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

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

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  • 1. Rock Physics Constraints on Seismic Signatures of Fractures Contract Number: DE-AC26-99FT40692 DOE Gary Mavko Rock Physics Laboratory Stanford University
  • 2. Objective Relate seismic attributes to fractures, … by quantitatively integrating •Multi-attribute seismic •Well logs •Geology … using Rock Physics
  • 3. Why Worry About Fractures? They dominate permeability: • Can make tight gas economical
  • 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. 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. 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. 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. Seismic Velocity
  • 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. Seismic Attenuation
  • 11. Attenuation Indicator of Fractures Unfractured Fractured Adding Fractures: changes amplitude & frequency
  • 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. Seismic Facies
  • 14. Fractures Often Prefer Certain Lithofacies Joint Sheared Joint
  • 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. Seismic Amplitude
  • 17. Amplitude Indicator of Fractures Fractures elastically soften the rock, changing the elastic impedance, and lowering reflectivity.
  • 18. Tight Limestone Anomalous Stack Amplitude at Fracture clusters 3D Seismic VSP
  • 19. Azimuthal Attributes
  • 20. Azimuthal Seismic Attributes 3D Surveys yield azimuthal variations in: •NMO N-S •Amplitude E-W •Frequency/Q
  • 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. Rock Physics Workflows
  • 23. Rock Physics Workflow Logs: Site-specific Rock properties Specify Compute •Fractures seismic •Fluids signatures •Lithology Fractured Interval Properties
  • 24. Unfractured Fracture Details Rpp Fractured below Seismic Resolution Incidence Angle Anomalous Amplitude at Fracture clusters
  • 25. AVO Computed Anomaly: Gas-filled Fractures Offset Offset Fracture indicator
  • 26. Will We See the Fractures? gas …at this site, AVO can distinguish gas- filled fractures, but not water-filled.
  • 27. Can We Distinguish Fractures from Shale? Increasing fractures Increasing shalines …at this site, AVO can distinguish gas- filled fractures from unfractured shale
  • 28. A Few Interpretation Pitfalls
  • 29. How are Faults and Fractures distributed?
  • 30. Joints Intermediate Faults Fault Zones Sheared Joints Incipient Faults Courtesy, Juan-Mauricio Florez-Nino
  • 31. x y z Fracture Strike Excellent Fractures along dip. Gas cdx = .1 cdy = 0 gas
  • 32. x y z Fracture Strike Excellent Fractures along strike. Gas cdx = .02 cdy = .08 gas
  • 33. x y ? z Gas-filled Fracture Water-filled Strike? Fractures! Excellent Fractures along dip, … but, with Water! cdx = .1 cdy = 0 gas
  • 34. x y z No Anisotropy No Fractures? Two Fracture sets! cdx = .05 cdy = .05 gas
  • 35. Quantify Signatures of Fractured reservoir, gas Various Fracture Styles xx yy zz
  • 36. Integration Methodology
  • 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. 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. Prior crack density distribution
  • 40. Updated crack density distribution
  • 41. Observed Reflectivity anisotropy 800 ft subperbins
  • 42. Fracture distribution from AVAZ 800 ft subperbins
  • 43. Comparison with FMI data
  • 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