2/5-6

General

Well 2/5-6 was drilled on the Siv structure, only 800 m northeast of the 2/5-4 discovery well. The Siv structure is a north-south trending anticline. The primary objective of well 2/5-6 was to establish the possibility of Jurassic sands being present along the west side of the Mandal High and to evaluate the hydrocarbon potential of these sands on the Siv structure. Sands of this age had previously been encountered in the BP block 7/12 further to the north. In addition, the well was planned to appraise the Chalk reservoirs found to be oil-bearing by Amoco well 2/5-4.

Operations and results

Wildcat well 2/5-6 was spudded with the semi-submersible installation Norskald on 14 May 1978 and drilled to TD at 4132 m in the Triassic Skagerrak Formation. A bentonite slurry used while drilling the first two intervals, 30" casing was set at 166 m and 20" casing at 540 m. The 17 1/2" hole was drilled using a seawater native solids mud. Problems were encountered as casing point was approached, sloughing shale and tight hole proved troublesome and the mud weight was increased to 10.6 lb/gal before running 13 3/8" casing to 1852 m. The 12 1/4" hole was drilled initially with a gypsum CMC mud. Tight hole was a significant problem until a depth of approximately 2900 m. At 2950 m the mud was converted to a dispersed lignosulphonate system to obtain more stable rheological properties. This mud was used until a depth of 3967 m. Tight hole was experienced on trips at a depth of 2000 m. The pipe was stuck at 3640 m and a fish was left in the hole. The 9 5/8" casing was set above the fish at a depth of 3560 m and a technical sidetrack was performed. The 8 1/2" hole was drilled using the same mud to a depth of 3967 m. At this point, in a Middle Jurassic sand, a salt water flow high in magnesium and calcium delayed operations considerably. The mud was converted to a brine polymer type system and the mud weight was raised to