Anisotropic seismic waves in systems with fluid‐saturated fractures

Abstract

The seismic wave speeds observed in exploration reflection surveys of oil and/or gas reservoirs may be dominated by the elastic properties in the dry state of very flat cracks or fractures, containing relatively little porosity. In contrast, the majority of work on effective medium theories (both estimates and bounds) has concentrated on systems whose elastic behavior depends on the volume fractions of the fluid constituents. Although a considerable amount of work has already been done in this area, there remain controversies regarding the use of the differential and self‐consistent schemes for property estimation in these systems, and these controversies are known and well documented. When the reservoirs contain vertical fractures, these systems exhibit anisotropic seismic wave speeds. Methods are known that permit relatively simple calculation of these anisotropic properties from theoretical estimates made on randomly isotropic systems of fractures. When these methods are appropriately combined with weak anisotropy approximations associated with the Thomsen seismic‐wave parameters, it is possible (at least in principle) to estimate fracture orientations and densities. The relationships between these forward and inverse problems will be treated and discussed with special emphasis on the question of uniqueness of the resulting interpretations.