The Schiehallion field lies in water depths of up to 500m and is situated on the United Kingdom continental shelf some 200km west of the Shetland Islands. The reservoir was discovered in 1993 and has been appraised with five wells. Development of the field was sanctioned in 1996, development drilling began later that year, and first oil is expected in 1998. During 1995, an extended well test (EWT) was conducted at well 204/20–5. This well was drilled horizontally through the T3la formation, which is composed of a sequence of channel sand bodies which have variable thickness and quality. The purpose of the EWT was to test the connectivity of the reservoir.
Initial transient analysis and reservoir simulation of the EWT suggested that the overall pressure behaviour was close to 'infinite acting', and no reservoir boundary effects were evident. This suggested that the well was accessing a larger connected volume than indicated by the seismically derived thickness and structure maps. Recognising that this interpretation was difficult to reconcile with the static data, the reservoir development plan positioned injector and producer locations in a manner robust to a range of reservoir connectivity outcomes.
This paper describes the objectives, conduct and initial interpretation of the EWT. It then goes on to describe how an alternative explanation of the EWT behaviour was derived by considering the potential uncertainties in the PVT data, and the results from the ongoing development well data acquisition programme. The revised interpretation requires less connected volume than the first, but the overall reservoir development philosophy of targeting producers into the main channel sands remains sound.
The Schiehallion field is one of the largest oil fields to be developed on the United Kingdom continental shelf in the past ten years with mean recoverable reserves of 340 MMstb. It is the second development west of Shetland following the Foinaven Field, and is located approximately 200km west of the Shetland Islands in water depths of 300 to 500m (Fig. 1).
The Schiehallion accumulation was discovered in late 1993 by well 204/20-1. Oil is trapped in submarine slope reservoir sands of Palaeocene age. The discovery well identified an oil-water contact at 2064m TVDss. P. 603^
