Optimising Performance of Mature Reservoir: An Innovative Use of Coiled Tubing Drilling Technology to Tap Unswept Reserves, Alwyn North UKCS

Abstract

This paper describes the rationale, the planning and the preliminary results of the first Coiled Tubing Drilling (CTD) experience for TotalFina. The objective was to enhance production from a high water-cut oil well producing under unstable conditions and located in an area of possible upsides. This well was drilled in 1990 on the edge of a fault-scarp degradation complex in the Brent East panel of the Alwyn Field. The geological uncertainties in this complex area needed to be reduced and it was necessary to reassess seismic, core and reservoir data to progress the current knowledge base. Analytical calculations supported 3D reservoir modelling techniques to constrain some of the uncertainties. Drilled in 1999, well 3/9a-N30Y was the first coiled tubing sidetrack from a producing well on Alwyn. The motherbore, 3/9a-N30, was still producing around 380 stb/d, although production had recently become intermittent. The well path presented a challenging trajectory due to the location of the unswept area in relation to the direction of the motherbore. After drilling 240m a series of operational issues arose, necessitating the drilling of the world's deepest open hole sidetrack with coiled tubing, but ultimately the BHA was left in hole. The coiled tubing was then utilised as an uncemented liner, perforations using slim guns were added along the liner, and the well was brought onto production. The CTD drain alone produced at an initial rate of 2900 stb/d, which stabilised out at 1200 stb/d after an initial rise in water cut. The successful outcome of 3/9a-N30, which was drilled in a complex offshore environment, confirmed the viability of CTD as a technique for accessing smaller pockets of remaining oil, whilst some of the operational difficulties encountered highlighted the need to simplify future trajectories to maximize the chance of success. With the possibility of drilling future CTD wells, further increases in productivity and reserves will be achieved at a significantly lower cost compared to conventional drilling. Introduction and Background Field Discovery, Appraisal and Development. The Alwyn North field (blocks 3/4a and 3/9a) is owned at 100% and operated by TotalFinaElf Exploration UK (Fig.1). It is situated in the South-eastern part of the East Shetland Basin in the UK North Sea, approximately 140km east of the Shetland Isles. The field was discovered in 1975, with gas and condensate in the Upper Triassic and Lower Jurassic Statfjord Formations, and undersaturated oil in the Middle Jurassic Brent Group. These reservoirs are separated by the Lower Jurassic Dunlin Group shales. The Brent reservoir pressure was initially 453bar at 3231mTVDMSL, and the Statfjord 496bar at 3580mTVDMSL. The field is divided into distinct compartments by a major NNW-SSE trending fault and by secondary ENE-WSW transverse faults. The major "spinal" fault separates the field into East and Western panels, while the transverse faults subdivide the western panels into North, North-West, Central-West, and South-West panels. Oil production commenced in November 1987 from the Brent reservoirs, followed by gas and condensate from the Statfjord reservoir two months later. Field Discovery, Appraisal and Development. The Alwyn North field (blocks 3/4a and 3/9a) is owned at 100% and operated by TotalFinaElf Exploration UK (Fig.1). It is situated in the South-eastern part of the East Shetland Basin in the UK North Sea, approximately 140km east of the Shetland Isles. The field was discovered in 1975, with gas and condensate in the Upper Triassic and Lower Jurassic Statfjord Formations, and undersaturated oil in the Middle Jurassic Brent Group. These reservoirs are separated by the Lower Jurassic Dunlin Group shales. The Brent reservoir pressure was initially 453bar at 3231mTVDMSL, and the Statfjord 496bar at 3580mTVDMSL. The field is divided into distinct compartments by a major NNW-SSE trending fault and by secondary ENE-WSW transverse faults. The major "spinal" fault separates the field into East and Western panels, while the transverse faults subdivide the western panels into North, North-West, Central-West, and South-West panels. Oil production commenced in November 1987 from the Brent reservoirs, followed by gas and condensate from the Statfjord reservoir two months later.