Ythan offshore field is operated by EnQuest. Credit: EnQuest.
Stena Spey semi-submersible rig drilled the first production well at the Ythan offshore field. Image courtesy of Alfvanbeem.
The field will be developed as a tie-back to the Don South West infrastructure in Block 211/18A.

The Ythan offshore oil field lies in blocks 211/18A, 211/18E and 211/19C in the North Sea in a water depth of approximately 170m. The field is operated by EnQuest (60%) in partnership with Ithaca Energy (40% working interest) and is being developed in phases as a subsea tie-back to the Don South West (Don SW) subsea distribution unit located in Block 211/18A. The estimated investment in the development is $28m.

Approval was received for the Ythan development plan from the UK Department of Energy & Climate Change (DECC) in 2014. The field came online at the end of May 2015 and is expected to operate for nine years, up to 2024.

The produced oil is sold in the market, whereas the gas is used partly for powering the platform equipment and lighting flares and the remaining is exported to the Dunlin field.

Location and reservoir details of Ythan offshore field

The field is situated 144km from the Scottish coastline, 9.6km from the UK / Norwegian median line, and roughly 2km to the east of the Don South West drill centres.

The field consists of two reservoir units, namely the Brent Group and the Statfjord Formation, possessing different fluid properties. Its crude has been classified as Group II light oil-based on reservoir fluids that are estimated to be between 33.8° API and 35.1° API crude with a gas-in-oil rate between 168 and 361 standard cubic feet per barrel (scf/b).

Being a part of the Don North East licence area, the initial production well of the Ythan field targets the same Brent reservoir sequence as the Don SW field.

Ythan offshore field development and infrastructure details

The field is planned to be developed in phases with the first well drilled in the Don SW field infrastructure using the Stena Spey semi-submersible rig.

Ythan is divided into two regions, namely the core area and the NE extension, that are partitioned by a fault. The initial oil-producing well, 211/18a-JT, was drilled in the core area. A water injection well, 211/18a-AA, will be drilled for maintaining pressure and production.

The development plan includes two subsea production and two subsea water injection wells that will be tied to the existing subsea infrastructure. A 3in gas lift flowline, an 8in production flowline and an 8in water injection flowline measuring approximately 40m-long will be used for the tie-back.

The gas lift spool will comprise three 3in carbon-steel spool pieces. The 8in production and water injection flowlines will be made of approximately three 8in duplex spool pieces.

The proposed Xmas trees of Ythan field will be linked to the Don SW subsea distribution unit by a control umbilical. The Northern Producer floating production facility (FPF) will receive the production from the Ythan field while existing pipelines will be used for gas lift and supplying water for injection from the Northern producer. The Northern Producer FPF has been providing topside production support at the Don SW and West Don locations since 2007.

A diving support vessel will be used to conduct the pipelay operations to position and tie-in the pipeline spool pieces and control jumpers.

Details of Don offshore field

The Don field located in blocks 211/18A, 211/13A, 211/14A and 211/19A was initially developed by BP in 1989 and produced until 2002. The Don South West area was purchased in 2006 by Petrofac Energy Developments Ltd (PEDL) and JV partners Valiant.

Subsea wells were drilled to develop West Don and redevelop Don SW in 2009 that were tied back to the Northern Producer floating production facility. BP and partners gave up the southern area of the Don field in the fourth quarter of 2013.

The area was awarded through an out-of-round licence to EnQuest and its joint venture partner Ithaca in the first quarter of 2014 and was renamed as 211/18E and 211/19C.