31/6-1

General

The primary objectives of the wildcat 31/6-1 was to test Late and Middle Jurassic sandstones on the main culmination of the Troll east structure in the western part of block 31/6. Secondary objectives were Middle to Early Jurassic and Triassic sandstones and a possible sandstone reservoir of pre-Triassic age within a set of tilted fault blocks buried by Triassic strata. The Troll east structure is a large tilted fault block containing Jurassic and older strata. The closure

at Sognefjord Formation level is delineated by a flatspot covering most of the 31/6 block. The flatspot was expected to be a gas/fluid contact evident on the seismic and penetrated by nine wells in the Troll West area to the NNW in block 31/2. Planned TD of the well was 3800 m into rocks of pre-Triassic (? Devonian age).

Operations and results

Wildcat well 31/6-1 was spudded with the semi-submersible installation Nortrym on 15 July 1983 and drilled to TD at 4070 m in pre-Devonian basement rock. Problems with getting logging tools past the 30" casing shoe occurred due to gumbo problems. Further drilling went forth without major problems. The well was drilled with spud mud down to 425 m, with seawater / prehydrated gel and hi-vis pills from 425 m to 749 m, and with KCl/polymer/PHPA mud from 749 m to TD

The total net sand in the Viking Group (1352-1805 m) found to be 424.25 m giving a net/gross ratio of 0.94 m with an average porosity of 27.5%. The Sognefjord Formation (1352-1488 m), the Heather Formation Unit B, (1488-1517.5 m) and the upper part of the Fensfjord Formation were found gas bearing from 1352 to 1571 m where the gas/oil contact was found. Below this a thin (3 m) oil zone was present with an oil/water contact at approximately 1574 m. The hydrocarbon-bearing reservoir consisted of very fine to fine grained sandstones, occasionally medium to coarse. They were generally friable to loose with a few tight calcite cemented stringers. The