02 Resistivity Measurement

Schlumberger

Contents

B1.0 RESISTIVITY OF THE FORMATION.....................................................................................1 B1.1 INTRODU INTRODUCTIO CTION N ............................................ ................................................................... ............................................... ......................................1 ..............1 B1.2 FORMATION WATER RESISTIVITY R W ......................................................................3 B1.3 FORMATION FORMATION RESISTIV RESISTIVITY ITY MEASUREME MEASUREMENTS NTS .........................................................3 .........................................................3 Chart Gen-9: Resistivity of NaCl Solutions..................................................................4 B1.4 TO SUMMARIZE SUMMARIZE ........................................... .................................................................. ............................................... ......................................6 ..............6 B1.5 THE DRILLING PROCESS AND PERMEABLE BEDS.................................................5 Invasion Invasion Profiles Profiles ........................................... .................................................................. ............................................... ......................................5 ..............5 Chart Gen-3: Symbols Used in Log Interpretation.................................. Interpretation......................................................7 ....................7 B1.6 SPONTANEOUS SPONTANEOUS POTENTIAL (SP) CURVE ................. ......... ................ ................ ................ ................ ................ ............... ....... 8 Chart SP-1: R weq Determination from E SSP (Clean Formations)..................................1 Formations)..................................1 3 Chart SP-2: R w versus and Formation Temperatu Temperature.................... re..........................................1 ......................1 4 R w weq weq

B2.0 MEASUREMENT OF R t BY INDUCTION PRINCIPLES........................................................15 B2.1 INTRODU INTRODUCTIO CTION N ............................................ ................................................................... ............................................... ....................................1 ............1 5 B2.2 INDU INDUCTIO CTION N LOGGING LOGGING PRIN PRINCIPL CIPLES.................................. ES......................................................... ......................................1 ...............1 5 B2.3 SPHERICA SPHERICALLY LLY FOCUSED FOCUSED LOG PRIN PRINCIPL CIPLES................................................ ES..........................................................1 ..........1 6 B2.4 DUAL INDUCTION - SPHERICALLY FOCUSED LOG................................................17 B2.5 PHASORPHASOR-INDU INDUCTIO CTION N SFL TOOL ........................................... .................................................................. ..................................2 ...........2 3 B3.0 MEASUREMENT OF R t BY LATEROLO LATEROLOG G PRI PRINCIP NCIPLES....................................... LES....................................................2 .............2 9 B3.1 DUAL LATEROLOG LATEROLOG....................... .............................................. .............................................. .............................................. ............................2 .....2 9

B4.0 MEASUREMENT OF R XO BY MICR MICRO-RE O-RESIST SISTIVIT IVITY Y PRINCIP PRINCIPLES LES .....................................3 .....................................3 5 B4.1 INTRODU INTRODUCTI CTION ON ........................................... .................................................................. .............................................. ....................................3 .............3 5 B4.2 MICROLOG.............................................................................................................36 B4.3 MICR MICRO-SP O-SPHER HERICAL ICALLY LY FOCUSED LOG.......................................................... LOG..................................................................3 ........3 8

B5.0 WORK SESSION SESSION ............................................ ................................................................... .............................................. ...........................................41 ....................41

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Introduction to Openhole Logging

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Schlumberger

B1.0

Resistivity of the Formation

B1.1 INT INTRODUC ODUCTI TIO ON The resistivity of a formation is a key parameter in determining hydrocarbon saturation. Electricity can pass through a formation only because of the conductive water it contains. With a few rare exceptions, such as metallic sulfide and graphite, dry rock is a good electrical insulator. Moreover, perfectly dry rocks are seldom found. Therefore, subsurface formaform ations have finite, measurable resistivities because of the water in their pores or absorbed in their interstitial clay.

For the purposes of our discussions we will divide substances into two general categories, conductors or insulators. Conductors are substances that pass electrical current (e.g., water, shales, mud). Insulators are substances that do not allow electrical current flow (e.g., hydrocarbons or rock matrix).

The measured resistivity of a formation depends on

opposite faces of a unit cube of that substance at a specified temperature. The meter is the unit of length and the ohm is the unit of electrical resistance. In abbreviated form, resistivity is R = r A/L, where R is resistivity in ohm-metres, r is is resistance in ohms, A is area in square metres, and L is length in metres. (See Figure B1)

The units of resistivity are ohm-metres squared per meter, or simply ohm-metres (ohm-m). Conductivity is the reciprocal of resistivity and is expressed in mhos per meter. To avoid decimal fractions, conductivity is usually expressed in millimhos per meter (mmho / m), where 1000 mmho/m = 1 mho/m C = = 1000/R.

- resistiv resistivity ity of the the formatio formation n water water - amoun amountt of water water pres presen entt - pore structur structuree geom geometry etry.. The resistivity (specific resistance) of a substance is the resistance measured between

Formation resistivities are usually from 0.2 to 1000 ohm-m. ohm-m . Resistivities higher than 1000 ohm-m are uncommon in permeable formations but are observed in impervious, very low porosity formations (e.g., evaporites).

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R = =

ra L

OHM-METERS METER

2

R

= resistivity a = area L = length r = resistance

Figure B1: Principles of Resistance and Resistivity

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Schlumberger

B1.2

FORMATION WATER RESISTIVITY RW As previously indicated, formation matrices are insulators; thus a formation’s ability to conduct electricity is a function of the connate water in the formation. Several factors must be considered:

- volume of the water (porosity) - pore space arrangement (type of porosity) - temperature of the formation - salinity of the water. a) Water Salinity As salinity increases, more ions are available to conduct electricity, so Rw (water resistivity) decreases. b) Water Temperature As water temperature is raised, ionic mobility increases and resistivity decreases. Chart Gen-9 (Figure B2) in the Log Interpretation Chart book illustrates these relationships. c) Water Volume As water-filled pore space in a rock is increased, resistivity decreases. If some water is displaced by hydrocarbons (insulators), water saturation decreases; resistivity increases.

B1.3

FORMATION RESISTIVITY MEASUREMENTS If we consider a formation with pore space that contains only water, its true resistivity is called Ro. We know that an important relationship exists between formation resistivity and the resistivity of the saturating water, Rw. The ratio of these two values, F , is called formation resistivity factor, or more commonly formation factor, which is a constant, where: F = Ro / Rw

For example, if the salinity of the connate water increases, Rw will decrease. This will in turn allow current to flow more easily through the formation, thus lowering Ro and maintaining F at a constant value. This is what we should expect as F is an inherent formation characteristic. Formation factor can be related to formation porosity by the general formula F = a / φm where a = constant m = cementation factor

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Resistivity of NaCl Solutions Conversion approximated by R2 = R1 [(T1 + 6.77)/(T2 + 6.77)] F or R2 = R1 [(T1 + 21.5)/(T2 + 21.5)] C °

°

m p p

F 5 7 t a l a g / s n i a r G

2 0 0

10

3 0 0

15

4 0 0

20

5 0 0

25

°

10 8 6 5 4 3 2

6 0 0 7 0 0 8 0 0

1

1 0 0 0 1 2 0 0 1 4 0 0 1 7 0 0 2 0 0 0

0.8 ) m m h o ( n o i t u l o s f o y t i v i t s i s e R

0.6 0.5 0.4 0.3

0.1 0.08 0.06 0.05 0.04 0.03 0.02

3 0 0 ,0 0 0

0.01

F 50 C 10

75

° °

20

30

100 40

125 150 200 50 60 70 80 90 100 Temperature ( F or C) °

°

Chart GEN-9 Figure B2

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40 50

100

3 0 0 0

150

4 0 0 0

200

5 0 0 0 6 0 0 0 7 0 0 0 8 0 0 0 1 0 ,0 0 0 1 2 ,0 0 0 1 4 ,0 0 0 1 7 ,0 0 2 0 0 ,0 0 0

0.2

30

250 300 400 500

1000

3 0 ,0 0 0

1500

4 0 ,0 0 0 5 0 ,0 0 0 6 0 ,0 0 7 0 0 ,0 0 8 0 0 ,0 0 0 1 0 0 ,0 1 2 0 0 0 , 1 4 0 0 0 0 ,0 0 0 1 7 0 ,0 2 0 0 0 0 , 2 5 0 0 0 0 2 8 ,0 0 0 0 ,0 0 0

2000

250 300 350 400 120 140 160 180 200

2500 3000 4000 5000

10,000 15,000 20,000

) l a g / s n i a r g r o m p p ( n o i t a r t n e c n o c l C a N

Schlumberger

B1.4 SUMMARY 1. Dry rock formations are an insulator. 2. Formations conduct current because of water in the pore spaces. 3. Knowledge of water resistivity ( Rw) is essential for log interpretation. 4. Resistivity used rather than resistance. 5. Formation resistivity factor (F ) is a porosity-related formation characteristic. 6. Relationships a. F = ( Rt / Rw) = ( Ro / Rw)

100% water saturated porous rock b. F = a / φm 7. Symbols Rw - resistivity of connate water Rt - true formation resistivity R xo - resistivity of flushed zone a - constant m - cementation factor. B1.5

DRILLING PROCESS AND PERMEABLE BEDS Before proceeding to a discussion of methods of obtaining formation resistivity, let us examine what happens to a permeable formation when it is penetrated by the drill bit. (Refer to Chart Gen-3 [Figure B3] in this section or the Log Interpretation Chart book .)

Under normal conditions, the hydrostatic head of the mud column is greater than formation pressure. This differential pressure forces filtrate from the mud system into the formation pore spaces, leaving solid particles or mudcake buildup on the borehole wall. Eventually this impervious mudcake will seal off further invasion (unless it is removed by some mechanical process; e.g., removing the drill bit).

Mudcake thickness is symbolized by hm c. Invasion Profiles: 1. Flushed Zone. Adjacent to the borehole the invasion process flushes out the original water and some of the hydrocarbons (if any were present). The resistivity of this zone is termed R x o; the water saturation is called S x o where FR mf S xo = 2

R xo

(for clean formations only) Plotting R xo as a function of radial depth into the formation yields (Figure B4). 2. Transition Zone. Further from the borehole the flushing action of the mud filtrate may create a variety of situations. If the flushing proceeds as a uniform front, we call this a step profile of invasion (Figure B5[a]). If the intermingling of formation fluids is gradual, we call this a transition zone (Figure B5[b]). Sometimes in oil- or gas-bearing formations, where the mobility of hydrocarbons is greater than the connate water, the oil or gas move out leaving an annular zone filled with connate water (Figure B5c). If Rmf > Rw, then the annular zone will have a resistivity lower than R xo and Rt and may cause a pessimistic saturation calculation.

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Symbols Used in Log Interpretation

Resistivity of the zone Resistivity of the water in the zone Water saturation in the zone Mud Rm

Adjacent bed Rs

hmc Rmc

Uninvaded zone Flushed zone

dh

(Bed thickness)

Mudcake h

Zone of transition or annulus

R xo

Rt Rw Sw

Rmf Sxo Rs

di d j Adjacent bed (Invasion diameters) ∆r j

dh Hole diameter

Chart GEN-3

Figure B3

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Schlumberger

tion S w. Plotting R xo , Ri and Rt as a function of invasion gives us Figure B4.

3. True Unaffected Zone. This is the zone that we want to analyze—it is the formation undisturbed by the drilling process. Its resistivity is termed Rt , water resistivity Rw and water satura-

R xo

D i Figure B4: Invasion Process

R xo R i

R

R xo

R xo R i R t

R

R t

R

R t R i

D i

(a)

D i D 2

(b)

D i

(c)

Figure B5

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B1.6

SPONTANEOUS POTENTIAL (SP) CURVE

a) Introduction The SP curve is a continuous recording (versus depth) of the difference in potential between a moveable electrode in the borehole and a fixed (zero) potential surface electrode. Units used are millivolts.

The SP was discovered quite by accident in the early days of electrical logging. In some of the first test wells logged by Schlumberger using the point-by-point technique, it was noted that a small natural potential was present in the well even when the current source was turned off. This spontaneous potential is due to a combination of two phenomena: an electrokinetic potential is usually negligible and an electrochemical potential is composed of a membrane potential and a liquid-junction potential. The membrane potential is about 5 times bigger than the liquid-junction potential.

Figure B6: Electrokinetic Potential of SP

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b) Electrokinetic Potential If a solution is forced by differential pressure to flow through a membrane, an electrical potential will appear across the membrane (Figure B6). A similar situation occurs when the mud filtrate flows through the mudcake because of the differential pressure between the mud column and the formation. This electrokinetic potential ( E kmc ) is generally small.

In a low-permeability formation, where the mudcake is only partially built up, this electrokinetic potential may be as high as 20 mV. This situation is, however, rare and in general the total electrokinetic potential can be neglected. c) Electrochemical Potential This potential is created by the contact of two solutions of different salinity, either by a direct contact or through a semipermeable membrane such as shales.

Figure B7: Electrochemical membrane potential of SP

Schlumberger

1) Membrane Potential An ideal cationic membrane because of its physico-chemical composition is permeable to positive ions (cations) only. Shales are ideal membranes as long as they are not too sandy or too limy. In a borehole, a shale section usually separates salty water (generally the connate water of the virgin zone) from a less salty liquid (generally the mud) (Figure B7). There is migration of the positive ions (Na + ) from the salty water (formation) to the less salty water (mud). When an equilibrium is reached: - Positive ions that have already crossed the shale membrane exert a repelling force on the positive ions in the mud. - Negative ions left behind in the formation exert an attractive force on the positive ions which cannot travel any more into the shale.

where amf and aw are the electro-chemical activities of mud filtrate and connate water, respectively. 2) Liquid Junction Potential The liquid junction potential takes place at the boundary between the flushed zone and the virgin zone. There is no shale separating the two solutions. Anions as well as cations can transfer from one solution to the other (Figure B8) because of the higher salinity of the formation water and both Na+ cations and Cl – anions will migrate toward the mud filtrate. The Na+ ion is comparatively large and drags 4.5 molecules of water. The Cl– ion is smaller and drags only 2.5 molecules of water. Hence, the anion Cl – will migrate more easily than the Na + ions.

The difference of potential appearing between the two solutions is given by the formula: E m = K ;og

amf aw

Figure B8: Electrochemical Liquid Junction Potential of SP

Figure B9: SP Circuit Path

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The result is an increase of positive charges left behind in the formation water. These positive charges restrict Cl– migration toward the flushed zone. A difference of potential appears at the boundary between the two solutions: amf

′

E j = K log aw d) Spontaneous Potential (SP) The total potential of the whole chain is thus the algebraic sum E m + E j , which is also called the Static Spontaneous Potential (SSP). Electrokinetic potential is neglected. The SP is the

drop of potential measured across the current lines in the borehole. Along its path the SSP current has to force its way through a series of resistances, both in the formation and in the mud (Figure B9). This means that the total potential drop (which is equal to the SSP) is divided between the different formations and mud in proportion to the resistances met by the current in each respective medium. The SP, which is the measure of the potential drop in the mud of the borehole, is only part of the SSP. In general, it is a large portion because the electrical resistance offered by the borehole is, in general, much greater than that offered by the formations.

SSP = -K log

R mf

= R w

R mf

R mfe R we

R mf > R w

FRESH MUD

Figure B10: The SP Deflection and its Rmf -Rw Dependency

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Schlumberger

So, we can write: SP ≅ SSP = (K + K ′) log

amf aw

The SP curve is generally presented in track 1, and usually recorded with resistivity surveys, assuming a conductive mud is in the borehole. Opposite a permeable formation, the SP curve shows excursions from the shale baseline. In thick, clean beds the SP deflection tends to reach an essentially constant deflection defining a clean line. The deflection may be either to the left (negative) or to the right (positive) depending mostly on relative resistivity of the formation water and of the mud filtrate (Figure B10). The magnitude of SP deflections is always measured from the shale line and for a clean, water-bearing formation containing a dilute sodium chloride solution is given by SSP = –K log( Rmfe / Rwe )

The constant K depends on the temperature and salt types in formation water (K = 71 at 25°C for NaCl).

In practice, the SP is affected by a number of factors, all of which tend to reduce its magnitude. The maximum available SP in a thick, clean, water-bearing zone is called the SSP (Figure B10). The SP is reduced by the shale in a shaly zone, and the deflection is called the pseudostatic spontaneous potential (PSP). The ratio of these two values, termed α = PSP/SSP, can be used as a shale indicator in sands. An approximation of the SSP in a shaly sand is SSP = PSP / (1 – V sh ) where the volume of shale (V sh ) is estimated from the gamma ray deflection, which is discussed later. e) Uses of SP The SP can be used to - detect permeable beds (a qualitative indication only) - determine Rw, formation water resistivity - give an indication of zone shale content - indicate depositional environment.

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f) Factors Affecting the SP - Bed thickness†: SP decreases when bed thickness decreases. - Invasion†: Reduces SP. - Shaliness: Shale reduces SP‡. - Hydrocarbons: Hydrocarbons in slightly shaly formations reduce the SSP. - Mud filtrate: The magnitude and direction of SP deflection from the shale baseline depends on relative resistivities of the mud filtrate and the formation water. - Fresh mud: negative SP (Figure B8). Rmf > Rw - Saline mud: positive SP (Figure B8). Rw > Rmf Rw = Rmf : zero SP (Figure B8).

g) Solution of Rw from SP

Because of its dependence on Rmf and Rw, the magnitude of SP deflection enables us to solve for the Rw of the formation when Rmf is known. This method, when applied in clean matrix, is generally accurate. 1. From the log heading, get Rmf at surface temperature. 2. Convert Rmf to formation temperature using chart Gen-9 (Figure B2). 3. Convert Rmf at formation temperature to Rmfe using: Rmfe = 0.85 × Rmf (approximation)

If Rmf is below .03 ohm-meter or above 1.5 ohm-meter at formation temperature, use chart SP-2m (Figure B12) to get Rmfe . 4. Calculate static SP from log at zone of interest. 5. Enter chart SP-1 (Figure B11) with static SP, formation temperature and Rmfe to get Rwe at formation temperature. 6. Enter chart SP-2m (Figure B12) with Rwe and formation temperature to get R w.

† ‡

corrosion charts are available to correct for these factors. Pyrite in the formation produces a positive SP.

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Schlumberger

Rweq Determination from E SSP (CLEAN FORMATIONS)

This chart and nomograph calculate the equivalent formation water resistivity, R weq, from the static spontaneous potential, E SSP, measurement in clean formations. Enter the nomograph with ESSP in mV, turning through the reservoir temperature in ° F or °C to deﬁne the R mfeq /R weq ratio. From this value, pass through the R mfeq value to deﬁne Rweq. For predominantly NaCl muds, determine Rmfeq as follows:

Example: SSP = 100 mV at 250°F

R mf = 0.70 ohm-m at 100°F or 0.33 ohm-m at 250°F Therefore, R mfeq = 0.85 × 0.33 = 0.28 ohm-m at 250°F R weq = 0.025 ohm-m at 250°F E SSP = –K c log(R mfeq /R weq ) K C = 61 + 0.133 T°F

a. If R mf at 75°F (24°C) is greater than 0.1 ohm-m, correct R mf to formation temperature using Chart Gen-9, and use R mfeq = 0.85 R mf .

Rweq (ohm-m) 0.001

K C = 65 + 0.24 T°C

b. If R mf at 75°F (24°C) is less than 0.1 ohm-m, use Chart SP-2 to derive a value of Rmfeq at formation temperature.

0.005

Rmfeq /Rweq 0.3

0.3

0.4

0.4

0.5 0.6

0.6

0.8

0.8

1

1

Rmfeq (ohm-m) 0.01

0.01

0.02

0.02

0.04 0.06 2

2

e w

R / e f m

R r o

f m

a / w a

3

0.1

0.05

0.2

4

4

5 6

6

8

8

10

10

0.4 0.6

0.1

1

2 5 0 C 2 0 1 0 C 5 5 0 0 0 5 1 0 0 0 C 4 F C C 0 3 0 F 0 1 2 0 0 0 C 0 0 0 F

30

°

Formation temperature

°

°

0.2

4 6

°

20

2

20

10

°

0.5

°

20

°

°

40 50 +50

°

°

°

F

0

–50

–100

F

–150

ESSP, static spontaneous potential (mV)

–200

40

40 60

1.0

100 2.0

© Schlumberger

SP-1 Figure B11

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Rw versus Rweq and Formation Temperature 0.001 250°C 200°C 0.002

150°C 100°C 75°C

0.005

50°C 25°C

0.01

Saturation 0.02 ) m m h o ( q e f m

0.05

R r o q e w

R

0.1

0.2 2 50 C 2 00 ° C 1 5 0 ° C 1 0 0 ° C °

0.5

7 5 C 5 0 C 2 5 C

N a C l a t 2 5 C

1.0

°

°

°

°

2.0 0.005

0.01

0.02 0.03

0.05

0.1

0.2

0.3

0.5

1.0

2

3

4 5

Rw or Rmf (ohm-m)

Gyp-base mud filtrates EXAMPLE: Rweq = 0.025 Ω• m at 120oC. From chart, Rw = 0.031 Ω• m at 120oC Special procedures for muds containing Ca or Mg in solution are discussed in Reference 3. Lime base muds usually have a negligible amount of Ca in solution; they may be treated as regular mud types.

SP-2m Figure B12

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Schlumberger

B2.0

Measurement of R t by Induction Principles

B2.1 INTRODUCTION We have two different types or classes of tools designed for the two most common borehole environments:

1. Nonconductive boreholes - including fresh mud systems, invert mud systems and air-filled holes. a. Dual-Induction SFL tool (no longer in service) b. Phasor-dual Induction SFL tool c. Array Induction Imager tool (AIT) 2. Conductive boreholes - including saline to salt saturated mud systems Dual laterolog. B2.2

INDUCTION LOGGING PRINCIPLES The induction logging tool was originally developed to measure formation resistivity in boreholes containing oil-base muds and in airdrilled boreholes. Electrode devices did not work in these nonconductive muds, and attempts to use wall-scratcher electrodes were unsatisfactory.

Experience soon demonstrated that the induction log had many advantages when used for logging wells drilled with water-base muds. Designed for deep investigation, induction logs can be focused to minimize the influences of the borehole, surrounding formations and invaded zone. Principle Today’s induction tools have many transmitter and receiver coils. However, the principle can be understood by considering a sonde with only one transmitter coil and one receiver coil (see Figure B13).

A high-frequency alternating current of constant intensity is sent through a transmitter coil. The alternating magnetic field created induces currents in the formation surrounding the borehole. These currents flow in circular ground loops coaxial with the transmitter coil and create, in turn, a magnetic field that induces a voltage in the receiver coil. Because the alternating current in the transmitter coil is of constant frequency and amplitude, the ground loop currents are directly proportional to the formation conductivity. The voltage induced in the receiver coil is proportional to the ground loop currents and, therefore, to the conductivity of the formation.

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There is also a direct coupling between the transmitter and receiver coils. The signal originating from this coupling is eliminated electronically. The induction tool works best when the borehole fluid is an insulator—even air or gas. The tool also works well when the borehole contains conductive mud unless the mud is too salty, formations are too resistive or borehole diameter is too large.

B2.3 SPHERICALLY FOCUSED LOG PRINCIPLES The SFL device measures the resistivity of the formation near the borehole and provides the relatively shallow investigation required to evaluate the effects of invasion on deeper resistivity measurements. It is the short-spacing device used in the Phasor induction SFL tool.

The SFL system differs from previous focused electrode devices. Whereas those systems attempt to focus the current into planar discs, the SFL system establishes essentially constant potential shells around the current electrode.

Figure B13: Basic two-coil induction log system

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Schlumberger

The SFL device is able to preserve the spherical potential distribution in the formation over a wide range of wellbore variables, even when a conductive borehole is present. To accomplish this, the SFL device is composed of two separate, and generally independent, current systems (Figure B14). The bucking current system serves to plug the borehole and establish the equipotential spheres. The io survey current system causes an independent survey current to flow through the volume of investigation; the intensity of this current is proportional to the formation conductivity.

The first sphere is about 9 in. away from the survey current electrode; the other is about 50 in. away. A constant potential of 2.5 mV is maintained between these two spherical surfaces. Because the volume of formation between these two surfaces is constant (electrode spacing is fixed) and the voltage drop is constant (2.5 mV), the resistivity of this volume of formation can be determined by measuring the current flow. B2.4

DUAL INDUCTION— SPHERICALLY FOCUSED LOG This is the most basic of induction devices and was the reference resistivity induction device for more than 20 years until its retirement in 1990. The tool supplies three focused resistivity curves: two induction and a shallow investigating spherically focused curve plus the spontaneous potential (SP). Each curve has a different depth of investigation (Figure B15).

Spherically focused log—a shallow reading device affected mainly by the flushed ( R xo ) zone (radial distance ≅ 30 cm). Medium induction (ILM)— depending on the invasion diameter and profile the ILM may be influenced by the R xo or Rt zones or both. (radial distance ≅ 60 – 80 cm). Figure B14: Electrode array of SFL tool and schematic representation of surveying current (i o) lines (dashed) and focusing current (io) lines (solid).

The SFL device consists of current-emitting electrodes, current-return electrodes and measure electrodes. Two equipotential spheres about the tool’s current source are established.

Deep induction (ILD) —mostly affected by Rt , unless invasion is very deep. Either or both induction curves may be influenced if an annulus is present (radial distance ≅ 1.2 – 1.5 m).

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DUAL INDUCTION - SP/SFL FILE 2

ILM 0.2000

(OHMM)

0.2000

(OHMM)

0.2000

(OHMM)

2000.0000

ILD

SP -150.0000

(MV)

SFLU 0.0000

600

Figure B15

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2000.0000

2000.0000

Schlumberger

a) Log Presentation a. Logarithmic: A 1:240 scale is presented with the resistivity curves on a logarithmic scale. This is the preferred presentation for log analysis (Figure B15). b. Log-lin: The 1:600 scale presents two resistivity curves, the SFL (averaged) and the ILD on the linear resistivity scale. Also included is the equivalent ILD conductivity curve. This presentation is primarily for correlation purposes. Both presentations are recorded simultaneously. b) Tool Characteristics and Applications 1. The Dual-Induction SFL tool is most effective when used in holes drilled with moderately conductive mud (e.g., where Rmf / Rw > 2.5). 2. Vertical focusing is good, and reliable values of Rt may be obtained where bed thickness is > 4.0 m. 3. Because this tool actually measures formation conductivity and converts the values to resistivity, results are most accurate in zones of low resistivity. 4. The recording of three curves that investigate different amounts of formation volume enable us to study invasion profiles and where invasion is deep, make the correction to obtain Rt . 5. Because the two induction devices produce their signals by inducing a magnetic field in the formation, they can be run in air-drilled wells or wells drilled with nonconductive mud. (The SFL tool requires a conductive mud path to the formation and cannot be presented.) A gamma ray curve is usually recorded in place of the SP.

Correction charts are available for the influence of: - borehole (diameter and mud resistivity) - bed thickness - invasion. c) Limitations 1. The logging of large diameter holes drilled with saline mud should be avoided, particularly in high-resistivity formations. Large borehole signals will add to the formation signals, producing anomalously low apparent resistivities. 2. In zones of high resistivity (low conductivity), e.g. in excess of 250 ohmm, errors in measurement can occur.

These problems may be minimized by a system of downhole calibration checks. A thick zero-porosity zone (e.g., limestone or anhydrite) is used for this purpose. Thus, if difficulties in producing a good DIL are expected, it is often advantageous to run a porosity-caliper log before the DIL. (Note that these changes were only made to the DIL logs in the remarks section of the log heading.) d) Log Responses (Figure B16) For wells drilled with fresh muds ( Rmf / Rw > 2.5, R xo / Rt > 2.5) the following general conclusions can be reached by log inspection:

- When SFL = ILM = ILD; Rt = ILD, this indicates zero or very shallow invasion. - When SFL > ILM = ILD; Rt = ILD this indicates moderate invasion. - When SFL > ILM > ILD; and if R xo = SFL, then Rt < ILD, which indicates deep invasion.

(05/96) B-19


When SFL = ILM > ILD and if R xo = SFL chart Rint-2c must be used (Figure B17) to obtain Rt . This response indicates very deep invasion. In general, the closer the medium curve is to the SFL, the deeper the invasion. The result of correcting for invasion is to obtain an Rt that is lower than the ILD. Hence, by using ILD without correction, you will obtain an optimistic S w. e) Summary Benefits: 1. Dual-Induction SFL tool can most effectively be used in holes filled with moderately conductive mud, nonconductive mud, and air-drilled holes. 2. Vertical focusing is good and gives reliable values of Rt for beds thicker than 3 m.

(05/96) B-20

3. It measures low resistivities (less than 10 ohm-m) accurately. 4. Recording of three focused resistivity logs, which investigate different volumes of formation, enables us to study invasion profile and good Rt values in the case of deep invasion. Correction charts are available for - borehole - bed thickness - invasion. Disadvantages: 1. Not reliable for resistivities > 250 ohm-m (use a dual laterolog) 2. Large hole and saline mud results in large borehole signals give an unusually low apparent resistivity. (use DLL in this case).

Schlumberger

DUAL INDUCTION INVASION PROFILES ILM 0.2000

(OHMM)

0.2000

(OHMM)

0.2000

(OHMM)

2000.0000

ILD

SP -80.0000

(MV)

2000.0000

SFL 20.0000

2000.0000

NO INVASION

SHALLOW INVASION

MODERATE INVASION

VERY DEEP INVASION

Figure B16

(05/96) B-21


DIL* Dual-Induction - SFL* Spherically Focused Log ID - IM - SFL

Thick beds, 8-in. [203-mm] hole, skin-effect corrected, DIS-EA or equivalent 40 Rxo /Rm ≈ 100 30

Rxo

20

i n .) d i (

50

40

60

Rt

30

70 80 90

30

25

25

20

20 10

15

9

15

8 7 RSFL /R ID

m ) d i ( 0.75

6

10

0.63 5 0.50 4

1.27

1.01

0.95

1.52

0.90

0.80

1.4

1.5

2.03

1.0

Rt RID

7

0.38 5

3

3

2

2

1 1.0

1.1

1.2 RIM /R ID

Rint-2c Figure B17

(05/96) B-22

1.3

1.7

1.9

Schlumberger

B2.5 PHASOR-INDUCTION SFL TOOL The Phasor-Induction SFL tool (Figure B18) uses a conventional dual induction-SFL array to record resistivity data at three depths of investigation (see Chart B1). In addition to the usual in-phase (R-signal) induction measurements, the tool makes a high-quality measurement of the induction quadrature signal (Xsignals). These measurements are combined with new advances in signal processing to provide an induction log with thin-bed resolution down to 2 ft [60 cm]. Full correction for such environmental distortions such as shoulder effect and borehole effect are also performed.

Central to this development is a nonlinear deconvolution technique that corrects the induction log in real time for shoulder effect and improves the thin-bed resolution over the full range of formation conductivities. This algorithm, called Phasor Processing, requires the use of the induction quadrature signals, or Xsignals, which measure the nonlinearity directly. Phasor Processing corrects for shoulder effect and provides thin-bed resolution through Enhanced Processing down to 60 cm in many cases.

Since its introduction in the early 1960s, the dual induction tool has evolved into the primary logging service for openhole formation evaluation in fresh and oil-base muds. Previous tools have, however, produced logs with response limitations. These limitations have usually required tedious hand correction. In extreme cases tool response limitations have produced features on logs that were mistaken for geological features. Although distortions of the formation resistivity caused by resolution effect and shoulder effect are fully predictable from electromagnetic theory, automatic correction algorithms were not successful before now because of the nonlinearity of the Rsignal measurement, which was the only measurement made in the older tools. New developments in electronics technology, work on computing the response of the induction tool in realistic formation models and modern signal processing theory have combined to allow the development of a newer tool that is able to overcome the limitations of previous tools.

Figure B18: Schematic of the Phasor-Induction SFL tool

(05/96) B-23


By adding borehole geometry measurements in the same tool string, borehole effect can also be corrected in real time. With these environmental effects removed, a real-time inversion of the data into a three-parameter invasion model can be done at the wellsite. The Phasor induction design provides several additional advantages over existing tools. These include improvements in the calibration system, sonde error stability, SFL response and a reduction of signal and cable noise. Each of these improvements contributes toward providing more accurate formation resistivity measurements over a wider range of resistivity and borehole conditions. a) Phasor Tool Description and Features The Phasor-Induction SFL tool can be combined with other cable telemetry tools. Measurements returned to the surface include deep (ID) and medium (IM) R-signals, ID and IM X-signals, SFL voltage and current, SFL focus current, spontaneous potential (SP), SP-toArmor voltage and array temperature. All measurements except SP are digitized downhole with high-resolution analog-to-digital converters, and all measure channels are recalibrated every 6 in. [15 cm] during logging.

The operating frequency of the induction arrays is selectable at 10, 20, or 40 kHz, with a default frequency of 20 kHz. The tool also provides measurements of important analog signals and continuous monitoring of digital signals as an aid to failure detection and analysis. Depths of investigation and vertical resolution of the measurements are listed.

(05/96) B-24

b) Log Presentation The same presentation format is used for both generations of induction tools. The two logs can be identified by the following differences (Figure B19): 1. Deep induction (IDPH)—the log inserts use the IDPH acronym to identify Phasor Processing. 2. Medium induction (IMPH)—the log inserts use the IMPH acronym to identify Phasor Processing. 3. There is a hash mark up the right side of the depth track. c) Tool Characteristics, Improvements, and Applications 1. The Phasor-Induction SFL tool can be most effectively used in holes filled with moderately conductive mud, nonconductive mud and airdrilled holes. 2. Vertical focusing is good and gives reliable values of Rt for beds thicker than 2.5 m with no shoulder bed corrections required. 3. Low resistivities are measured accurately. 4. The recording of three focused resistivity logs investigates different volumes of formation. 5. It is reliable for resistivities up to 1000 ohm-m versus 250 ohm-m with the normal induction tool. 6. Accurate readings are obtained in boreholes up to 66 cm in diameter Rm < 1000). ( R / t 7. Varying transmitter frequencies improve the signal-to-noise ratios. 8. Digital transmission techniques are used to improve accuracy of calibration and measurement.

Schlumberger

Correction charts are available for - borehole - bed thickness - invasion (chart Rint-11a).

Phasor-Induction SFL tool Median Depth of Investigation 1.

Tool

Depth

Above 100 ohm-m, homogeneous formation

ID IM SFL

62 in. 31 in. 16 in.

[1.58 m] [0.79 m] [0.41 m]

ID IM SFL

48 in. 26 in. 16 in.

[1.22 m] [0.66 m] [0.41 m]

2. At 0.1 ohm-m, homogeneous form ation

Phasor-Induction SFL tool Vertical Resolution

Vertical resolution bed thickness for full Rt determination—no i nvasion

IDPH IMPH IDER† IMER IDVR‡ IMVR SFL

8 ft 6 ft 3 ft 3 ft 2 ft 2 ft 2

ft

[2.46 m] [1.85 m] [0.92 m] [0.92 m] [0.61 m] [0.61 m] [0.61 m]

†ER—enhanced resolution phasor tool ‡VR—very enhanced resolution phasor tool

Chart B1

(05/96) B-25


PHASOR INDUCTION - SFL

SFQF 0.0

10.000 IMQF

0.0

10.000 IDQF

0.0

10.000 SFLU(OHMM) .20000

TENS(N )

2000.0 IMPH(OHMM)

0.0

20000.

.20000

SP(MV )

2000.0 IDPH(OHMM)

-80.00

20.000

.20000

2000.0

IDPH QUALITY

IMPH QUALITY

SFLU QUALITY

PHASOR PROC.

CP 32.6

FILE

8

08-JUN-1992 17:03

INPUT FILE(S) CREATION DATE 1 18-MAY-1992 10:33

1/240

1450

---TENS ---SFLU SP-- ---IMPH ---IDPH ---SFQF ---IMQF ---IDQF 1475

Figure B19

(05/96) B-26

Schlumberger

Phasor* Dual Induction-SFL Spherically Focused Log ID Phasor - IM Phasor - SFL

Thick beds, 8-in. [203-mm] hole, skin-effect and borehole corrected Rxo /R m 100, DIT-E or equivalent, frequency = 20 kHz ≈

200 25

100

50

40

30

50

di (in.) 60

70

20 15 200

0.95

0.9

Rt 0.8 RIDPH

80

90

100 120

0.7

0.6

0.5 0.4

20

160 0.3

140

200

100 70

RSFL /RIDPH 10

50 40 30

5

20

1

15

2

10 7 5

1

Rxo 3 2 Rt 1

2

3

4

5

RIMPH /RIDPH

These charts (Rint-11) apply to the Phasor induction tool when operated at a frequency of 20 kHz. Similar charts (not presented here) are available for tool operation at 10 kHz and 40 kHz. The 20 kHz charts do provide, however, reasonable approximations of Rxo /R t and Rt /RIDPH for tool operation at 10 kHz and 40 kHz when only moderately deep invasion exists (less than 100 inches). All Phasor* Induction invasion correction charts are applicable to Enhanced Resolution Logging (ERL*) and Enhanced Resolution Analysis (ERA*) presentation.

Rint-11a

Figure B20

(05/96) B-27


(05/96) B-28

Schlumberger

B3.0

Measurement of Rt by Laterolog Principles

B3.1 DUAL LATEROLOG Broadly speaking, borehole fluids used during drilling operations are broken into conductive and nonconductive categories. Each poses particular challenges in measuring formation resistivities. The dual laterolog is a current emitting electrode device that performs best in Rm >>> 100, Rmf / Rw saline muds (i.e., where R / t < 2.5). It is designed to extract Rt by measuring resistivity with several arrays with different depths of investigation.

a) Description and Features These requirements resulted in the development of the dual laterolog MicroSFL tool with simultaneous recordings. Figure B21 illustrates the focusing used by the deep laterolog device (LLD, left) and by the shallow laterolog device (LLS, right). Both use the same electrodes and have the same current-beam thickness, but have different focusing to provide their different depth-of-investigation characteristics.

Measurements responding to three appropriately chosen depths of investigation usually approximate the invasion profile sufficiently well to determine Rt . For best interpretation accuracy, a combination system should have certain desirable features: - Borehole effects should be small and/or correctable. - Vertical resolutions should be similar. - Radial investigations should be well distributed (i.e., one reading as deep as practical, one reading very shallow and the third reading in between).

Figure B21: Dual Laterolog Deep and Shallow Current Patterns

(05/96) B-29



The DLL tool has a response range of 0.2 to 40,000 ohm-m, which is a much wider range than covered by previous laterolog devices. To achieve accuracy at both high and low resistivities a constant-power measuring system is employed. In this system both measure current (io) and measure voltage ( V o) are varied and measured, but the product of the two V oio (i.e., power) is held constant. The deep laterolog measurement (LLD) of the DLL tool has a deeper depth of investigation than previous laterolog tools and extends the range of formation conditions in which reliable determinations of Rt are possible. To achieve this, long guard electrodes are needed; the distance between the extreme ends of the guard electrodes of the DLL-R xo tool is approximately 28 ft [8.5 m]. The nominal beam thickness of 2 ft [60 cm], however, insures good vertical resolution. Radial investigation is 4–5 ft [1.2–1.5 m].

b) Log Presentation The DLL MicroSFL log presentation is similar to that of the Phasor Induction. Differences include an expanded resistivity scale (0.2–200,000 ohmm) and the addition of gamma ray and caliper (if MicroSFL is used). See the log in Figure B23. c) Tool Characteristics and Applications 1. The dual laterolog performs most effectively in saline mud (high Rt / Rm Rw < 2.5 (Figure ratios) or where Rmf / B22). 2. The tool has an excellent resistivity range; by utilizing a unique design, resistivity resolution from 0.2 to 40,000 ohm-m is possible.

The shallow laterolog measurement (LLS) has the same vertical resolution as the deep laterolog device at 2 ft [60 cm], but it responds more strongly to that region around the borehole normally affected by invasion. It uses a type of focusing called the pseudolaterolog, wherein the focusing current is returned to nearby electrodes instead of to a remote electrode. This causes the measure current to diverge more quickly once it has entered the formations, thus producing a relatively shallow depth of investigation of 20 to 24 in. [50 to 60 cm].

Figure B22: Preferred Ranges of Applications of Induction Logs and Laterologs

(05/96) B-30

Schlumberger

Schlumberger DUAL LATEROLOG - MSFL FILE 16

LLD 2000

BS 125

(MM)

(OHMM)

LLS 375

2000

(OHMM)

TENS 50000

(MM)

0

0.2

(OHMM)

375

0.2

(OHMM)

(GAPI)

150

0.2

(OHMM)

2000

LLD

GR 0

200000

MSFL

(N)

CALS 125

200000

2000

LLS 2000

2550

2600

Figure B23

(05/96) B-31



3. Vertical resolution is excellent. Rt can be obtained in beds as thin as 2 ft [60 cm]. 4. The LLD has very little borehole effect in large holes. 5. When combined with an R xo measurement, the LLD and LLS curves may be used to study invasion profiles and compute a more accurate Rt . See Chart Rint-9 (Figure B24). 6. Assuming borehole conditions are suitable, the separation of the LLS and LLD curves may be used to give quicklook indications of hydrocarbons; particularly in salt mud. In salt muds R xo / Rt will be less than 1 so the better the zone, the greater the separation between the LLS and LLD.

d) Limitations 1. The tools should not be used in fresh Rw > 2.5). muds ( Rmf / 2. The tools requires good centralization to minimize borehole influence on the LLD. 3. If invasion is deep, a good value of R xo (e.g., from a microspherically focused log) is required to correct LLd for invasion influence to obtain an accurate value of Rt .

Correction Charts are available for the influence of - borehole (diameter and mud resistivity) - invasion. (Chart Rint-9b, Figure B24) - bed thickness.

(05/96) B-32

Schlumberger

Schlumberger

Dual Laterolog -R xo Device DLT-D/E LLD - LLS - Rxo Device

Thick beds, 8-in. [203-mm] hole, no annulus, no transition zone, Rxo /Rm = 50, use data corrected for borehole effect 100

20

80

30

40

50

100

0.50 0.75

60

80

1.01

1.27

70 40

60

1.52 2.03

120

50 3.04

Rt

30

di (in.)

Rxo 1.1

30

1.2

20

di (m)

1.3

15

100

1.4 1.6

20

1.8 15

10 8

Rt

10

RLLD

6 7 RLLD /Rxo

4

5

3 3 2 2 1.5

1.5

1 0.8

Rt

di (in.)

Rxo

di (m) 0.6 100 2.54 60 0.4 0.3

0.2 0.4

0.4

1.52 40 30 1.01 20 0.2 0.75 0.50 0.6

0.8 1.0

1.5

2

3

4

6

8

10

15

20

30

40

50

RLLD /RLLS

Rint-9b Figure B24

(05/96) B-33



(05/96) B-34

Schlumberger

B4.0

Measurement of R xo by Microresistivity Principles

B4.1 INTRODUCTION As has been mentioned, a measurement of flushed-zone resistivity R xo is an important input when attempting to define invasion diameter. Because the flushed zone may extend only a few centimetres from the borehole, a shallow-reading device is required. Such tools are the microlog, microlaterolog, proximity log and the MicroSFL log. All are pad-type devices that are pressed against the borehole wall to make their measurements.

Today, the microlog MicroSFL log are completely combinable with all main logging services. The microlaterolog and proximity log have been discontinued because of their limitations in design; hence, explanations of their measurements are not provided. Another service, the EPT (Electromagnetic Propagation Tool), also provides an excellent R xo measurement. This service is an advanced device and is not discussed in this book. For more information, refer to Schlumberger Log Interpretation Applications/Principles.

To measure R xo , the tool must have a very shallow depth of investigation. Because the reading should be affected by the borehole as little as possible, a sidewall-pad tool is used. Currents from the electrodes on the pad must pass through the mudcake to reach the flushed zone. Therefore, microresistivity readings are affected by mudcake; the effect depends on mudcake resistivity Rmc and thickness hmc. Moreover, mudcakes can be anisotropic, with mudcake resistivity parallel to the borehole wall less than that across the mudcake. Mudcake anisotropy increases the mudcake effect on microresistivity readings so that the effective, or electrical, mudcake thickness is greater than that indicated by the caliper.

(05/96) B-35


B4.2 MICROLOG With the microlog tool, two short-spaced devices with different depths of investigation provide resistivity measurements of a small volume of mudcake and formation immediately adjoining the borehole.

Comparison of the two curves readily identifies mudcake, which indicates invaded and, therefore, permeable formations. a) Principle The rubber microlog pad is pressed against the borehole wall by arms and springs (Figure B25). The face of the pad has three small inline electrodes spaced 1 in. [2.5 cm] apart. With these electrodes a 1- by 1-in. microinverse ( R1" x1" ) and a 2-in. [5.1 cm] micronormal ( R2" ) measurement are recorded simultaneously. The currents emitted from these electrodes are totally unfocused and hence flow by the path of least resistance (Figure B26).

Figure B25: Microlog

(05/96) B-36

As drilling fluid filters into the permeable formations, mud solids accumulate on the hole wall and form a mudcake. Usually, the resistivity of the mudcake is slightly greater than the resistivity of the mud and considerably lower than the resistivity of the invaded zone near the borehole. The 2-in. micronormal device has a greater depth of investigation than the microinverse. It is, therefore, less influenced by the mudcake and reads a higher resistivity, which produces positive curve separation. In the presence of low-resistivity mudcake, both devices measure moderate resistivities, usually ranging from 2 to 10 times Rm . In impervious formations, the two curves read similarly or exhibit some negative separation. Here the resistivities are usually much greater than in permeable formations (see Figure B27).

Figure B26: Microlog

Schlumberger

MICROLOG

ACCUMULATED INTEGRATION VALUES SUMMARY: Integrated Hole Volume: 2.07418 M3

FROM 2039.87 M

TO 1995.07 M

EVENT MARK SUMMARY: OUTPUT

INTERVAL DEPTH TRACK BETWEEN PIPS EDGE

Integrated Hole Volume

.100000 M3

LEFT EDGE

MCAL(MM ) 125.00

375.00 TENS(N )

50000.

0.0 SGR(GAPI)

BMNO(OHMM)

0.0

150.00

0.0

40.000

BS(MM )

BMIN(OHMM)

125.00

375.00

CP 32.6

FILE

3

0.0

40.000

00- -1941 00:39

INPUT FILE(S) CREATION DATE 61 02-JUN-1992 15:15

1/240

2000

2025

MCAL-- ---BMNO ---BMIN TENS-- ---SGR ---BS

Figure B27

(05/96) B-37


Under favorable circumstances the microlog can be used to obtain R xo but it is generally considered a good qualitative indicator of permeability, rather than an R xo measurement.

This eliminates the need for a separate logging run to obtain R xo information. See Figure B23 for a log example of the MicroSFL tool with dual laterolog.

b) Microlog Limitations Rmc must be less than about 15. - R xo / - Mudcake thickness < 1.2 cm - Depth of flushing > 10 cm, otherwise the microlog readings are affected by Rt .

The second improvement is in the tool’s response to shallow R xo zones in the presence of mudcake. The chief limitation of the microlaterolog measurement was its sensitivity to mudcakes. When mudcake thickness exceeded about 3 / 8 in., the log readings were severely Rmc contrasts. The influenced at high R xo / proximity log, on the other hand, was relatively insensitive to mudcake, but it required an invaded zone diameter of about 100 cm to provide direct approximations of R xo .

B4.3 MICROSPHERICALLY FOCUSED LOG The MicroSFL tool is a pad-mounted, spherically-focused logging device that has replaced the microlaterolog and proximity tools. It has two distinct advantages over the other R xo devices. The first is its combinability with other logging tools, including the PhasorInduction SFL, the AIT (Array Induction Imager and dual laterolog tools).

The solution was found in an adaptation of the principle of spherical focusing in a sidewall-pad device. By careful selection of electrode spacings and bucking-current controls, the MicroSFL measurement was designed for minimum mudcake effect without any undue increase in the depth of investigation. Figure B28 illustrates, schematically, the current patterns (left) and the electrode arrangement (right) of the MicroSFL tool. By forcing the measure current to flow directly into the formation, the effect of mudcake resistivity on the tool response is minimized; yet, the tool still has a shallow depth of investigation.

Figure B28: Current Distribution of MicroSFL device (left) and Electrode Arrangement (right)

(05/96) B-38

Synthetic microlog curves can also be computed from MicroSFL parameters. Because the measure current sees mostly the flushed zone and the bucking current sees primarily the mudcake, it is possible to mathematically derive micronormal and microinverse curves.

Schlumberger

B5.0 Work Session 1a. Given Rmf = 2.5 ohm-m at 10oC, find Rmf at 52oC, using Chart Gen-9 (Figure B2). Rmf =

b. What is NaCl concentration of the mud filtrate in ppm?

2a. Given a solution salinity of 80,000 ppm, find the solution resistivity at 121oC. Rm =

b. Given a solution salinity of 10,000 ppm at 20oC, find the solution resistivity at 50 oC. Rm =

3.

Given Rm = 0.74 at 20 oC, what does Rm equal at BHT if the total depth is 2400 m and the geothermal gradient is 2oC/100 m (surface temperature is 20 oC) ?

Rm = __________________________ at __________________ oC

(05/96) B-41


4. SP(MV ) -150.0

0.0

15 -|---|+

From the SP in Figure B30, calculate Rw. Formation temperature is 63oC. Rmf = 0.79 at 20 oC.

a) Rmf = at formation temperature CP 32.6

FILE

1

01-APR-1941 17:28


b) SP =

mV

c) Rmfe = at formation temperature

1/240

d) Rwe = at formation temperature 2150

e) Rw = at formation temperature

f) Rw =

g) Formation NaCl concentration =

at25oC

ppm

Note: Use charts SP-1 and SP-2m (Figures B11 and B12).

SP---

2175

Figure B30

(05/96) B-42

Schlumberger

5. GR(GAPI) 30.000

130.00

Calculate Rw for the zone from 2326 to 2340 m in Figure B31.

SP(MV ) -150.0

Rmf = 0.110 at 20 oC Formation temperature = 58.9oC

0.0

15 -|---|+

CP 32.6

FILE

3

01-APR-1941 18:05

Rw =


at25oC

1/240

6. 2325

Using the log examples in Figure B32 calculate

a)

Depth of invasion at A and B and b) Rt (ILD corrected) at A and B

---GR SP---

7.

Calculate Rw for the example of the dual induction SFL in Figure B15. Rm Given: = 3.05 at 17oC = 2.60 at 17 oC Rmf BHT = 23oC

2350

Figure B31

(05/96) B-43


ILM(OHMM) .20000

2000.0

GR(GAPI) 0.0

ILD(OHMM) 150.00

.20000

2000.0

SP(MV )

SFL(OHMM)

-150.0

0.0

CP 32.6

FILE

8

.20000

2000.0

09-JUN-1992 14:42


1/240

A

1800

---GR ---SP ---ILM ---ILD ---SFL

1700

B

---SP ---ILM ---ILD SFL---

1725

Figure B32

(05/96) B-44

Schlumberger

8. Calculate Rw for both zones in Figure B33 Rm = 1.18 at 25oC Rmf = 0.93 at 16oC BHT = 59oC

SP(MV ) -80.00

20.000

10 -|---|+

CP 32.6

FILE 4 01-APR-1941 18:13 INPUT FILE(S) CREATION DATE 1 05-JUN-1992 08:41 1675

a. Top zone: 1685 m to 1695 m

Rw =

at 59oC

Rw =

at 25oC

b. Bottom zone: 1695 m to 1717 m 1700

Rw =

at 59oC

Rw =

at 25oC

---SP

c. What are possible reasons for the difference?

1725

Figure B33

(05/96) B-45

02 Resistivity Measurement

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