Microseismic monitoring and other stuff

1. Microseismic monitoring and other stuff A Comparison of Geomechanical Effects at 3 ‘Megatonne’ CCS Sites: Sleipner, Weyburn and In Salah James Verdon University of Bristol, U.K. Bristol University Microseismicity ProjectS BUMPS

2. Acknowledgements: • Anna Stork, Mike Kendall (UoB) • Andy Chadwick (BGS) • Don White (GSC) • Rob Bissell (BP)

3. Carbon Capture and Storage: • AKA: CCS, carbon sequestration, clean coal, geologic CO2 storage • A total of 10 billion bbls of CO2 storage needed per year

4. Leakage Risks: • For both economic viability and social acceptability, we must ensure the CO2 is trapped at depth for tens of thousands of years • Possible leakage pathways include: • Along well bores • Through faults and fractures • Diffusion through cap-rock • Hydrodynamic flow

5. Injection and Deformation: Terzaghi (1943): Hooke’s Law (1660): (uttensio, sic vis) • Pore pressure change causes changes in effective stress, and thereforeproduces strain, or deformation. • Magnitude of deformation controlled by Pfl, α, and Cijkl

6. CCS and Geomechanical Deformation:

7. Monitoring Deformation:

8. McGarr and Induced Seismicity: MMAX = GDV

9. McGarr and Induced Seismicity: Sleipner In Salah MMAX = GDV

10. McGarr and Induced Seismicity: MMAX = GDV ? ?

11. Case Studies: Weyburn Sleipner In Salah

12. Sleipner: • Gas from the Sleipner field contains ~10% CO2. Must be reduced to <2% for sale • Stripped CO2 was flared, until in 1996 the Norwegian government imposed a tax on offshore CO2 flaring • CO2 is now re-injected into a saline aquifer overlying the gas reservoirs • Over 13 MT CO2 injected

13. Sleipner Pressure Performance: • Utsira pore volume = 6x1011m3 • Total injected CO2 volume = 18x106m3, = 0.003% of pore volume • Utsira flow properties: ϕ = 35%, κ = 1-3 D • No pressure increase expected • No increase in injection pressure for constant rate injection. • No indication of pressure increase in seismic data

14. Sleipner – Take-Home Message: • If a reservoir is very large, with excellent flow properties, injection will not increase pressures. • If pressures do not increase, geomechanical deformation is unlikely to be an issue. • Sleipner is a great example of CCS done well. But is it an anomaly? Can we find enough Sleipners (in the places where storage potential is needed) to accommodate 10 billion bbls of CO2 per year?

15. Weyburn: • Located in Saskatchewan, Central Canada • Size = 70 miles2 • OOIP = 1.4 billion bbls • Production began in 1955 • Oil recovered so far = 370 million bbls • Extra oil from CO2 injection = 130 million bbls • CO2 injection rate ~ 3 MT/year • Total expected CO2 stored = 30 MT

16. The Weyburn Reservoir: • Palaeozoic carbonate reservoir: lower limestone and upper dolostone layers • ϕ ~ 0.1 – 0.25, κ ~ 3 – 50mD • Primary caprock: thin evaporite. • Regional seal: 200m of Mesozoic shale • Structurally monotonous

17. Weyburn Wells:

18. Weyburn Pressure History:

19. Weyburn Microseismic Monitoring • CO2 injection initiated in 2000 • Microseismic monitoring initiated in 2003 • 1 downhole array, 8 3C geophones close to reservoir depth • CO2 injection in a nearby vertical well initiated in Jan 2004 • Several nearby producing wells

20. Weyburn Microseismic Monitoring: • Sept 2003 – Jan 2010

21. Weyburn Microseismic Monitoring: • Sept – Oct 2010 (post shut-in)

22. Reservoir: Overburden:

23. Weyburn – Take-Home Message: • CO2 EOR can be an excellent commercial opportunity – CCS ‘for free’ • Mature reservoirs may have ‘space’ to put CO2 without increasing pore pressures above initial conditions • Mature reservoirs may have a complex pressure history, which needs to be taken into account through numerical modelling • Coincident injection and production can lead to more complex stress effects (stress arching, transfer, hysterisis, etc.)

24. In Salah: • Gas production began in 2004. CO2 stripped from gas is re-injected into water leg • To date, 3.85 MT have been injected • Reservoir is 20m thick fractured sand at ~1900m depth • 950m thick shale caprock

25. In Salah: • Injection through 3 horizontal wells. Production through 4 wells • ϕ ~ 0.13, κ ~ 1 mD • Little apparent pore pressure communication between injectors and producers • ~7 – 10MPa pore pressure increases

26. Deformation at In Salah: From BP • Initial observations of deformation came from InSAR • Bi-lobate uplift pattern above KB-502 indicated a fracture at depth • Fracture location from InSAR matches a feature in the seismic data From TRE Vasco et al. (2010)

27. Microseismic Monitoring at In Salah • Microseismic array installed 2009 in shallow borehole above ‘fracture zone’ • 8 3C geophones from 80 – 500m. However, technical problems mean that only 1 has provided useable data • >700 events identified in first months of 2010

28. In Salah Microseismicity • A single geophone cannot provide accurate locations. • S – P differentials and arrival angles are used to characterise events • Two clusters identified: • S – P = 0.6 and incidence angle <20° • S – P = 1.0 and incidence angle = 20 – 30° • Azimuths are consistent with fracture zone/regional stress • Cluster 1 consistent with deformation in or just above the reservoir. • Cluster 2 more difficult to constrain

29. In Salah – Take-Home Message: • Without either EOR or Sleipner-scale aquifers, CO2 injection can substantially increase pore pressure • Elevated pore pressures (even below HF pressure) can cause deformation that might be detrimental to storage integrity • A fracture zone appears to have been created or reactivated, extending ~100 – 200m above the reservoir • The caprock at In Salah is 950m thick. Therefore storage integrity is not compromised. Nevertheless, important lessons have been learned • InSAR monitoring can be a useful early-warning technique • A better MS array would have been able to more fully image the deformation

30. Discussion – CCS: • The size of the pore pressure increase is crucial • The best targets are large with good flow properties, but we need 10 billion bbls of storage per year, in the right places: can we find enough Sleipners? • A long history of pressure change in a mature field can give you surprising results (hysteresis etc.) • A role for water removal in CCS pressure management? • If deformation is likely, a thick caprock gives you added security • Geomechanical monitoring and modelling is ESSENTIAL

31. Discussion – Monitoring Methods: • InSAR • Quick and cheap, potentially retrospective. Especially effective where bare rock is exposed • Not subsea. Vegetated areas may require reflective structures • Need to invert surface displacement for reservoir processes via numerical methods • 4D Reflection Seismic • Likely to be done at most CCS sites to image fluids • Velocities are known to be stress sensitive, but techniques to invert for stress are still immature. • Use of attributes to detect fracture zones • Microseismic • Provides a direct indication of rock fracturing • Covering a full field may be challenging. • Continuous recording (data intensive, but a potential early warning system)

32. Bristol University Microseismicity ProjectS BUMPS www1.gly.bris.ac.uk/BUMPS/