MAASP constant while kill fluid displaces original mud in open hole. MAASP decreasing while kill fluid displace original mud inside casing with same value as the decrease in casing pressure.


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03.08 Wait and Weight Method or Engineer's Method

This method is also called the "balance method" and is described below. The well is shut in and pressure values are observed. The drilling fluid is increased to the weight necessary to kill the well. While keeping constant casing pressure the pumps are slowly brought up to slow circulating rate and when running at the desire rate the drill pipe pressure is the ICP. The primary objective while killing the well is to keep constant BHP, but as the heavy drilling fluid fills the drill string the ICP cannot be held constant due to the change in hydrostatic head. In the annulus the gas expands as it rises, so therefore casing pressure cannot be held constant either. However, the choke can be manipulated in such a way that standpipe pressure can be gradually decreased as the heavy drilling fluid is pumped down the drill string. How much and how often it is decreased can be decided in the following way. Like the Driller's Method we begin to circulate with a standpipe pressure equal to PSIDPP + RRCP and when the drill string is full of heavy drilling fluid the pressure will be equal to the new RRCP with heavy drilling fluid inside the string or FCP. This change in standpipe pressure occurs over a certain period of time that depends on the total number of strokes it takes to pump the drill string full of heavy drilling fluid. (Surface-to-bit). The easiest way is to represent this graphically. The following graph will show standpipe pressure changes in relation to pump strokes combined with a table that shows the new standpipe pressure for every 100 strokes. See Fig 58 The figures used for the graph and table apply to example in Driller’s Method and W&W Method. Pressure change per 100 strokes is calculated in the following way: ∆P/100 strk =

(ICP – FCP) x 100 ----------------------------Surface to bit strokes

Using the previous example we will get:

∆P/100 strk =

(1180 – 692) x 100 ----------------------------1812

=

27psi/100 strk

Circulation of the heavy drilling fluid from surface-to-bit can now proceed by regulating the choke after the table, so the bottom-hole pressure will remain constant.


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As soon as the drill string is full of heavy drilling fluid (after 1812 strokes) no change will occur of drilling fluid density and drilling fluid column in the drill string, which means that standpipe pressure (692 psi) can be held constant for the rest of circulation. PRESSURE 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1812

1180 1153 1126 1099 1072 1045 1018 991 964 937 910 883 856 829 802 775 748 721 694 692

1100 900 700 500 300

100 500

1000

1500

2000

STROKES

Fig 58

When killing a kick by the Weight and Wait Method, there are four phases that are described below. Phase 1.

Mix the required kill fluid immediately. When the kill fluid is ready, start pumping and open choke slowly while the pump is brought up to speed holding casing pressure constant at this initial start-up. As the drill string is gradually filled with kill fluid, circulation pressure is regulated with the choke to follow the values of the curve, until the calculated FCP with kill fluid at the bit is reached. (At this stage the drill pipe should be dead).

Phase 2.

Continue pumping until the gas is at the choke keeping constant drill pipe pressure.

Phase 3.

Continue pumping until all the gas is out. At this stage the annulus will be full of kill fluid, minus the capacity of the drill string which is light drilling fluid.

Phase 4.

Continue pumping until the annulus is full of heavy drilling fluid. At this stage the well should be dead.


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Wait and Weight Method: Well Kick Data: Hole size Hole depth TVD/MD Casing (9 5/8 in) TVD/MD Drill pipe 5 in capacity Heavy Wall pipe 5 in Capacity Drill collars 6¼ in Capacity Drilling fluid density Capacity open hole x collars Capacity open hole x drill pipe/HWDP Capacity casing x drill pipe Fracture fluid density at the casing shoe SIDPP SICP Mud pumps displacement Slow Circulating Rate Pressure at 30 SPM Pit gain

8½ 11536 9875 0.01741 600 0.00874 880 0.00492 14.0 0.03221 0.04470 0.04891 16.9 530 700 0.1019 650 10.0

in ft ft bbl/ft ft bbl/ft ft bbl/ft ppg bbl/ft bbl/ft bbl/ft ppg psi psi bbl/strk. psi bbl

With the following date given the kill sheet can be filled out and the necessary information be required to kill the well: Internal strokes from surface to bit: Total annulus from bit to surface: Open hole from bit to shoe: Kill fluid density: Initial circulation pressure Final circulation pressure: Initial MAASP with drilling fluid density: New MAASP with kill fluid density: Pressure drop/100 strk Influx gradient Height of influx around DC Height of influx around DP

1812 5360 620 14.9 1180 692 1489 1027 27 0.178 310 204

strokes strokes strokes ppg psi psi psi psi psi/100 strk psi/ft ft ft

Situation Fig 59 shows the start of circulation. The standpipe pressure is equal to PSIDPP plus RRCP (Reduced Rate Circulation Pressure). The pressure at the casing head Pa is equal to PSICp.

While keeping constant casing pressure the pumps are slowly brought up to slow circulating rate, in this case 30 SPM. When the pumps are running at 30 SPM and pressures have stabilized change to ICP and then keep DP pressure on schedule. ICP = 530 psi + 650 psi = 1180 psi Shoe pressure = 7189 psi + 700 psi = 7889 psi MAASP = 8678 psi - 7189 psi = 1489 psi


SIDPP + RRCP Phshoe + SICP Max Shoe Pressure - Phshoe

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PDP

PA

ANNULUS

Shoe

DRILL STRING

OMW OMW

ANNULUS

DRILL STRING

OMW

KMW

PDP

ITEM

JNO/HES

PREPARED BY


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OMW

Shoe GAS

BHP

Fig 59

BHP

GAS

Fig 60

Situation Fig 60 shows kill fluid fills the drill string while gas is circulated a way up the annulus.

Drill Pipe Pressure is kept on schedule while gas is being pumped up through the open hole section and the top of the gas bubble reach the shoe. The gas is expanding allowing the pressure inside bubble to decrease. Drill Pipe Pressure is kept on schedule with 27 psi drop per 100 strokes pumped. DP P = ICP - (470 x 27) 1053 psi 100 Casing pressure increasing due to the expanding gas is displacing drilling fluid. CSG P = BHP - (Phmud + Phgas) 775 psi Shoe pressure is increasing with same value as the casing pressure and reach max. value when gas reaches the shoe. Shoe P = Phshoe + Csg P 7964 psi MAASP remains constant due to no change in Ph inside the casing. MAASP = Max Shoe Pressure - Phshoe 1489 psi Situation Fig 61 shows kill fluid fills the drill string while gas is circulated inside the casing.

The gas is expanding allowing the pressure inside bubble to decrease. Drill Pipe Pressure is kept on schedule with 27 psi drop per 100 strokes pumped. DP P = ICP - (620 x 27) 1013 psi 100 Casing pressure increasing due to the expanding gas is displacing drilling fluid. CSG P = BHP - (Phmud + Phgas) 785 psi M:\IWCF Surface\3\1\Section 5.doc

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PA

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PDP

PA

GAS Shoe

ANNULUS

OMW KMW

ANNULUS

DRILL STRING

OMW

DRILL STRING

OMW

0

CHAPTER

KMW

PDP

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JNO/HES

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GAS

Shoe OMW

BHP

Fig 61

BHP

Fig 62

Shoe pressure is decreasing while gas moves from below the shoe until all gas inside the casing. Shoe P = BHP - Phopen hole 7718 psi MAASP start increasing from the first gas enters the casing due to change in Ph inside the casing. MAASP = Max Shoe Pressure - Phshoe 1685 psi Situation Fig 62 shows kill fluid has filled the drill string.

The gas is expanding allowing the pressure inside bubble to decrease. Drill Pipe Pressure is kept on schedule with 27 psi drop per 100 strokes pumped and reach FCP when kill fluid at bit. DP P = ICP - (1812 x 27) 692 psi 100 Casing pressure increasing due to the expanding gas is displacing drilling fluid. CSG P = BHP - (Phmud + Phgas) 1050 psi Shoe pressure constant due to no change in Phopen hole Shoe P = BHP - Phopen hole 7718 psi MAASP increasing with the same value as the Csg P due to change in Ph inside the casing. MAASP = Max Shoe Pressure - Phshoe 1950 psi Situation Fig 63 shows kill fluid has displaced the top of the gas to the choke.

The gas is expanding allowing the pressure inside bubble to decrease. M:\IWCF Surface\3\1\Section 5.doc

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Drill Pipe Pressure constant at FCP after kill fluid reach the bit. DP P = FCP

85

692 psi

Casing pressure increasing due to the expanding gas is displacing drilling fluid, but slower due to kill fluid is displacing original mud. CSG P = BHP - (Phmud + Phgas + Phkill mud) 1278 psi Shoe pressure remains constant after kill fluid enter casing due to no change in Phopen hole. Shoe P = BHP - Phopen hole 7641 psi MAASP increasing with the same value as the Csg P due to change in Ph inside the casing. MAASP = Max Shoe Pressure - Phshoe 2178 psi PDP

PA

PDP

PA

OMW

ANNULUS

KMW

ANNULUS

Shoe

DRILL STRING

OMW

DRILL STRING

KMW

GAS

KMW

Shoe

KMW

BHP

BHP

Fig 63

Fig 64

Situation Fig 64 shows kill fluid has displaced all the gas out of the well bore.

Drill Pipe Pressure constant at FCP after kill fluid reaches the bit. DP P = FCP

692 psi

Casing pressure decreasing while gas is displaced out of the well bore. CSG P = BHP - (Phmud + Phkill mud) 180 psi Shoe pressure remains constant after kill fluid enter casing due to no change in Phopen hole. Shoe P = BHP - Phopen hole 7641 psi MAASP decreasing while gas is displaced out of the well bore. MAASP = Max Shoe Pressure - Phshoe 1204 psi Situation Fig 65 shows kill fluid has displaced the remaining original drilling fluid out of the wellbore.

Drill Pipe Pressure constant at FCP after kill fluid reaches the bit. M:\IWCF Surface\3\1\Section 5.doc

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692 psi

Casing pressure decreasing to 0 psi while kill fluid displaces original mud out of the well bore. CSG P = BHP - Phkill mud 0 psi Shoe pressure remains constant after kill fluid enter casing due to no change in Phopen hole. Shoe P = BHP - Phopen hole 7641 psi MAASP decreasing while kill fluid displace original mud out of the well bore. MAASP = Max Shoe Pressure - Phshoe 1027 psi PA

ANNULUS

DRILL STRING

KMW

PDP

KMW

Shoe

BHP

Fig 65

Fig 66 shows the pressure relationships between drill pipe and casing. Drill Pipe Pressure: Drill Pipe Pressure decreasing from ICP to FCP while kill fluid fills the drill string. Drill Pipe Pressure constant at FCP the remaining circulating time. Casing Pressure: Casing pressure constantly increasing until gas reach the choke. Casing pressure decrease rapidly while gas is displaced from the well bore. Casing pressure decreasing to 0 psi while original mud is displaced with kill fluid. Shoe Pressure: Increase while gas is moving up in the open hole section. Decrease while gas enters the casing. Constant until kill fluid reaches the bit. Decrease while kill fluid is moving up the open hole section. Constant after open hole has been displaced to kill fluid. MAASP Pressure: Constant while gas is moving up in the open hole section. Increase rapidly while gas enters the casing. Increase until gas reaches the choke. M:\IWCF Surface\3\1\Section 5.doc

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Decrease rapidly while gas is displaced from the well bore. Decrease while original mud is displaced with kill fluid.


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CASING PRESSURE

DRILL PIPE PRESSURE 1500 1400 1300 1200 1100 1000 PRESSURE

900 800 700 600 500 400 300 200 100 0

1000

2000

3000

4000

5000

6000

7000

8000

STROKES

Fig 66

Annular pressure will fall to 0 PSI as soon as heavy drilling fluid appears at the choke. When this is observed the well should be dead. A flow check can now be made. If no flow is observed the blow-out preventer can be opened, and the drilling fluid can be circulated and conditioned as necessary, a trip margin can be added if it was not added to the kill drilling fluid at the start of the operation. Tripping or drilling can now take place again.


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04.08 The Concurrent Method

This is the most complicated of the three methods and its main value lies in the fact that the killing operation can be started as soon as the closed in pressures etc. have been recorded. Instead of waiting until the surface drilling fluid has all been weighted up to the kill drilling fluid weight, circulation at the reduced rate is started and the drilling fluid weight is increased while circulating. The rate of increase will depend on the mixing facilities available on the rig. The complication here is that the drill pipe can be filled with fluids of different densities, making calculation of the bottom well hydrostatic pressure difficult. However, provided adequate supervision is available on the rig this could be the most effective way of killing a kick. Procedure for Concurrent Method

When all the kick information has been recorded, open up the pump slowly while adjusting the choke until the initial circulating pressure has been reached at the reduced circulating rate. The drilling fluid should he weighted up at the maximum rate available with the rig equipment and, as the drilling fluid weight changes in the suction tank the choke operator is informed. He checks the pump strokes gone when the new drilling fluid weight starts on his chart, similarly with each change of drilling fluid weight, adjusting his choke pressure to suit the new drill pipe conditions as pre-recorded on his surface to bit graph. When the final kill drilling fluid reaches the bit the final circulating pressure will be reached and from this point onwards the pressure should be kept constant until the operation is completed.


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05.08 Advantages and Disadvantages of the Three Methods The Driller's Method is the simplest. The only calculations required is the kill drilling fluid weight, the capacity of the drill pipe and the capacity of the annulus. While circulating out the kick the drill pipe pressure is kept constant by regulating the choke. On the second circulation while the kill drilling fluid is filling the drill pipe, the annulus pressure is kept constant. When the drill pipe is full of kill drilling fluid control is switched back to the drill pipe while the annulus is being killed. The Wait and Weight Method on the other hand, requires the added calculation of the pump strokes required to fill the drill pipe and the subsequent reduction in circulating pressure as the pipe is filled. The Concurrent Method has the added complication of possibly two or more drilling fluid weights being present in the drill pipe at the same time.

Fig. 66 shows a comparison of casing pressures under killing according to the used method. DRILLER’s METHOD

WAIT and WEIGHT METHOD

CONCURRENT METHOD

1500 1400 1300 1200 1100 1000 PRESSURE

900 800 700 600 500 400 300 200 100 0

1000

2000

3000

4000

5000

6000

7000

8000

STROKES

Fig 66


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ADVANTAGES & DISADVANGES METHOD DRILLER’S METHOD

ADVANTAGES Simplest to teach and understand. Very few calculations. In case of saltwater the contaminant is moved out quickly to prevent sand settling around drilling assembly

WAIT and WEIGHT METHOD

Lowest casing pressure. Lowest casing shoe pressure. Less lost circulation (if not over killed). Killed with one circulation if influx doesn’t string out in washed out sections of the hole.

CONCURRENT METHOD

Minimum of non-circulating time. Excellent for large increases in mud weight (underbalanced drilling) Mud condition (viscosity and gels) can be maintained along with mud weight.

DISADVANTAGES Higher casing shoe pressure if long open hole section (gas kick). Higher annular pressure (gas kick). Takes two circulations.

Requires the longest noncirculating time while mixing heavy mud. Pipe could stick due to settling of sand, shale, anhydrite or salt while not circulating. Requires a little more arithmetic.

Arithmetic is more complicated. Requires more on-choke circulating time. Higher casing and casing shoe pressure than Wait and Weight Method.

Less casing pressure than Driller’s Method. Can easily be switched to Wait and Weight Method.


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06.08 PRESSURE CONTROL SCHEMES General

When a kick occurs there are a lot of facts to record and analyse. These facts are recorded in a so-called work sheet, a pre-planned scheme. These work sheets, when completed will give us a complete picture of the conditions and calculations in a kick situation. These work sheets can differ greatly from company to company but they all have the same basic content. Contents of Work Sheets

A work sheet will contain, in one way or another the following facts: Information that is Previously Known Equipment:

Drill string: Drill collars:

Dimensions, capacity, etc. Inner and outer measurements, length, etc.

Bit diameter: Casing: Open Hole:

Dimensions, depth measure and true vertical, capacity, etc. Total measured depth, true vertical depth.

Pumps:

Normal circulation rate and pressure. Reduced Rate Circulating Pressure. Pump output per stroke. Facts after the Well is Shut In

Standpipe pressure (SIDPP). Annulus pressure (SICP). Pit level increase (kick gain). Calculations

By using the previous mentioned work sheet (Kill Sheet) all required calculations to circulate out a kick on a safe manner is easily done. See Fig 67 Surface to bit strokes. Bit to surface strokes. Bit to shoe strokes. Kill mud weight. Initial Circulating Pressure. Final Circulating Pressure.


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Fig 67a M:\IWCF Surface\3\1\Section 5.doc

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CALCULATIONS OF DENSITY AND PRESSURE GRADIENT OF AN INFLUX

01.09 General

It is always important to analyse what the influx actually is when a kick is taken into the wellbore. It can be decided what the influx is (gas, oil or water) by making a calculation with height and pressure. The height of the influx is easy enough to find by the measured pit gain at the surface (bbl) and the annulus capacity that is already known in bbl/ft. hi =

kick gain (bbls) = (ft.) annulus capacity (bbls/ft.)

If we call the depth of the well H, drilling fluid weight MWm, we can work out the different pressures in the annulus and drill string and furthermore bottom hole pressure (formation pressure). See Fig 68. SIDPP

SICP

OMW

ANNULUS

DRILL STRING

OMW

H - Hi

H

BHP

Hi

Shoe

INFLUX

H x MWm x 0.052 + SIDPP = Pf = (H-Hi) x MWm x 0.052 + Hi x Wi x 0.052 + SICP

Fig 68

Pressure from drill string ==

Formation Pressure

H x MWm x 0.052 + SIDPP ==

F.P. == (H – Hi) x MWm x 0.052 + Hi x Wi x 0.052 + SICP

Hi

:

Height of influx


Wi

:

== Pressure from annulus

Density of influx © MTC

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The equation can now be reduced to the following formula: Wi=

P sidp - P sia + MW m hi x 0,052

As the influx is either gas, oil, water or a mixture of same the density of the influx is lower than the drilling fluid with result that SICP is greater than SIDPP the formula can be expressed as follow: W i = MW m -

P sia - P sidp (ppg) hi x 0,052

When knowing the density of the influx pressure gradient can then be calculated as follows: Gi = Wi x 0,052 psi/ft

or in the following way: Gi = Gm -

P sia - P sidp psi / ft hi

02.09 Examples Example #1:

Influx OH – DC capacity Drilling fluid density TVD SIDPP SICP

25 bbl. 0.042 bbl/ft 15 ppg 7600 ft 265 psi 660 psi

Height of Influx: hi =

25 (bbl) = 595 ft. 0.042 (bbl / ft)

Gradient of Influx: G i = 15 x0.052 -

660 - 265 psi / ft = 0.116 psi/ft 595

Comparing the pressure gradient with table Fig 06 it can be seen that the influx is gas or a mixture of gas/oil. Example #2: Same as example #1 except that SICP is 450 psi. M:\IWCF Surface\3\1\Section 5.doc

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Gradient of influx: G i = 15 x0.052 -

450 - 265 psi / ft = 0.469 psi/ft 595

Comparing the pressure gradient with table Fig 06 it can be seen that the influx is water. We should be aware that there could be margin in these calculations of error. The accurate annular or DC-OH capacity is not known due to wash out etc. and the results are therefore quite unsure and shall not be used for anything else than to get a rough index of what the influx is. The circulation of a kick is also not dependent on what the influx is. Therefore this particular calculation is not relied upon to any great extent. At the same time it can be advantageous to know whether it is a gas kick or oil/water kick that we have to deal with.

If it is a gas kick we can be prepared for the high casing pressure and pit volume increase towards the last stage of circulation, which will not occur if the influx is a fluid. The pressure gradients for influx are as noted below. Between 0.47 to 0.52 psi/ft. the influx is saltwater. if the influx is less than 0.16 psi/ft. the influx is gas. Between 0.31 and 0.42 psi/ft. the influx should be oil, but it could also be a mixture. See Fig 06. If we are in doubt about the result, treat the kick as a gas kick, and in this way the most dangerous situation will be expected.


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Lost circulation

01.10 General

Lost circulation is one of the most serious problems that occur in rotary drilling. Lost circulation is defined as loss of drilling fluid into the formation, which can be total. As most wells are drilled there is experienced a lesser or greater loss of drilling fluid to the formation. Lost circulation is both expensive and time-consuming (price of drilling fluid and lost rig time). In connection with well killing operations lost circulation is extremely dangerous. See Fig 69. Losses can best be described as unintentional transfer of fluid from the borehole into the formation.

When describing losses, the duration for which they occur needs to be taken into account, e.g. a 10 bbl loss that stops after 5 min, should not be reported as 120 bbl/hr losses! It should also be recognised that the rate of losses will change under static or dynamic conditions. The description of losses can differ from operator to operator but falls into the following categories: a. No losses - less than 2 bbl/hour. b. Seepage Losses - between 2 and 5 bbl/hour. c. Partial Losses - between 5 and 50 bbl/hour. d. Severe Losses - greater than 50 bbl/hour. e. Complete Losses - unable to maintain a fluid level at surface with the desired mud weight, regardless off pumping rate f. Static Losses - The losses that occur when the well is not being circulated and the drill string is stationery. g. Dynamic Losses - The losses that occur when the well is being circulated, or the drill string creating surge pressure. Fig 69

When a kick occurs and the well is shut in, the drill pipe pressure informs us of the extra pressure required balancing the formation. The problem is that when the well is shut in it is difficult to decide if in fact lost circulation has, or is occurring. The biggest problem is therefore the uncertainties. 02.10 Causes of lost circulation

The three most common causes that lost circulation arise in connection with a kick. l.

Bad cementing job.

2.

Caused formation breakdowns.

3.

Fissured and Fractured Formations.


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Bad cementing job

One of the most common causes of lost circulation in a kick situation is a bad cement job at the base of the last casing string. Most operators insist that the cement is pressure tested after drilling the shoe, to test its strength and bond to the casing. The test pressure shall take into account the highest drilling fluid weight that is to be used in the next phase of drilling or shall follow legal requirements. A bad cement job is dangerous because it can allow gas to escape up the side of the casing to the surface. Large gas blowouts have occurred in oilfields because of this. Formation breakdown

This cause of lost circulation is most common. The breakdown can be caused by large pressure fluctuations, the use of drilling fluid that is too heavy or from blowout conditions. In most cases after the pressure falls the breakdown in the formation will close itself up in a relatively short time. Such a formation breakdown often occurs around the casing shoe and in effect is exactly the same as a bad cement job. Fractured and fissured formations

In hard formations fractures and fissures can be the cause of serious lost circulation problems. It can be difficult to stop these formations taking drilling fluid. In many cases these kind of formations are the actual reservoir beds, and the pressure which is used to balance the reservoir is often very close to the pressure that will cause breakdown and resultant lost circulation. 03.10 Well control with partly lost circulation

In most cases the first sign that lost circulation is occurring is a fall in the drilling fluid pit level. Fig 70 and Fig 71 shows the relationships in such a situation. PIT LEVEL

PDP NATIONAL

1100

PA BOP

900 700 500 LOST CIRCULATION 300

WEAK FORMATION

100 TIME

Fig 70 Fig 71


LOST CIRCULATION

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Fig. 70 shows a graph of pit. The solid line shows how the pit level changes during a gas kick, from when the kick is taken to when it is killed. The dotted line could show how the pit level will change if part-lost circulation occurs while killing a gas kick. Fig. 71 shows the situation in the well. The gas is circulated a way up the annulus and breaks out of the wellbore into a relatively weak zone. This zone cannot withstand the pressure of the choke combined with the hydrostatic pressure, and therefore drilling fluid will flow into the formation. If the fluid column falls under circulation, there are several methods that can be used to combat this problem, and are as follows: l.

If the lost volume is not too great and the drilling fluid volume can be made up by mixing new drilling fluid, go ahead. The pressure on the weak zone will decrease as the gas bubble passes upwards. The problem solves itself. When circulating with part-lost circulation the pressure at the choke will be the highest casing pressure that the formation can withstand. Every 30 minutes the choke is closed partly so the pressure in the well bore increases 100 psi. If the annular pressure does not increase, open the choke to the same setting as before and continue circulating out the influx. If the well bore pressure increases check for similar increase on the drill pipe pressure. If the drill pipe pressure does not increase, open the choke to the same setting as before and continue circulating out the influx. If both the drill pipe and annulus pressure increases the losses are decreasing and the formation is healing itself. If so shut the well in and record the new SIDPP:

2.

Stop the pump and close in the well. Give the well from 30 minutes to 4 hours to heal itself up. Hold SIDPP constant by regulating the choke. If the casing pressure rises by more that 100 psi continue to circulate out the influx.

3.

Decide on a lower circulation rate and a new initial circulation pressure. Consider the well as being closed in and proceed as follows: a.

By manipulating the choke keep casing pressure constant while the pumps are brought up to the new lower circulating rate.

b.

Adjust the choke until the annular pressure is the same as when the well was shut in (this method not good for sub-sea wellheads). Proceed accordingly now that a new initial circulation pressure of drill pipe is known.

4.

Mix a pill of lost-circulation material of a type, which will be effective on the formation in question. Normally lost circulation material is more effective in hard formations and less effective in softer plastic formations.

5.

If the losses continue after the above-mentioned solution has been tried a barite or barite/diesel plug can be pumped in attempt to seal off the weak zone.


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04.10 Well control with total lost circulation

Standard blowout control procedure cannot be used if the well cannot be circulated. With total lost-circulation gas can rise up to the surface, but there is also the danger of an underground blowout. The only way to solve the problem is first to stop loss of drilling fluid to the formation so the well can be killed with the help of standard procedure. l.

Barite Plug: The best solution with a gas kick is to try and plug the gas zone with a barite plug and proceed to seal the lost circulation zone. In the meantime it is possible that there can occur a high speed underground flow of formation fluid/gas into the weak zone. This flow could possibly wash away the barite plug, so to try to prevent this a plug as large as 300 ft in height should be used. See Fig 72. BARITE PLUG MIXTURE for 300 ft. 180

15”

100 0s xB

160

Water in bbl.

140

- 15 0 lb

12-1/4”

120 For a

100 9-7/8”

80 60 40 20

arit e

700 sx 17-1 /2

8-3/4” 7-7/8”

335 sx B ari

15

sph ate

Bari te - 1 00 lb Pho ” ho sph le us ate e tw ice t he m ix fo ra

425 sx Barite - 50 lb 270 sx B ari

6-1/2”

P ho

te - 50 lb

te - 35 lb

185 sx Barite -

ole

Phosp hate Phospha te Phospha te

25 lb Phosph

ate

17

16

12-1 /4” h

19

18

Mud Weight -lb/gal Fig 72

Mixing procedure for barite plug: Add water, the Phosphate and finally Barite. Adjust pH to 9.0 using Caustic Soda. Use fresh clean water only.


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Gunk Plug: A gunk plug is a plug consisting of bentonite mixed with diesel fuel and is a very fast solidifying plug that is especially effective for water flows. The plug will not start to become stiff until it comes into contact with water, so there is no danger of premature setting. When the plug is pumped down in the bottom of the well the diesel is washed away from the solids, which begin to set as they come into contact with the water. A large plug shall be used, about 300 ft. in height. An oil plug shall be pumped before and after the gunk plug to prevent contamination of the plug with drilling fluid to avoid premature swelling of the bentonite. For gunk plugs in Oil Based drilling fluid Geltone II (MI product name) or similar is used. See Fig 73.

Prior to pumping gunk plugs all lines must be flush and cleaned.

GUNK MIX for 300 ft COLUMN HOLE SIZE inch

DIESEL OIL bbl

BENTONITE sacks

TOTAL VOLUME bbl

6-1/2” 7-7/8” 8-3/4” 9-7/8” 12-1/4” 15” 17-1/2”

9 13 14 20 33 50 66

27 40 49 62 98 150 200

12 18 22 28 44 66 89

Fig 73

A thick mixture of rough lost circulation material can sometimes be pumped down the annulus via the kill line, to seal off the thief zone. HALLIBURTON

Fig. 74 illustrates the conditions in the well with regard to the pumping of a plug to contain the kick zone, and illustrates also the manner in which lost-circulation material is pumped down into the weak zone.

PA BOP

KILL LINE Lost Circulation Material

WEAK FORMATION

PLUG


INFLUX

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VOLUMETRIC WELL CONTROL.

01.11 General

Volumetric Method is used, if gas or gaseous influxes for one or another reason cannot be circulated out. Examples of such a situation can be: - Prior to pumping kill fluid with conventional method. - Pipe off bottom. - Drill string or bit plugged. - Drill string out of hole. - Wash out in the drill string. - If drill string have been cut and left in hole. - Repairs to pumps or other equipment failure such that normal kill procedure cannot be exercised. When the gas bubble is down hole, the Volumetric Method can be used to allow the bubble to expand while it migrates up the hole, keeping bottom hole pressure constant. The basic of this method is the knowledge that every bbl of fluid gives a certain bottom hole pressure. This pressure can be measured in psi/bbl by dividing the fluid gradient psi/ft with annular volume in bbl/ft or the volume of the well bore in bbl/ft if there is no drill string in the hole. As the gas migrates up the annulus, the annular capacity usually changes. It is therefore necessary to estimate the location of the gas, calculate the correct annular volume and control the casing pressure accordingly. In general with pipe in the hole casing/drill pipe capacity is used due to this is the longest section the gas has to migrate. The amounts of fluid which are to be bled off or pumped (lubricated) into the well, must be measured precisely enabling us to have exact control of pressure in the well bore. A migration rate exceeding 1000 ft/h (300 m/h) makes the Volumetric Method a fair alternative, but keep in mind that recent research has show that gas is able to migrate as fast as 10.000 ft an hour under ideal condition even in highly deviated wells. 02.11

Volumetric Method Specification required

1. Closing the well When the well is shut-in, take accurate readings of the shut-in casing pressure (SICP) and pit gain. Make note of the time when readings are taken. An increase in closed-in pressures confirms that the influx is gas and migration is taking place. 2. Estimating the migration rate When a gas influx is taken, the large density difference between gas and drilling fluid will cause the gas bubble to migrate up the hole. As gas migrates, without expansion being permitted, pressure throughout the wellbore increases. M:\IWCF Surface\3\1\Section 5.doc

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The velocity of gas-migration depends on hole size, gas and fluid densities, fluid viscosity and whether gas influx is one big bubble, or distributed as many smaller bubbles. A common rule of thumb is to assume a gas migration velocity of between 500 ft to 1000 ft per hour. If oil base drilling fluid is in use, gas migration may be limited by solubility or gas/oil miscibility effect. The following discussion of gas migration applies to water base drilling fluid only. The distance that gas has migrated and the rate of migration may be estimated as follows See Fig 75: 300 psi

PA

12.5 ppg 10000 ft

GMD

P2 - P1 = -------------------------MWG

GMR

GMD = -------------------------T 2 - T1

Where:

GAS

GMD = Gas migration distance MWG = Mud gradient P1 = Surface pressure at time T1 P2 = Surface pressure at time T2 GMR = Gas migration rate ( feet per hour) T1 = Time 1 (hour) T2 = Time 2 (hour)

6500 psi Fig 75

03.11

Volumetric Method Handling

1. Shut the well in and record the initial shut-in casing pressure SICP, Pit Gain and Initial Shut-in Time. 2. Allow the casing pressure to increase by approx. 100 psi (P1) above the original shut-in pressure for safety factor. The safety factor is used because the pressure will always fluctuate a little depending on the man at the choke, so by using a safety factor we make certain that the bottom hole pressure does not drop below the formation pressure so further influx is taken into the wellbore. P2 = SICP + P1


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3. Allow a new pressure increase 50-100 psi (P3), but do not exceed the fracture pressure at the casing shoe. This pressure is called working range and will determine the amount of fluid in the well bore that represents this pressure. P4 = SICPP + P1 + P3 = P2 + P3

4. Calculate the migration distance, corresponding to P3. P3 GMD = -----------MWG

5. Calculate the volume of fluid corresponding to the pressure increase, i.e. volume to bleed off (Vm). P3 x Cap Vm = Cap x h = -------------MWG

Cap = Hole capacity refers to the capacity directly above the bubble. In practice it is usually acceptable to use capacity of the casing below BOP’s for the following reasons: Open hole capacity is generally close to casing capacity. Only small volumes of fluid are bleed from the well, when the bubble is in open hole. Most of the increase in surface pressure and associated fluid occurs as gas approaches surface, where hole capacity is known accurately. 6. As the annulus pressure increases above P4 bleed of the calculated volume (Vm) gradually maintaining the pressure P4 at the choke. 7. After having bleed off the calculated volume (Vm), let the pressure build up to P5: P5 = P4 + P3

8. Repeat points 6 and 7 until casing pressure stabilises as the gas reaches the surface. 9. As gas is bled out of the hole the bottom hole pressure will decrease. Additional fluid should be pumped (lubricated) back into the well bore to maintain a constant bottom hole pressure to prevent an additional kick. 04.11 Lubrication Technique

This method is used to reduce the casing pressure when gas is at the surface so that another operation such as stripping or snubbing can be performed. 1. Calculate the hydrostatic pressure, which will be exerted by a certain volume of drilling fluid in the annulus. If we use the same working range as before the volume will be the same. M:\IWCF Surface\3\1\Section 5.doc

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2. Slowly pump the given volume of fluid into annulus through the kill line. Allow the fluid to ”fall” through the gas (by gravity). Low yield point fluids are preferable. A small pressure increase (∆P) may occur due to compression of the gas bubble. 3. Bleed gas from annulus until the surface pressure is reduced by an amount equal to the hydrostatic pressure of the fluid pumped in. Do not bleed off drilling fluid.

If the annulus pressure increases during ”pumping in” procedure, the amount of this increase (∆P) should be bled off in addition to the pressure bled for hydrostatic pressure increase. If drilling fluid starts coming back shut-in the choke and wait for the gas to percolate to the surface before continuing to bleed off. 4. Repeat this procedure until all gas has been bled off or the desired surface pressure reached. Lubrication

During the pumping and gas bleeding, it will usually be necessary to decrease the volume of fluid to be pumped before the gas is bled of completely. This is because the annular volume occupied by the gas decreases with each pumping and bleeding sequence. If the Volumetric Method is going to be used it is important that we have the right equipment and drills have been carried out with all the crews. See Fig 76. 5

BOP

HALLIBURTON

1

PA 2

KILL LINE

1. Accurate pressure gauge on annulus side. 2. Adjustable choke (manual). 3

3. Trip or strip tank with accurate measurement.

4

4. Pump to empty strip/trip tank.

PUMP GAS

5. HP pump with accurate displacement tanks. Fig 76


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05.11 Volumetric MethodExample

Casing: Hole: EFD/Leak off: 1.

TVD/MD 5.000 ft - 9-5/8” - 47lb/ft - N-80 - Cap. 0.073 bbl/ft TVD/MD 10.000 ft - 8-1/2” - Cap. 0.070 bbl/ft 17.5 ppg MW: 10 ppg Pit V: 600 bbl

Shut in data:

SICP: 2.

243 psi

Pit gain: 3 bbl

Overbalance(SF): (P1 = Approx 100 psi)

P2 = SICP + P1 3.

P2 = 243 + 100 =

Pressure increase: (P3 = Approx 50 psi)

P4 = P2 +P3 2.

P4 = 343 + 50 =

393 psi

Height of gas in Annulus corresponding to P3

P3 H = -----------MWG 5.

343 psi

50 H = ----------- = 10x0.052

96 ft

Vm = 0.07 x 96 =

6.73 bbl

Volume to be bled off

Vm = Cap x H

Fig 77

BOP HALLIBURTON

Vm

PA Vm

KILL LINE

6

GAS

3 P3

P3 Vm 5 P3 Vm 4 Pa

P3 6

2

P1 1 5

GAS

4

GAS

S I C P BLEED OFF

3 2

GAS GAS

1

GAS


LUBRICATE

P3 P1

BHP

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1.

Influx into the wellbore and SICP recorded to 243 psi.

2.

The gas bubble is allowed to percolate without expansion increasing the annulus and BHP with the safety factor (P1)100 psi to P2.

3.

The gas bubble is allowed to percolate further without expansion increasing the annulus and BHP with the working range (P3) 50 psi to P4. P4 = 243 + 100 + 50 = 393 psi

4.

While keeping 393 psi on the annulus 6.73 bbl of drilling fluid is bleed off allowing the gas bubble to percolate and expand. While bleeding off the 6.73 bbl the BHP pressure will decrease 50 psi (P3).

5.

After first bleed off, the well is shut in and the gas bubble is allowed to percolate further without expansion increasing the annulus pressure and BHP with the working range (P3) 50 psi to P5. P5 = 243 + 100 + 50 + 50 = 443 psi

While keeping 443 psi on the annulus 6.73 bbl of drilling fluid is bleed off allowing the gas bubble to percolate and expand. While bleeding off the 6.73 bbl the BHP pressure will decrease 50 psi (P3). 6.

After second bleed off, the well is shut in and the gas bubble is allowed to percolate further without expansion increasing the annulus pressure and BHP with the working range (P3) 50 psi to P6. P6 = 243 + 100 + 50 + 50 + 50 = 493 psi

While keeping 493 psi on the annulus 6.73 bbl of drilling fluid is bleed off allowing the gas bubble to percolate and expand. While bleeding off the 6.73 bbl the BHP pressure will decrease 50 psi (P3). By continue this procedure the gas is brought to surface and the operation is reversed so 6.73 bbl of drilling fluid is lubricated into the well bore after witch (P3) 50 psi + ∆P is bleed off. See Fig 77. Fig 78 shows a work sheet to be used to keep control when using the Volumetric Method and Fig 79 illustrate the pressures in the wellbore while using the Volumetric Method.


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Worksheet for the Volumetric Method Pit volume increase Vm

Psi/bbl

Pressure inc. P3

Original pressure P4

New pressure

Total pit volume

Fig 78

PRESSURE

Gas bubble pressure Bottom hole pressure Annular pressure Drill pipe pressure

BLEED OFF

LUBRICATE

TIME Fig 79


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06.11 Low Choke Method Dynamic Kill

This method of well control is occasionally proposed for handling shallow gas kicks. If it is anticipated that shutting-in a kick will result in surface pressure above the maximum allowable, the well is allowed to flow through the choke (and kill) line and surface pressure is maintained slightly below the maximum allowable value. In this way the rate of influx may be sufficiently slowed to allow well control to be regained by circulating kill fluid down the drill string. There may be circumstances under which this technique can be implemented successfully, however there are inherent dangers. Initially bottom hole pressure is maintained at a value below the kicking formation pressure and inflow will therefore continue. The continued influx will reduce bottom hole pressure further as the annulus is unloaded. Only if kill fluid can be circulated into the annulus at a sufficient rate to overcome this unloading effect and increase the bottom hole pressure will well control be regained. The low choke method is an attempt to out run a kicking well, and should not be attempted except for handling shallow gas kicks. 07.11 Bullheading

Bullheading is generally recommended in the following circumstances: 1. If a kick is taken with the drill string far off bottom, or with no pipe in the hole. With the pipe close to bottom, stripping in should be considered. The decision to strip, as well as the stripping procedure, must allow for the effects displacing the influx up-hole and for the effect of gas migration. If the upward force (closed in pressure multiplied by the cross-sectional area of the closed-end drill pipe) exceeds the string weight, it will not be possible to strip in. 2. If the influx has the potential for containing H2S. 3. If circulating the kick out could result in excessive gas rates through the well control system. 4. If the influx is very large, resulting in excessive surface pressures. After shutting in the well on a potential kick, the decision of whether to bullhead or circulate out the kick must be made very quickly after considering the following subjects. See Fig 80

1. Stabilised SIDPP and SICP – Do the pressures stabilise very quickly, indicating a kick from a high permeability formation? Is gas migration evident? 2. Influx volume and fluid type. 3. What are the estimated fracture pressure gradients for the shales and any exposed sand(s) in the open hole? How do they relate to the shoe strength (LOT)? How are shales and sands distributed in the open hole? Is fracturing the hole (with potentially ‘charged’ formations) an acceptable consequence? M:\IWCF Surface\3\1\Section 5.doc

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4. Pressure limitations of pumping equipment, wellhead equipment, and casing shoe tests. SIDPP

SICP HALLIBURTON

SICP

SICP BOP

KILL LINE

GAS INFLUX

HALLIBURTON

KILL LINE

SIDPP

SICP BOP

BULL HEADING

INFLUX

Fig 80

5. If a gas influx is suspected (shut in pressure continues to rise indicating migrating gas in water base system), pumping rate for bullheading must be fast enough to exceed the rate of gas migration. If pump pressures increase instead of decreasing, this is an indication that the pumping (injection) rate is too slow to be successful. This can be a problem in a large diameter hole. 6. The possibility of breaking down the formation of long open hole sections beyond the last casing shoe rather than the producing formation. This could provoke the development of an underground blowout. Bullheading Procedure

1. 2. 3.

4.

Ensure that sufficient fluid of the current weight is available for the operation and that the line to the kill pump suction is clear. Line up BOP and choke manifold to pump down lower kill line. Pressure test the surface equipment to above the maximum injection pressure. Start the bullheading operation at a sufficiently slow rate such that the volume versus rate relationship can be monitored. Attempt to keep the rate constant during the operation and plot up volume versus rate as per leak off graph. Allow for the compressibility of the drilling fluid as the pressure is brought up to the injection pressure. As bullheading continues, the surface pressures should theoretically decrease as lower density influx is displaced by higher density fluid. Surface pressures should


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be monitored and plotted at regular intervals to check that the influx is being bullheaded away. If the injection pressure does not fall it may be as a result of fluid being injected into a formation above the influx. See Fig 81 The injection pressure may increase during the operation as the permeability of the reservoir is damaged. If the injection pressure approaches the maximum allowable surface pressure, stop the pumps and allow the pressure to stabilise. Recommence at a slower rate keeping within the maximum pressure limitations. If it becomes impossible to bullhead without exceeding maximum pressure limitations i.e. fracture pressure, the decision to continue bullheading operations in excess of this pressure will depend upon the volume of the remaining influx and the position of the bit in the hole. Once the calculated volume of influx has been bullheaded back to the formation, bleed off trapped pressure and shut in the well to monitor drill pipe and casing pressures. If the shut-in pressures have fallen, then it is a fair assumption that the operation has been partially successful. It should be remembered that if the kick was taken whilst drilling. It is unlikely that the drill pipe and casing pressures will read the same due to the dissemination of the influx in the fluid. If bullheading was seen to be successful, then it should be continued until the drill pipe and casing pressure are similar. The subsequent well kill operation to secure the well will depend on how the kick was taken. If the influx was taken whilst drilling, then the well can be killed using the wait and weight method utilising the original shut in pressure information. If the pipe is off-bottom, then it will be necessary to strip back to bottom using standard stripping procedures. A circulation should then be performed, maintaining constant bottom hole pressure, to clear the hole of disseminated gas. If the procedure is not seen to be successful, then consideration will have to be given to: Stripping back to bottom if necessary and circulating out the influx at a rate dependent on its size and the limitations of the surface equipment. Beginning operations leading to the suspension of the well.


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1300 Formation Strength 1100 900

1

Pump Pressure

700

2

500

3 300

100

Volume pumped Fig 81

1:

Bullheading taking place over the influx with plugging of the formation taking place.

2:

Bullheading taking place over the influx.

3:

Bullheading influx into the formation as pump pressure reduces.


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Kick with bit off bottom

01.12

Introduction During drilling, completion and work-over operations it sometimes becomes necessary to trip tubular through the BOP’s under pressure. The procedures used are called Stripping and Snubbing. Stripping: This procedure is used when the pipe weight is sufficient to overcome the upward force created by well pressure acting on the cross-sectional area of the pipe. Snubbing: This procedure is used when the pipe weight is not sufficient to overcome the upward force created by well pressure on the cross-sectional area of the pipe. In this case an external force must be applied to move the pipe through the BOP’s. 02.12

Stripping

Stripping is an emergency well control procedure. It requires good planning, proper training of personnel and careful execution. The primary objective of the stripping operation shall be to maintain a constant bottom hole pressure, thus preventing a build up of excessive wellbore pressures or influx from exposed permeable zones. The following are guidelines for carrying out a successful stripping operation: 1. Pressure control is based on a volume balance. This means that for every barrel of pipe stripped into the hole, a barrel of mud must be bled off. Since it is necessary to install an Inside BOP before stripping, total displacement must be considered, including both pipe displacement and internal capacity. 2. Mud bled from the annulus must be accurately measured in order to maintain the correct volume balance. 3. Annulus pressure should not be constant while stripping pipe into the hole. It should gradually increase as the pipe is stripped into the lower density kick fluid. This is due to the increased length or height of the influx fluid in the annulus and the resultant loss of hydrostatic pressure. 4. When stripping through the annular preventer, the closing pressure on the preventer must be adjusted to allow a small amount of leakage to lubricate and reduce wear on the sealing element. The mud, which is allowed to leak past the annular preventer, should be measured along with the mud bled through the adjustable choke. 5. Drill pipe with casing wear protectors should never be stripped through the annular preventer, because excess friction and wear would be generated due to the rubber to rubber contact. 6. If stripping is to be carried out with two sets of pipe rams, then a side outlet is required between the rams. This is necessary to enable the pressure to be equalised, before opening the rams. Opening rams without equalising the pressure will shorten the life of the sealing element and create excessive pressure surge in


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the BOP. Pressure from the well must not be used to equalise the pressure across the rams. The objective in all stripping operations is to maintain a constant bottom hole pressure, slightly greater than the formation pressure, throughout the entire operation. 03.12 Closing Procedures

Stripping procedures must be adjusted to suit the well conditions and the equipment, which is available. A specific procedure should be developed for each situation. The following guidelines provide a basis for the design of detailed procedures closing in the well on a kick, with pipe off bottom and stripping back to bottom: To avoid excessive surface pressures, the correct closing in procedure as outlined should be adopted, i.e. Close in the well at the first indication of flow. 1. 2. 3. 4. 5. 6. 7. 8.

Install a FOSV on the drill pipe, in the open position. Close the FOSV. Close the annular preventer. Open the HCR valve. Close the automatic choke (if not already closed). Make up Topdrive. Open the FOSV. Record and monitor the drill pipe and the casing pressure.

Ensure that the above steps are executed as quickly as possible. The Gray IBOP can be installed when ready to strip in. 04.12 Rig layout for combined stripping and volumetric method

In general, the annular preventer is used for stripping pipe into or out of the hole. The annular preventer allows the use of one preventer and permits the tool joints to pass through the packing ACCUMULATOR BOTTLE PRECHARGE @ 400 PSI

OPEN

ANNULAR PREVENTER BALL VALVE CLOSE


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element without creating excessive pressure surge in the well bore. To minimise the wear, the pipe should be well lubricated with grease and closing pressure applied to the annular preventer kept to a minimum. A surge bottle should be installed as close to the annular preventer as possible. See Fig 82. Regardless of the method used to strip pipe into the hole and enable effective pressure control, it is very important to measure all of the fluid that comes out of the well bore. Formation fluid that has entered the well bore may be gas and during stripping operation migration may take place, so it is essential that rigs are suitably rigged-up to immediately implement the volumetric method. See Fig 83. 1.

Annular preventer. See Fig 82

2.

Accurate pressure gauge.

2

1

3.

Adjustable choke.

4.

Piping from choke manifold to trip tank.

5.

Calibrated trip tank.

6.

Calibrated stripping tank.

3

4

5

6

Fig 83

05.12 Procedure 1: After closing in the well, determine the influx volume and record pressures at two minute intervals. After closed in pressures have stabilised; further record pressures at five minute intervals.

2:

Determine a convenient working pressure increment Pw

3:

Convert the working pressure Pw of say 50 psi into an equivalent working volume V in the OH/DC annulus (the volume of fluid to be used for volumetric control steps). See Fig 84. PW x Cap V = Cap x h = -------------MWG


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Fig 84

117

Fig 85

V H2 Expansion of gas influx V 4:

H1

Determine the extra back pressure Ps to compensate for the loss of hydrostatic pressure as the bit and drill collars are run into the influx. If the influx is assumed to be in the open hole beneath the bit, an increase in surface pressure will be required to maintain BHP above Pf when this event occurs. It is unknown when the extra back pressure will be required since the exact position of the influx is unknown; it is therefore advisable to adopt a suitable safety factor from the very start of the stripping operation. Since overbalance (trip margin) will exist in nearly all wells which kick during round tripping, it is not possible to use closed in annulus pressure SICP to make an accurate estimate of the magnitude of the influx and thus the additional back pressure required to compensate for the previous mentioned loss of hydrostatic head. It is therefore essential to accurately measure the influx volume gained at surface, and by application of a factor based on the ratio open hole to DC/OH annulus, calculate the expected loss of hydrostatic head as the DC’s enter the influx. See Fig 85. (Mud Gradient - Influx Gradient) x Influx Volume Ps = ---------------------------------------------------------------------DC/OH Capacity

5:

Adjust the closing pressure on the annular preventer to a minimum, but avoid leakage. Whilst reducing closing pressure check continuously for flow.

6:

Allow annulus pressure to build up to PCHOKE whilst stripping the first stand. PCHOKE = SICP + PS + PW Where SICP = Initial closed in annulus pressure before second build up.


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=

Allowance for the loss of hydrostatic head as DC’s enter the influx. PW = Working pressure increment. See Note #1 Maintain PCHOKE constant whilst further stripping pipe. The volume increase due to closed end displacement of drill pipe is purged into the trip tank and after stripping the entire stand bleed off into the stripping tank the volume equal to the closed end displacement of one stand. The increase in the trip tank volume is due to the expansion of the gas influx only and reflects the loss of hydrostatic head in the well. See Note #2

8:

Avoid excessive surge pressures by adjusting the pipe lowering rate to allow chokeman to maintain PCHOKE constant.

9:

Maintain PCHOKE constant at the above value until a volume of mud V bbl has accumulated in the trip tank while simultaneously strip pipe in the hole.

10:

When the additional mud volume V bbl has accumulated in the trip tank (at constant choke pressure), PCHOKE is allowed to increase again by value PW and now becomes PCHOKE1.

Pchoke + Pw= Pchoke1

V

PCHOKE1 = PCHOKE + PW See Fig 86

11:

Fill each stand run and file off any sharp edges or tong marks from the pipe body and tool joints. Coat drill pipe with grease prior to stripping in the hole.

Pw

Expansion of gas influx

Fig 86

12:

By repeating this cycle, as often as necessary gas is able to percolate upwards and expand while a nearby constant BHP is maintained.

13:

Values of pressure and volume should be recorded in table throughout the stripping exercise.

With the bit on bottom the well can be killed using the “Driller’s Method” first circulation, but first ensure that the entire string is full of mud.


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Note 1:

A length of the first stand will be stripped against the closed in well until the required stripping choke pressure, PCHOKE, has been reached. Only the remainder of the stand, which is stripped at constant choke pressure, should be considered when bleeding off the closed end displacement volume. For example, if the required PCHOKE is reached after stripping two singles of the first stand, only one third of the closed end displacement volume should be bled off into the stripping tank. The same principle will of course apply when PW increment are added. Note 2:

Should, during the stripping operation, bottom hole pressure inadvertantly drop below formation pressure (BHP < PF), a second influx will take place. The method makes allowance for this eventuality and re-established the required PCHOKE by overcompensating for the loss of hydrostatic pressure caused by the new influx. This is achieved automatically due to the manner in which PW has been calculated. PW compensates for loss of hydrostatic pressure assumed opposite the DC’s. A second influx will enter in the open hole section resulting in a volume gain at surface, where it will be interpreted as a volumetric step. The well will be closed in and PCHOKE allowed to increase by PW. The effect, of course, will be to overcompensate the underbalance that existed in the well. In other words it is impossible to loose hydrostatic control of the well since the method is self correcting.

06.12 Snubbing

Snubbing involves moving pipe in and out of a well under pressure, while maintaining constant bottom hole pressure. The operation is very similar to stripping except that the pipe will not move into the well under its own weight and must be forced in through application of external force at the surface. Snubbing operations are much more dangerous than stripping operations and always involve the use of specialised equipment and personnel. Two types of snubbing system are generally employed namely mechanical snubbing units and hydraulic snubbing units. Mechanical snubbing units require the use of the drilling rig’s hoisting system, while hydraulic snubbing units are self contained.


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Gas Cut Drilling Fluid

01.13 General

Gas cut drilling fluid is the term used when the drilling fluid contains a percentage of gas in the form of small bubbles, when it returns to the surface. Generally gas cut drilling fluid does not decrease the hydrostatic pressure so much as to cause underbalance/kick situations. This is because the gas content in the drilling fluid is mostly compressed, except very close to the surface. Every atmosphere (14.7 psi) reduces the gas volume by half. Therefore the drilling fluid weight considerably reduces the volume of the gas. If the volume of the gas in the drilling fluid is very small the reduction in bottom well pressure will also be very small. Fig. 87 shows a typical example of pressure reduction bottom well caused by gas cutting of drilling fluid. 20

-9

50% cut

pg

-5 10 p pg

ppg

ppg

33.3% cut

18 p

3

ppg 18 ppg - 13 .5 ppg 10 ppg - 6.66 ppg 18 ppg - 12 pp g

4

25% cut

10 ppg - 7.5

5

18 ppg - 16.2 ppg

7 6

10% cut

10 ppg - 9 ppg

DEPTH in 1000ft

10 9 8

2

1 0

20

40

60

80

100

120

DECREASE IN BHP (psi)

Fig 87

It is very important to understand that the gas expanding as it nears the immediate surface causes almost all the bottom well pressure reduction. Therefore flow line drilling fluid weight can be very low in some cases. 02.13 Causes of gas cut drilling fluid

Gas cut drilling fluid can occur because of three reasons:


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1.

When a gas bearing formation is penetrated the cuttings will always release an amount of gas into the drilling fluid. This will be the first gas to register at the surface, and is a positive indication that a gas bearing formation has been penetrated. This type of gas will not cause a drilling fluid weight reduction, but if there is any doubt, pick up the drill string, shut down the drilling fluid pumps and check for flow.

2.

Another cause of gas cut drilling fluid is that some formations with a very low permeability have a pore pressure, which is bigger than the hydrostatic pressure from the drilling fluid column. So long as the drilling fluid is circulated there is a small overbalance in the well because of pressure loss in the annulus. When circulation is stopped a small underbalance will occur, ant this causes varying amounts of gas to intrude into the wellbore. This often occurs when the pumps are shut down during a connection or during a trip and these conditions are respectively called Trip Gas and Connection Gas.

3.

The third cause of gas cut drilling fluid can be a washout in the wellbore. This washout or cavity acts as a trap for old gas cut drilling fluid which is picked up by the drilling fluid at a later period in time and transported to the surface.

Calculations to estimate the change in hydrostatic pressure caused by gas cut drilling fluid

Reduction of bottom well pressure caused by gas cut drilling fluid can be calculated by using the following formula: ∆ P = 2.3 x N x Log BHP ∆P

=

Reduction in pressure in physical atmospheres where 1 ATM = 14.7 psi.

N

=

Original MW – Gascut MW Gascut MW

BHP =

Bottom hole pressure in physical ATM.

Example:

Well depth OMW GMW

12.000 ft 14 ppg 7 ppg

12.000 x 0.052 x 14 = 6.37 ATM 14 - 7 x log------------------------------P = 2.3 x ----------14.7 7

94 psi

The pressure in the well (bottom well) is therefore reduced by 94 PSI which answers to a change in drilling fluid weight of 0.15 ppg.


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A more practical and precise method for calculating bottom hole pressure reduction is reached by using the volumetric method. The volumetric method is used in the following way. The volume increase in the drilling fluid tanks that has flowed back due to gas cut drilling fluid is measured. This figure can be used to calculate the change in hydrostatic pressure in psi/bbl units by the following formula: ∆P=

∆ Pit volume x (0.052 x OMW) Annular capacity at surface

Fig 87 shows that even with a flow line weight reduction of 50% through gas cutting, bottom hole pressure is not seriously affected, as the reduction is less than the change that is caused through pressure loss in the annulus. Although pressure reductions from gas cut seldom cause underbalance, there are other factors that can lead to dangerous situations. Foremost gas cut drilling fluid is an indication of (possible) low drilling fluid weights, and pump effectiveness can be seriously reduced by gas cut drilling fluid. If drilling fluid becomes seriously gas cut the pump output is seriously decreased, that can lead to a following fall in annulus pressure loss, fall in bottom hole pressure and therefore risk of influx and blowout. It is therefore most important that gas cut drilling fluid is de-gassed (gas content extracted) before it is pumped down hole again. It may be that he most common fault in connection with gas cut drilling fluid is the tendency to maintain the original drilling fluid weight with barite without removing all the gas from the drilling fluid. When a moderate gas cutting gives a relatively small change in hydrostatic pressure, it is possible that addition of barite to increase drilling fluid weight can lead, in extreme cases, to lost circulation.


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Gas Kicks in Oil Based Mud

Early detection of gas kicks in oil based mud is of particular importance. The behaviour of hydrocarbon gases in an oil based drilling fluid is fundamentally different from their behaviour in a water based drilling fluid. These differences must be understood to allow safe handling procedures to be followed. The solubility of methane in diesel oil is approximately 100 times greater than in water, and therefore comparatively large gas flows (10 MMSCFD) can be taken into solution when circulating an oil based drilling fluid. The volume of the resulting solution is approximately equal to the sum of the gas and oil components, and therefore an influx will result in both a pit gain and an increase in return flow rate, as for a water based fluid. As shown in Fig 88 the expansion of a gas in oil solution, with decreasing pressure, is different from the expansion of the gas that occurs when a water based fluid is in use. Diesel alone

0

0 2.000 PRESSURE (PSI)

PRESSURE (PSI)

2.000 Methane alone 4.000 6.000

4.000 6.000

8.000

8.000

10.000

10.000 1

10 RELATIVE VOLUME

100

4% Methane in diesel

1

10

100

RELATIVE VOLUME

Fig 88

When a water-based fluid is in use, gas expansion occurs continuously, and the kick is therefore comparatively easy to detect. With an oil based mud there is negligible expansion until the solution reaches the bubble point, but at pressures below the bubble point the expansion is very rapid. The bubble point can be very difficult to determine due to a lot of unknown factors, but Fig 89 shows a typical phase equilibrium. Zone A: Zone B: Zone C: Zone D:

For pressure above the bubble point line and below the critical temperature the material in the reservoir is a liquid. For pressure above the dew point line the material in the reservoir is gas. For material outside the dew line the material is always gas. For material within the phase envelope the material is a 2 phase equilibrium mixture of free gas and its associated liquid.


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Zone C 3

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FREE GAS

1 Critical Point

6000

Lin e Bu bb le P oin t

10%

25%

2

e t Lin Poin

PRESSURE PSI

Dew

2-Phase Zone D

4

40%

0 - 200

400

TEMPERATURE F

800

Fig 89

If a hydrocarbon liquid at point 1 is expanded down line from 1 to 2, the pressure reach the bubble point line and the liquid starts to evaporate (boil) and bubbles of gas appear within the liquid. As the expansion proceeds more gas is produced at the expense of liquid. If hydrocarbon gas at point 3 is expanded down a line to point 4, the pressure is reduced to the dew point line and droplets of liquid starts to appear in the gas (the gas condenses). As the expansion proceeds, more liquid is produced at the expense of gas.


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04.13 Influx volume

In all previous calculations in well control we have presumed that the measured pit gain after shutting in the well was equal to the size of influx in the well bore. This prediction is due to that liquid is incompressible, witch is in fact not quit right. When performing a leak off test a certain amount of fluid is required to obtain pressure in the well bore. The greater the annular capacity is and the higher pressure applied the more volume has to be pumped into the well bore. This pressure/volume effect will also be applicable when looking on the size of an influx into the wellbore. As we drill deeper and longer the annular capacity is greatly increased and compression of the fluid has to be taking into consideration to determine if the handling capacity of our surface equipment is sufficient. Fig 90 shows the compressibility of a water base drilling fluid system: Applied pressure

500

600

Barrels of Water-base drilling fluid pressurised 700 800 900 1000 1100 1200 1300

500 1000 1500 2000 2500 3000 3500 4000

0.75 1.50 2.25 3.00 3.75 4.50 5.25 6.00

0.90 1.80 2.70 3.60 4.50 5.40 6.30 7.20

1.05 2.10 3.15 4.20 5.25 6.30 7.35 8.40

1.20 2.40 3.60 4.80 6.00 7.20 8.40 9.60

1400

1500

1.35 1.50 1.65 1.80 1.95 2.10 2.70 3.00 3.30 3.60 3.90 4.20 4.05 4.50 4.95 5.40 5.85 6.30 5.40 6.00 6.60 7.20 7.80 8.40 6.75 7.50 8.25 9.00 9.75 10.50 8.10 9.00 9.90 10.80 11.90 12.60 9.45 10.50 11.55 12.60 13.65 14.70 10.80 12.00 13.20 14.40 15.60 16.80

2.25 4.50 6.75 9.00 11.25 13.50 15.75 18.00

Fig 90

Example: A 10 bbl measured influx in a water base drilling fluid system of 1400 bbl and a SIDPP of 1000 psi. Influx to handle on surface: 10 bbl + 4.20 bbl = 14.20 bbl

This means that the 10 bbl influx measured as pit level increase is actual a 14.20 bbl influx, witch means that the volume of gas we have to handle on surface is 42% higher than expected.

When drilling with oil base drilling fluid the problem increases considerably and especially in the HPHT wells we are drilling to day we have to take this fluid compression seriously when evaluating handling method of an influx into the well bore. See Fig 91.


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Example: A 10 bbl measured influx in an oil base drilling fluid system of 1400 bbl and a SIDPP of 1000 psi. Influx to handle on surface: 10 bbl + 7.00 bbl = 17.00 bbl

Applied pressure 500 1000 1500 2000 2500 3000 3500 4000

500

600

Barrels of Oil-Base drilling fluid pressurised 700 800 900 1000 1100 1200 1300

1.25 1.50 1.75 2.50 3.00 3.50 3.75 4.50 5.25 5.00 6.00 7.00 6.25 7.50 8.75 7.50 9.00 10.50 8.75 10.50 12.25 10.00 12.00 14.00

2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

2.25 4.50 6.75 9.00 11.25 13.50 15.75 18.00

2.50 5.00 7.50 10.00 12.50 15.00 17.50 20.00

2.75 5.50 8.25 11.00 13.75 16.50 19.25 22.00

3.00 6.00 9.00 12.00 15.00 18.00 21.00 24.00

3.25 6.50 9.75 13.00 16.25 19.50 22.75 26.00

1400

1500

3.50 7.00 10.50 14.00 17.50 21.00 24.50 28.00

3.75 7.50 11.25 15.00 18.75 22.50 26.25 30.00

Fig 91

This means that the 10 bbl influx measured as pit level increase is actual a 17.00 bbl influx, witch means that the volume of gas we have to handle on surface is 70% higher than expected.


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Deviated and Horizontal Well Control

01.14 Introduction

From its early beginnings in the 1920s when it was regarded as a “black art”, directional and horizontal drilling has evolved to the point where it can truly be regarded as a science, although not always an exact science. The offshore and onshore drilling industry is founded on directional and horizontal drilling. Without the use of directional drilling techniques, it would not be economical to produce oil from most offshore fields. Improvements in directional drilling tools and techniques coupled with advances in production techniques have led to a steady increase in the production of wells drilled directionally and horizontal rather than vertically. As the search for oil and gas extends into ever more hostile and demanding environments, this trend will continue. This also means that normal well control practice used in vertical wells have to altered to meet the new demand for deviated/horizontal well control. The true vertical depth of the wells drilled to day is getting less while the measured depth is increasing making it harder to control bottom hole pressure and ensure that the influx is circulated out. The normal preferred method in circulating out an influx is the “Wait and Weight” witch means that the increasing hydrostatic pressure causes the drill pipe pressure to fall when circulating kill fluid from surface to bit. Pump strokes represent a certain measured length of fluid in the drill pipe. In a vertical well, the measured length of the fluid is the same as the vertical length of the fluid. In a deviated well, the vertical length of the fluid is less than the measured length of the fluid. This means that the pressure will drop less in a deviated well than in a vertical well per stroke. By calculating an average pressure drop across both the vertical and deviated sections of the well, the pressure will drop too slowly in the vertical section of the well. This means that by using our regular kill sheet in a deviated or horizontal well we tend to overpressure the well, which can lead to stuck pipe and lost circulation. These problems not only exist in deviated wells, but can also be created in vertical wells. We will have a look on a few pressure developments while circulating out an influx using the “Wait and Weight” method.

5 bbl

Ph

5 bbl

Ph

In a drill string with different size tubular (tapered string) the internal diameter change witch means that 5 bbl drilling fluid in a 5” drill pipe does not create the same column height as if the 5 bbl was contained in a 3-1/2” drill pipe. This means that the hydrostatic pressure created when pumping kill fluid through the 5” drill pipe per stroke is less than through a 3-1/2” drill pipe. This requires that the pump pressure must be reduced faster per pump stroke in smaller size pipe. See Fig 92.

H

H


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ICP

With a tapered string in the well bore using the “Wait and Weight” method the red dotted line represent the theoretical pressure graph when circulating kill fluid to the bit from ICP to FCP. The blue line represents the true pressure graph to follow using “Wait and Weight” method. By using the theoretical pressure graph it can be seen that while kill fluid is circulated to the bit the well is in underbalance creating further influx into the well bore with resulting higher annulus pressures. See Fig 93. FCP

Fig 93

ICP

FCP

In a deviated well with a uniform string using the “Wait and Weight” method the red dotted line represent the theoretical pressure graph when circulating kill fluid to the bit from ICP to FCP. The blue line represents the true pressure graph to follow using the “Wait and Weight” method. By using the theoretical pressure graph it can be seen that while kill fluid is circulated to the bit the well is in overbalance witch can lead to further serious well control problems like lost circulation that again can lead to underground blow-out. See Fig 94.

Fig 94


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ICP

In a “S” shaped well with a uniform string using the “Wait and Weight” method the red dotted line represent the theoretical pressure graph when circulating kill fluid to the bit from ICP to FCP. The blue line represents the true pressure graph to follow using the “Wait and Weight” method. By using the theoretical pressure graph it can be seen that the well is first in overbalance witch can result in losses and then later the well becomes underbalanced taken in more influx with resulting higher annulus pressures. See Fig 95.

FCP

Fig 95 ICP

FCP

Fig 96

In a horizontal well with a uniform string using the “Wait and Weight” method the red dotted line represent the theoretical pressure graph when circulating kill fluid to the bit from ICP to FCP. The blue line represents the true pressure graph to follow using the “Wait and Weight” method. By using the theoretical pressure graph it can be seen that while kill fluid is circulated to the bit the well is in extreme overbalance witch can lead to serious additional well control problems. See Fig 96. By using the deviated well control sheet the true pressure graph can be calculated. M:\IWCF Surface\3\1\Section 5.doc

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02.14 Complications

When taking a gas influx into a horizontal well bore some associated problems might be encountered and have to be taking into consideration. The density of the gas is lower than the density of the drilling fluid with the result that the gas will accumulate in the top of the well bore in the horizontal section. See Fig 97.

Fig 97

When a gas influx is taken in the horizontal part of the well bore it can be hard to detect. The gas will not percolate and expand before it reaches the deviated section. An undetected swabbed gas kick in a horizontal section can be dangerous due to that no surface pressure will be observed and the first indication will take place when new tubular are run into the well bore or circulation is resumed. See Fig 98. Fig 98

Open hole sections are not looking like a gun barrel due to that there will be angle deviations, hole enlargement and the well can be inverted with the result that gas influx in horizontal section will be accumulated in these pockets. To be able to flush the gas out of the well bore the annular velocity must be so high gas moves in the horizontal section. See Fig 99.

Fig 99

Attempt to circulate gas out of the horizontal section with RRCP will not be successful due to the flow is laminar and the high density kill fluid will have a tendency to flow along the lower part of the well bore. See Fig 100. Fig 100

A gas kick taking in a horizontal well section is most likely due to drilling through a fault with the result that there is a great chance for losses at the same time due to the difference in formation pressure. M:\IWCF Surface\3\1\Section 5.doc

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03.14 Horizontal well control example

KOP TVD/MD 4200 ft

SHOE

Hole size Hole TVD Hole MD KOP MD/TVD EOB TVD EOB MD Csg 10-3/4” TVD Csg 10-3/4” MD BHA DP cap. DC cap OH/DC cap OH/DP cap Csg/DP cap SIDPP SICP Influx volume RRCP

8-3/4” 6130 ft 16330 ft 4200 ft 5470 ft 6178 ft 6200 ft 8200 ft 80 ft 0.01755bbl/ft 0.0066 bbl/ft 0.03014 bbl/ft 0.04896 bbl/ft 0.07373 bbl/ft 410 psi 450 psi 8.2 bbl 450 psi

TD TVD 6130 ft MD 16330 ft

EOB TVD 5470 ft MD 6178 ft

Fig 101

By using the example in Fig 101 the kill sheet can be filled capacity/stroke data be obtained: Internal Surface to KOP 73.7 bbl KOP to EOB 34.7 bbl EOB to BHA 176,8 bbl BHA 0.5 bbl External BHA/OH 2.4 bbl DP/OH 394.1 bbl DP/Csg 604.6 bbl

out and the following 703 stks 331 stks 1687 stks 5 stks 23 stks 3760 stks 5769 stks

Calculations: Calculate the required kill fluid density:

(A)

SIDPP 410 KMW = OMW + ----------------= 14.3 + -----------------= 15.6 ppg TVD x 0.052 6130 x 0.052


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(B)

ICP = RRCP + SIDPP = 450 + 410 = 860 psi

Calculate final circulating pressure:

(C)

KMW = 450 x ----------15.6 = 491 psi FCP = RRCP x -----------OMW 14.3

Calculate dynamic pressure loss at kick off point:

(D)

4200 = 461 psi RRCP at KOP = RRCP + (FCP - RRCP) xKOPmd ---------- = 450 + (491 - 450) x ----------16330 TDmd

Calculate remaining SIDPP at kick off point:

(E)

SIDPP at KOP = SIDPP - (KMW - OMW) x 0.052 x KOPtvd = 410 - (15.6 - 14.3) x 0.052 x 4200 = 126 psi

Calculate circulating pressure at kick off point:

(F)

CP at KOP = (D) + (E) = 461 + 126 = 587 psi

Calculate dynamic pressure loss at end of build:

(G)

6178 = 466 psi RRCP at EOB = RRCP + (FCP - RRCP) xEOBmd ---------- = 450 + (491 - 450) x ----------16330 TDmd

Calculate remaining SIDPP at end of build:

(H)

SIDPP at EOB = SIDPP - (KMW - OMW) x 0.052 x EOBtvd = 410 - (15.6 - 14.3) x 0.052 x 5470 = 40 psi

Calculate circulating pressure at end of build:

(I)

CP at EOB = (G) + (H) = 466 + 40 = 506 psi

Calculate pressure drop per 100 strk from surface to KOP: (B - F) x 100 (860 - 587) x 100 = 38 psi Pdrop = -------------------------= -----------------------strokes 703

Calculate pressure drop per 100 strk from KOP to EOB: (F - I) x 100 - 506) x 100 = 24 psi Pdrop = -------------------------= (587 -----------------------strokes 331

Calculate pressure drop per 100 strk from EOB to TD: (I - C) x 100 - 491) x 100 = 1 psi Pdrop = -------------------------= (506 -----------------------strokes 1692


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04.14 Wait and Weight Method

The “Wait and Weight” method is also called the “balance method” witch means that the kill fluid is pumped to the bit holding BHP constant by adjusting the choke to keep the precalculated drill pipe pressure on schedule according to the graph. To use “Wait and Weight” method in horizontal wells is not recommended due to that it requires a lot of calculations and the kill fluid has to be pumped to the bit at reduced rate to control the drill pipe pressure, with the result that the influx will stay trapped in the horizontal section. The following graph shows the drill pipe and casing pressure while circulating out the influx at the previous example using “Wait and Weight” method. See Fig 101.

1200 1100 1000 900

ICP

800

KOP

700 500

EOB

PRESSURE

600

FCP

400

Csg. Pressure

300

Gas at EOB

200 100 000 0

1500

3000

4500

STROKES

6000

7500

9000

10500 12000

Fig 101

05.14 Driller’s Method

This method is also called the “constant drill-pipe pressure method” and consist of two steps. First step to circulate out the influx without changing drilling fluid density and second to displace OMW with the KMW. This method does not require the same calculations as the “Wait and Weight” method and are therefore more simple, but not recommended due to using reduced rate for circulating the result could be that the influx will stay trapped in the horizontal section. The following graph shows the drill pipe and casing pressure while circulating out the influx at the previous example using “Driller’s Method”. See Fig 102.


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1200 1100 1000 DP Pressure

900

700 600 500

PRESSURE

800

400

KMW at EOB Csg. Pressure

300

Gas at EOB

200 100 000

STROKES

0

3000

6000

9000

12000

15000 18000 21000 24000

Fig 102

06.14 Horizontal well kill method

To circulate out a influx in a horizontal well bore the annular flow must be so high that the flow becomes turbulent and test have showed that a annular velocity of at least 100 ft/min is required. The industry recommendation is to use the “Driller’s Method” with modification to handle an influx in a horizontal well and the following is only guidelines. Prepare calculations for using “Driller’s Method”. Calculate open hole strokes from bit to end of horizontal section. Keep constant casing pressure while bringing pumps to required SPM to give minimum 100 ft/min annular velocity. Keep constant drill pipe pressure while flushing influx out of horizontal section. Keep constant casing pressure while bringing pumps down to reduced circulating rate. M:\IWCF Surface\3\1\Section 5.doc

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Continue circulating influx out of the well bore using “Driller’s Method” With influx out of well bore keep constant casing pressure while pumping kill fluid to bit using reduced circulating rate. With kill fluid at bit keep constant casing pressure while bringing pumps to required SPM to give minimum 100 ft/min annular velocity. Keep constant drill pipe pressure while flushing light drilling fluid out of the horizontal section. Keep constant casing pressure while bringing pumps down to reduced circulating rate. Continue circulating light drilling fluid out of the well bore using “Driller’s Method”.


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Running/Pulling Pipe.

01.15 Introduction

Most well control incidents take place during tripping pipe for different reasons, but in general they can all be considered to be negligence from the drilling team. The negligence could be due to a single circumstance or a combination of several circumstances. The reasons for well control incidents during tripping could for the following reasons: 1.

The effect of pumping a slug.

2.

Inadequate hole fill.

3.

Hole not taking correct amount of fluid.

4.

Hole not giving correct amount of fluid.

02.15

Pumping Slug

Pumping a slug prior to pulling out of hole is a well-known procedure in the drilling industry. The slug is a heavy pill of drilling fluid with a density higher than the drilling fluid used during drilling. The slug is pumped into the drill pipe prior to start pulling the drill string out of hole and due to its higher density will create a U-tube effect allowing the fluid level inside the drill string to drop. The drill string can then be pulled dry avoiding any pollution on the rig floor, so the roughnecks do not get in contact with the drilling fluid. Prior to pumping a slug it is important that calculation are made to determine the amount of fluid that will be drained back into the trip tank due to the U-tube effect as this will be indicating if the well is in balance. Tripping should not start before the U-tube effect is finished and the correct amount of this effect has been measured in the trip tank. As the amount of slug pumped is known together with the internal capacity of the drill pipe in use the following formulas can be used to determine the level drop inside the drill pipe and the volume to be drained back into the trip tank:

Slug pumped (bbl) Length of slug = ----------------------------------DP Capacity (bbl/ft) Slug MW (ppg) Level Drop = Length of slug (ft) x -------------------------- — Length of slug (ft) Original MW (ppg) Trip tank return = Level Drop (ft) x DP Capacity (bbl/ft) M:\IWCF Surface\3\1\Section 5.doc

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Example:

Drilling fluid density Slug density Slug volume Drill pipe capacity

12.6 14.5 22 0.01776

ppg ppg bbl bbl/ft

The slug is pumped and the surface lines displaced by original drilling fluid. The Topdrive is disconnected and the slug allowed to drop. See Fig 103 Calculate:

Length of slug. Level drop. Trip tank return

22 bbl Length of slug = ----------------------- = 0.01776 (bbl/ft)

1239 ft

14.5 (ppg) Level Drop = 1239 (ft) x ------------------- — 1239 (ft) = 187 ft 12.6 (ppg) Trip tank return = 187 (ft) x 0.01776 (bbl/ft) = 3.32 bbl

PDP NATIONAL

PDP NATIONAL

Level drop Length of slug Length of slug Trip tank increase

Fig 103


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Effect on Bottom hole Pressure Pumping a slug prior to pulling pipe out of hole will not have any immediate effect on the bottom hole pressure due to the U-tube effect where pressure hydrostatic inside the drill pipe and in the annulus equalise. The bottom hole pressure will first be affected when the pipe is pulled so far out of the wellbore that the slug is starting getting out of the bit and the heavy slug density get mixed with the drilling fluid density. The increase in bottom hole pressure is not very high and is normally not taken into consideration. In HPHT wells where a very small margin exist between Pore pressure and Fracture pressure and a lot of tripping takes place consideration must be made to the effect on the bottom hole pressure when pumping a slug. Effect of pumping slug when running tapered string Slug calculations are normally based on pumping a heavy slug into a uniform string i.e. 5” drill pipe from surface to the BHA. Several time this is not the case due to that drilling takes place through 7” liners where either 4-1/2” or 3-1/2” drill pipe is used in the lower part of the drill string. The effect of pumping a slug into a tapered string are many time not understood by the drilling crew and the well shut in for the wrong reason with loss of rig time and unnecessary concern. To understand the problems with a slug in a tapered string it must be understood that every feet of drilling/slug fluid represent a certain pressure hydrostatic in the wellbore. When a slug goes from a 5” drill pipe into a smaller diameter drill pipe the length of the slug will increase and thereby also the pressure hydrostatic created by the slug. This will result in a further level drop and a trip tank increase. If this effect is not understood by the drilling crew and the increase in the trip tank is not calculated before hand this increase could be interpenetrated as an influx and the well shut in. Example: Drilling fluid density 12.6 ppg Slug density 14.5 ppg Slug volume 22 bbl Drill pipe capacity ( 5” ) 0.01776 bbl/ft Drill pipe capacity ( 4-1/2” ) 0.0142 bbl/ft 22 bbl Length of slug in 5” DP = ----------------------0.01776 (bbl/ft) 22 bbl Length of slug in 4-1/2” DP = ----------------------0.0142 (bbl/ft) Trip tank return = (1549 ft - 1239 ft) x 0.0142 bbl/ft

= 1239 ft = 1549 ft = 4.4 bbl

or Ph of slug in 5” DP =

1239 ft x 14.5 x 0.052

= 934 psi

Ph of slug in 4-1/2” DP = 1549 ft x 14.5 x 0.052

= 1168 psi

(1168 psi - 934 psi) x 0.0142 bbl/ft Trip tank return = --------------------------------------------- = 4. 4 bbl 14.5 ppg x 0.052


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As can be seen on the calculation an expected extra return into the trip of 4.4 bbl must be expected when slug enter the 4-1/2” drill pipe and this volume needs to be calculated prior to tripping to avoid any misunderstandings when pulling pipe out of hole. 03.15

Inadequate hole fill.

When pulling pipe out of hole it is important that the hole is kept full all the time to maintain the pressure hydrostatic in the well bore. If the pipe is pulled without adequate hole fill the level will drop in the well bore with the result of a decrease in the bottom hole pressure. If the decrease in the bottom hole pressure gets severe the well might get in underbalance resulting in an influx into the well bore. As all displacement and capacity figures is known for the pipe in use on a drilling rig the pressure drop for tripping pipe can easily be calculated using the following formulas: Pulling dry Drill Pipe: Drilling fluid density(ppg) x 0.052 x DP metal displacement(bbl/ft) Pressure drop per ft. pulling dry pipe = ------------------------------------------------------------------------------------------Casing capacity (bbl/ft) - DP metal displacement (bbl/ft)

Pulling wet Drill Pipe: Drilling fluid density(ppg) x 0.052 x DP closed end displacement(bbl/ft) Pressure drop per ft. pulling wet pipe = ------------------------------------------------------------------------------------------------Annular capacity (bbl/ft)

Pulling dry Drill Collars: Length of DC (ft) x DC metal displacement (bbl/ft) Level drop for pulling dry DC = -------------------------------------------------------------------------Casing capacity (bbl/ft)

Pulling wet Drill Collars: Length of DC (ft) x DC closed end displacement (bbl/ft) Level drop for pulling wet DC = -------------------------------------------------------------------------------Casing capacity (bbl/ft)

Example:

Drilling fluid density Drill pipe capacity ( 5” ) Drill pipe metal displacement ( 5” ) Drill Collar capacity ( 6-3/4” ) Drill Collar metal displacement ( 6-3/4” ) Length of Drill Collars Casing capacity ( 9-5/8” – 47 lbs/ft )

12.6 0.01776 0.00852 0.00768 0.03658 450 0.07287

ppg bbl/ft bbl/ft bbl/ft bbl/ft ft bbl/ft

Calculate the pressure drop per ft. pulling dry Drill pipe: 12.6 x 0.052 x 0.00852 ----------------------------------- = 0.0867 psi/ft 0.07287 - 0.00852 M:\IWCF Surface\3\1\Section 5.doc

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Calculate the pressure drop per ft. pulling wet Drill pipe: 12.6 x 0.052 x (0.01776 + 0.00852) ------------------------------------------------ = 0.3696 psi/ft 0.07287 - (0.01776 + 0.00852)

Calculate level drop for pulling dry Drill Collars: 450 x 0.03658 ----------------------- = 226 ft 0.07287

Calculate level drop for pulling wet Drill Collars: 450 x ( 0.03658 + 0.00768) ------------------------------------- = 273 ft 0.07287

For different reasons it could be necessary drop the level in the annulus, but by doing so the bottom hole pressure will be reduced. If pulling pipe without filling the hole the amount of pipe that can be pulled before the well loses its overbalance can be calculated by using the following formulae: Pulling dry Drill Pipe: Overbalance (psi) x (Casing Capacity - DP metal displacement) Pipe to pull before well starts to flow (ft) = ------------------------------------------------------------------------------------------Drilling fluid density x 0.052 x DP metal displacement

Pulling wet Drill Pipe: Overbalance (psi) x (Casing Capacity - DP closed end displacement) Pipe to pull before well starts to flow (ft) = ------------------------------------------------------------------------------------------------Drilling fluid density x 0.052 x DP closed end displacement

Example:

Drilling fluid density Drill pipe capacity ( 5” ) Drill pipe metal displacement ( 5” ) Drill Collar capacity ( 6-3/4” ) Drill Collar metal displacement ( 6-3/4” ) Length of Drill Collars Casing capacity ( 9-5/8” – 47 lbs/ft ) Depth of well (TVD/MD) Formation gradient


12.6 0.01776 0.00852 0.00768 0.03658 450 0.07287 10000 0.6995

ppg bbl/ft bbl/ft bbl/ft bbl/ft ft bbl/ft ft psi/ft

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Dry pipe to pull before the well starts to flow: 10000 x (0.6995 - 12.6 x 0.052) x (0.07287 - 0.00852) -------------------------------------------------------------------------- = 5107 ft 12.6 x 0.052 x 0.00852

Wet pipe to pull before the well starts to flow: 10000 x (0.6995 - 12.6 x 0.052) x [(0.07287 - (0.00852 + 0.01776)] ---------------------------------------------------------------------------------------- = 1198 ft 12.6 x 0.052 x (0.00852 + 0.01776)

04.15

Hole not taking correct amount of fluid.

As mentioned before then all displacement and capacity of the tubular used on a drilling rig should be known so the correct calculated amount of fluid can be measured filling the hole as the pipe is pulled out of the hole. This measurement is carried out by the use of the Trip tank where the fluid volume can be measured either visual or by an electronic measuring devise. Any deviation in the correct calculated amount of fluid means that some abnormalities is taking place down hole and tripping must be stopped and the problem evaluated and rectified. If the hole is taking more fluid than calculated per stand tripped out of hole either wet or dry some dynamic losses is taking place and the situation needs to be evaluated. Minor dynamic losses when pulling out of hole could turn into severe losses when running back into the hole due to surge pressure created. Consideration should be made to run back to bottom and cure losses prior to comments tripping. If the hole is taking less fluid than the calculated per stand tripped out of hole either wet or dry indicates that the hole could be swabbing. The reason for swabbing could be due to balled bit/ BHA or due to very high viscosity combined with a low BHA annulus capacity. The less amount of fluid that the hole has taken could be formation fluid that has been swabbed into the well bore and depending on the amount of swabbed fluid the bit should be run or strip back to bottom and the well circulated clean prior to commence tripping or alternative pumping out of hole. If the formation is tight the swabbed fluid could come from the drill pipe with the result that the fluid level inside the drill pipe has been lowered and thereby the pressure hydrostatic. If the level drop inside the drill pipe becomes severe the hydrostatic pressure inside the drill pipe might drop below formation pressure. This result could be that the formation starts producing and formation fluid could enter the drill string creating complications. Prior to tripping it is of extreme importance that swab pressure is calculated and the correct pulling speed found to avoid swabbing tendency. 05.15

Hole not giving correct amount of fluid.

When running the drill string into the well bore it is of equal importance that the fluid coming back for the pipe run is measured as any deviation from the calculated amount


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means that some abnormalities is taking place down hole. The tripping has to stopped and the problem rectified prior to continue running in. If the volume coming back is higher than the calculated volume this could indicate that the bit or drill string is plugged and that the pipe is not filled as running in. Circulation has to be established and the bit/string unplugged before continue tripping. If a drill pipe float is installed in the drill string circulation should be broken every 1500 ft to avoid excessive collapse pressure on the drill string. If the volume coming back is less than the calculated volume the reason could be that the tripping speed is to high and excess surge pressure is created on the formation with result in losses. Prior to tripping it is of extreme importance that surge pressure is calculated and the correct running speed found to avoid breaking down the formation.


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