7/12-9

General

Well 7/12-9 was the ninth appraisal well to be drilled on the Ula Field in the North Sea. The primary well objectives were to prove sufficient mobile oil in place in the SE-sector to support further development in this field area. In order to complete a full evaluation of the Ula Formation reservoir, the well was deepened with a 6" hole into the lower reservoir zones 3B, 4 and 5, including about 50 m of the Triassic. A secondary objective of this well was to be a possible future water injector should development of this area proceed.

Operations and results

Appraisal well 7/12-9 was spudded with the semi-submersible installation Ross Isle on 17 March 1990 and drilled to TD at 3820 m in the Triassic Skagerrak Formation. After the 30" conductor was set at 166 m, a 17 1/2" pilot hole was drilled without riser to 950 m. Shallow gas was observed between 677 - 680 m. No significant problem was reported from the operations. The well was drilled with spud mud down to 1008 m, with Petrofree mud down to 3684 m, and with Aker oil based mud from 3684 m to TD.

The Ula Formation came in at 3701 m, 20.5 m shallower than predicted. The thickness was 58.5 m, which was 20.5 m thinner than expected. The uppermost reservoir zones were absent in the well. Wire line logging, RFT pressure measurements fluid samples proved oil down to ca 3735 m, and that the lowermost Ula Formation and the Triassic section were water bearing. Virgin pressure conditions were confirmed below 3750 m (Ula reservoir zones 4 and 5).

One core was cut from 3689 m in the lowermost Farsund Formation to 3721.5 m in the middle of the Ula Formation reservoir zone 2B. The cored interval was 32.5m with 100% recove