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The 3-D seismic survey technique has been used to monitor the progress of an enhanced oil-recovery project in which production is stimulated by in-situ combustion driven by injected gas. A baseline 3-D data volume was recorded previous to the initiation of the combustion program. After combustion had been allowed to proceed for some time, the 3-D survey was repeated. Since the basis for tracking the effects of the combustion process is comparison, great care was taken to duplicate field geometry, recording parameters, and data processing. VSP data were also recorded to locate precisely the target sand reflection time and character.
Previous to the analysis of the 3-D data, synthetic traces were generated from well log data modified in several ways to simulate the effects of the combustion process. The target sand is characterized seismically by an impedance contrast due to low density. The predicted changes in reflection character are primarily due to changes in density caused by increased gas saturation. Complex trace attributes were computed to examine amplitude and other waveform changes. Comparison of preburn to postburn data shows differences that can be explained by increased gas saturation.
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