Chestnut

Location and Geology

• The Chestnut oilfield is situated in block 22/2a of the Central North Sea, approximately 290 kilometers east of Aberdeen, Scotland, in a water depth of 120 meters.

Discovery and Development

• The field was originally discovered in 1986 but was deemed unviable as a standalone development by its initial owners, a consortium including Amerada Hess, Premier Oil Exploration, Roc Oil UK, Fina Petroleum Development, and AGIP UK.
• Venture Production Plc acquired a 5% stake in the field in 2001 and later increased its holding to just under 70% after the original consortium deemed it unviable.

Ownership and Operators

• At the time of development, Venture Production Plc was the operator with a 69.9% interest, while Bow Valley Petroleum held 15.1%, and Atlantic Petroleum held 15%.
• By the time of decommissioning, the ownership had changed, with Spirit Energy North Sea Oil Limited holding 82.206% and Dana Petroleum Bow Valley United Kingdom (BVUK) Limited holding 17.794%.

Production and Reserves

• The Chestnut field has estimated net recoverable reserves of 6.8 million barrels of oil equivalent. Following reservoir modeling in 2009, the overall recoverable reserves were increased by 2-5 million barrels of oil equivalent.
• Production began in late September 2008, with an initial average net rate of 6,000-10,000 barrels of oil equivalent per day (boepd). A second production well increased the production rate to 12,000 boepd by March 2009.

Field Development and Infrastructure

• The field development involved drilling a new vertical water injection well and reusing a horizontal production well drilled by the original consortium.
• The wells were tied back to the Sevan Hummingbird (later renamed Hummingbird Spirit) Floating Production Storage and Offloading (FPSO) unit. This FPSO was the second Sevan Marine 300 unit built and the first cylindrical FPSO deployed in the North Sea.
• The FPSO had an oil storage capacity of 300,000 barrels, an oil processing capacity of 30,000 barrels per day, and a water injection capacity of 20,000 barrels per day. It was contracted to Venture for five years at approximately $365 million.

Operations and Management

• Aberdeen-based Wood Group provided operations management for the project under a contract valued at about £8 million per year.

Decommissioning

• Production from the Chestnut field ceased in March 2022, with the Cessation of Production (CoP) formally accepted by the North Sea Transition Authority (NSTA) in November 2021.
• The Hummingbird Spirit FPSO departed the field in June 2022, marking the beginning of the decommissioning phase. The decommissioning activities include the disconnection and removal of the FPSO, riser systems, flowlines, and umbilicals. The wells, including the water injection well, have been decommissioned, and the flowlines and umbilicals have been flushed and cleaned.

Environmental Considerations

• The decommissioning process involves a thorough environmental appraisal to assess the impact of the activities on the marine environment. This includes pre-decommissioning environmental surveys, metocean conditions assessment, seabed sediments analysis, and marine flora and fauna studies.

In summary, the Chestnut oil project is an example of the development and production of a small, stranded oil field in the North Sea, utilizing innovative infrastructure like the Sevan Hummingbird FPSO, and now undergoing decommissioning to ensure environmental compliance.