The goal of an oil field development project is to accelerate the hydrocarbon production and optimize the recovery at the lowest cost. For a thin oil rim reservoir with a large gas cap on top and a strong aquifer below, achieving such goal can be very challenging since recovery of both oil and gas needs to be optimized. A successful project may entail plan first to accelerate the oil production with re-injection of the produced gas at the gas cap, maximizing the oil recovery prior to the start-up of the gas cap blow-down (Ref. 1-2).The maximum oil recovery factor achievable in thin oil rim reservoirs was evaluated for a Malaysian thin oil rim reservoir based on dynamic flow properties. The force balance between the gas cap expansion, aquifer expansion and viscous withdrawal (volumetric fluid production) was demonstrated by showing the model simulated water-oil and gas-oil contact movement. The understanding of the force balance progressively guided the field development project team to selectively re-activate some of the idle wells, to selectively place new additional infill horizontal wells, and to plan selective water and gas injection in key reservoir sectors.In this paper, the impact of changing force balance on oil recovery by increasing viscous withdrawal with increasing idle well re-activation and infill horizontal well drilling at their respective optimum produced gas re-injection was reported. This effort showed a potential of improving the current recovery factor from 35% up to 48%, at and above the estimated theoretical maximum oil recovery efficiency.

This paper describes a two-year effort by ExxonMobil and PETRONAS to develop a simulation model of a complex Seligi group J-sand, a major reservoir of this Malay Basin giant. This joint study demonstrated the successful collaboration between ExxonMobil and PETRONAS to enhance the existing depletion plan and identify improved oil recovery opportunities. This J-sand contains roughly 750 to 850 million stb of oil originally in place in this maturing offshore reservoir in the Malay Basin comprising multiple interdependent closures with relatively thin oil columns and large gas caps. The reservoir communicates with nine producing fields via a common regional aquifer encompassing an area of more than 7000 square kilometers. Seligi was challenging to model because of its large size (8km x 12km), long history (30 years), significant gas and water coning issues, and its interaction with the regional aquifer. This paper will highlight techniques that were critical in creating an integrated simulation model of the field and regional aquifer, managing large quantities of data, and evaluating history match improvements due to the large well count and interdependent closures. The integrated simulation model was created from two geomodels of different vintages by seamlessly integrating the coarsely gridded Regional Aquifer Model, required for capturing the material balance impact of neighboring fields, and the finely gridded Seligi model to simulate field performance. Advances in reservoir simulation capabilities allowed a large single integrated model to be built, significantly reducing turnaround time from the previous version, where two models were run in tandem. This study resulted in the recommendation to increase the OOIP by nearly 20% and has been the catalyst for focusing on production enhancement opportunities. The model is currently being used to identify and highgrade opportunities to maximize economic hydrocarbon recovery.

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.