Breaking Oil Recovery Limit in Malaysian Thin Oil Rim Reservoirs: Water Injection Optimization

Abstract

Abstract The goal of an oil field development project is to accelerate the hydrocarbon production and maximize the recovery at a 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 shall be maximized. A successful project shall entail plan first to accelerate the oil production maximizing the oil recovery prior to the gas cap blow-down. The maximum oil recovery factor achievable in thin oil rim reservoirs was evaluated for a Malaysian thin oil rim reservoir. The force balance between the gas cap expansion, aquifer expansion and viscous withdrawal 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 concept and strategy of water injection for a subject thin oil rim reservoir was delineated. Placement of a system of water injectors at the selected sector periphery and at the gas oil contact was studied. Together with optimization of infill well placement and the selected idle well reactivation, this effort showed a potential of improving the current recovery factor from 35% up to 51%, way beyond the maximum theoretical vertical displacement efficiency. In addition, it can be shown that successful water injection can improve productivity of production wells due to increasing near wellbore oil saturation. Introduction For thin oil rim reservoirs in Malaysia, the key force balance is between the gas cap and aquifer expansion, and the fluid withdrawal by well production, in addition to the capillary and gravitational forces. The understanding of the force balance in the reservoir will lead towards a better optimization of reservoir oil and gas production, achieving a higher recovery factor, and generating a good investment plan (Ref. 1). The study in this paper will focus on the Seligi field in Malaysia. It is located offshore Terengganu and was progressively developed starting in 1978 as an oil field. It is a combination of stratigraphic and heterogeneous structural traps (Figure 1). It has a huge regional aquifer and 5 distinctive gas caps (Figure 2) at the time of discovery/development with a significant amount of oil in place (Ref. 2). Seligi field was developed with the concept of produced gas re-injection at the gas caps. Typically, 75% of the produced gas was re-injected via 15 gas injectors at the 5 gas caps. However, the subject oil rim reservoir pressure has been declining gradually as oil being produced (Figure 3).