The Brent field was an oil and gas field located in the East Shetland Basin of the North Sea, north-east of Lerwick in the Shetland Islands, Scotland, at a water depth of . The field operated by Shell UK Limited was discovered in 1971 and was once one of the most productive parts of the UK's offshore assets but has reached the stage where production is no longer economically viable. Decommissioning of the Brent field is complete with the exception of Brent C, which is producing from another field. The discovery well 211/26-1 was drilled in 1971 by the semi-submersible drilling rig "Staflo". This was a major surprise at the time as the nearest land in Scotland and Norway is composed of granite and other non reservoir metamorphic rocks.
Name
Shell initially named all of its UK oil fields after seabirds in alphabetical order by discovery – Auk, Brent, Cormorant, Dunlin, Eider, Fulmar and so on. Brent refers to the brent goose, and in turn gave its initials to the geologic subdivisions of the Middle Jurassic-age Brent Group that make up the field: Broom, Rannoch, Etive, Ness and Tarbert formations, with each name representing a loch in the Scottish Highlands.
Geology
Situated in the East Shetland Basin, the Brent is the archetype for many of the fields in the area, consisting of a tilted fault block exposing the eponymous Brent formation, next to bounding faults which allowed migration from deeper adjacent "kitchen" areas where the Kimmeridge Clay Formation becomes fully mature and releases hydrocarbons. Unusually on a worldwide scale (but common in this basin), the seal or cap rock for the reservoir (which stops the hydrocarbons from migrating further towards the surface) is also the Kimmeridge Clay, or technically the Heather Formation immediately below.
The reservoir depth is .
Production
Production started on 11 November 1976, and on 13 December 1976 the first tanker was loaded. The Brent field oil was extracted by four platforms in an irregular SSW-NNE line. The first in place was the concrete-legged "Condeep" Brent Bravo, built by Norwegian Contractors in Stavanger, in August 1975. This was followed by the steel-jacket Brent Alpha built by Redpath Dorman Long in Methil and installed in May 1976; the concrete-legged Brent Delta, again built by Norwegian Contractors in Stavanger, in July 1976; and the concrete-base Brent Charlie, built by McAlpine/Sea Tank in Ardyne Point, installed in June 1978. As of 2004, the field was still producing oil through a manifold (all Brent Alpha fluids are produced across to Brent Bravo). A fifth installation, the floating Brent Spar, served as a storage- and tanker-loading buoy and was installed early in the field's construction. The "spar" design of this installation led to the name by which it became the best known of the Brent installations (outside the oil industry). The field also included a remote flare, the "Brent Flare", which was used to flare off excess gas before gas handling and export facilities were installed in the field, also used as a vent in winter 1995–96. This unit was decommissioned and removed using a heavy lifting barge in 2005.
The topsides for Brent Charlie were designed by Matthew Hall Engineering which was awarded the contract in January 1974. Initially there were facilities for 19 oil production wells, nine water injection wells, six gas injection wells and three spare slots. The production capacity was per day and 8.5 million standard cubic metres of gas per day. There were three production trains each with four stages of separation with the first stage separators operating at a pressure of 9.6 bar. The 14 subsea storage cells had a capacity of . Electricity generation was powered by three 12 MW Rolls-Royce Avon gas turbines. The topside accommodation was for 120 people. There were 14 topsides modules and the topsides weight was 34,000 tonnes.
The initial design parameters of the Brent installations is summarised in the table.
The field supplied oil via the Brent System pipeline to the terminal at Sullom Voe, while gas was piped through the FLAGS pipeline ashore at St Fergus on the north-east coast of Scotland. The production of oil from the field (in 1000s barrels) was:The field underwent a £1.3 billion upgrade project in the mid-1990s, which involved depressurising the entire reservoir and making extensive modifications to three of the four Brent platforms. This converted them to low-pressure operation, which unlocked significant quantities of natural gas from the reservoir and extended the field life beyond 2010.