The first horizontal oil producer in the Oseberg Field was drilled in 1992, and to date a total of 22 horizontal wells have been completed including one multi-lateral well. The extensive use of long reach, multiobjective horizontal wells is vital to prolong the plateau production period and to achieve high recovery. The expected ultimate oil recovery is currently estimated at about 60 percent. This paper discusses the experiences with the Oseberg horizontal wells such as drilling, completion, production logging and well performance.
The horizontal wells are located some 3-8 meters above the oil-water contact as observed in the pilot wells. Locating the horizontal sections close to the oil-water contact enables drainage of downflank oil and delays gas breakthrough from an advancing gas front. The high productivity reduces the effect of water coning, and water production has not been a problem. Gas breakthrough has been experienced in some horizontal wells earlier than predicted, although reservoir monitoring indicated a global gas front some 50 meters above the horizontal well sections. The early gas breakthrough is interpreted to be a result of gas underrun due to stronger flow barriers than assumed in the simulation models. Stronger gas cusping due to lower than assumed permeability upflanks may also be an explanation.
The Oseberg horizontal wells are completed with cemented, partially perforated liners. This provides increased production flexibility and improved zonal drainage which is particularly important in wells penetrating formations with large permeability contrasts. Only a portion of the well may be perforated for an initial production phase. The second phase involves production from selective perforations along the entire horizontal section. Sequential production in Oseberg horizontal wells is important for maximizing oil reserves.
The production logged inflow profiles indicate that the selective perforation strategy has been successful. Field examples included in the paper demonstrate that oil inflow can be achieved along the entire horizontal section (of 1500-2000 m) from the well heel to the well toe if the perforation intervals are designed properly.
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. 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 (Fig. 1). Gas caps exist in all three structures. The initial vertical gas column in Alpha was 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 field contains initially about 520x106 Sm3 free oil and 65x109 Sm3 free gas located in seven partly communicating reservoir segments. The initial gas-oil contact is located at about 2500 m true vertical depth (TVD) and initial oil-water contact in the Oseberg, Rannoch and Etive (ORE) formations at about 2715 m TVD. The current expected ultimate reserves is 315.4 MSm3 including 17 MSm3 condensate. The expected recovery is above 65 percent in some reservoir units.
The Tarbert formation contains sand of fair to good quality and thickness in the south, but 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 several Darcy. The permeability in the thin Etive formation is in the range 500-1500 ml). The Rannoch formation consists primarily of interbedded fine to medium grained sandstone and acts as a flow restriction between the Etive and Oseberg formations.
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