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Eight Years of Passive Seismic Monitoring at a Petroleum Field in Oman: A Case Study

Eight Years of Passive Seismic Monitoring at a Petroleum Field in Oman: A Case Study. Sudipta Sarkar, M. Nafi Toksöz , Sadi Kuleli , Haijiang Zhang Massachusetts Institute of Technology and Fahad Al- Kindy , Oghale Ibi , Nasser Al- Touqi Petroleum Development Oman.

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Eight Years of Passive Seismic Monitoring at a Petroleum Field in Oman: A Case Study

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  1. Eight Years of Passive Seismic Monitoring at a Petroleum Field in Oman: A Case Study Sudipta Sarkar, M. NafiToksöz, SadiKuleli, Haijiang Zhang Massachusetts Institute of Technology and Fahad Al-Kindy, OghaleIbi, Nasser Al-Touqi Petroleum Development Oman SEG, Las Vegas – November 12, 2008

  2. Introduction • Microearthquakes are induced by gas and oil production, and water injection in this field. • Two independent networks – shallow and deep - have recorded thousands of events over the past 8 years. • Analysis of these events has helped us understand the causes of induced seismicity and its relation with reservoir dynamics. • Location of these events show reactivation of preexisting faults and identify new faults and fractures.

  3. Outline Field Overview Importance of velocity model Location methods for reservoir-induced seismicity Spatial and temporal analysis of induced seismicity Reservoir Imaging Conclusion

  4. Schematic Cross Section of the Field

  5. Microseismic Monitoring Network Shallow Network 10/99 Present ~ 150 m below surface Deep Network 08/03 02/02 ~ 750 – 1300 m below surface

  6. Shallow Network Events: 1999 - 2007

  7. Induced Seismicity vs. Gas Production / Water Injection

  8. Correlation of Seismicity with Production/Injection

  9. Importance of Velocity Model in Induced Event Location • In petroleum fields, subsurface velocity models could change abruptly with depth due to layered sedimentary rocks. • Alternating High/Low velocity layers give rise to complex ray paths, thereby affecting travel time calculation. • An accurate velocity model is important for hypocenter determination. • Focal depth is more strongly affected by velocity model.

  10. Comparison of Two Velocity Models Sze, 2005 Sarkar et al., 2006

  11. Depth Distribution of Events located using a “Smooth” Velocity Model

  12. Depth Distribution of Events located using our Improved Velocity Model

  13. Depth Validation: Poor fit for Check-shot Velocity Model

  14. Depth Validation: Good fit for Log-derived Velocity Model

  15. Microseismic Event Location • Picked and located seismic events: • shallow network / 8 years of data/ ~ 1500 events • deep-borehole network / 11 months of data / ~ 5400 events (only ~30% of total event detected) • Location methods: • NonLinLoc (Lomax et al., 2000) : A probabilistic, global-search method that gives complete solution PDFs for hypocentral parameters. • Station-Pair-Differencing (Sarkar & Toksöz, 2008) : An exhaustive grid-search method that minimizes travel time differences between station pairs. • Double-difference (Waldhauser & Ellsworth, 2000) : A relative event location method that minimizes travel-time differences between event pairs.

  16. NonLinLoc Sta-Pair-Diff Double-Diff

  17. Induced Seismicity Map: Shallow Network • Showing locations of ~ 700 events, from Nov. 1999 – Feb. 2007. • Seismicity is spread along the NE-SW fault corridor. • Depth of events show large scale fault reactivation across the entire reservoir, and the overburden.

  18. C B A D 3D Interpretation of Faults B A D C

  19. Fault/Fracture Monitoring over Time • Seismicity has continued to occur in the same depth range. • Events repeating on the same structure. • # of deeper events has increased in recent years • Important for reservoir management decisions

  20. Microseismic Monitoring: Deep-borehole Network • 5 monitoring wells, 8 levels of sensors in each. • Operational during 02/02 – 08/03. • ~ 15,800 event data received from 10/02 – 08/03, ~ 5400 picked and located.

  21. Induced Seismicity Map: Deep Network • Distribution of seismicity consistent with results obtained from Shallow Network • Seismicity is spread along the NE-SW fault corridor. • Depth of events show large scale fault reactivation across the entire reservoir.

  22. 3D Interpretation of Faults • Detail analysis of event locations reveal features that were unmapped by surface seismic – • e.g. a E-W trending sharp feature crossing the two major faults is identified

  23. Tomography using Induced Seismicity Data • Method: Double-difference Tomography (tomoDD) – Zhang & Thurber, 2003. • Data: Events from the deep network. • Results: Preliminary, qualitative.

  24. Conclusions • Locations of induced events are improved by using an accurate velocity model, and differencing methods (station-pair differencing and double-difference) • Microseismicity in this field is primarily induced by depletion/compaction of the gas reservoir • Relatively few events are induced by water injection • Location of events from both deep and shallow networks have identified preexisting and new faults and fractures • Induced events have the potential to be used for mapping and monitoring of reservoir properties • 3D and 4D induced-seismic tomography

  25. Acknowledgement We thank Petroleum Development Oman (PDO) for providing field data and funding support for this research. We thank PDO and Ministry of Oil and Gas Oman for permission to present this work.

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