Reservoir Geomechanics In situ stress and rock mechanics mechanics applied applied to reservoir reservoir processes processes
Mark D. Zoback Professor of Geophysics
Week 5 – Lecture 10 Failure of Deviated Wells Chapter 8
Outline Section 1 • Failure of Deviated Wells • Determination of Stress from Failure of Deviated Wells Section 2 • Distinguishing Distinguishing Drilling-induced Drilling-induced Tensile Fractures from Natural Fractures Section 3 • Determination of Stress Orientation from Shear Velocity Anisotropy Measured with Dipole Sonic Logs 2
Some Key Points Failure depends on magnitude of all 3 principal stresses and the orientation of the well relative to the stress field. Observations of failure in deviated wells is a powerful technique of constraining the magnitudes and orientations of principal stresses Tensile fractures (often appearing as en echelon sets) are expected to be fairly common in inclined wells. The good news is that they do not affect hole stability and are extremely useful for assessing the in situ stress state. The bad news is that they could be mis-interpreted as natural preexisting fractures. There are a number of aspects of inclined well failure that may be counterintuitive. Trust your intuition at your own risk. For example, the initiation of failure and the severity of failure may not be correlative (Chapter 10).
3
Borehole Wall Stresses
Figure 8.1 a – pg. 237
4
Principal Stresses at the Wellbore Wall
Figure 8.1 c – pg. 237
5
Breakouts in Deviated Wells SHmax azimuth 145°
55º/235º
vertical well
100º/280º tangential stress
100º/280º well inclined 70° at an azimuth of 280 °
6
Tensile Fractures in Deviated Wells
North
Azi
East
Borehole image Horizontal
posTF
Fracture trace
Dev Bottom side
incTF
incTF Wall fracture
posTF Bottom Down
z
Top
Bottom
Hole axis
7
Wellbore Coordinate System
Figure 8.1 b – pg. 237
8
Stresses at the Wall of an Arbitrarily Oriented Wellbore — 1
Far-Field Stress Tensor
"S $ $0 $ #0
1
Ss
Rs
=
0
S2 0
% ' 0 ' ' S & 0
3
& cos " cos # ( = cos " sin # sin % $ sin " cos % ( ( ' cos " sin # cos % + sin " sin %
" xs % " X% $ ' $ ' $ ys ' Rs$ Y' $ ' $ ' z # s& # Z& =
$ sin # ) + sin " sin # sin % + cos " cos % cos # sin % + + sin " sin # cos % $ cos " sin % cos # cos % * sin " cos #
Equations 8.1-8.3 – pg. 238
9
Stresses at the Wall of an Arbitrarily Oriented Wellbore — 2
" xb % " X% $ ' $ ' $ yb ' Rb $ Y ' $ ' $ ' z # b& # Z&
Rb
=
Sg Sb
=
RTs SsRs T
=
# ij
#
zz
T
RbRs SsRsRb
=
Sij
" ! ij P p
$ ##
=
=
= $
11
% " cos # cos $ " sin # cos $ ' " cos# ' sin # ' sin # sin $ & cos# sin $
sin $ (
* 0 * * cos $ )
$ 2" (# 11 $ # 22 )cos 2! $ 4"# 12 sin 2!
# 33
+ $
22
$ !z
(
)
" 2 $ 11 " $ 22 cos 2# " 4$ 12 sin 2# " !P
=
(
2 " 23 cos ! # "13 sin !
" rr
=
)
!P
Equations 8.4-8.7 – pg. 238 10
Stresses at the Wall of an Arbitrarily Oriented Wellbore — 3
" t max
" t min
=
1& (" zz 2'
=
1& (" zz 2'
+ "
+
##
+ "
##
$
" rr
=
("
zz
("
zz
$ "
##
$ "
##
)
) 2
2
) + 4% + z * 2
#
) + 4% + z * 2
#
!P
Equations 8.8 – pg. 239 11
Representing Drilling Trajectories
Figure 8.1 d – pg. 237
12
Tendency for Breakout Initiation for Different Stress Regimes*
3 km Depth, Hydrostatic Pp
Figure 8.2 a,b,c – pg. 240
*Don’t use this plot for wellbore stability 13
Tendency for Tensile Fracturing for Different Stress Regimes*
3 km Depth, Hydrostatic Pp *Don’t use this plot for lost circulation
Figure 8.3 a,b,c – pg. 241 14
Visund Field
15
Constraining Stress Magnitudes
Figure 8.10 – pg. 250
16
Stress Magnitudes
17
Disappearing Tensile Fractures Confirm S Hmax Magnitudes
Figure 8.9 a,b – pg. 249 18
Breakout Orientation With Deviation Direction and Azimuth
Figure 8.5 a,b – pg. 244
19
Tensile Fracture Orientation and Well Deviation
Figure 8.5 c,d – pg. 244
20
Failure Orientation as Function of Deviation
Figure 8.4 a,b – pg. 243
21
Estimating Stress and Rock Strength from Observations of Breakouts in Inclined Wells Observations of failure in inclined boreholes are extremely valuable for assessment of in situ stress and strength 1. Because failure depends on magnitude of all 3 principal stresses and the orientation of the well relative to the stress field, the following is often determinable with observations of only the orientation of failure in a single deviated well: Known Shmin Stress Orientation
Determinable SHmax magnitude and Stress Orientation Shmin and SHmax magnitudes
2. Observations in multiple wells are very helpful as long as you are confident that the stress field is uniform (both stress orientation and magnitudes) . 3. Once stress magnitudes have been constrained, it is possible to estimate upper (and lower) bounds of compressive rock strength. 22
South Eugene Island Pathfinder Stress Study
Observations • Wellbore breakouts 17° (clock-wise) from the bottom of well • Wellbore azimuth of 35° • Wellbore inclination of 32° • Sv = 42.9 MPa (density log) • Shmin = 37.1 MPa (leak-off test) • Pp = 29.0 MPa (equal to mud weight)
Objective •To find an azimuth and magnitude of S Hmax which are consistent with the observations above.
23
Pathfinder Stress
(psi)
(psi)
24
South Eugene Island, Block 330 Area
Figure 8.11 b – pg. 252 25
Key Seating in Deviated Wells, GOM Example
26
Drilling Induced Tensile Fractures KTB Pilot Hole, Germany
Figure 8.6 a,b – pg. 245 27
Development of En Echelon Tensile Fractures
Figure 8.7 a,b,c – pg. 246
28
Modeling Drilling-Induced Tensile Fractures
Geothermal Well, Japan
Figure 8.8 a,b – pg. 248
29
Known Parameters Brazil 5819m TVD (6110m MD)
30
Brazil 5819m TVD (6110m MD). Modified Lade Criteria for breakouts
31
Tensile Fractures and S Hmax Orientation
SHMax . 00
SHMax . 90
SHMax . 30
S HMax . 120
SHMax . 60
SHMax . 150
32
SHMax Azimuth = 120 deg
33
Outline Section 1 • Failure of Deviated Wells • Determination of Stress from Failure of Deviated Wells Section 2 • Distinguishing Drilling-induced Tensile Fractures from Natural Fractures Section 3 • Determination of Stress Orientation from Shear Velocity Anisotropy Measured with Dipole Sonic Logs 34
Natural Fractures, Drilling Induced Fractures and Drilling Enhanced Fractures
Figure 8.12 a,b,c – pg. 253
35
Development of En Echelon Tensile Fractures
Figure 8.7 a,b,c – pg. 246
36
Theoretical Tensile Fracture Growth
Figure 8.13 a – pg. 254 37
Drilling Induced Tensile Wall Fracture, Argentina
Figure 8.13 b – pg. 254
38
Drilling Induced Tensile Wall Fractures Soultz, France
Figure 8.13 c – pg. 254 39
Drilling Enhanced Fractures
40
Outline Section 1 • Failure of Deviated Wells • Determination of Stress from Failure of Deviated Wells Section 2 • Distinguishing Drilling-induced Tensile Fractures from Natural Fractures Section 3 • Determination of Stress Orientation from Shear Velocity Anisotropy Measured with Dipole Sonic Logs 41
Shear Velocity Anisotropy Shear velocity anisotropy has been studied widely in seismology
Reflection Seismology Cross-Dipole Logs Earthquake Seismology
But what controls shear velocity anisotropy?
Bedding? Stress? Fractures? 42
Stress-induced anisotropy Highly disordered system
!
!
!
Preferential closure of fractures in response to S Hmax Stress parallel fast direction Decreasing anisotropy with depth as stress increases.
43
Structural anisotropy Highly ordered system
Aligned macroscopic features (e.g., fault fabric or bedding) Fast direction parallel to structure !
!
44
Dipole Sonic Shear Logs Measure Anisotropy in Plane Normal to the Wellbore
Structure Stress
UBI and FMI Logs
45
Stress Induced Acoustic Anisotropy, Compared to Borehole Breakout Orientations FAST SHEAR SLOWNESS
240
40
0 FEET
(us/ft)
SLOW SHEAR SLOWNESS 240
ANISOTROPY [ani]
AZIMUTHAL ANISOTROPY MAP 40 0
AVG. ANISOTROPY [ania] 40
40 (us/ft)
(ANISOTROPY %)
0 (%)
0 0 4 7
360
(%)
0
4
S T R A T I G R A P H Y C O L U M N
B 3 D b a r
Average Anisotropy Azimuth
N
S
N
7404
A
N69
7411
7417 N63 5 2 4 7
4 D b a r A
7423.5
7433 N64
7439
46
Scott Field
Figure 8.14 – pg. 257
47
Comparison of Stress Orientations
Figure 8.15 a,b – pg. 258
48
Stress and Shear Anisotropy SAFOD Pilot Hole
(Boness & Zoback, GRL, 2004)
49
What will be the affect of bedding on cross-dipole logging data?
Figure 8.16 – pg. 259 50
Sedimentary Bedding Electrical Conductivity Image
Bed Orientation
2000
2200
SAF strike
) 2400 m ( h t p e D 2600
2800
3000 51
Effect of Anisotropy on Well Logs
Figure 8.17 a,b,c – pg. 260 52
Apparent Fast Directions For a borehole with azimuth from North, ! , and inclination from the vertical, ! , the vector, Bn that defines the axis of the borehole from an arbitrary origin is given by: Bn
2 & . , ) , ) - I ' ' = $sin(/ ) 1 + * sin* 2 $ (( + + %
, , . ) ) cos(/ ) 1 + * sin* - I ' ' (( + +2
2
# , . )! - sin* - I ' +2 (! "
Equation 8.9 – pg. 261 where all angles are in radians. Given the dip, ƒ d , and dip direction, ƒ " , of the true fast plane we compute three discrete points, F 1, F 2 and F 3, in the fast plane that has a corner at the origin used to define the borehole. The normal to the fast plane, F n, may now be computed using A = F 1 - F 2 and B = F 2 - F 3 , thus giving
F n
=
A ! B
The vector, ƒ a, that describes the apparent fast direction, ƒ ad (defined to be in the dip direction), and the apparent fast dip, ƒ a" , from the origin is then found by computing the line that is perpendicular to the borehole and perpendicular to the normal to the fast plane (i.e. in the fast plane) such that a
f
=
B
!
F
53
Bedding Plane Geometry and Anisotropy
Figure 8.18 – pg. 262 54
