Abstract

The Reservoir Characterization Project at Colorado School of Mines acquired a three-dimensional (3-D) multicomponent survey over Silo field in southeastern Wyoming with the objective of imaging reservoir heterogeneity. A 3-D shear-wave survey resolved spatial variations in the fracture distribution of Niobrara chalks by detecting small percentages of anisotropy induced by fractures in chalks of the Niobrara reservoir. In addition, the compressional-wave survey imaged structural drape over a zone of deeper salt dissolution, which fractured the brittle chalks.Rotation analysis of the shear-wave survey took advantage of its 3-D nature to identify an azimuthal pattern of anisotropy associated with vertical fractures, known as extensive dilatancy anisotropy (EDA). The shear-wave data were sorted by shot-to-geophone azimuth to search for the orthorhombic pattern of anisotropy that might be expected from the combined effects of sedimentary layering and vertical fractures, but it was not found at Silo Field. Although groundroll contaminated some of the 'pie slices' of azimuth in the rotation analysis, the redundancy of ten 'pie slices' enabled us to determine an overall fracture orientation of N 58 degrees W.The 3-D shear-wave survey yielded a picture of fracture variability that could not be determined from well control alone. Fracture-identification logs and production records were used in interpreting the anisotropy determined from the shear waves. Large positive values of anisotropy in the Niobrara interval, up to 5 percent, coincided with good producers in the field, while smaller magnitudes of anisotropy were tied to poor producers. Combining the multicomponent seismic recording with 3-D survey techniques added lateral resolution to the reservoir description and rendered a more complete understanding of the pattern of anisotropy resulting from fractures in the reservoir.

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