Converted Shear-Wave Seismic Fracture Characterization Analysis at Pinedale Field Wyoming
Multicomponent 3D 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.
A 3D 3-component (one vertical and two horizontal geophones) survey from Wyoming is presented, acquired over the southern tip of the Pinedale field. The targets are naturally fractured gas sand reservoirs in the Lance formation. From the analysis of fast and slow S-waves a regional direction of anisotropy was observed. Layer-based analyses confirmed the presence of azimuthal anisotropy in the overburden, which required compensation during the processing to isolate S-wave splitting properties at reservoir depths. Results from the layer-stripping analysis suggested areas of increased fracturing in the overburden as well as at target levels that are associated with faults over the crest and along the limbs of the Pinedale anticline. Although FMI logs show mostly bedding planes near horizontal dip, at many levels where dip is larger, the dip direction is qualitatively in agreement with the principal S-wave directions.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005