Multicomponent Processing and Fracture Characterization Analysis of 3-D PS-Wave Onshore Seismic Surveys

Abstract

Abstract Multicomponent 3-D surveys where downgoing compres-sional (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 Pinedale 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 S-wave splitting in the overburden as well as at target levels. Azimuthal anisotropy sweet spots observed in the datasets suggest more intense fracturing, and are typically associated with other structural features such as lineaments, faults and folds. At Pinedale this occurs over the crest and along the limbs of the anticline. Also, the strike of near vertical fractures observed on an FMI log is in agreement with the fast S-wave direction.