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

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