The Tern Field, Block 210/25a, UK North Sea

Abstract

Abstract The Tern Oilfield is situated 150 km northeast of the Shetland Islands in Block 210/25a in the UK sector of the northern North Sea. The discovery well 210/25-1 was drilled in 1975 in a water depth of about 541 ft. The trap is defined at around 8000 ft TVSS by a tilted horst-structure. The hydrocarbons are contained in reservoirs belonging to the Middle Jurassic Brent Group sands deposited by a wave-dominated delta system in the East Shetland Basin. Complex faulting of the structure is responsible for the division of the field into two areas with different original oil-water contacts: the Main Area of the field with an oil-water contact at 8260 ft TVSS, and the Northern Area with a possible oil-water contact at 8064 ft TVSS. Reservoir quality is good with average porosities ranging from 20-24% and an average permeability of 350 md. The expected STOIIP and ultimate recovery of oil are 452 and 175 MMBBL, respectively which represents a recovery factor of 39%. The initial stage of the development plan calls for ten wells, five oil producers and five water injectors, to be drilled from a single platform, Tern Alpha. Development drilling started in February 1989 and first oil was produced on 2 June 1989. The oil is evacuated via the North Cormorant and Cormorant Alpha platforms into the Brent System pipeline for export to the Sullom Voe terminal. To date, two producers have b een drilled and total cumulative production is 6.4 MMBBL (1 January 1990). Ultimate recovery is estimated to be some 175 MMBBL.