Multicomponent Processing and Fracture Characterization Analysis at Pinedale Field and Washakie Basin, Wyoming

Abstract

Abstract Multicomponent 3-D surveys provide a practical means for analyzing fracture properties where downgoing compressional (P) waves convert to upgoing shear (S) waves at interfaces. The unique characteristics of S-wave azimuthal anisotropy induced by vertical fracturing may provide valuable information for delineating naturally fractured reservoirs and monitoring production performance. 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 3D three-component (one vertical and two horizontal geophones) surveys from Wyoming are presented: one acquired over the southern tip of the Pinedale Field in the Antelope area and the other in the Washakie Basin. The targets are naturally fractured gas sand reservoirs. From the analysis of fast and slow S-waves, the same regional direction of anisotropy was observed in both areas. Layer-based analyses measured anisotropy 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 surveys and the volumes indicated areas of increased fracturing in the overburden as well as at target levels. Introduction Recent interest in the use of PS-waves to help characterize fractured reservoirs has prompted the acquisition of several multicomponent surveys around the industry. Ata and Michelena (1995) used three 2D lines centered over a well to quantify fracture information. Although the spatial coverage was sparse, two fracture systems appeared to cause azimuthal anisotropy. A small 3D/3C survey collected in the Wind River Basin in Wyoming to calibrate a larger P-wave effort had some measure of success in characterizing fracture anisotropy (Gaiser, 1999; Grimm et al., 1999). Ocean-bottom cable surveys from the North Sea (Olofsson et al., 2002) and the Adriatic Sea (Loinger et al., 2002) showed that it was important to characterize overburden azimuthal anisotropy before deeper targets were analyzed. The objective of these studies was to use a PS-wave seismic survey in the Pinedale field and Washakie Basin in Wyoming to quantitatively identify fractured areas in a naturally fractured Cretaceous sandstone reservoir at depths between 3,000 and 4,500 m. 3D/3C surveys were designed and acquired to provide wide azimuth and offset coverage at the target. In the Pinedale survey, receiver lines were oriented N70°E and the shot lines were oriented at 45° to the receiver lines in a NW-SE orientation to yield a CMP fold of approximately 50 (P-wave) and 50 (PS-wave) over 25 sq km. At Washakie Basin the receiver lines were oriented E-W and a diagonal brick shot pattern was acquired to yield a CMP fold of approximately 98 (P-wave) and 98 (PS-wave) over 50 sq km. A detailed processing methodology is developed to preserve the effects of S-wave birefringence and prepare the data volume for further fracture analysis. 2Cx2C (read, 2C by 2C) Alford (1986) rotation adapted for PS-waves (Gaiser, 1999) provides a means to combine the multiazimuth data into a single 2Cx2C volume. This four-component matrix has terms: PS11, PS12, PS21, and PS22. Minimization of the off-diagonal terms PS12, and PS21 identifies the principal axes of the azimuthal anisotropy that are related to fracture orientation, and measuring the time delays between the fast, PS11, and slow, PS22, diagonal terms quantifies the magnitude of anisotropy related to fracture density.