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Abstract

The Braefields,operated by Marathon Oil (UK) Limited, are situated in Block 16/7 in the UK, NorthSea. The first of the Brae fields was discovered in 1975 with the testing of a rich gas condensate accumulation, later named North Brae. Further drilling led to the discovery of three oil fields, West, Central and South Brae, and appraisal drilling concentrated on South Brae.

After four wells had been drilled to appraise South Brae a fan delta sedimentary model was developed to explain the geology. The South Brae plan of development was based on this model. Early development drilling in 1983/4 came up with major surprises and indicated that the reservoir system was more complex than originally assumed. As the planned production profile appeared difficult to achieve a combined study involving geophysical, geological and reservoir engineering was instigated to understand the geological model and achieve optimal well locations. The available 2D seismic although lacking in quality and resolution was an essential part of this study. After a new geological model had been developed further development wells were more successful. This enabled the field to achieve predicted production rates and enabled an effective waterflood to be put in place to improve recovery.

By 1982 North Brae had been appraised and the development plan for this reservoir was assessed using a compositional reservoir simulator. Gas cycling with partial pressure maintenance was selected as the best development option. Prior to the platform being installed in 1987 a 3D seismic programme was completed. With experience gained from South Brae and integrating geological and geophysical interpretations a refined geological model was rapidly and efficiently developed. Subsequent development drilling was highly successful and the field produced at plateau rates earlier than expected. Reservoir simulation incorporating the complex geology confirmed that gas cycling was indeed the best option and that excellent recoveries could be expected.

The multi-disciplinary study on South Brae successfully achieved its objectives and using the experience gained from this study enabled the development of North Brae to be rapidly optimised with excellent results.

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