The Forties Field - 20 Years Young

Abstract

Summary This paper describes how the development of the Forties field has evolved over the past 20 years; the current issues and activities; and the direction for the future. Introduction In October 1970, BP' s semi-submersible drilling rig SeaQuest struck oil in Upper Palaeocene (Tertiary) sandstones of UKCS Block 21/10 (Figure 1). Four appraisal wells were drilled in 1970 and 1971, which delineated a large oil reservoir at a depth of 2135 metres, covering an area of 93 km2. The maximum thickness of the oil-bearing sandstone was estimated at 187 metres. Reserves were calculated to be 1800 mmstb from an oil-in-place of 4600 mmstb, and production was projected to continue until the year 2000. In 1971 Shell/Essodrilled a successful well, 22/6-1, which demonstrated the south-eastern extension of the field into Block 22/6. The field is a four way dip closed anticline which overlies the Forties-Montrose Ridge and is situated close to the junction of the Central, South Viking and Witch Ground Grabens (Figure 2). The reservoir occurs in thick Upper Palaeocene sandstones deposited as a major sand-rich submarine fan sequence. The field is underlain by a large active aquifer. The development plan evolved during 1972-75. The first major oil field development in the North Sea, it comprised four fixed steel drilling/production platforms, installed in Block 21/10 in 1974 and 1975, with a total capacity for108 wells, a production system capable of handling 500 mbd of oil, later increased to 600 mbd, with 100 mbd of produced water, and a sea water injection system with a capacity of 600 mbd. A schematic diagram of the production system is shown in Figure 3. Development drilling began in June 1975 and production started on 12th September 1975. Plateau production of 500 mbd was reached in1978 and lasted until 1981. A fifth platform, smaller and without processing facilities, was added over the south-eastern extension (South East Forties -SEF) during 1985, with production beginning in March 1987. For more detail on the reservoir reasoning behind the original development scheme see Reference1. Forties is currently producing 111 mbd, the field water cut has only recently passed 60% and 90% of the reserves have been produced. Reservoir Management Reservoir performance has been characterised by excellent vertical sweep through clean sands by the underlying, very mobile aquifer, supported by peripheral sea water injection. There are sufficient permeability restrictions between the three parts of the field, namely SEF, the Main Sand and the Charlie Sand, to cause significant pressure differences between them (Figure 4). Other localised isolated regions have been identified in the FD area during recent infill drilling. The management of the reservoir has been based on treating the three regions separately in terms of pressure support requirements. The primary thrust of reservoir management, common to all areas, has been to track the movement of water within the field to maximise vertical and areal sweep. This has been achieved by performing water shut-off when advantageous to oil flow rate, applying artificial lift when possible, and targeting infill wells at poorly swept areas. With the onset of sea water breakthrough in production wells, bariumsulphate scale deposition has increased in down-hole tubulars and surface facilities (Reference 2). Extended life scale inhibitor squeeze treatments into the formation have been required to inhibit scale deposition. Calcium carbonatescale has also been a problem in gas lifted wells. Generally the deposition of scale is inhibited, but in some cases inhibitor deployment has not been successful. Acid (for carbonate) or chelator treatments have had limited success, while in severe cases wells have been successfully restored to production by milling with coiled tubing (Reference 3).