Analysis of Anisotropic Velocities in a Core Sample and AVOA from a Fractured Vuggy Carbonate Reservoir
Stephen A. Hall, Robert R. Kendall, J-Michael Kendall, Carl Sondergeldx, 2001. "Analysis of Anisotropic Velocities in a Core Sample and AVOA from a Fractured Vuggy Carbonate Reservoir", Anisotropy 2000: Fractures, Converted Waves, and Case Studies, L. Ikelle, A. Gangi
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We investigate the effects of fracture- and inclusion-induced seismic anisotropy in a carbonate reservoir rock and the resulting influence this anisotropy may have on surface seismic data. Whole-core velocity measurements made on a carbonate sample from the Gulf of Mexico show evidence of elastic anisotropy. Constraints on the style of this anisotropy are obtained from comparisons with effective medium modeling. The core exhibits monoclinic symmetry, which is interpreted as being caused by the combined effects of vertically-aligned drilling-induced fractures and oriented ellipsoidal vugs inclined at an of angle roughly to the vertical. The in situ anisotropy is believed to be orthorhombic, as there is evidence of natural fractures oriented orthogonally to the vugs. Surface seismic modeling is used to investigate amplitude variations with offset and azimuth (AVOA) effects due to such anisotropy. Our model is somewhat hypothetical, but consistent with velocities from the reservoir logs and the inferred in situ anisotropy. Our results suggest that for this model, P-wave AVOA will show significant azimuthal variation only at far offsets (near critical reflections). In fact, the onset of critical reflections will be dependent on the orientation of the seismic line with respect to the fracture direction. Shear modes will be more sensitive to fracture orientation at near offsets. In addition, we find that the P-P and P-S AVOA are sensitive to the presence of aligned vuggy porosity and so could provide a tool for identifying highly productive zones where fractures connect vugs.
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“This volume contains 25 papers that represent most of the best work in seismic anisotropy in 1998 and 1999. Fracture characterizations and processing of converted waves are the two main topics covered in this volume. They are addressed from both theoretical and practical viewpoints. Also included are papers describing the historical roots of seismic anisotropy.”