RIASSUNTO
Abstract
Shell U.K. Exploration and Production, operator in the U.K. sector of the North Sea for Shell and Esso, is developing the tight gas reservoirs of the Barque and Clipper fields, located in the Sole Pit Basin of the U.K. Southern North Sea. Well productivity is fundamentally dependent on the intersection of dune slipface sands and/or sparsely distributed open natural fracture systems. The initial development plans were based on the use of conventionally deviated wells with cemented liners across the pay section, relying on massive hydraulic fracture stimulation, if required, to achieve economic production rates. This concept has progressively evolved towards the use of horizontal wells which increase the probability of intersecting the productive features in the reservoir. Twelve horizontal wells were drilled up to mid 1992, out of a total of twenty six development wells. The main findings are: - average initial deliverability of horizontal wells is significantly higher than that of conventionally deviated wells; Productivity Improvement Factors of between 2 and 7 have been realised; - horizontal wells decline more slowly Om fracture stimulated wells; - Ultimate Recovery has increased by 20% relative, which is to a large extent due to horizontal wells; - the well count can be reduced by some 25 %; this has, in combination with well cost savings through the use of slim casing schemes and the increase in UR, significantly improved the project economics; - additional reserves from low relief flank areas can be accessed.
Introduction
Shell U.K. Exploration and Production, operator in the U.K. sector of the North Sea for Shell and Esso, is simultaneously developing the Barque and Clipper fields, located in the Sole Pit Basin of the U.K. Southern North Sea (see Fig. 1). The Barque and Clipper fields were discovered in 1971 and 1969 respectively, but because of the tight nature of the reservoir and the poor well test results, the prospects were deemed sub-economic. Appraisal was delayed until 1983 when techniques had advanced and commercial development appeared feasible. In that year a follow-up appraisal well produced at high rates after encountering natural fractures. An intensive appraisal campaign and associated studies culminated in 1988 in the start of the development of both fields, which came on stream in 1990.
Barque and Clipper are NW-SE oriented horst/domal structures, bounded by major faults on their NE and SW flanks and dip closed to the NW and SE. The elongated Barque field is 25 km along axis by 3 km wide. The Clipper field is a broad, low relief structure with multiple culminations and measures 12 km by 5 km. The gas-bearing Permian Leman Sandstone Formation (Rotliegend Group) is found between 7500-8500 ft tvss with a maximum gas bearing column of 900 ft.
The Rotliegend sandstone is subdivided into three zones, based on lithofacies distribution (aeolian, fluvial and sabkha) which in turn influences reservoir quality. The uppermost ""A"" zone is a mixture of mainly dune, interdune and sabkha, with occasional Weissliegend sandstone at the very top and is generally of poor quality.
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