7/8-4

General

Wildcat well 7/8-4 was drilled on the northern flank of the Central Graben. The prospect was a structural/stratigraphic trap situated on the north flank of a high standing Triassic fault block. The objective was to test the hydrocarbon potential of a prognosed Late Jurassic sand body at 3840 m. Isolated sands within the Triassic were also postulated as hydrocarbon traps and potential secondary targets.

Operations and results

Well 7/8-4 was spudded with the semi-submersible installation Nortrym on 11 December 1984 and drilled to TD at 4400 m in Early Triassic sandstones and shales of the Smith Bank Formation. The overall operation took 72 days from spud. Problems with gumbo and keeping the hole clean were experienced. A weighted KCl polymer mud was used to keep the hole open and prevent shale spalling. From 4192 m onwards, problems of tight hole was experienced with the drill string becoming permanently stuck at 4400 m. Differential sticking was thought to be the problems, so the mud weight was reduced to compensate for this. While attempting to free the pipe the well started to flow but this was quickly stopped by raising the mud weight. While pulling out of the hole the drill pipe got stuck with the bit at 4382 m. The pipe was unscrewed at 4250 m and during fishing operations two more sections of pipe were lost in the hole, one with the top at 4192 m, the other at 4183 m. The operator then decided on a sidetrack, and the hole was cemented back to 4130 m and a sidetrack was kicked off from 4140 m. Eleven days were lost due to the plug back and sidetrack operation. The well was drilled with seawater down to 619 m and with KCl polymer mud from 619 m to TD.

Two cores were cut from 3825.5 m to 3859 m in what was believed to be the Ula sandstone of the Late Jurassic, the primary objective. Subsequent analysis revealed the Lower Cretaceous to unconformably overlie Middle Triassic formations with no Jurassic rocks present at this