Characterizing a Fractured Carbonate with an Embedded Multicomponent Seismic Test
While the orientation and distribution of fractures cannot be directly imaged by seismic surveys, characteristics related to fracturing can be inferred from the azimuthal variations of seismic attributes. The same physical phenomena that create azimuthal amplitude variation (AVOA) in primary (P) waves will generate azimuthal velocity variation (shear-wave birefringence) in secondary or shear (S) waves. These two independent techniques – P-wave AVOA and converted-wave birefringence – are complementary methods of evaluating anisotropy caused by the fractured medium. Converted-wave birefringence, which can only be evaluated through full-wavefield recording with multicomponent receivers, also resolves the inherent 90-degree ambiguity in fracture orientations determined by P-wave AVOA analysis alone.
In this US example, the fundamental goal was to determine the essential practicality of acquiring converted-wave data in gas-bearing hydrothermal dolomites at a depth of approximately 9000 ft. This is achieved through an embedded multicomponent test, where 544 three-component digital accelerometers are co-located with production single-component geophone arrays over a subset of the survey area. The embedded test demonstrates the expected characteristics of converted-waves in azimuthally anisotropic media, and provides a clear illustration of the use of shear-wave birefringence in a land dataset. The method of analysis is discussed and the results compared directly to AVOA results from the larger, conventional, P-wave 3-D survey, and verified with well-bore imaging and production data.