7/12-11

General

Well 7/12-11 was designed to test 7/12-JU6 prospect, a Late Jurassic Ula Formation sand in a structural prospect some 7 km east of the Ula Field. Well 7/12-11 was located close to the edge of the perceived Late Jurassic Ula Trend fairway.

The prospect was defined as a structural trap with fault closure to the east and north, and dip closure to the southwest. Lateral seal to the east was thought to be provided by cross-fault seal of Late Jurassic reservoir against Triassic shales and siltstones. The key element of risk was trap effectiveness, as the prospect relied on a shattered sub-seismic fault zone in the northeastern corner. The results might have implications for further prospectivity in the licence.

The primary objective for the 7/12-11 well was to prove a volume of oil that was commercial as a tieback development to the Ula Platform. Secondary objectives were the uppermost part of the Triassic rocks and the Middle Jurassic Bryne Formation.

Operations and results

Wildcat well 7/12-11 was spudded with the semi-submersible installation Ross Isle on 31 August 1991 and drilled to TD at 3865 m in the Triassic Skagerrak Formation. A 9 7/8" pilot hole was drilled to 950 m, and some shallow gas was detected by ROV sonar and observation of some surface bubbles at 518 m. The 17 1/2" hole was drilled with the experimental mud system RCS/DF+. In the 12 1/4" hole this mud was gradually diluted and displaced to a KCl/IOBOND mud. Drilling proceeded without significant problems to TD.

The Ula sandstones came in at 3787.5 m, 39.5 m deeper than prognosed. It was only 12.5 m thick which was 78 m thinner than prognosed. The Bryne Formation was absent. Very weak hydrocarbon fluorescence was observed in sand stringers of the Lista Formation. Minor gas shows were observed in the Mandal Formation. Occasional oil stained grains with very weak cut fluorescence were seen in the cuttings of the Ula Formation.