ABSTRACT

Two 3D seismic surveys are analyzed to monitor production changes within the reservoir. The base data set was acquired prior to start of in-situ combustion process in four injection wells while the monitor survey was acquired 2 years later. Fluid substitution studies, based on acoustic well logs, predicts a seismic amplitude decrease at top reservoir and an increase at the base reservoir and a relative velocity slow down of up to 8%. Both the amplitude dimming at top reservoir and the brightening at the base reservoir are observed in the field data. However, the extent of the most pronounced 4D anomaly (50m) is lower than reservoir engineering expectation (100m). This difference is probably caused by the lateral shift between the gas zone and the hot zone within the combustion front. The anomalies are laterally shifted towards northwest of the injector location, which might be explained by the fact that the reservoir sands are dipping upwards in this direction. No clear evidence of air leakage into other sand layers, neither above nor below the reservoir sand, is observed. This does not necessarily mean that all the injected air is following the reservoir sand, especially if the thief sand layers are thin, which might be difficult to observe on seismic data.

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