The Prudhoe Bay Field, Alaska, is produced by a number of recovery processes. A miscible gas (MI) injection pilot was studied to see if repeated seismic surveys could detect the progress of the MI gas. Gassmann's equation was used on the injection, producing and monitor wells where a detailed reservoir simulation was available. Time-varying saturations of the three fluid phases and the pressure were used to calculate the expected velocity of the reservoir at different stages of the injection. The differences between the modeled velocities at two extremes of gas saturation after the water-after-gas (WAG) range up to 500 ft/s (150 m/s). It was concluded that it would have been possible to detect the fluid saturation had a baseline survey been collected early in the field's development. Unfortunately, initial production introduced 2% gas into the fluid, muting later attempts to map changes in saturation that varied from between 30% and 60%. Additionally, the use of a WAG process further complicated the gas mapping by both increasing and decreasing the reservoir fluid velocities. Collecting new seismic data over this pilot area was not recommended. The modeling exercise highlighted a number of issues that are important in monitoring other reservoirs. Amongst these are the timing of data collection and the weakness of the petrophysical models caused by the numerous assumptions that are required in the absence of field observations. It was demonstrated that modeling exercises can both save unnecessary field expenses and provide considerable insight in reservoir behavior.

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