Azimuthal variation of AVO response for fractured reservoirs
- Colorado School of Mines, Golden, CO (United States)
Amplitude variation with offset (AVO) analysis has been successfully used in a variety of exploration contexts. However, its application to fractured reservoirs was hampered by the difficulty in relating AVO attributes to crack parameters. The presence of vertical parallel cracks in an isotropic matrix makes the medium transversely isotropic with a horizontal symmetry axis (HTI). Here, we present a concise analytic approximation for reflection coefficients in HTI media and discuss the relation between the azimuthal AVO signature and the anisotropy parameters. We show that in the limit of weak anisotropy and small velocity and density contrasts at the interface, the difference between the P-wave AVO gradients in two symmetry planes of HTI media becomes a simple function of the shear-wave anisotropy parameter {gamma}. The coefficient {gamma} is close to the crack density, an important parameter in reservoir characterization. The second anisotropy coefficient that contributes to the azimuthal variation in the AVO gradient is equivalent to Thomsen`s (1986) parameter {delta} and can be obtained from P-wave moveout data. These results make it possible to use the difference between the P-wave AVO gradients in two symmetry planes of HTI media in the inversion for the crack density. Our algorithm becomes particularly simple in the special case of thin cracks and negligible equant porosity, typical for fractured coals. Similar analytic approximations were obtained for the gradients of the reflection coefficients of split shear waves in the symmetry planes of HTI media. We also demonstrate that AVO processing in fractured media should include correction for the distortions of the AVO response caused by the wave propagation in azimuthally anisotropic overburden.
- OSTI ID:
- 542940
- Report Number(s):
- CONF-951013--
- Country of Publication:
- United States
- Language:
- English
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