Brage Field, Lessons Learned After 5 Years of Production

Abstract

Abstract The Norwegian North Sea Brage field is currently on decline from its plateau oil rate of 19 400 std m3/d. This paper summarizes five years of production experience from the two highly different reservoirs that have been developed so far. The fluvial, Lower Jurassic Statfjord formation consists of 1000 to 4000 md sandstone reservoir, with excellent vertical and lateral communication properties. A recovery of approximately 64% is expected from waterflooding the highly undersaturated 36 API oil. The shallow marine Fensfjord formation, of Middle Jurassic Age, is a stratified sandstone reservoir with average zone permeabilities in the range of 1 to 200 md. Calcite layers and high permeability strikes, with up to 5000 md permeability, amplify the heterogeneity and complexity of this reservoir. The oil has similar properties as in Statfjord, but a recovery of only about 32% is expected. The principal recovery mechanism is aquifer support aided by water injection from the flanks. The initial Fensfjord development was based on vertical wells with perforated cemented liner. Early production experience proved sand production to be a major problem. To avoid sand production, two wells were hydraulically fractured with proppants. This concept has later been left in favour of horizontal wells with sand screen, sliding sleeves and external casing packers (ECP's). New water injection wells are horizontal. In these wells the horizontal section is divided into two parts: the toe part is completed with a sand screen and accessed through tubing; the heel part is a perforated liner and accessed through the annulus. In this way the two parts can be controlled as two individual injectors. An additional benefit is the possibility of producing oil from the toe part of the well while at the same time injecting in the annulus section. A water-alternating-gas (WAG) injection pilot was established in the Fensfjord reservoir in 1994. Following a successful one-year pilot, with emphasize on performance monitoring through the use of tracers and mapping of high permeability streaks, the WAG process has been gradually expanded to include six injectors. To avoid sulphate scale, injection of seawater has been replaced by injection of formation water produced from Tertiary sands in the shallow Utsira formation. P. 103