An independent method for estimating initial reservoir water saturation in the Gyda Field has been investigated. Further application of recent tracer technology was used in describing the initial fluid saturations in the reservoir. This was achieved by using a deuterium tracer technique to quantify aqueous mud-filtrate invasion to core and which enabled 'native' water saturation determinations to be made. The volume of extracted core-water was translated into native reservoir water saturations after correcting for mud-filtrate invasion, reservoir overburden, pressure and temperature effects.

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Summary Underbalanced coiled tubing drilling was identified as the best method to access additional reserves in the Ula oil field. The first operation involved deepening a live well through the existing production tubing with flow routed to the platform facilities. This was carried out simultaneously with other platform activity and the technical challenges, operational experience and drilling performance are described. The operation was ultimately successful and the new hole section presently contributes an additional 700 STB/D of oil to the field production. Introduction The Ula Field is situated some 280 km southwest of Stavanger in the Norwegian sector of the North Sea (Fig. 1). The field has been on production since 1986 and plateau oil production has been up to 150,000 STB/D. The field is now off plateau and current oil production is ca. 60,000 STB/D with increasing water production. In the search for opportunities to extend the field life, coiled tubing drilling (CTD) was considered a viable option. Development of the technique began in 1991, Ref. 1 and it was recognised that the method could be used to economically increase production from existing wellbores In addition, CTD can be carried out underbalanced on a live well, giving improved well productivity and drilling performance. The Ula Jurassic sandstone reservoir includes a low permeability zone at the top. This horizon contains a relatively large volume of unswept oil and is one of the main drivers for BP Norge to develop the use of CTD. This will provide a means of accessing the upper horizon with horizontal drainholes without having to pull the production tubing. The underlying Triassic formation contains potential additional reserves in poorer quality formation. The Triassic reservoir was tested at 2300 STB/D during field appraisal. These Triassic reserves were the target of the first operation which involved deepening an existing well, underbalanced through the production tubing. Ula well A02 was selected as the ideal candidate since it was positioned over the best quality Triassic reservoir and had watered out in the Jurassic. The objectives of the first Ula CTD operation were:Obtain maximum production from the Triassic reservoir by cleaning out the existing well and deepening it by 100 metres, using underbalanced coiled tubing drilling.Evaluate the Triassic reservoir connectivity and oil-in-place.Prove up CTD technology for economical use in other Ula candidate wells. Technical Challenges Many technical and operational challenges were faced in planning the underbalanced coiled tubing drilling operation both due to the complex nature of the well conditions and the limited space on the platform.