Abstract

Seismic reflection data were used to monitor the progress of an in-situ combustion, enhanced oil recovery process. Three sets of three-dimensional (3-D) data were collected during a one-year period in order to map the extent and directions of propagation in time. Acquisition and processing parameters were identical for each survey so that direct one-to-one comparison of traces could be made. Seismic attributes were calculated for each common-depth-point data set, and in a unique application of seismic reflection data, the preburn attributes were subtracted from the midburn and postburn attributes. The resulting "difference volumes" of 3-D seismic data showed anomalies which were the basis for the interpretation shown in this case study. Profiles and horizon slices from the data sets clearly show the initiation and development of a bright spot in the reflection from the top of the reservoir and a dim spot in the reflection from a limestone below it. Interpretation of these anomalies is supported by information from postburn coring. The bright spot was caused by increased gas saturation along the top-of-reservoir boundary. From postburn core data, a map of burn volume distribution was made. In comparison, the bright spot covered a greater area, and it was concluded that combustion and injection gases had propagated ahead of the actual combustion zone. The dim spot anomaly shows good correlation with the burn volume in distribution and direction. Evidence from postburn logs supports the conclusion that the burn substantially decreased seismic velocity and increased seismic attenuation in the reservoir. Net burn thicknesses measured in the cores were used to calibrate the dim-spot amplitude. With this calibration, the dim-spot amplitude at each common depth point was inverted to net burn thickness and a map of estimated burn thickness was made from the seismic data.

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