The Alwyn North Triassic Development: Developing a Large Gas-Condensate Reservoir While Making Maximum Use of Existing Facilities

Abstract

Abstract The Alwyn North Triassic reservoir is a deep gas condensate accumulation underlying the currently producing Alwyn North Brent and Statfjord reservoirs in quadrant 3 of the Northern North Sea. Two early exploration wells in quadrant 3 had encountered Triassic gas-bearing sands with only the second one, a subsea well, tested but at a non economical rate. However, in 1995 a third well, drilled from the Alwyn North platform, successfully tested at 1.3 Mscm/d of gas and 3,300 stb/d of condensate. A platform extended well test (EWT) was then performed by simply connecting the well to the Alwyn North gas manifold and gas processing train to obtain long term production performance. Following encouraging results from this well, an extensive appraisal programme was embarked on consisting of drilling three further platform wells and connecting them to production to provide information on long term reservoir performance. In addition, sand frac jobs were performed on two of the wells to assess productivity improvement, access to additional reserves and technical feasibility. Successful results from the appraisal programme were then used as the basis for a full field development plan for the Triassic, the main element of which is the drilling of additional wells to optimise recovery of the Triassic fluids. The new platform wells will be challenging (total drilled lengths of up to 7800m with depleted sands above Triassic) and an upgrade of the Alwyn North drilling facilities is required. The first platform development well was spudded in April 1999 and peak Triassic production rate is anticipated to be around 5 Mscm/d. Given the relatively poor reservoir quality and well productivity and encouraging results from sand frac jobs performed during appraisal, improved recovery techniques of hydraulic fracturing and multi-lateral wells are being considered in the development plan. All Triassic wells will be produced via the existing Alwyn North production facilities which require only minor modifications. It is also planned to debottleneck the high pressure gas train on the Alwyn North platform to ensure that the significant volumes of additional gas brought by the Triassic development can be produced without facilities constraint. This paper describes how information from the extensive appraisal programme was used to define and optimise the full field development plan whilst making full use of the existing Alwyn North facilities, and upgrading where necessary. Integrated field development aspects (reservoir, drilling and facilities) are presented in this paper.