24/9-1

General

Wildcat well 24/9-1 is located in the Vana Sub-basin in the Southern Viking Graben, about 11 km east of the border to British sector. The well was drilled to test a large, dip-closed structure located in the centre of the Viking Graben. The target was Late Jurassic sandstones. Two sandstone intervals were expected in the well, with a possible gross thickness of about 250-300 m. The prognosis was based on a correlation with the UK wells 16/7-1, 16/8-1 and the Norwegian well 15/3-1 which are situated in a similar position as 24/9-1 relative to the eastern boundary fault of the Viking Graben.

The well is Reference Well for the Svarte, Tryggvason, Kyrre, and Jorsalfare Formations.

Operations and results

Well 24/9-1 was spudded with the semi-submersible installation Ross Rig on 29 February 1976 and drilled to TD at 4907 m in the Late Jurassic Heather Formation. Below the 20" casing at 758.3 m the Formations drilled contained abundant sand and was drilled extremely fast. This created problems as the solids removal system on the rig was incapable of removing the sand as fast as it was drilled. In addition the polymer mud system chosen did not provide sufficient fluid loss control in the porous formation and large amounts of fluids were lost to the formation. From ca 2440 m to TD at 2752 m in the 17 1/2" hole low penetration rates was a problem. Of 126 rig days 20 % was counted as lost time, but the total time spent on the well was still less than the originally estimated 139 days. Problems related to tight/sloughing hole and down hole tool failure accounted for roughly half of the lost time. The well was drilled with seawater down to 256 m, with lime / lignosulphonate from 256 m to 3983 m, with Dextrid / Lignosulphonate from 3983 m to 4613 m, and with lime / lignosulphonate from 4613 m to TD.

Tertiary sandstones were encountered in the Grid Formation (1131 m to 1420 m), The Heimdal Formation (2202 m to 2497