Tolmount Prospect, North Sea, United Kingdom

location of tolmount prospect

Tolmount prospect lies in Block 42 / 28d of the P1330 licence, approximately 40 miles east of the Yorkshire coast in the southern North Sea, UK.

Block 42 / 28d was first awarded to Dana Petroleum Exploration & Production (E&P) in December 2005.

E.ON E&P farmed into the project in 2010 by purchasing operatorship and 50% equity of the project.

E.ON sold its entire UK exploration and production (E&P), and transferred ownership and operatorship of the prospect, to Premier Oil in January 2016.

Premier Oil and Dana Petroleum E&P each own 50% interest in the field.

Tolmount is one of the biggest gas prospects discovered in the UK since Breagh, and has significant upside potential.

First gas from Tolmount is targeted for 2020 at a peak production rate of 200 million cubic feet a day (mmcfd).

Discovery of Tolmount prospect

The complex geology of the block presented a number of challenges to E.ON and Dana Petroleum while exploring the prospect.

Dana drilled the first exploration well 42 / 28d-10 in 2008, but failed to reach the reservoir due to mechanical rig problems.

E.ON then drilled another well (42 / 28d-11) and a side-track (42 / 28d-11z), which also failed due to borehole collapse.

The 42 / 28d-12 exploration well was drilled by the Ensco 92 jack-up drilling rig in 2011, after extensive planning and redesign.

The well was targeted at the Permian Leman Sandstone Formation and successfully encountered a 200ft gas column in the reservoir.

A well test was performed after drilling, which achieved a flow rate of 50.2 million standard cubic feet of gas a day.

Tolmount prospect geology and reserves

Tolmount lies in the Lower Leman Sandstone Formation Play Fairway, which is characterised by Aeolian dunes, fluvial sands and local sabkha. The reservoir is linear with parallel faulting.

The prospect is estimated to contain up to one trillion cubic feet (bcf) of gas, with the main field containing more than 500 billion cubic feet and the Tolmount East and Tolmount Far East fields containing up to 450bcf.

Appraisal drilling at Tolmount prospect

Tolmount was appraised in 2013 with the drilling of the 42 / 28d-13 and 42 / 28d-13z appraisal wells.

The wells confirmed the presence of high-quality hydrocarbons in the Lower Leman Sandstone Formation reservoir.

Tolmount prospect development details

E.ON was already carrying out pre-development activities before transferring ownership of the prospect to Premier Oil, including drilling studies, offshore surveys and sub-surface activities to design the development plan for the field

Two development options were considered for the field: a standalone normally unmanned installation (NUI), and export to the shore. The second option included development through a 12in, 14km subsea tie-back to nearby infrastructure.

Premier Oil announced in January 2017 that the development of Tolmount was in advanced stages and the development concept of NUI was finalised.

The initial design of the platform includes 1,456t topsides and six slot facilities installed at a depth of 52m.

Subsea infrastructure will include three platform wells, one subsea well at Tolmount and another at Tolmount East.

The subsea well will be tied-back to the platform through a 4km-long, 8in-diameter infield pipeline along with a 4km-long, 3in-diameter methanol line and control umbilical.

A 45km-long, 18in-diameter pipeline along with a 3in-diameter methanol line will transport the produced hydrocarbons to an onshore terminal.

The initial phase of development will focus on the main Tolmount prospect, while subsequent phases will focus on the Tolmount East prospect.

Vertical / low angle deviated wells are planned to be drilled on Tolmount and Tolmount East reservoir sands, with consideration given regarding the complex geology of the reservoir.

"The prospect is estimated to contain up to one trillion cubic feet (bcf) of gas, with the main field containing more than 500 billion cubic feet."

Premier Oil plans to complete concept selection in the first quarter of 2017 with FEED contracts to be awarded thereafter.

Final investment decision is planned for late 2017.

Contractors involved

Fugro was contracted to perform geophysical, geotechnical and environmental surveys at the prospect, along with laboratory testing.

Petroxin carried out the field reservoir study for the prospect. Core Completions completed a peer review of the field development concept.