7/12-6

General

Well 7/12-6 was drilled as an appraisal well on the Ula Field in the southern North Sea. The objective was to determine sand distribution in northern part of the Ula field, and production and injection properties of reservoir and aquifer.

Operations and results

Appraisal well 7/12-6 was spudded with the semi-submersible installation Sedco 707 on 10 April 1981 and drilled to TD at 3700 m in the Triassic Skagerrak Formation. Some hole problems and excessive cavings due to underbalanced drilling occurred in the 12 1/4" section between 2633 and 3353 m, otherwise the well was drilled without significant technical problems or delay. The well was drilled with gel/seawater and fluid loss additives down to 480 m and with gypsum/lignosulphonate and fluid loss additives from 480 m to TD.

The Ula Formation sandstones were penetrated at 3406 m. The sandstones were 115 m thick, very fine to fine grained with porosities from 10 to 25% and permabilities from 1 to 2000 mD. The reservoir was oil bearing throughout, no OWC was established. An unexpected Triassic reservoir of good quality was found below the Ula Formation. The well drilled 179 m into it without reaching the OWC. The reservoir consisted of various types of micaceous sandstones with porosities from 4 to 23% and permabilities from 0.03 to 2600 mD. The first hydrocarbon indication was recorded as a weak oil show at 2440 in siltstone. Further shows were recorded in the Late Cretaceous between 3010 m and 3060 m, however the logs did not indicate any significant mobile hydrocarbon. Hydrocarbon shows were recorded throughout the Upper Jurassic Ula Formation between 3407 m and 3521 m. Patchy shows were recorded in the Triassic interval between 3521 m and TD.

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