Modelling Stress Path and Fracture Pressure Hysteresis for CO2 Storage in Depleted Reservoirs - presentation by Thomas Lynch of the University of Leeds at the UKCCSRC meeting Monitoring of the deep subsurface, 23 October 2014

1. T. Lynch1, Q.J. Fisher1, D. Angus1, P. Lorinczi1
Modelling Stress Path and Fracture Pressure Hysteresis for CO2 Storage in Depleted Reservoirs
Centre for Integrated Petroleum Engineering and Geoscience, Institute of Applied Geoscience, University of Leeds

2. Overview Summary
Context of research – Stress path and fracture pressure hysteresis Modelling approach – Coupled geomechanical-fluid flow modelling Results – Fracture pressure hysteresis in a faulted depleted gas reservoir, stress changes around the reservoir Conclusions

3. Context of Research Stress Path and Fracture Pressure hysteresis
Principle:
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Stress path hysteresis leads to reduction of predicted fracture pressure upon re-injection in depleted fields
Evidence:
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Based on observations by Santarelli, Tronvoll et al 1998 and Santarelli, Havmoller et al 2008
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Few studies on phenomenon in literature

4. Context of Research Stress path hysteresis
a) Classic textbook pore pressure response

5. Context of Research Stress path hysteresis
b)/c) Reservoir scale pore pressure/stress coupling

6. Context of Research Stress path hysteresis
d) Stress path hysteresis e.g Field data from: Santarelli, Tronvoll et al 1998 and Santarelli, Havmoller et al 2008

7. Context of Research Stress Path Parameters
(푎푎) 훾훾푣푣= Δ휎휎푣푣 Δ푝푝 푏푏 훾훾ℎ= Δ휎휎ℎ Δ푝푝 풄풄 푲푲풔풔풔풔= Δ흈흈흈풉풉 Δ흈흈흈풗풗 = 휸휸풉풉−휶휶 휸휸풗풗−휶휶

8. Context of Research Stress Path and Fracture Pressure hysteresis
Santarelli et al. 1998 report:
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γh = 0.7 during depletion
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γh = 0 during injection
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Clear implications for CO2 storage if the fracture pressure is reduced during injection
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Equally situations where fracture pressure is higher than expected are beneficial.

9. Coupled Modelling Approach Coupled modelling
- Initial grid - Initial porosity - Pressures
TEMPEST
ELFEN
ELFENRS
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Modified pore volume
Universities of Leeds and Bristol and Rockfield Software Ltd. developed coupled methodology ELFEN (geomechanics) Tempest (flow)

10. Coupled Modelling Approach Current capabilities and future developments
- Initial grid - Initial porosity - Pressures
TEMPEST
ELFEN
ELFENRS
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Modified pore volume
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SR3 (soft rock 3) – Cam Clay type const. model – large strains, range of initial stress, plastic deformation
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Geometry - complex
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Uplift and surface deformation
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Seismics and Microseismics
Future:
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Temperature – through manual pass through from Tempest, hope to automate

11. Coupled Modelling Approach Fluid flow modelling
0.0
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Relative Permeability kr
Gas Saturation Sg
CO2 - Host
Brine - Host
CO2 - Fault
Brine - Fault
From Tueckmantel 2010
Tempest black oil flow simulation – simplification - cut down run times
Using experimentally derived relative permeabilities – available for fault and host rock
Message passing interface – allows efficient coupling without write files

12. Model Set-up Generic “Two fault” model
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Investigated effect of model parameters on stress path using a generic gas reservoir with depletion - reinjection
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Poorly lithified sandstone reservoir, shale over/sideburden – from compilation of literature data
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Fault transmissiblity – sealing and non-sealing
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Fault movement also controlled with friction parameter

13. Results Generic “Two fault” model
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p’-q plots of the stress path
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1 – 2 elastic depletion
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2 – 3 plastic depletion
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3 – 4 elastic injection
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Minimal impact of fault transmissibility and movement on the stress path
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Lower frac. Pressure indicated by increment to failure between points 1 and 4
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q (MPa)
p’ (MPa)
High TM Low Friction p-q
High TM High Friction p-q
Low TM Low Friction
Low TM High Friction
Yield Surface Initial
Yield Surface Final
1
2
4
3

14. Results Generic “Two fault” model
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q (MPa)
p' (MPa)
High TM Low Friction K = 0.9
High TM Low Friction K = 0.5
High TM Low Friction K=0.7
1
3
1
1
4
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Biggest impact on stress path – initial stress ratio - K (not to be confused with Ksp represent the same ratio)
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High K leads to greatest fracture pressure reduction
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Also leads to similar stress path parameters to those observed in literature.

15. Results Generic “Two fault” model
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Shear Stress (MPa)
Normal Stress (MPa)
0.35 Poisson's ratio
3.8 Gpa reservoir
13.8GPa reservoir - soft overburden
13.8 Gpa reservoir - side
3.8 Gpa reservoir - side
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Other important factors are those that ‘flatten’ the stress path, greater reduction in fracture pressure observed with:
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Increased Poisson’s ratio in reservoir
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Less stiff overburden
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Marginal location within the geometry

16. Results Generic “Two fault” model
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Shear Stress (MPa)
Normal Stress (MPa)
Initial
Depletion End
Injection Start
Injection End
Final Yield
s'-t
0 Horizontal Stress Parameter
s'-t 0 Horizontal Stress Path Parameter
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Shear Stress (MPa)
Normal Stress (MPa)
Edge
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After K ratio edge or geometric effects have the largest effect on stress path
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In current work reservoir edge flat interface modelled
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Other geometries would be interesting e.g. lense like
Centre

17. Results Generic “Two fault” model
0
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10
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Shear Stress (MPa)
Normal Stress (MPa)
Initial
Depletion End
Injection Start
Injection End
Final Yield
s'-t
0 Horizontal Stress Parameter
s'-t 0 Horizontal Stress Path Parameter
0
2
4
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Shear Stress (MPa)
Normal Stress (MPa)
Edge
Centre
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The Mohr circle/stress path shows that:
• γh = 0.8 during depletion
• γh = 0.6 injection in the centre
• γh = 0.43 injection in the edge

18. Results Generic “Two fault” model
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Shear Stress (MPa)
Normal Stress (MPa)
Initial
Depletion End
Injection Start
Injection End
Final Yield
s'-t
0 Horizontal Stress Parameter
s'-t 0 Horizontal Stress Path Parameter
0
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4
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0
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Shear Stress (MPa)
Normal Stress (MPa)
Edge
Centre
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The Mohr circle plots show that the models do not fully replicate the behaviour observed by Santarelli et al.
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Temperature is suggested as a significant factor in a later paper by Santarelli
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Do show inherent potential for hysteresis even w/out temp. effects
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Both a risk and potential opportunity

19. Results Generic “Two fault” model
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Model stresses around the reservoir in the overburden for seismic modelling/monitoring – shows different behaviour depending on reservoir stiffness due to redistribution of stress during plastic deformation
Low stiffness reservoir (3.8 GPa)
High stiffness reservoir (13.8 GPa)

20. Conclusions
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Shown potential for fracture pressure to change during the lifetime of a field – particularly important in CO2 injection ( generic modelling of the Bunter – 20% reduction in the fracture pressure reduced capacity by 60%)
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Important factors other than the initial stress ratio of the field are the geometry of the field and the temperature effects of CO2 injection
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Further modelling of phenomenon in more complex geometries with temperature effects would be interesting
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Meshing complex geometries remains a significant challenge which requires automation
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Important crossovers with monitoring - with plastic compaction and depletion/re-injection – apparent divergent response in seismic attributes