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A New Approach for Location of Induced Earthquakes in Petroleum Fields

A New Approach for Location of Induced Earthquakes in Petroleum Fields. Sudipta Sarkar, M. Nafi Toksöz , William L. Rodi Massachusetts Institute of Technology. Outline. Overall Seismicity in the Region. Induced Seismicity and the Petroleum Field. Induced Earthquake Data.

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A New Approach for Location of Induced Earthquakes in Petroleum Fields

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  1. A New Approach for Location of Induced Earthquakes in Petroleum Fields Sudipta Sarkar, M. NafiToksöz, William L. Rodi Massachusetts Institute of Technology

  2. Outline

  3. Overall Seismicity in the Region

  4. Induced Seismicity and the Petroleum Field

  5. Induced Earthquake Data

  6. Importance of Velocity Model in Induced Earthquake 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 because of depth/origin time trade-off.

  7. Comparison of Two Velocity Models

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

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

  10. Challenges Faced in Location of Reservoir-Induced Seismicity • Very high accuracy requirement • Large uncertainty in S picks • Poor station coverage • An earth structure with alternating high and low velocities in sedimentary sections • Conventional algorithms based on increasing layer velocities/smooth velocity gradients cannot handle many thin layers with high and low velocities.

  11. Our Approach to Location of Reservoir-Induced Seismicity Use highly accurate, detailed velocity model (e.g. derived from well-logs) Pre-calculate 3D travel times using finite difference [Podvin and Lecomte (1991)] and store on disk Compute travel time differences between two stations for each event Minimize residuals of travel-time differences for all station-pairs Use exhaustive grid search method

  12. Our Approach to Location of Reservoir-Induced Seismicity Arrival Time: Event to Station Arrival Time Difference: Event to Station Pairs Travel Time Difference Grid between Station Pairs Find (X,Y,Z) for which errors are “minimum” Data: Model: Objective Function:

  13. Location of Induced Earthquakes

  14. Location of Induced Earthquakes

  15. Comparison Epicenter locations from an advanced non-linear, 3D location method

  16. Comparison Epicenter locations from our location method

  17. Comparison Depth locations from an advanced non-linear, 3D location method Depth locations from our location method

  18. Conclusions

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