Intervention-Free Completion Suspension and Installation West of Shetland
Authors: Elliot G. , Makin G.D. , Ferguson S.E. , Moyes P.B. 
Published: October 2008

Foinaven is the first field to be developed in the deep water, West of Shetland. A sub-sea development, in 500 metres of water, Foinaven will have fourteen production wells. Most of these wells will have extended reach horizontal sections, completed with sand control screens. The challenging weather conditions mean that a number of the wells are suspended once the screens are in place, with the upper completions being run when the weather is less severe.

The suspension operations required plugs to be set using drill pipe or coiled tubing, from a semi-submersible rig capable of operating in the West of Shetland environment. The high day rates associated with these vessels mean that a major cost saving can be realised by eliminating these interventions.

This paper describes the planning, development, and field deployment of a deep-well barrier system which is installed as part of the sand control completion. The clear understanding of the well objectives led to the creation of a system whose benefits exceeded simply reducing the cost of the completion operations, namely:–deep well suspension without well intervention;–protection against fluid losses immediately after completing the reservoir section;–improved well productivity;–less operational risk during the completion phase;–reduced personnel exposure to harm due to less rig-up operations;–shorter well clean-up times;–ability to run the completions underbalance.

At the time of writing this paper, seven Foinaven wells have been completed using elements of this system.

Background

Located 180 km West of Shetland in the Atlantic Ocean, the Foinaven oilfield was discovered in 1992 by BP Exploration. It is located across Block Nos 204/19 and 204/24a in 500 metres water depth with a shallow reservoir of Palaeocene sands separated by shales at 2,000-3,000 metres TVD. A phased sub-sea development was sanctioned in late 1994 and first production occurred in November 97. As of January 1999 there is well capacity for circa 95,000 barrels per day. There is no present infrastructure in the area to support oilfield developments and the area is exposed to challenging weather conditions and unpredictable current regimes.

The sub-sea architecture is designed around 2 drill centres with a manifold on each feeding to a floating production, storage and off-loading vessel (Petrojahl Foinaven) designed for up to 140,000 dry barrels per day annual average oil production, with full water injection, gas compression and produced water handling facilities. A total of 22 reservoir penetrations are planned for the development of which 14 are oil producers, 7 are water injectors and 1 is a gas injector disposal well. Many of these wells are at deviations greater than 80 degrees, which require coiled tubing for deep well plugging operations. As of January 1999 there are 10 completed producers, 3 water injectors and 1 gas disposal well operating.

Foinaven represents a very high cost operating environment during the well installation phase, owing to the requirement for fourth generation semi-submersible drilling units for drilling and completion operations.

Well Design

Foinaven completion design is discussed in papers SPE 36909 and SPE 50678. The design philosophy was centred around longevity and serviceability for both installing the completion and for future re-entry. The use of 5-1/2" 17 lb/ft tubing in 13% Cr material - despite the relatively benign fluids - coupled with the minimised use of elastomeric seals adds to the life of the completion. No expansion joint is required due to the low reservoir temperature.