For a thin oil rim reservoir with a large gas cap on top and a strong aquifer below, the art of optimizing the oil recovery is to keep the oil rim in continuous contact with the producing wells in the oil rim. Therefore, the management of gas-oil contact and water-oil contact movement is extremely critical. To achieve the maximum results, force balance between aquifer drive, gas cap expansion, and viscous withdrawal (production) shall be carefully studied for a given reservoir at various stages of the production life cycle.

In this paper, the risk of losing oil rim oil to the gas cap when the force balance is impaired by weakening gas cap energy due to gas production is illustrated. This condition can also be observed due to impaired water injection or the improper placement of water injection wells.

For thin oil rim reservoirs with significant oil reserve, the strategy of hydrocarbon exploitation should be producing the oil rim oil first and then blowing down the gas-cap later to maximize hydrocarbon recovery (Ref. 1). For this reason, most of the produced gas shall be re-injected to the reservoir to keep the proper force balance and to enhance the improved oil recovery (IOR) options such as infill wells, water injection and enhanced oil recovery. Model simulation studies can demonstrate that the IOR benefit can severely reduced by producing gas-cap gas. A correlation between IOR recovery factor and gas cap gas withdrawal volume, for a particular thin oil rim reservoir in Malaysia, is shown and proposed.

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