The giant Gullfaks Main Field comprises Statfjord, Cook and Brent Formations of Early to Middle Jurassic. The reservoir is complex due to large number of faults and extreme permeability contrast ranging from several Darcies in the Tarbert to milli-Darcy in the Cook. The highly productive sands are poorly consolidated causing sand production problem. Reservoir fluid in some of the areas contains high H2S. Uncertainties associated with structures, degree of communication, extreme contrast in reservoir properties and effective control of sand and H2S pose a great challenge for reservoir management.
Despite the challenges, the recovery factor on Gullfaks Main Field is high. A total of 335 Sm3 of oil has so far been produced, which amounts to an overall recovery factor of 56% (60% in the Brent Formation). This high recovery factor is attributed to effective reservoir management. The management strategy involves conservation of reservoir energy, implementation of simple and advanced strategies, systematic and sustained collection of data, and continuous application of improved recovery technologies.
Conservation of energy is achieved through water and gas injection. Simple and advanced strategies include selective perforation of wells, sand control, zone isolation, multi-target wells, controlled drainage through DIACS technology, through-tubing drilling, etc. Data collection involves 3D and 4D seismic, core and well log, RFT/MDT pressure, PLT, RST saturation, well completion, production and injection, etc. Improved recovery techniques, studied and some of them implemented, consist of infill-drilling, water and WAG injections, polymer assisted surfactant flooding, microbial injection, CO2 injection, etc. The current IOR initiatives are meant to extend the production life of the field to 2030 and thus meet the ambition of recovering 400 MSm3 of oil.
This paper summarizes the reservoir management challenges, techniques and technologies applied to evaluate and monitor the reservoir performance, and the strategies to enhance oil production.
Introduction
The Gullfaks field is currently owned 70% by StatoilHydro and 30% by Petoro. StatoilHydro is the operator. The field is located mostly in block 34/10 in the Norwegian sector of the North Sea (Fig. 1). The Gullfaks area with field, discoveries and prospects are shown in Fig. 2. The area includes nine production licenses. The red dotted line divides the area into two: Gullfaks main and Gullfaks satellites. Gullfaks satellites consist of Gullfaks Sør, Rimfaks, Gullveig, Skinfaks and Gulltopp. Gullfaks main represents the main reservoir containing 78% of the total in-place oil volumes and 88% of the recoverable reserves. This paper solely deals with reservoir management of the main field and hence no more discussion will be made on the satellites. Hereafter, if not stated otherwise, the main field will be referred to as the Gullfaks field.
Block 34/10 was awarded to Statoil, Norsk Hydro and Saga Petroleum in June 1978. The Gullfaks field was discovered in the same year by the first exploration well 34/10–1, which encountered a 160m oil column in the Brent Group and penetrated water-bearing Cook and Statfjord formations. Exploration wells 34/10–3 to 6 appraised the western part of the field and established the oil-water-contact (OWC) in the Brent Group. A deeper hydrocarbon system in the Cook formation was discovered by 34/10–7, whereas well 34/10–11 in the north-eastern part of the block showed a deeper OWC and a new oil-bearing system in the Statfjord formation. The appraisal phase of the main field ended in 1983, while the appraisal of the satellites continued up to 2002. More than 20 exploration and appraisal wells were drilled to assess the full potential of the field.
Based on structural understanding from seismic and well data, a 2-phase development plan was proposed 1. Following the commerciality report in late 1980, the authorities approved a field development plan (Phase-I) in October 1981 allowing the production of Brent Group reserves in the western part of the field from two concrete gravity base platforms. The field was set on production in December 1986 from five pre-drilled subsea wells connected to Gullfaks A-platform (GFA). Gullfaks B platform (GFB) was commissioned in February 1988. The authorities approved the development of the eastern part (Phase-II) in 1985 from a third concrete gravity base platform. Gullfaks C platform (GFC) was put on production in January 1990.
