This paper describes the reservoir engineering aspects of the CO feasibility study for Department of Energy Class II project, at the West Welch San Anders Unit. The DOE Class II demonstration simulation effort for the CO feasibility study uses an 11 component equation of state (EOS) for surface facility design. Relative permeability data measured in the laboratory indicated two different waterflood residual oil saturations. The low reservoir temperature causes the fluid phase behavior to exhibit a two liquid hydrocarbon phase region with CO present Fluid characterization steps taken to virtually eliminate the two liquid hydrocarbon phase region is described. The problems these changes created and adjustments made to the EOS to improve the simulation are reviewed. We show that the EOS needs to predict slim tube recovery compositions as well as the total slim tube recovery to adequately predict field scale oil recovery. The fluid characterization procedure, described here, made the full field simulations more closely match field CO recovery.

Two types of wettability were identified from relative permeability curves at the project in the Welch field. The intervals of different wettability were identified from resistivity logs to aid in placing these intervals in the proper model layers. The geologic model with both relative permeability rock types led to overpredicting oil recovery. However, the low waterflood residual oil saturation appears more significant in the higher permeability rock. Thus, the relative permeability tests from core samples with higher permeability led to significant differences in model results. Subsequent history matching efforts, incorporating 3D seismic, showed that the relative permeability rock type with low residual oil saturation to water was not volumetrically significant. A separate paper, SPE 39809, describes the incorporation of 3Dseismic into the geologic model.

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