35/11-2

General

Block 35/11 is situated at the boundary between the Horda Platform and the Viking Graben just north of the Troll Field and south of block 35/8 where two gas/condensate discoveries had been made. The work obligation for Block 35/11 included three exploration wells to be drilled to 4000 metres or to Triassic sediments, one of which must test an Early Cretaceous stratigraphic play. The first of these wells, 35/11-1, was drilled in 1984 to a depth of 3361 m in rocks of Triassic age. The primary objective of that well was to assess the hydrocarbon potential of the Middle to Upper Jurassic sands in the "A" structure. No hydrocarbons were present in any of the objectives although good quality Brent sands were encountered. The second well to be drilled on the block was 35/11-2. It was designed primarily to test an Early Cretaceous stratigraphic play, interpreted seismically as a fan development with a 130 km3 closure, and reservoir thickness of 270 m. A secondary objective was the "B" structure at Brent level.

Operations and results

Wildcat well 35/11-2 was spudded with the semi-submersible rig Treasure Scout on 20 July 1987 and drilled to TD at 4025 m in Early Jurassic rocks. The hole was drilled to setting depth for 20" casing without a riser. MWD was used, but the resistivity unit was destroyed after few meters. Conventional logs did not get past 760 m, and while attempting this; there was an intrusion of formation fluid into the hole. There was no sign of shallow gas, and heavier mud was used. The hole was opened to 26" without logging below 760 m. The reason for the problems around 760 m was probably washed-out zones. When preparing the setting of 20"casing, fluid was again flowing into the hole, and the mud weight was increased. Further dril