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ABSTRACT: Cross-Well Tomography in a Producing Oil Field: Seventy-Six West

MCDONALD, J. A., H. W. ZHOU, J. JECH, and C. A. LINK, Allied Geophysical Laboratories, University of Houston, Houston, TX

The Seventy-Six West field is a partially depleted, barrier/strandplain sandstone reservoir of the Eocene Jackson-Yegua trend located in Duval County, Texas. Over a period of two years, five cross-well experiments have been conducted in this field by the AGL group.

The first four experiments consisted of cross-well measurements with a maximum well spacing of 600 ft and depths to 1000 ft. The fifth experiment was designed to collect data from below the producing zone at approximately 1350 ft; maximum well spacing was 1215 ft. In addition to cross-well data, inverse VSP data were recorded from the downhole source into surface geophones.

Three energy sources were tested: a piezoelectric "bender," an electrical arc discharge device or "sparker," and an airgun. Measurements show that the relative energy output for the sources tested is highest for the airgun followed by the sparker and finally the bender. Conversely, signal frequency content is highest for the bender followed by the sparker and then the airgun.

Compressional velocity tomograms have been produced for two cross-well setups using airgun data. Combined iterations of curved ray-tracing and constrained conjugate-gradient inversion are used to construct the tomograms. Theoretical studies show that iterations are essential for this type of shallow-depth tomography to constrain the velocity range of the inversion allowing suitable convergence. The tomogram of the automatically picked data set is quite similar to the tomogram of the hand-picked set, but has much better coverage.

A layered average profile of our velocity tomograms correlates quite well with a sonic log from a nearby well; unfortunately, sonic logs are not available for the wells used in the experiments.


AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)