HARRIS, J. M., R. NOLEN-HOEKSEMA, J. W. RECTOR, III; M. VAN SCHAACK, and S. K. LAZARATOS, Department of Geophysics, Stanford University, Stanford, CA

ABSTRACT: High Resolution Crosswell Imaging of a West Texas Carbonate Reservoir

Crosswell seismic tomography and reflections were combined to produce high resolution images of a producing West Texas oil reservoir. The technology was being tested as part of a miscible CO2 flood pilot site located on the Eastern Margin of the Central Basin Platform. The target was a reservoir zone approximately 30 meters thick within the Permian aged Grayburg formation at a depth of nearly 1000 meters. Project objectives included (1) developing a pre-injection tomographic image of the velocity variations between wells for reservoir characterization purposes; (2) testing the resolving capability of a new crosswell reflection imaging methodology; (3) monitoring the vertical and lateral movement of injected CO2. This paper discusses pre-injection results. The crosswell data were recor ed with a high frequency downhole source and an array of hydrophone receivers. Field operations were distinguished by the fact that two imaging profiles of nearly 80,000 seismic traces were recorded in approximately four days. Other pre-injection data available included surface seismic, logs, and nearly 100 meters of core on which both geophysical and petrophysical measurements were run. The processed crosswell images show 1-2 meter vertical resolution of stratigraphy inside the reservoir and subtle lateral variations in velocity possibly related to porosity. The images also reveal features not resolved bysurface seismic, including the Grayburg-San Andres unconformity below the reservoir and the Queen formation that forms the reservoir seal from above. The tomographic images provide inte well velocities that fall within 1% of the log values at the wells. P and S waves are used to estimate Vp/Vs ratio of the major formations, including the reservoir zone. Overall, the crosswell images complement other data by filling the gap in resolution and coverage between logs and surface seismic.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.