31/4-8

General

Well 31/4-8 was drilled on the Brage field in the North Sea to appraise the Statfjord Group oil discovery made in well 31/4-7. The Cook Formation of the Dunlin Group was the secondary target. This sandstone was found water bearing in well 31/4-7. Chances for finding hydrocarbons in well 31/4-9 were good since the formation would be penetrated in a structurally higher position. Prognosed TD was 2565 m or 50 m into Triassic rocks.

Operations and results

Appraisal well 31/4-8 was spudded with the semi-submersible installation Treasure Hunter on 11 May 1986 and drilled to TD at 2611 m in the Late Triassic Lunde Formation. The 13 3/8" casing got stuck at 1855 m. Diesel and spot fluid was pumped down the hole and the casing was worked free. The well was drilled with spud mud down to 900 m and with KCl/polymer mud from 900 m to TD. From 2030 m to ca 2400 m the KCl/polymer mud contained 1% diesel as a result of the problem with the stuck casing.

The well 31/4-8 encountered hydrocarbon bearing sandstones and siltstones in the Viking Group, the Statfjord Group and the Lunde Formation. Of these, only the Statfjord reservoir is produceable. The Viking Group consisted of a thin Draupne Formation from 2085 m to 2088 m and a 29 m thick Fensfjord Formation with poorly developed, generally very fine-fine sandstones grading into and interbedded with siltstones and claystones. Some residual hydrocarbons were encountered in the best sands. Net pay in the Fensfjord Formation was 0.61 m, water saturation was 59.9% and the average porosity was 25.5%. The Cook Formation was found water bearing without traces of shows. Average porosity in the Cook sand was 17.7%. The Statfjord Group had a gross oil column of 60 m from 2346 to the OWC at 2406 m.