Multicomponent Processing and Fracture Characterization Analysis of Two Wyoming 3-D PS-Wave Surveys: Jonah Field and Washakie Basin
Gaiser, J.E., R.R. Van Dok
WesternGeco, Denver, CO
Multicomponent 3-D surveys where downgoing compressional (P) waves convert to upgoing shear (S) waves at interfaces provide a practical means for analyzing fracture properties. This is particularly important for delineating naturally fractured reservoirs by exploiting the unique characteristics of S-wave azimuthal anisotropy induced by vertical fracturing. In the presence of fractured media, S-waves split into a fast wave that is polarized parallel to fractures and a slow wave that is polarized normal to fractures. The amount of splitting (time difference between the two S-waves) is proportional to fracture intensities. To investigate this phenomenon we utilize a wide range of source-receiver azimuths in the processing and analyze the fast and slow S-waves to extract fracture information.
Two 3-D 3-component (one vertical and two horizontal geophones) surveys from Wyoming are presented: one acquired over a portion of the Jonah Field and the other in the Washakie Basin. The targets are naturally fractured gas sand reservoirs. From the analysis of fast and slow S-waves a regional direction of anisotropy was observed in both areas. Layer-based analyses confirmed that much of the anisotropy was in the overburden, which required compensation during the processing to isolate the variations at reservoir depths. Eight limited-azimuth volumes were created for the two horizontal geophone components. These volumes were analyzed to determine the time-variant anisotropy within the survey and indicated areas of increased fracturing in the overburden as well as at target levels.