In-situ Redistribution of Dormant Gas Reservoir Energy to Maximise Oil Recovery

Abstract This paper discusses a novel development concept of internal gas injection, in which the energy of gas reservoirs is used to maximise production and ultimate recovery of depleted oil reservoirs. The South West (SW) Ampa Block 11 field in Brunei is characterised by highly stratified formations containing numerous, often areally extensive, stacked thin reservoirs alternated with sealing shales. The sequence contains a variation of oil, gas, oil rim and water reservoirs. Due to faulting and reservoir shale-out, Block 11 oil reservoirs are isolated and therefore lack support from an active aquifer. The pressures in the oil reservoirs have declined severely, resulting in low production rates and a low ultimate recovery (current recovery factors range from 20% to 40% for individual reservoirs). A novel development concept of internal gas injection is now proposed to provide energy to the oil reservoirs at low unit cost. Block 11 contains gas reservoirs that underlie the more shallow oil reservoirs, and contain an abundance of energy. The internal gas injection scheme will be implemented by drilling two "injection" wells. Within each well-bore gas will be produced from the gas reservoirs and injected directly into the oil reservoirs. The process will be controlled using intelligent well completions. The scheme will substantially increase the oil production rates and reserves. The injected gas will be back-produced both during the proposed development and after economic oil production has ceased, without loss of gas ultimate recovery. The proposed internal gas injection scheme is believed to be an industry first.