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

摘要

Multicomponent 3-D surveys where downgoing compressionalrn(P) waves convert to upgoing shear (S) waves atrninterfaces provide a practical means for analyzing fracturernproperties. This is particularly important for delineatingrnnaturally fractured reservoirs by exploiting the uniquerncharacteristics of S-wave azimuthal anisotropy induced byrnvertical fracturing. In the presence of fractured media, Swavesrnsplit into a fast wave that is polarized parallel tornfractures and a slow wave that is polarized normal to fractures.rnThe amount of splitting (time difference between the two Swaves)rnis proportional to fracture intensities. To investigaternthis phenomenon we utilize a wide range of source-receiverrnazimuths in the processing and analyze the fast and slow Swavesrnto extract fracture information.rnTwo 3-D 3-component (one vertical and two horizontalrngeophones) surveys from Wyoming are presented: onernacquired over a portion of the Pinedale Field and the other inrnthe Washakie Basin. The targets are naturally fractured gasrnsand reservoirs. From the analysis of fast and slow S-waves arnregional direction of anisotropy was observed in both areas.rnLayer-based analyses confirmed that much of the anisotropyrnwas in the overburden, which required compensation duringrnthe processing to isolate the variations at reservoir depths.rnEight limited-azimuth volumes were created for the twornhorizontal geophone components. These volumes werernanalyzed to determine the time-variant anisotropy within thernsurvey and indicated areas of increased S-wave splitting in thernoverburden as well as at target levels. Azimuthal anisotropyrnsweet spots observed in the datasets suggest more intensernfracturing, and are typically associated with other structuralrnfeatures such as lineaments, faults and folds. At Pinedale thisrnoccurs over the crest and along the limbs of the anticline.rnAlso, the strike of near vertical fractures observed on an FMIrnlog is in agreement with the fast S-wave direction.