Presentation by Mark Zoback, given to SPWLA Abu Dhabi Chapter on 9th Dec 2009

1. Determination of the State of Stress With
Applications to Wellbore Stability and
Fracture Flow in Reservoirs
Mark Zoback
Professor of Geophysics
Stanford University
1

2. Geomechanics Through the Life of a Field
E xploration A ppraisal D evelopment H arvest A bandonment
P
r Wellbore Stability
o Pore Pressure Prediction
Fault Seal/ Fracture Permeability
d
u Sand Production Prediction
c Compaction
t Casing Shear
i Subsidence
Coupled Reservoir Simulation
o
Fracture Stimulation/ Refrac
n Depletion
Geomechanical Model
Time

3. Geomechanics Through the Life of a Field
E xploration A ppraisal D evelopment H arvest A bandonment
P
r Wellbore Stability
o Pore Pressure Prediction
Fault Seal/ Fracture Permeability
d
u Sand Production Prediction
c Compaction
t Casing Shear
i Subsidence
Coupled Reservoir Simulation
o
Fracture Stimulation/ Refrac
n Depletion
Geomechanical Model
Time

4. Middle East and
Caspian Sea
GMI
Dubai
LEGEND
Wellbore Stability
Fracture Permeability
Fault Seal
Pore Pressure
Sand Production
Stress Direction
Last Update:
1/10/09

5. Topics
 How to Determine the State of Stress in
Oil and Gas Wells (and How Not To)
 Wellbore Stability Applications
 Fluid Flow in Fractured Reservoirs
 3D/4D Geomechanics

7. Get the Stress Right!
Principal Stresses at Depth
Sv – Overburden
SHmax – Maximum horizontal
Sv principal stress
Shmin – Minimum horizontal
principal stress
Additional Components of a
Geomechanical Model
UCS Pp – Pore Pressure
Pp
UCS – Rock Strength (from logs)
Fractures and Faults (from Image
Shmin SHmax Logs, Seismic, etc.)
7

8. Developing a Comprehensive Geomechanical Model
Parameter Data
z0
Vertical stress Sv (z0 ) = ∫ ρ g dz
0
Least principal
stress Shmin ⇐ LOT, XLOT, minifrac
Max. Horizontal
Stress SHmax magnitude ⇐ modeling
wellbore failures
Stress
Orientation Orientation of Wellbore failures
Pore pressure Pp ⇐ Measure, sonic, seismic
Rock Strength Lab, Logs, Modeling well failure
Faults/Bedding Wellbore Imaging
Planes

9. Compressional and Tensile Wellbore Failures
Well A
UBI Well A FMI Well B

10. Borehole Wall Stresses for a Particular Trajectory

11. Breakouts in Deviated Wells
SHmax
azimuth
145°
55º/235º
100º/280º
vertical well
tangential
stress
100º/280º
well inclined 70° at
an azimuth of 280

12. Stereo Plot for Deviated Wells
Easy and functional display
of wellbore stability or risk
for wells of any orientation.

13. Wellbore Failure Orientation in Deviated Wells

14. Pre-Salt, Brazil - SHMax Azimuth?

15. Wellbore Failures – South America

16. Geomechanics Through the Life of a Field
E xploration A ppraisal D evelopment H arvest A bandonment
P
r Wellbore Stability
o Pore Pressure Prediction
Fault Seal/ Fracture Permeability
d
u Sand Production Prediction
c Compaction
t Casing Shear
i Subsidence
Coupled Reservoir Simulation
o
Fracture Stimulation/ Refrac
n Depletion
Geomechanical Model
Time

17. Similar Diagrams for Nahr Umr Shale

18. Don’t Calculate Stress From Poisson’s Ratio
Assumptions: However...
•Sv applied instantaneously •Observations indicate that the
•No other sources of stress exist horizontal stresses are not equal,
•No horizontal strain (Bilateral
•Model doesn't explain SH > Sh > Sv,
Constraint)
•Material is elastic, homogeneous •Global tectonic activity indicates that
and isotropic from the time Sv is the crust is not tectonically relaxed
applied to the present
ν
SH - Pp ~ 1− ν
(Sv - Pp)α
Utilizing an Effective
Poisson’s Ratio and
Adding Tectonic Stress
Does Not Make Model
Correct
Lateral Constraint
(horizontal strain = zero)

19. Don’t Calculate Stress from Poisson’s Ratio!

20. Topics
 How to Determine the State of Stress in
Oil and Gas Wells (and How Not To)
 Wellbore Stability Applications
 Fluid Flow in Fractured Reservoirs
 3D/4D Geomechanics

21. The Key to Wellbore Stability is Controlling the
Width of Failure Zones

22. Design for Variations in Strength
Increase Mud Weight as Needed

23. Frac Gradient
“Collapse
Pressure”
Pore Pressure

24. Tendency for Breakout Initiation for
Different Stress Regimes
3 km Depth, Hydrostatic Pp

25. Mud Weight Needed to Maintain 30º Breakouts
Normal Strike-Slip Reverse
Stress States Same as Previous Slide
Medium Strong Rock UCS = 7250 psi

26. Example - Stability of Uncased Multi-Laterals
Key Questions:
• Is it possible to leave short sections
(~15’), of laterals uncased near the
parent well?
• Will such intervals be stable as the
reservoir is produced?
• Could producing too fast
exacerbate sand production and
stability problems?

27. Calibrated Rock Strength Log
C o, K psi
0 5 10 15 20
9500
• Triaxial tests in laboratory
9600
• Relate strength to P-wave
modulus
9700
• Use ∆T and density to compute
UCS
9800
• Caution - should not be used in
hydrocarbon zones
9900
10000

28. Wellbore Stability Plot
N
Less stable
Required mud weight
Required Strength
Breakout Width W E
More stable
S
S H m ax
Lower hemisphere stereographic projection of well orientation

29. Previously Unknown
Drilling Experience
M O NO PO D
K-2 6 -9
80
0'
0'
70
-9
0'
60
-9
in g
B ay
Fa
u lt
-9
00
0'
Well X
'
00
ad
-92
'
00
Tr
0'
-94
-960
0'
-980
-9800'
Drilled at 335 degrees,
KING SALMON
-9
60
0'
G-1 5 RD -9
-9600'
40
0'
-9400'
maximum deviation 108 degrees.
-94
00
'
-9200'
-96 00'
00
' -92
-920
0'
0'
60
-9
Successfully drilled and
0'
-900
-9600'
-940
M-3 1
0'
0'
-940
0'
-920
completed
0'
GRAYLING
-900
00'
-94
0'
20
-9
STEELHEAD
-920
0'
-940
0'
-920
0'
-9
60
0'
0'
-940
Well Y
0'
60
-9
0'
80
-9
Drilled at 31 degrees,
0'
20
-9
DOLLY VARDEN
deviation 88 degrees.
-9
40
0'
-9
60
-9400'
0'
0'
-960
0'
-980
Wellbore collapsed in
open-hole section

30. Moderate Drawdown / Damage
• Decreased pressure drop
• Damage zone less
important
Pore pressure distribution during drawdown

31. Moderate Drawdown / No Damage
Smaller pressure drop
10000
Uniaxial compressive strength [psi]
Lower stress at wellbore
8000
6000
→Relatively more stable
4000 →Total BO’s ~ 100o
2000
0

32. Rapid Drawdown / Damage
• Large pressure drop near
the well
• Exacerbated by damage
zone
Pore pressure distribution
during drawdown

33. Rapid Drawdown / Damage
Large pressure drop
10000
Increased stress at wellbore
Uniaxial compressive strength [psi]
8000
→Unstable well
6000
→Total BO’s > 180o
4000
2000
0
Strength required to prevent failure is too high → excessive breakouts

34. Example 2
• Severe wellbore instabilities in
the Fortune Bay shale led to
abandonment of original PG-2 Side track
well and required drilling a side track
• The side track was completed abandoned
successfully by switching to oil
PG-2
based mud and raising the mud
weight to 12 ppg in the Fortune
Bay shale.
Objective for future wells
• Optimization of wellbore stability
in deviated and horizontal wells
• Feasibility of drilling highly
deviated wells with a maximum
mud weight of ~11.5 ppg

35. Orientation of SHmax
Hibernia
World stress map data
superimposed with mean SHmax
Newfoundland orientation (red arrow) derived from
St. John’s 4-arm caliper and UBI breakout
analysis in vertical wells of the
Terra Nova field
Terra Nova

36. Pore Pressure and Stress in the Terra Nova Field
Pressure/Stress [bar]
0 200 400 600 800 1000
0
Pp[bara] wet sand
Pp water
500 Pp[bara] sand
Pp oil wet
LOT (C-09)
Hydrostatic
Hydrost. [bara]
Overburden
Sv [bara]
1000
Test Pres.[bara]
FIT
LOT
X-LOT
1500
SSTVD [m]
2000
2500
1.117
Sv = 0.0848*SSTVD
3000
X-LOT (GIG-3)
X-LOT (PG-2)
3500
Pp = 0.098*SSTV LOT (C-23)
4000
Shmin = -15.889 + 0.19416*SSTVD

37. Breakouts from UBI log in PG-2
Azimuth [deg]
Fortune
0 90 180 270 360
Shale
•
3800
Total breakout
Bay
no data
Low er FBS
3850 E sand
ED shale
length: 32 m
Dc sand
3900
•
Db shale
Da sand
D congl.
Mean breakout
3950
UC2 sand
width: 40° (±11°)
LC2 shale
Jeanne d’Arc
Reservoir
4000
LC2 sand
C2C1 shale
4050
C1 sand
4100
C1B shale
4150
B sand
B Rank shale
no data Rankin Mbr.
4200
Breakout azimuth
Azimuth (deg)
Breakout (deg)
Width width

38. Breakouts from UBI log in PG-2
N
Lc2 shale within the
W E
Jeanne d’Arc reservoir
S
Isotropic compressive failure
C1 sand within the
Jeanne d’Arc reservoir

39. Breakouts from EMS 6-arm caliper log in PG-2
Jeanne d’Arc reservoir Fortune Bay shale
Isotropic failure Anisotropic failure
The difference in failure behavior between the Fortune Bay shale
and the Jeanne d’Arc reservoir is similar to the UBI images

40. Breakouts from UBI log in PG-2
Lowermost Fortune Bay shale
Anisotropic compressive failure

41. Modeling anisotropic breakouts in the Fortune Bay shale
with the given in situ stress state
Anisotropic failure Anisotropic failure
Bedding plane properties:
• dip = 8° (from core data)
• Azi = 23° (from core data)
• S0 = 4.8 MPa (from lab data)
• µs = 0.21 (from lab data)
MW = 10.5 ppg MW = 12 ppg Result:
The in situ stress tensor
Isotropic failure Observed
derived in this study and the
bedding plane properties
measured in the lab can
account for the anisotropic
breakouts seen in the Fortune
Bay shale

42. Predicting stability in the
Fortune Bay shale for well GIG-3
C0 = 55 MPa
wBO = 75° MW = 12 ppg
Assuming anisotropic behavior
• There exists a steep stability gradient for deviations between 25° and 45 °
• Well PG-2 is oriented less favorably in the current stress field
• Well GIG-3 is oriented more favorably in the current stress field
• Severe stability problems can be avoided for GIG-3 with a maximum
mud weight of 11.5 ppg if deviation < 30 °

43. Business impact
• Petro-Canada successfully drilled well
GIG-3 through the Fortune Bay Shale successful
by limiting deviation to 27° and
mud weights to 10.5 ppg – 11 ppg
abandoned
• Petro-Canada avoided costly stability
PG-2
problems by following GMI’s
recommendations for this well
successful
GI
G-
3
Graben structure at base of reservoir

44. Topics
 How to Determine the State of Stress in
Oil and Gas Wells (and How Not To)
 Wellbore Stability Applications
 Fluid Flow in Fractured Reservoirs
 3D/4D Geomechanics

45. Characterizing Hydraulically-Permeable
Fractures and Faults
But which ones control fluid flow and
how do we take advantage of this?

46. Hydraulically Conductive Fractures are Shear Faults
Active (or Activated) in the Current Stress Field
From Townend and
Zoback (2001)

47. Active Faults Maintain Permeability Through Time
Faulting is key to maintaining permeability

48. Temperature Anomalies and
Permeable Faults in the KTB Borehole
Zoback and Townend (2001)
Ito and Zoback (2000)

49. Mechanical Lithosphere
Zoback, Townend and Grollimund (2002)
High Stress, Critically-Stressed Crust
Ductile Lower Crust and Upper Mantle
Is This Model Quantitatively Correct?

50. Broad-Scale Stresses and Distributed Seismicity

52. Gas Leakage Along Faults

53. Active Strike-Slip Faults Conduct Fluids

54. ~5cm/yr
Examples -Critically-Stressed
Faults in Damage Zones

55. Fault Damage Zones and Directional Permeability
Damage
zone

56. Strong Directional Flow Near Dormant Normal Faults
Preferential flow along the faults from
interference and tracer test

57. Current Strike-Slip Stress State
Stratigraphic
Permeability Model
Paul, Zoback and Hennings (2009)

58. Need For a Better Model to Match Reservoir Flow
Permeability Model Does Not Match
Pressure Data in
Producers or Injectors

59. No Wells Directly in Damage Zones
Dynamic Rupture Propagation to Calculate Damage Zones
Depth ~2700m
0 2000 N
m
Origin point of rupture
8
x 10
Damage Intensity 1 .5
sxx
Damage zone sxy
syy
1 szy
szx
s t r e s s m a g n it u d e ( P a )
Rock strength szz
Horizontal Plane 0 .5 S1
S2
S3
oct shear
0 to ta l o c t s h e a r
Fault Plane
-0 .5
Cross Section View Along -1
0 50 100 150 200 250 300
Strike of Normal Fault d is t a n c e f r o m r u p t u r e f r o n t ( m )

60. Calculated Damage Zone Width
At reservoir depths from
100 simulations:
Simulation 1 Mean of DZ width ~50-90m
Simulation 2
Process Zone Width, m
Simulation 3
Fault Zone Length, m
Simulation 4 Vermilye and Scholz (1998)
2km

61. Utilizing the Dynamic
Rupture Model to
Predict Width of
Damage Zone and
Anisotropic Permeability

62. Improved
Damage Zone Model Matches Model
Pressure Data in
Producers and Injectors
Base
Model

63. Breakout Orientation Fluctuations Due to Fault Slip
Shamir and Zoback (1992)

64. Geomechanics Through the Life of a Field
E xploration A ppraisal D evelopment H arvest A bandonment
P
r Wellbore Stability
o Pore Pressure Prediction
Fault Seal/ Fracture Permeability
d
u Sand Production Prediction
c Compaction
t Casing Shear
i Subsidence
Coupled Reservoir Simulation
o
Fracture Stimulation/ Refrac
n Depletion
Geomechanical Model
Time