A Risk Sharing Alliance Breathes New Life into a Mature North Sea Field
Authors: Comrie Paul , Olsen Thomas 
Published: December 2007

The North West Hutton field is located in the UK sector of the North Sea. Because of the complexity of the layered and faulted Brent reservoir, recovery from the field is less than 25% of the approximately 500 million bbls STOIIP. Recent studies have suggested that wellbore damage may have contributed to the poor performance of the field, if this can be successfully treated then new technology may offer the potential to improve oil recovery. However, in a field where historical performance and prediction have been poor, a cautious approach to further investment is required.

The method adopted by the N.W. Hutton owners to investigate this opportunity resulted in a risk and reward sharing alliance with an Integrated Service Company. The programme included:–A 6 million well intervention programme aimed at accelerating production, culminating in tip screen-out (TSO) fracturing of the multi-layered Brent reservoir.–A 6–9 month reservoir study to identify and quantify the opportunities to increase ultimate recovery.

This paper presents the methodology used in the review process and the opportunities that were identified. It details how these opportunities were appraised for risk and value in an environment of inadequate reservoir understanding. Then how a team from both alliance partners planned an intervention campaign that minimised risk while pursuing the opportunity to realise value and stave off decommissioning.

History

The N.W. Hutton field is located in the Northern North Sea 300 miles North East of Aberdeen Scotland. Production started in April 1983 using a 27,000 tonne self contained steel platform in 450 feet of water. The field is operated by Amoco (UK) Exploration Company, and partners in the development are Petrobras UK Ltd, Mobil North Sea Ltd and Cieco UK Ltd. There are currently 18 production wells and 7 injection wells in service.

Reservoir Description. In common with many fields in this area, the reservoir is comprised of the Brent Group; a multilayered sequence deposited in shallow to marginal marine environments during the Middle Jurassic period.

The Brent Group records the northerly progradation of a major delta system. The initial phases of this sequence are marked by the sudden change in depositional style from the deep marine shales of the Lower Jurassic Dunlin Formation, to the marginal marine, coarse grained sands of the Broom Formation, see Figure 1. The Broom forms the basal part of the Brent Group, and is classically described as a "sub-littoral sand-sheet", portraying its general sheet-like geometry. The top of the Broom is capped by a thin but areally extensive shale, marking another sea-level change, which in this case was a slight rise. The resultant deposition of pro-delta clays mark the arrival of the Brent Delta "proper" in the N.W. Hutton area.

The Rannoch and Etive formations are deposits of the delta shoreface, and exhibit strong evidence of shallow marine deposition, in both their sedimentary structures and fossil assemblages. The geometry of these units is generally sheetlike as exposure to storm and wave processes on the delta front promoted sediment recycling. In broad terms these two formations record a coarsening-up sequence, produced as a result of the Brent Delta's continued northerly progradation The Ness formations record the shallowing from transitional marine/estuarine environments to delta-top fluvially dominated environments, with their associated channel geometry. The Ness sequence is split by a major flooding event, which deposited the Mid-Ness shale. This unit is fieldwide and forms a major vertical pressure barrier within the reservoir. The top of the Ness is marked by a similar flooding event, which effectively drowned the Brent Delta, ceasing its deposition in the N.W. Hutton area. P. 735^