7/12-10

General

Well 7/12-10 was drilled a few km north of the Ula Field, on the north-eastern side of the Cod Terrace in the North Sea. The primary objective was to prove a commercial volume of oil in the Late Jurassic Ula formation for tie back to the Ula platform. Secondary reservoir potential existed in both the middle Jurassic and the uppermost part of the Triassic.

Operations and results

Wildcat well 7/12-10 was spudded with the semi-submersible installation Ross Isle on 8 July 1991 and drilled to TD at 3667 m in the Triassic Skagerrak Formation. After the 30" conductor was set at 167 m, a 17 1/2" pilot hole was drilled without riser to 950 m. No indications of shallow gas were observed. The well was drilled with seawater and hi-vis slugs down to 948 m, with Ancoquat water based mud from 948 m to 2730 m, and with KCl/polymer mud from 2730 m to TD. The inhibitive Ancoquat mud system was tested for the first time on the Norwegian Continental Shelf in this well.

Shows of oil were observed from 3010 m to 3100 m in the Tor limestone. Subsequent intermediate logs however indicated that there was not sufficient quantity of oil to warrant testing the limestone. Top Mandal formation was encountered at 3531 m drilled depth/3538 m logged depth. Top Ula formation was seen at 3620m drilled depth/3627 m logged depth. This was 56 m deeper than prognosed, and the formation was also ca 30 m thinner than expected. The Ula sandstone was found water bearing with no shows and it was not in pressure communication with the main Ula field. Top Triassic was identified at 3641.5 m logged depth. It was water bearing with no shows.

Two conventional cores were cut from 3616 m to 3628 m and 3628 m to 365