34/10-23

General

Well 34/10-23 was drilled on the Gullfaks Gamma structure, south-southeast of the Gullfaks Sør Field in the Northern North Sea. The objectives were to prove a significant hydrocarbon accumulation, with the Brent Group as the primary target and the Early Jurassic as secondary target. The first well on the structure, well 34/10-20, had the same targets as 34/10-23, but was terminated in the Early Cretaceous due to technical problems.

Operations and results

Wildcat well 34/10-23 was spudded with the semi-submersible installation Dyvi Delta on 6 May 1985 and drilled to TD at 4764 m into Sinemurian /possibly Rhaetian age sediments of the Statfjord Formation. The well took 28 days more than prognosed to drill. The main part of the extra time came in the 8 1/2" section, where among many other things, the BOP stack was pulled for inspection and repairs. A flow occurred at 4080 m just above top Brent Group, and approximately three days were used to bring the well under control. At 4409 m (4406.5 m TVD RKB) the deviation exceeded 5 deg. The last survey, at 4514 m (4510.8 m TVD RKB) m showed 9.1 deg deviation. If this deviation is extrapolated the vertical depth at TD is 4756.2, ca 8 m short of measured depth. The well was drilled with seawater and hi-vis pills down to 1252 m, with gypsum/lignosulphonate mud from 1252 to 3118 m, and with gypsum/lignosulphonate/lignite mud from 3118 m to 4525 m. At 4509 m the pipe stuck, and imco-spot/pipelax pills with 3% diesel were placed to free it. The final section from 4525 m to TD was drilled with a gel/lignosulphonate/lignite/Anco Resin mud system.

The pore pressure increased through the entire Cretaceous section and reached a maximum of 1.96 g/cc at the top of the Brent Group. Gas bearin