We conducted a core analysis program to provide supporting data to a series of crosswell field experiments being carried out in McElroy Field by Stanford University's Seismic Tomography Project. The objective of these experiments is to demonstrate the use of crosswell seismic profiling for reservoir characterization and for monitoring CO 2 flooding.For these west Texas carbonates, we estimate that CO 2 saturation causes P-wave velocity to change by -1.9% (pooled average, range = -6.3 to +0.1%), S-wave velocity by +0.6% (range = 0 to 2.7%), and the P-to-S velocity ratio by -2.4% (range = -6.4 to -0.3%). When we compare these results to the precisions we can expect from traveltime tomography (about + or -1% for P- and S-wave velocity and about + or -2% for the P-to-S velocity ratio), we conclude that time-lapse traveltime tomography is sensitive enough to resolve changes in the P-wave velocity, S-wave velocity, and P-to-S velocity ratio that result from CO 2 saturation.We concentrated here on the potential for CO 2 saturation to affect seismic velocities. The potential for CO 2 saturation to affect other seismic properties, not discussed here, may prove to be more significant (e.g., P-wave and S-wave impedance).

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