Locating the Remaining Oil in the Brent Field Prior to Depressurisation

Abstract As the Brent Field approaches the 25th anniversary of its discovery and the 20th anniversary of first production, three of the four platforms are being completely refurbished in preparation for a major change in the operation of the field. Starting in 1997, the reservoir pressure will be gradually reduced in order to recover the large volumes of gas dissolved in the residual and bypassed oil. Initially depressurisation will be accomplished by stopping water injection, but, from the year 2000, it will be enhanced by using Electric Submersible Pumps (ESPs) to back produce aquifer water. Despite the lower wellhead pressures after refurbishment, the declining reservoir pressure will cause oil wells to loose lift from 2000, and by 2004 oil production will virtually cease. This sets the time frame for optimising production of the remaining oil reserves (360 MMstb at 1.1.96). A major project has been undertaken to Locate The Remaining Oil (LTRO) in unswept and bypassed areas of the Brent Field. This involves a multi-disciplinary effort encompassing petroleum engineering, well engineering and research. A detailed geological assessment, followed by inventorisation and quantification of the remaining oil pools. formed the basis of the project. Once drilling targets were identified, the integrated team approach was used to optimise the well paths, usually requiring a compromise between the desired and the practical approach. As the Brent Field matures, the development of the remaining reserves becomes more challenging. Bypassed accumulations tend to be smaller than traditional development targets. have a greater range in volume uncertainty and are generally more difficult to access. The latest developments in horizontal and extended reach drilling have been used to drill complicated well paths, connecting up targets in several reservoir layers at minimum cost. The LTRO project in Brent is well advanced and reserves of over 70 MMstb have been identified. The first LTRO wells have been drilled and the successes to date have proved the concepts which were applied to identify and to economically develop the remaining oil. Introduction The Brent Field was discovered in July 1971 and brought on stream just over five years later on 11th November 1976. In the year of the 25th anniversary of discovery and the 20th anniversary of first production, the Brent Field is undergoing major surgery in preparation for a change of life. Located 186 km north-east of Lerwick, Shetlands Islands, the Brent Field has a STOIIP of 3.8 MMMstb and a GIIP of 7.5 Tscf in two reservoirs; Brent and Statfjord. The field consists of a westerly dipping monocline, the West Flank, and, at both reservoir levels, the slumps, which are a zone of tilted and rotated blocks created by crestal failure when the structure began to form (Fig. 1). The field reached a maximum annual average oil production of 410 Mb/d in 1984. Since the mid-80s, oil production has been declining but, because of the high solution GOR (ranging from 250 to 980 v/v) substantial gas reserves still remain, dissolved in the residual and bypassed oil. Since the outset of field development there have been conceptual ideas on how to recover these large gas volumes and give this giant oil field a second lease of life. As oil production started to decline, IOR/EOR options were systematically evaluated. In 1992 the decision was taken to go ahead with the largest ever offshore brownfield development. The ageing facilities on three of the four Brent platforms are being completely refurbished in order to optimise recovery by lowering the first stage separator pressure, to implement Cullen safety upgrades, to extend the platform life and to reduce operating costs. Each platform is being shut down for up to a year while field production is maintained from the other platforms. The justification for depressurisation has been extensively reported. P. 171