Predicting changes in seismic response within thermally produced reservoirs provides an important tool for time-lapse seismic survey interpretation. As a step towards linking thermal reservoir simulators to synthetic seismogram generation, we have studied the effects of substitution of thermal fluids in a synthetic reservoir that is based on the Pikes Peak field. We used the Gassmann equation and the fluid property relations of Batzel and Wang to predict the effects of fluid substitution. Fluid substitution was used to investigate the changes in seismic response at the Pikes Peak field. The predicted velocity decrease due to the steam injection process is 17 percent and is similar to the lab test results. The synthetic seismic time-lapse analysis gives similar results to the time-lapse analysis for the real seismic survey. The difference section shows a significant difference within the reservoir zone that had experienced thermal fluid substitution. Large differences under the changed reservoir zone are due to the time delay caused by fluid substitution and the changes are similar to the response on the real seismic time-lapse difference section. Non-cancelled energy appearing outside of the effected reservoir zones is also observed on the real seismic time-lapse difference. We suspect that these changes are processing artifacts. The behaviour of the synthetic P-wave traveltime change has similarities to the traveltime changes from the time-lapse seismic surveys, and the major differences may be due to early production that was not accounted for in the modelled results. An overall increase in traveltime within the Waseca formation in the synthetic seismograms may be due to processing artifacts and needs further investigation.
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