Premier Oil Shelley field, United Kingdom

The Shelley field is located on Blocks 22/2b and 22/3a in the central North Sea, about 192km east of Fraserburgh, Scotland. It lies in 95m of water in a Paleocene Forties reservoir.

The field was discovered in 1984 by the 22/2-2 well and then plugged for 20 years until it was awarded to Oilexco subsidiary Oilexco North Sea Ltd (ONSL) in 2005.

In October 2007, Diamond Offshore’s Ocean Guardian semi-submersible drilled eight well penetrations from a single subsea well bore to appraise the discovery. Several appraisal wells were then drilled to further define the area.

The appraisal well 22/2b-13 reached a true vertical depth of 4,221m. It tested at a rate of 3,082 bopd at more than 31º API. Further appraisal drilling was carried out on well 22/2b-13 to determine the size of the reservoir, which was found to measure 7km².

The final appraisal well, 22/2b-13t, was tested through 13m of perforations representing five true vertical metres of perforated reservoir from the top of 10 vertical metres of oil pay. The well tested at a flow rate of 3,082bopd at a 36/64in choke. The findings confirmed the previous discovery from 22/2b-13.

Estimated total recoverable reserves for the field are 20Mmbbl of oil.

In 2008, the Sedco 712 semisub, which is owned and managed by Transocean, began Shelley’s development drilling, and in the same year Technip was awarded a $242m contract for the engineering, installation and commissioning of pipelines for the field. The contract covered the control umbilicals and jumper, the flexible risers, the subsea manifold and an attendant flowline.

Production, which began in early August 2009, is via Sevan Marine’s Sevan Voyageur FPSO. Built at the Yantai Raffles shipyard in Shandong, China, it was transported to Bergen Group’s Hanøytangen shipyard for rigging up and engineering of various structures, then to Rotterdam for topside hook-up and commissioning in the last quarter of 2007. It arrived at the field in November 2008 to be hooked up to a pre-laid mooring system.

The Sevan Voyageur, at 66m long and 60m wide, has a displacement of 50,000t. It has a production capacity of 35,000bopd and a storage capacity of 300,000 barrels of oil.

Sevan Marine says that, as a cylindrical FPSO, it can operate more efficiently in harsher climates on fields of various sizes because of its flexible design and motion characteristics. The round vessel has less hull surface, which reduces the amount of steel needed for construction, but it contains the same storage volume as that of traditional FPSOs.

In May 2009, Premier Oil took over as the operator and owner of Shelley as part of its acquisition of ONSL for $505m. Earlier, in April 2009, Sevan had entered into a memorandum of understanding with Premier for the continued provision and operation of Sevan Voyageur, which Oilexco had contracted to operate for a fixed term of five years. The memorandum granted Premier or one of its affiliates continued use of the vessel in the event Oilexco or the Shelley field was sold.

Premier has developed the field using two horizontal subsea production wells equipped with downhole electric submersible pumps. The wells are connected via a short flowline, and a control umbilical, to Sevan Voyageur. The wells and infrastructure are monitored, controlled and operated from the FPSO.

The oil is separated from gas and water on the FPSO. Water injection was considered at one stage but ultimately dropped, so produced water is cleaned up to relevant standards and discharged into the sea. Part of the gas is used as fuel to generate power and the rest is flared off. The oil is stored in the FPSO, then transferred to a shuttle tanker for transport onshore to an unnamed refinery.

Production is at an initial rate of 10,000-12,000bopd. With an expected life of 25 years, production from Shelley is forecast to peak at 35,000bopd, although because of the complex geology of the area the longer term performance is less certain.

Premier says it is evaluating other possible accumulations in the area. Exploration drilling will depend on the results of that work and the performance of the main Shelley reservoir.