Seismic signature of Fractures

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very nice presentation about:Rock Physics Constraints on Seismic Signatures of Fractures I hope you find it interesting

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Seismic signature of Fractures

  1. 1. Rock Physics Constraints on Seismic Signatures of Fractures Contract Number: DE-AC26-99FT40692 DOE Gary Mavko Rock Physics Laboratory Stanford University
  2. 2. Objective Relate seismic attributes to fractures, … by quantitatively integrating •Multi-attribute seismic •Well logs •Geology … using Rock Physics
  3. 3. Why Worry About Fractures? They dominate permeability: • Can make tight gas economical
  4. 4. What do we need to know? Where to find “Sweet Spots” for drilling •Fracture location •Fracture intensity •Fracture orientation •Gas, oil, or water? •Permeability
  5. 5. Rock Physics Discover, understand relations between •Seismic Attributes: •Velocity, Impedance •AVO, Reflectivity •Attenuation •Rock and Fluid Properties: •Fractures •Gas vs. Oil vs. Water •Rock type, porosity, mineralogy •Stress, Pore, Pressure, Temperature
  6. 6. Fractures Can Have Many Seismic Attributes •Low P- and S-wave velocities •Anomalous reflectivity (Impedance) •Low Q (high seismic attenuation) •Low Poisson’s Ratio (Vp/Vs ratio) •Anomalous AVO •Azimuthal variation in velocity, AVO … the optimum choice will vary from site to site
  7. 7. Site-to-site variations result from: •Rock type •Fracture geometries •Fluids •Acquisition geometries •Business objective/constraints … there is no “silver bullet” that works everywhere.
  8. 8. Seismic Velocity
  9. 9. Velocity Indicator of Fractures 6 Fractured Limestone Bedford Limestone Water-saturated Sat. 5 V Velocity (km/s) P Dry Velocity (km/s) 4 More fractures 3 Dry Sat. V 2 Saturated S 1 0 100 200 300 Effective Pressure (bars) Effective Pressure (bars) Adding Fractures: •Lowers Seismic Velocities •Change Seismic Vp/Vs ratio
  10. 10. Seismic Attenuation
  11. 11. Attenuation Indicator of Fractures Unfractured Fractured Adding Fractures: changes amplitude & frequency
  12. 12. Frequency/Amplitude Tight Sands - Powder River Basin Line 2: Surface-consistent RMS Amplitude (event at 2.5 seconds) Stacking-chart display of the center frequencies of the pre-stack P-wave data on line 2. 1601 Low Frequency source station numbers 1601 Low Amplitude CDP 3004 - more fractures Area CDP 2928 1501 1501 CDP 2816 Source Station Numbers CDP 2666 1401 1401 11 40 9 1301 1301 7 35 High Frequency High Amplitude 5 1201 Area 30 1201 Area 3 1 25Hz 1101 1101 1101 1201 1301 1401 1501 1601 1101 1201 1301 1401 1501 1601 Receiver Station Numbers receiver station numbers
  13. 13. Seismic Facies
  14. 14. Fractures Often Prefer Certain Lithofacies Joint Sheared Joint
  15. 15. Seismic Indicator of Fracture-Prefered Lithofacies Fractures “prefer” rocks with higher Vp, Impedance Vp Few fractures: Lower Vp, Impedance Vs … and we might detect the facies more easily than the fractures
  16. 16. Seismic Amplitude
  17. 17. Amplitude Indicator of Fractures Fractures elastically soften the rock, changing the elastic impedance, and lowering reflectivity.
  18. 18. Tight Limestone Anomalous Stack Amplitude at Fracture clusters 3D Seismic VSP
  19. 19. Azimuthal Attributes
  20. 20. Azimuthal Seismic Attributes 3D Surveys yield azimuthal variations in: •NMO N-S •Amplitude E-W •Frequency/Q
  21. 21. Tight Gas - Powder River Basin Azimuthal variations at a CMP Superbin Far offset azimuthal P- Azimuth Traveltime variation Fracture Azimuth Sussex Niobrara Fracture 1st Frontier Intensity bottom ∆t between top Sussex and Far Offset (25-30o) P-wave bottom first Frontier sands
  22. 22. Rock Physics Workflows
  23. 23. Rock Physics Workflow Logs: Site-specific Rock properties Specify Compute •Fractures seismic •Fluids signatures •Lithology Fractured Interval Properties
  24. 24. Unfractured Fracture Details Rpp Fractured below Seismic Resolution Incidence Angle Anomalous Amplitude at Fracture clusters
  25. 25. AVO Computed Anomaly: Gas-filled Fractures Offset Offset Fracture indicator
  26. 26. Will We See the Fractures? gas …at this site, AVO can distinguish gas- filled fractures, but not water-filled.
  27. 27. Can We Distinguish Fractures from Shale? Increasing fractures Increasing shalines …at this site, AVO can distinguish gas- filled fractures from unfractured shale
  28. 28. A Few Interpretation Pitfalls
  29. 29. How are Faults and Fractures distributed?
  30. 30. Joints Intermediate Faults Fault Zones Sheared Joints Incipient Faults Courtesy, Juan-Mauricio Florez-Nino
  31. 31. x y z Fracture Strike Excellent Fractures along dip. Gas cdx = .1 cdy = 0 gas
  32. 32. x y z Fracture Strike Excellent Fractures along strike. Gas cdx = .02 cdy = .08 gas
  33. 33. x y ? z Gas-filled Fracture Water-filled Strike? Fractures! Excellent Fractures along dip, … but, with Water! cdx = .1 cdy = 0 gas
  34. 34. x y z No Anisotropy No Fractures? Two Fracture sets! cdx = .05 cdy = .05 gas
  35. 35. Quantify Signatures of Fractured reservoir, gas Various Fracture Styles xx yy zz
  36. 36. Integration Methodology
  37. 37. Independent Constraints Seismic amplitude Geologic rule Interpreted fault Time slice at the top of the Prior spatial distribution for the reservoir mean values of crack density
  38. 38. Integration Methodology Well Log Fracture modeling & Condition on Vp Stochastic PP reflectivity Prior simulations from seismic Geological data Info Rpp P(e) Vs P(e|Rpp) E [P(e)] = 0.05 ρ Rpp e: fracture E [P(e|Rpp)] = 0.02 density Crack Density
  39. 39. Prior crack density distribution
  40. 40. Updated crack density distribution
  41. 41. Observed Reflectivity anisotropy 800 ft subperbins
  42. 42. Fracture distribution from AVAZ 800 ft subperbins
  43. 43. Comparison with FMI data
  44. 44. Conclusions • No “Silver Bullet” for fracture detection; Anisotropy alone might not be the answer • Fracture mapping has many pitfalls • Rock physics can help reduce risk, by: •Quantifying signatures of fracture scenarios •Exploring effects of rock types, fluids •Integrating well log and seismic constraints •Finding optimum attributes

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