Designing Wells on a Lifecycle Basis: A Case Study from the Statfjord Field
Authors: Kostøl Petter , Nedregaard Rune 
Published: October 2008

Abstract
To achieve the goals of maximum lifecycle value of wells in the Stafjord field, continuous improvements in well design, completion and well intervention techniques have been made over the past 10 years, A major challenge has been to simultaneously optimise reservoir penetration, drilling and completion operations and later well interventions, This paper will review the development in the well design and completion techniques from the low angle (&lt;60°) wells with cemented liner and step down nipple profile completion to the horizontal and extended reach wells with monobore completions, This includes the lessons learned from the numerous workovers performed and how they lead to a change in completion design to ensure reliability, redundancy and optimised production. The impact of the well profile and how it should be optimised for future light interventions, i.e. coiled tubing, or wireline tractor will also be discussed. Finally, the advantages of working in multi-disciplinary teams from the early planning stage to completing the well will be addressed.

Introduction
The Statfjord Field is located in the North Sea on the Norway/UK boundary. The field is approximately 24 Ian long and 4 km wide and consists of the Upper and Lower Brent, Dunlin and Statfjord reservoirs. The Brent group is subdivided into five formations; Broom, Rannoch, Etive, Ness and Tarbert, while the Statfjord formation is subdivided into three members, Raude, Eiriksson and Nansen (Fig-1). The field has been developed with three concrete gravity base structure platforms, Each platform is a combined drilling and production platform with 42 slots divided between two wellhead areas, To date a total of around 156 wells have been drilled, 34 of which are sidetracks. The oil production from the field is currently at 65,000 std m3d with estimated total recoverable oil reserves of 650 × 106 std m3.

Drainage strategy.
The initial drainage strategy established for the field in 1977 was to develop the Brent reservoir with two lines of water injectors and two lines of oil producers. The Statfjord formation was developed with a line of gas injectors and a line of oil producers, This was based upon the following strategy:3Develop Upper and Lower Brent as separate reservoirs with water injection for pressure maintenanceHigh pressure gas flood of the Statfjord reservoir by injecting produced gas. In 1989 the drainage strategy was re-evaluated due to gas breakthrough in the Statfjord formation and water breakthrough in the Brent group. New lines of producers and injectors were established leading to a number of workovers and recompletions. To maintain production at a plateau level, the drainage lines were further optimised through the use of horizontal and ERD wells.3 Most new wells will be multitarget horizontal wells to penetrate unswept areas of the Statfjord and Brent main field reservoirs. Additional wells will also be drilled to develop the Dunlin reservoir and the very complex east flank of the field (Fig. 1&2). Some of these new wells will be drilled as multilateral wells to accelerate production of the remaining. reserves, Water alternating gas (WAG) injection in the Brent and tatfjord formation is also expected to give an improved sweep efficiency and increased recovery from the field. A number of old production wells have therefore been selected for WAG injection in the near future.