The Gullfaks Field: Creating Value by means of a Multidisciplinary Reservoir Management Approach

Abstract

Abstract Production from the structurally complex Gullfaks Field in the northern North Sea is now on decline, reduced by a third from the peak year 1994, when oil production exceeded 30 MSm3. Recoverable reserves are currently estimated at 319 MSm3, of which approximately 240 MSm3 have been produced to date. Approximately 80% of the recoverable reserves lie in the middle Jurassic Brent Group. An important goal for the Gullfaks license is to increase the recoverable reserves by 47 MSm3, of which the Brent Group isexpected to yield 30 MSm3 through a recently launched technology programme. Locating the remaining oil is an important part of this programme. Time lapse seismic data are considered to be a key element in this process, and a multidisciplinary team is now in the process of integrating 4D seismic data with other available reservoir and production information, in order to improve the static, as well as the dynamic, reservoir description. The project is also a testing ground for several recent R&D results in the field of geological modelling, reservoir and production engineering, assisted by modern visualization techniques to enhance communication of technical information and ideas. An example of how the multidisciplinary approach has improved our reservoir and drainage understanding will be demonstrated. This improved understanding provides a better basis for quantifying the field's IOR potential. Introduction Reservoir Description. The Gullfaks Field is located in the Norwegian sector of the North Sea, in block 34/10, approximately 175 km northwest of Bergen, where the Gullfaks Business Unit is based, on the west coast of Norway, see Figure 1. The reservoir units are sandstones of early and middle Jurassic age, around 2000m sub sea and measure several hundred meters thick (Ref. 1). The uppermost Brent sequence contains roughly 80% of the reserves, with the deeper Cook and Statfjord formations contributing the remainder. Reservoir quality is generally very high, with permeability ranging from few tens of mD to several Darcys depending on layer and location. Figure 2 shows a cross section indicating the quality and variability of the reservoirs. The reservoirs are over pressured, with an initial pressure of 310 bar at datum depth of 1850 m below mean sea level, and a temperature of 70 degrees C. The oil is undersaturated, with a saturation pressure of approximately 245 bar, depending on formation depth and location. The GOR ranges between 90 and 180 Sm3/Sm3, with stock tank oil gravity around 860 kg/m3. Structurally, the field can be divided into three regions(Ref. 2). The so-called 'Domino Area' with rotated fault blocks in the west, and a Horst area in the east. In between is a complex 'Adaptation Zone', characterized by folding structures. The north-south faults that divide up the field have throw up to 300 meters. In the western part the faults slope typically around 28 degrees downward to the east, whereas in the eastern horst they slope 60–65 degrees downwards to the west. The field is further cut by smaller faults, with throws of zero to few tens of meters, both in the dominant north-south as well as east-west direction. Many of these lesser faults have slopes of 50–80 degrees.