A Review of IOR/EOR Opportunities for the Brent Field: Depressurisation, the Way Forward

Abstract

SPE Member Abstract Under the current waterflood development, it has been predicted that gas production from the Brent Field will fall off plateau in 1998 and, at field abandonment approximately 55% of the 6.9 Tscf GIIP and 54% of the 3600 MMstb STOIIP would be recovered. A review of EOR/IOR techniques was carried out at an early stage to define which of the various processes might be applicable in the Brent Field and would warrant further evaluation. The scouting study considered microbial, thermal, chemical, gravity drainage and miscible processes as well as reservoir depressurisation. Of all the possibilities, surfactant flooding, immiscible (or near miscible) gas injection and depressurisation were found to be technically viable. However, based on an estimate of the costs involved It was established that only reservoir depressurisation was economically attractive under the current and projected oil price trend. With its high solution gas-oil-ratio (ranging from 1500 to 3000 scf/stb), depressurisation of the field, ultimately to 1000 psi, would liberate a significant proportion of the remaining solution gas and raise recovery by 1.5 Tscf. It was also established that, by extending the field's life, oil and condensate recovery would be increased by 30 MMstb compared to a continuation of the ongoing waterflood. Before the project could be initiated uncertainties such as aquifer influx, reservoir souring, compaction, subsidence and sand production had to be assessed and contingency measures identified. In addItion, the full range of engineering options for the four platforms had to be costed and evaluated. With a planned expenditure of 1.3 Billion, the Brent Field will be able to sustain oil and gas sales well into the next century. Introduction The Brent Field is located 150 km north-east of the Shetland Islands, and is one of the first and largest oil fields to be developed in the UK sector of the North Sea (Fig. 1). Discovered in 1971 in Shell/Esso block 21½9, it was brought on stream in 1976 and reached a peak oil production of 500 Mstb/d in 1984. The field has been developed with four installations and a remote flare. Brent Alpha is a steel piled jacket, while Brent Bravo, Charlie and Delta are concrete gravity structures (Fig. 2). In total, 83 oil producers have been drilled from the four platforms and a further 38 down-dip water injectors wells and 13 crestal gas injectors maintain reservoir pressures at approximately 5400 psi. Until the gas export pipeline was commissioned in 1982, produced gas was re-injected into the primary gas caps. After 1982, a sales agreement with British Gas commenced, and an average of 500 MMscf/d of dry gas has been supplied via the FLAGS line. Gas compression requirements are minimised by operating with a high first stage separator pressure (135 bar) which allows export directly into the pipeline to the onshore gas treatment plant at St. Fergus. Oil is exported via the Brent Systems pipeline to Sullom Voe. By January 1994 cumulative production amounted to 1530 MMstb of oil (78% of waterflood UR) and 2740 Bscf of gas (73% of waterflood UR). P. 329^