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ABSTRACT: Applications of Cross-Borehole Seismic Tomography to Monitor EOR Displacement Fronts in Heavy-Oil Fields of Kern County, California

P. R. Hutt, J. H. Justice, A. A. Vassiliou, P. S. Cunningham, S. Singh, J. Bulau

Cross-borehole seismic tomography has been applied to assist in the monitoring of an EOR (enhanced oil recovery) displacement front in a heavy oil field located in Kern County, California. Results from this survey indicate that tomographically processed cross-borehole seismic surveys can image variations in the velocity field of the subsurface. Several factors including the temperature, type, and physical state of the pore-filling material strongly affect the velocity field of a reservoir. Cross-borehole velocity profiles, referred to as tomograms, are interpreted to provide insight into the physical changes taking place in the reservoir as an EOR process progresses.

A cross-borehole seismic survey is acquired by placing a downhole seismic source system into one well bore and a downhole seismic receiver system into an adjacent well bore and recording seismic wavefield information for a series of shots from the source to the receiver. The time it takes the wavefront to propagate from the source to the receiver is associated with a raypath. When an adequate number of wavefronts are recorded to properly sample the subsurface profile from source well to receiver well, they may be tomographically processed to provide a velocity profile between the two well bores.

A tomographic survey acquired in a reservoir undergoing cyclic steaming stimulation resulted in a data set of both compressional and shear wave velocity profiles. These data were combined to produce a Poisson ratio tomogram for one of the two well-to-well surveys. The P-wave, S-wave, and Poisson ratio tomograms are interpreted to demonstrate variations in the type and physical state of the pore-filling material of the reservoir. The data may reveal information with respect to

heavy oil saturations as well as highlight zones of thermally heated reservoir. The interpretations are highly qualitative and require further acoustic core analysis to confirm and quantify the findings.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990