31/4-5

General

Appraisal well 31/4-5 was drilled on the 31/4-3 Discovery on the Bjørgvin Arch in the northern North Sea. The primary objective was to test "Intra Heather Formation" sands on the "B structure" in a crestal position, up dip from the discovery well 31/4-3. The well was planned to delineate known hydrocarbons in the "Intra Heather Sand I" (Sognefjord Formation) and to test the prospect for additional up dip oil and gas reserves in the "Intra Heather Sand II" (Fensfjord Formation). The well was planned to enter the "Intra Heather Sand I" reservoir approximately 20 m below the OWC in order to provide evidence of the extent of the reservoir. The lower oil bearing zone ("Intra Heather Sand II") was prognosed approximately 20 m higher than in well 31/4-3 and significant amounts of up dip oil were thought to exist. The Brent, Dunlin and Statfjord Groups were secondary objectives, possibly prospective in an up dip position.

Operations and results

Wildcat well 31/4-5 was spudded with the semi-submersible installation Nortrym on 27 May 1981 and drilled to TD at 2930 m in the Triassic Hegre Group. The well was drilled without significant problems. It was drilled with seawater and hi-vis pills down to 926 m, with KCl mud from 926 m to 1966 m, and with a lignosulphonate/XC polymer/Drispac mud from 1966 m to TD.

The well penetrated the Sognefjord Formation at 2070 m and the Fensfjord Formation at 2104 m. The Sognefjord Formation was 8.5 m thick and water bearing, but with good oil shows. The Fensfjord Formation was hydrocarbon bearing with a net sand of 60 m and a net pay of 37 m. The oil/water contact was not possible to define exact