Reservoir Management of the Oseberg Field During Eight Years' Production

Abstract

Abstract The Oseberg Field contains about 520×10 6 Sm3 of oil and 65×10 9 Sm3 of free gas. The field is developed from the Oseberg B and Oseberg C platforms. Oil production was initiated in December 1988, and by April 1997 about 64 percent of the expected reserves have been produced. The end of the plateau oil production period is expected to occur late 1997 at Oseberg B and early 1998 at Oseberg C. The Oseberg Field is developed primarily by full scale gas injection supplemented by water injection in some reservoir units. Re-cycling of produced gas, and gas imported from the Troll Field is used for pressure support. Production experience and reservoir monitoring have confirmed gravity stable gas front evolution. The expected ultimate recovery is about 60 percent of the original oil in place. The process capacity has been increased significantly at both platforms since production startup. An important reservoir management challenge has therefore been to fully utilize the increased production capacity without any negative impact on the ultimate recovery. A main concern has been the gas front evolution at high production rates. Extensive use of TDT (thermal decay time) logging has been performed to trace the gas front. The oil production from each reservoir unit has been adjusted to achieve a stable gas front movement. Advanced saturation monitoring such as tracers and sponge coring has been applied alining to obtain residual oil saturation in gas and water flooded areas. The field test results are given in this paper. Experiences with foam treatment of production wells to reduce gas coning are also discussed. The Oseberg Field was produced with deviated wells until 1992, then the first horizontal well was drilled. A total of 28 horizontal wells have been completed until June 1997 including three multi-lateral wells. The extensive use of horizontal wells located close to the oil-water contact is important to prolong the plateau oil production period and to achieve high recovery. The experiences with the horizontal wells including multi-laterals are summarized in this paper. The IOR (improved oil recovery) due to altered reservoir management is discussed. This involves IOR due to the use of horizontal wells, advanced completions, phased production and other initiatives. The future IOR potential is also discussed. Introduction The Oseberg Field is located in the Norwegian sector of the North Sea in block 30/6 and 30/9, 140 km west of Bergen (Fig. 1). The field was discovered in 1979 and declared commercial in 1983. Hydrocarbons are trapped in three major tilted fault blocks: Alpha, Alpha North, and Gamma. Gas caps exist in all three structures. The Oseberg Field is 25 km long and 7 km wide. The vertical gas column in Alpha is 380 m and the underlying oil column about 215 m. The structural dip is an average of 6-10 degrees. The reservoir belongs to the Middle Jurassic Brent Group: Oseberg, Rannoch, Etive, Ness and Tarbert formations (Fig. 2). The expected ultimate reserves are 319.3 MSm3 including 18 MSm3 of condensate. The plateau oil production is 81000 Sm3/D (510000 STB/D). Fig. 3 shows highlights from the Oseberg Field development. The Tarbert formation contains sand of fair to good quality. The thickness in the south can be 10–15 m, while in the northern part the thickness can exceed 40 m. The permeability has an average of 1000–4000 md in the good quality sand, and may be less than 100 md in the poor quality sand. The Ness formation consists of delta plain channel sandstones interbedded with overbank fine-grained sediments and coal beds. The permeability is in the range from less than 1 md and up to 5000 md. The permeability in the thin Etive formation is in the range of 500 md to 1500 md. The Rannoch formation consists primarily of finely laminated sands and silt, and acts as a flow restriction between the Etive and the Oseberg formations. The Oseberg formation is the main reservoir unit and contains medium to coarse grained, fan-delta sandstones of very good reservoir quality. P. 549^