Seismic signature of Fractures
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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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Seismic signature of Fractures Seismic signature of Fractures Presentation Transcript

  • Rock Physics Constraints on Seismic Signatures of Fractures Contract Number: DE-AC26-99FT40692 DOE Gary Mavko Rock Physics Laboratory Stanford University
  • Objective Relate seismic attributes to fractures, … by quantitatively integrating •Multi-attribute seismic •Well logs •Geology … using Rock Physics
  • Why Worry About Fractures? They dominate permeability: • Can make tight gas economical
  • What do we need to know? Where to find “Sweet Spots” for drilling •Fracture location •Fracture intensity •Fracture orientation •Gas, oil, or water? •Permeability
  • 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
  • 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
  • Site-to-site variations result from: •Rock type •Fracture geometries •Fluids •Acquisition geometries •Business objective/constraints … there is no “silver bullet” that works everywhere.
  • Seismic Velocity
  • 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
  • Seismic Attenuation
  • Attenuation Indicator of Fractures Unfractured Fractured Adding Fractures: changes amplitude & frequency
  • 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
  • Seismic Facies
  • Fractures Often Prefer Certain Lithofacies Joint Sheared Joint
  • 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
  • Seismic Amplitude
  • Amplitude Indicator of Fractures Fractures elastically soften the rock, changing the elastic impedance, and lowering reflectivity.
  • Tight Limestone Anomalous Stack Amplitude at Fracture clusters 3D Seismic VSP
  • Azimuthal Attributes
  • Azimuthal Seismic Attributes 3D Surveys yield azimuthal variations in: •NMO N-S •Amplitude E-W •Frequency/Q
  • 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
  • Rock Physics Workflows
  • Rock Physics Workflow Logs: Site-specific Rock properties Specify Compute •Fractures seismic •Fluids signatures •Lithology Fractured Interval Properties
  • Unfractured Fracture Details Rpp Fractured below Seismic Resolution Incidence Angle Anomalous Amplitude at Fracture clusters
  • AVO Computed Anomaly: Gas-filled Fractures Offset Offset Fracture indicator
  • Will We See the Fractures? gas …at this site, AVO can distinguish gas- filled fractures, but not water-filled.
  • Can We Distinguish Fractures from Shale? Increasing fractures Increasing shalines …at this site, AVO can distinguish gas- filled fractures from unfractured shale
  • A Few Interpretation Pitfalls
  • How are Faults and Fractures distributed?
  • Joints Intermediate Faults Fault Zones Sheared Joints Incipient Faults Courtesy, Juan-Mauricio Florez-Nino
  • x y z Fracture Strike Excellent Fractures along dip. Gas cdx = .1 cdy = 0 gas
  • x y z Fracture Strike Excellent Fractures along strike. Gas cdx = .02 cdy = .08 gas
  • x y ? z Gas-filled Fracture Water-filled Strike? Fractures! Excellent Fractures along dip, … but, with Water! cdx = .1 cdy = 0 gas
  • x y z No Anisotropy No Fractures? Two Fracture sets! cdx = .05 cdy = .05 gas
  • Quantify Signatures of Fractured reservoir, gas Various Fracture Styles xx yy zz
  • Integration Methodology
  • 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
  • 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
  • Prior crack density distribution
  • Updated crack density distribution
  • Observed Reflectivity anisotropy 800 ft subperbins
  • Fracture distribution from AVAZ 800 ft subperbins
  • Comparison with FMI data
  • 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