Hydraulic Fracture Stimulation of Permeable North Sea Oil Wells

Abstract

Abstract The vast majority of fracture stimulations conducted to date in the North Sea have been hydraulically propped fractures of tight sandstone gas reservoirs or acid fractures of carbonate oil reservoirs. This paper demonstrates that hydraulic fracturing technology is applicable to permeable sandstone formations and can enhance the economic viability of permeable sandstone formations and can enhance the economic viability of marginal North Sea oil reservoirs. In 1989, Mobil North Sea successfully conducted its first hydraulic fracture stimulation of a North Sea oilwell in the A45 well on the Beryl Alpha condeep production platform. The formation stimulated was the Triassic Lewis Unit 1 Sandstone, characterized by average permeability of 10 md (streaks over 100 md), and capable of producing 500 -1000 BFPD. The stimulation was successful in increasing the well's PI from 0.7 BPD/psi to 1.4 BPD/psi and resulted in post-frac production rate of 2400 BOPD. Technical highlights of the stimulation were the use of large propping material to achieve maximum fracture conductivity, and the use of resin coated proppant to minimise its flowback. The formation stimulated was at 16,155ft MD with wellbore inclination of 55deg.. Introduction The Beryl field is located 350 kilometers north-east of Aberdeen on the UK continental shelf. The field was discovered in 1972 and production commenced in 1976 from the Beryl Alpha condeep production platform, operated by Mobil North Sea Ltd. Annual production, which peaked at 99.2 MBOPD in 1980, is projected to average 44.0 MBOPD in 1990. The oil reserves accessed from the platform are contained in 13 different producing horizons, of which the four Beryl formations are the best quality. accounting for 85% of production to date. However, secondary reservoirs, which have contributed only 15% of total production, contain 45% of total field STOIIP. Concern exists that unless steps are taken to develop some of these reservoirs, a significant quantity of unproduced reserves will remain at the end of field economic life. Among the most significant of the secondary reservoirs is the Triassic A03/A22 fault block which contains over 10% of remaining Beryl A reserves in three distinct formations, the Lewis Units I, III, and IV (Figure 1). The Lewis Units Ill and IV are characterized by reservoir permeability averaging 50 - 100 md while the deeper Lewis Unit l has permeability averaging only 10 md This paper evaluates the technical viability and economic merit of hydraulically fracturing this Lewis Unit l formation. The zone is isolated from overlying oil horizons by a 50 ft shale barrier while a less extensive shale, in terms of thickness, provides isolation from the underlying aquifer. Reservoir parameters typical of the horizon are contained in Table l and are the basis for all numerical analyses in this paper. paper. P. 307