The Department of Energy has funded a three-year project that has acquired, processed, and interpreted multi-component 3D seismic data for fracture characterization in a naturally-fractured gas reservoir in the Wind River Basin, Wyoming. The principal objective of this project was to investigate and recommend cost-effective seismic technologies form characterizing the spatial distribution of natural fractures from which gas is produced from the Tertiary Lower Fort Union, at depths 5500-9500 foot.

Two wells with borehole image logs (BHI) in a 3D-3C survey provided control datapoints on the natural fractures' orientation and maximum horizontal stress azimuth: dominantly east-west (EW) fractures and maximum horizontal stress are seen. This is parallel to the dominant fault azimuths in the Lower Fort Union, as known from mapping in the larger 37 square mile survey. At these two wells, the fast P-wave velocity direction and the S1 polarization are east-west.

The 3D-3C P-P and P-S data were processed as four narrow-azimuth volume in order to produce volumes in which interpreters can map: 1) reflections of interest, 2) fracture directions, 3) relative fracture density (interpreted as proportional to the magnitude of the anisotropy). The P-P data were processed as two-azimuth volumes (NS and EW) as well as four-azimuth volumes (for Fractogram analysis by Western Geophysical). The P-wave fast interval-velocity direction and the magnitude of the interval velocity azimuthal variation were calculated The 3D P-S data were processed by Western Geophysical (Denver), using a 4C P-S layer stripping algorithm to produce 3D maps of the estimated S1 azimuth and the estimated magnitude of the S-wave anisotropy. The technique requires orthogonal (P-wave) source-receiver pairs with H1-H2 data (two horizontal phones) in which comparable energy exists on all four tracts. This requirement is fulfilled in the NE-SW raypaths and the NW-SE raypaths, but not the N-S and E-W raypaths.

The 3D-3C survey is contained within a larger 37 square mile P-P 3D survey that was processed ns two limited-azimuth volumes (EW and NS) through prestack time migration. Comparisons of the P-P interval velocity, interval average frequency, and interval amplitude in the reservoir interval reveal azimuthal variations which are related to fracture orientation, fracture density and fracture fill (gas versus water). At the well control with BHI logs, the fracture and stress orientations are parallel to the P-wave fast velocity direction (±15¯). The 9C VSP on the eastern side of the large 3D recorded the S-wave anisotropy: the S1 azimuth is interpreted to be N90-115E from 2000-7000 foot depths; from 8000-9300 foot, the S1 is interpreted to be N50E. The shear-wave birefringence is higher in the near surface (3%) than the indurated rock column (~1%) above the target; anomalously increased birefringence (10-12%) are present in various intervals at target depths. In the same well, a crossed dipole shear wave los was acquired; increased birefringence is observed in zones which either lost fluid or took fluid.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah