HIGH-RESOLUTION CROSSWELL IMAGING OF A WEST TEXAS CARBONATE RESERVOIR .1. PROJECT SUMMARY AND INTERPRETATION

A carbon dioxide flood pilot is being conducted in a section of Chevron's McElroy field in Crane County, west Texas. Prior to CO2 injection, two high-frequency crosswell seismic profiles were recorded to investigate the use of seismic profiling for high-resolution reservoir delineation and CO2 monitoring. These preinjection profiles provide the baseline for time-lapse monitoring. Profile #1 was recorded between an injector well and an offset observation well at a nominal well-to-well distance of 184 ft (56 m). Profile #2 was recorded between a producing well and the observation well at a nominal distance of 600 ft (183 m). The combination of traveltime tomography and stacked CDP reflection amplitudes demonstrates how high frequency crosswell seismic data can be used to image both large and small scale heterogeneity between wells: Transmission traveltime tomography is used to image the large scale velocity variations; CDP reflection imaging is then used to image smaller scale impedance heterogeneities. The resolution capability of crosswell data is clearly illustrated by an image of the Grayburg-San Andres angular unconformity, seen in both the P-wave and S-wave velocity tomograms and the reflection images. In addition to the imaging study, cores from an observation well were analyzed to support interpretation of the crosswell images and assess the feasibility of monitoring changes in CO2 saturation. The results of this integrated study demonstrate (1) the use of crosswell seismic profiling to produce a high-resolution reservoir delineation and (2) the possibility for successful monitoring of CO2 in carbonate reservoirs. The crosswell data were acquired with a piezoelectric source and a multilevel hydrophone array. Both profiles, nearly 80 000 seismic traces, were recorded in approximately 80 hours using a new acquisition technique of shooting on-the-fly. This paper presents the overall project summary and interpretation of the results from the near-offset profile.