1. Reservoir Stress-Sensitivity
BGD Smart
JM Somerville
M Jin

2. Reservoir Stress-Sensitivity
Reservoir properties and therefore behaviour influenced by changes in stress
Caused by either changes in pore pressure or temperature, or combination
Properties = permeability, dimensions, integrity

3. Stress-Sensitivity Scales
Near wellbore
permeability – (stress skin cf skin caused by invasion)
failure
Increasingly distant from the wellbore
permeability
Whole reservoir
permeability, directional floods
Field
compaction, subsidence, seal alteration

4. Stress-Sensitivity Scales
Near wellbore – Influenced by UBD
permeability – (stress skin, no skin caused by invasion)
failure
Increasingly distant from the wellbore
permeability

5. Reservoir Stress-Sensitivity: a multi-disciplinary challenge
More Realistic Reservoir Model
Better Decisions

6. Reservoir Stress-Sensitivity: a multi-disciplinary challenge
Better Decisions
More Realistic Reservoir Model
Stress Sensitivity

7. Better Decisions Re:- All impacting recovery factor and costs Reserves
Well design PI
Well locations
Production strategy
Reservoir management (inc 4D seismic)
Seal integrity
Compartmentalisation
Facilities
Efficacy of UBD technology and methodology
All impacting recovery factor and costs

8. HWYH-399 Key: Breakout from CBIL(A) Drilling-induced tension from STAR
The figure above shows a selection from a RECALL image analysis display. The data are from well HWYH-399 between 6324 and 6327 feet measured depth (MD).
From left to right, the tracks are:
natural Gamma Ray (GR) for the whole logged interval of the STAR+CBIL tool run
interpreted breakouts from the CBIL image and dips from the STAR resistivity image
the CBIL(A) acoustic image in amplitude mode
the STAR resistivity image. Both displayed images have dynamic gain correction selected.
Note:
the prominent high angle tension crack on both the CBIL(A) and STAR images between and feet MD. The crack has the same azimuth in both images.
Breakout is common in well HWYH-399, always on a near north-south axis.
The azimuth control of the two image logs seems to be in agreement. According to the GeoScience Phase-1 report, the strike of tension gash, or induced axial failures, parallel to the maximum principle stress of the regional Zagros trend should be around 150 - i.e. almost perpendicular to the trend shown.
However, Kes reports that “max principal stress Shmax over Ghawar has a mean direction of N55E, but there are examples of rotation, even to NW-SE ... [he] … would not be surprised by an Shmax azimuth of N90E in HWYH-399.” So the orientation processing for the HWYH-399 image logs may not be suspect. At least the two logs agree with each other.

9. HWYH-394 Key: Drilling-induced tension cracks Bed boundary Fracture
All from STAR
Bed boundary
Fracture
Unclassified,
possible
stylolite
The figure above shows a selection from a RECALL image analysis display. The data are from well HWYH-394 between 6201 and 6204 feet measured depth (MD).
From left to right, the tracks are:
natural Gamma Ray (GR) for the whole logged interval of the STAR+CBIL tool run
caliper, GR and interpreted dips from the STAR resistivity image
the CBIL(T) acoustic image in travel time mode for breakout analysis – though the crack is much more obvious in amplitude mode, where the mismatch between the two image logs stands out.
the STAR resistivity image. Both displayed images have dynamic gain correction selected.
Note the prominent sub-vertical tension crack on both the CBIL(T) and STAR images between 6202 and feet MD. The crack’s apparent azimuth differs between the two images:
CBIL(T): around 180, or midway between 0 and 360
STAR: at about 60 and 240
It seems that the azimuth control of at least one of the two data sets may be incorrect. The STAR tool image has been extensively picked for dips and the strike of tension gash, or induced axial failures. These are consistently around N080E, similar to that for HWY So, perhaps the CBIL image log remains incorrectly oriented after processing? It would be as well to check on the raw data files provided to HWU.

10. The Conceptual Model
The reservoir consists of blocks or layers of intact rock bounded by discontinuities
The reservoir is stressed in an anisotropic manner
The whole system exhibits hysteresis

11. Thinly-bedded interval in the Annot Sandstone
Thinly-bedded interval in the Annot Sandstone.This interval is underlain and overlain by more ‘massive’ sandstones.

12. The Reservoir
“Intact” Rock
Discontinuities

13. Boundary and Local Stresses within the Reservoir
Boundary or Regional Stresses sh sH sv
Local Stresses

14. Intact Rock Properties (stress-sensitive values where appropriate)
Ambient porosity and permeability
Elastic constants E and v
Biot’s coefficient
Failure (Fracture) Criteria
Vp and Vs velocities
Vp anisotropy at ambient conditions
Permeability at reservoir stress conditions
Palaeomagnetic trial

15. Tests with Specimen in Triaxial Cell
P and S waves
Fluid flowing at pressure
Stress-Sensitive Values of:-
Elastic Moduli
Biot’s Coefficient
Permeability
Vp,Vs
Failure Criterion
Tests with Specimen in Triaxial Cell

16. Single State Triaxial Testing
Failure e1
s2 = constant s2 s2 e1 s1
Single State Triaxial Testing

17. Failure Criterion - Triaxial Factor
s2’’’’ s1 s1 x x x
s2’’’ x x
Tan = Triaxial Factor x
s2’’ x x
s2’ e1 s2
Failure Criterion - Triaxial Factor

18. Multi-Failure vs Single State

19. Multi-Failure State Triaxial Testing
Tan = Triaxial Factor x x
s2’’ x x
s2’ e1 s2
Multi-Failure State Triaxial Testing

20. Multi-Failure vs Single State

21. Failure Criterion s1 s1 s2 s2 s2

22. Vp at 27MPa vs Porosity

23. Young’s Modulus at 27 MPa vs Porosity

24. Angle of Internal Friction vs Porosity

25. Sampling Rationale - Intact Rock
Wireline Log
Correlation
Rock Mechanics Property
Sample Core, then Test
Petrophysical Property

26. Populating Model - Intact Rock
Convert Reservoir Characterisation Model into a Geomechanical Model
Synthetic Rock Mechanics Log
Correlation

27. Discontinuity Properties
1. Mechanical
Pre-failure properties
Failure criterion
Post-failure properties
2. Petrophysical/Permeability Sensitivity to Stress and Strain

28. Types of Fracture Behaviour

29. Direct shear-test
F=const

30. Fracture Permeability Sensitivity to Stress and Strain
K/K0
Discontinuity dilating only
Dilation 1
1000
TYPE 1
Shearing under moderate normal stress
Shear Displacement
100
TYPE 2
TYPE 3
0.1
Shearing under high normal stress
Fracture Permeability Sensitivity to Stress and Strain

31. The Process Populate the Conceptual Model with properties and data
So create a Geomechanical Model of the reservoir (plus surrounding rock)
Impose process-induced changes on the Geomechanical Model using analytical or numerical solutions
Numerical offers more realism than analytical – hence coupled modelling

33. Coupled Modelling

34. Fluid Flow Simulator Stress-Analysis Simulator
More realistic simulation results
Fluid Flow Simulator
Change in Pore Pressure, Temperature, Saturations
Change in Permeability
Change in Effective Stresses
Rock Movements, Change in Stress and Strain
Stress-Analysis Simulator
Reservoir and o/b stresses, strains and displacements

35. Differentiating Filter (Synthetic)
Enhanced 4D Seismic Interpretation/Reservoir Management
Fluid Flow Simulator
Differentiating Filter (Synthetic)
Change in Pore Pressure, Temperature, Saturations
Saturation-Related changes in Impedance
Stress-Related changes in Impedance
Change in Permeability
Change in Effective Stresses
Rock Movements, Change in Stress and Strain
Changes in Velocity and Density
Stress-Analysis Simulator

36. Differentiating Filter (Synthetic) Fluid Flow Simulator
Stress-Analysis Simulator
Change in Pore Pressure, Temperature, Saturations
Change in Effective Stresses
Rock Movements, Change in Stress and Strain
Change in Permeability
Enhanced 4D Seismic Interpretation/Reservoir Management
Saturation-Related changes in Impedance
Stress-Related changes in Impedance
Changes in Velocity and Density
More realistic simulation results
Reservoir and o/b stresses, strains and displacements

37. UKNS, Perm Stress Sensitivity (ECLIPSE coupled with VISAGE)
Example 1
UKNS, Perm Stress Sensitivity
(ECLIPSE coupled with VISAGE)

38. Production Prediction: permeability reduction
The diagram shows the absolute reduction (k1-k18). The maximum reduction in permeability is in the central part of the field
Perm sensitivity modelled with hysteresis
(ECLIPSE Output)

39. Stress Sensitive Permeability with hysteresis
Depressurisation in Miller
Injection in Miller induced unloading
Injection in South Brae induced unloading in Miller Field

40. Comparison of GOPR Predictions
Oil Production Rate is sharply reduced because the permeability reduction in the area causes a reduction in BHP and leads to a increase in gas production
(ECLIPSE Output)

41. Horizontal Ground Displacements - 1

42. Horizontal Ground Displacements - 2

43. Horizontal Ground Displacements - 3

44. Stress Ratio vs. time
Between wells
Far from well
Close to well

45. Stress Status in p-q terms (anisotropy)
close to wells
far from wells

46. Stress Path Distribution

47. Permeability Stress Path Sensitivity
MATLAB
Excel

48. Compaction and subsidence
Compaction in 1987 1 2
Compaction IN 1995 in which the result of injection is shown

49. UKNS, Seismic Stress-Sensitivity (ECLIPSE, VISAGE, H-WU software)
Example 2
UKNS, Seismic Stress-Sensitivity
(ECLIPSE, VISAGE, H-WU software)

50. Features of a 2D flow model grid embedded
for coupled geomechanical simulation
Well
Overburden
Faults
Caprock
Sideburden
Model mesh of a gas-water reservoir
Flank to crest of a long structure transected by faults mostly parallel to the longitudinal direction - allows 2D modelling
Moderate to low porosity stiff frame sandstone reservoir with an evaporite sequence caprock - not a particularly good candidate 4D seismic (or any seismic in practice!)
Flow model grid embedded and conditioned for coupled geomechanical simulation
Gas , Water in the flow model grid

51. Displaced shape of the geomechanical model
Surface subsidence
Differential compaction
across faults in reservoir
Typical location of shear
strain on faults
Features of the displaced shape of the geomechanical model due to pore pressure changes
differential compaction across faults
surface subsidence - changes in the overburden
localized shear strain on faults
(VISAGE Output)

52. Mean effective stress distribution at the end of the simulation
Localized effects
at faults
Perturbed stress field
above and below reservoir
Unperturbed stress field
(constant gradient)
Apparent deepening of reservoir
due to decreasing pore pressure
Mean effective stress effects
unperturbed stress field away from reservoir
apparent deepening of resevoir due to decreasing pore pressure
perturbed stress field directly above and below reservoir
localized effects at faults
(VISAGE Output)

53. Time-lapsed compressional acoustic impedance
Changes in overburden/caprock
due to stress redistribution
Changes in reservoir
due to pore pressure decline
Top of caprock
Time-lapsed acoustic impedance from the acoustic model database with fluid change and stress change effects showing
changes in reservoir due to fluid movement at GWC
changes in reservoir due to pore pressure decline (increasing stress)
changes in overburden/caprock due to stress redistribution
Initial gas-water contact
Changes in reservoir
due to fluid movement
(VISAGE Output)

54. Initial Modelling: Before Production Begins

55. Time Lapse Model: Saturation Changes Only

56. Time Lapse Model: Saturation + Stress

57. Time-lapsed seismic trace model
Reflector at top of caprock
Perturbations at reflector event due to fluid change effects
Reservoir top
Time-lapsed seismic trace model generated from the acoustic model database showing
strong reflector event at the top of the reservoir caprock
slight perturbations at the GWC due to fluid change effects
pull-up in reflector event at the reservoir base due to stress change effects
These effects are better displayed dynamically
Reservoir base
Pull-up in reflector event
due to stress change effects

58. Where are we now?
Extreme examples of reservoir stress-sensitivity accepted: Ekofisk, HP/HT, Gulf of Mexico, Angola?
The processes required exist in usable form
Non-uniform levels of commitment
What about the more subtle reservoirs?

59. Technical Challenges Discontinuity distributions
Discontinuity properties
Rel perm stress-sensitivity
In situ stress state
Coping with anisotropy
Seamless software

60. Organisational Challenges
Realising the full value of the data we already have
Cost vs value of the process
Coping with multi-disciplinarity

61. Is this too much to ask for?
Better performance
Fully owned decisions
Shared belief
Shared analysis

62. Decision Making
Straight from the geomechanical model, aided possibly by some calcs, e.g.
fracture density = well locations for max PI
subsidence = yes or no
With the aid of coupled modeling, e.g
improvement of appraisal
impact of perm sensitivity = recovery, GOR etc
Ground movements and subsidence = threat to wells and facilities
4D seismic enhancement = better management

63. Thank You

64. What do we want to achieve today?
Overview of the main tasks of the project
Select candidate reservoirs for study
Set up communications
Agree next meeting date 17th August?

65. Hysteresis K
Increasing Stress

66. Hysteresis K X
Increasing Stress

67. Hysteresis K
Increasing Stress

68. UBD site history very important
Effective Stress around the wellbore
Failure Level
Time
Drilling Completion Production

69. Building the Geomechanical Model
*Structure and anisotropy analysis from Seismic
*Geomechanical Core Analysis
Multi-Disciplinary Tasks assembling data for Model
*Published and proprietary studies
*Log analysis
Basin process simulations
*Geomechanics of fracture genesis
*Genetic Units expertise
Analogue studies
Characterise Structural Setting of the Reservoir
Creation of the Geomechanical Model
Characterise Reservoir Rocks
Characterise Reservoir Faults & Fractures
Reservoir Geomechanical Model
feedback to
improve characterisation
feedback to improve characterisation
Stress-Sensitive Coupled Modelling
Stress-Sensitive Reservoir Modelling and Coupled Simulations (Ground movements, Fluid Flow and 4D seismic)
Deliverables
Better Decisions Reservoir Management