Clair Field, West of Shetland

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 97772, "Unlocking the Potential: A North Sea Heavy Oil Success Story," by J.J. Wylde, SPE, and G.D.M. Williams, SPE, Clariant Oil Services, and R.E. Cousins, BP plc, prepared for the 2005 SPE International Thermal Operations and Heavy Oil Symposium, Calgary, 1–3 November. The full-length paper details one of the newest producing fields in the North Sea, the West of Shetland development. Prestartup design and implementation challenges are described, as well as commissioning philosophy for the plant and chemical-injection facilities. Most of the full-length paper focuses on the first 6 months of production history, production-technology management, and how actual operation differed from the planned and designed specifications. Clair Field Background The Clair field is 40 miles west of the Shetland Islands and currently is produced from a single fixed platform in 460 ft of water. The field produces both oil and gas. The field produces 20°API crude with a 550 scf/bbl gas/oil ratio. The Clair reservoir extends over a 25×12.5-mile area in complex Devonian and Carboniferous units covering five license blocks. Current estimates of likely oil exceed 4 billion STB original oil in place (OOIP), making Clair the largest undeveloped hydrocarbon accumulation on the U.K. continental shelf and a key component of future U.K. production stategy. The Clair field was discovered in 1977. In the early 1990s, 3D-seismic data were acquired over the whole field, and two wells were drilled in 1991 and 1992. Although demonstrating commercial flow rates, the wells were not produced for sufficiently long to provide confidence in long-term reservoir performance. In 1996, an extended performance test was conducted on Well 206/8-10z in the core area. Flowing at an average rate of 10,000 B/D for 23 days, with a 18.5-B/D peak, the well performance changed the perception of the Clair reservoir by demonstrating sustainable crude-oil delivery. Because of its extent, the Clair field is to be produced as a phased development. The first phase builds on the successful 1996 well test and targets development of the Core, Graben, and Horst areas. The reservoir is divided into nine fault-bounded segments having a common free-water level and 1,969-ft maximum oil column. A gas cap is present in the structurally elevated ridge segments. The reservoir depth is 6,070 ft true vertical depth subsurface, and initial reservoir pressure was 2,736 psi with a 151°F temperature. Work is continuing to define this first development. The challenge for Clair is understanding the issues of reservoir deliverability, well productivity, and managing the cost base while achieving a sustainable development. Clair will be the third West of Shetland development, following Foinaven and Schiehallion.