31/2-3

General

Well 31/2-3 was drilled in the Troll West area, approximately 8 km NNE of the Troll Discovery well 31/2-1. The well was drilled to appraise the Troll Discovery. It should evaluate reservoir parameters along the axis of maximum gross hydrocarbon column; prove maximum hydrocarbon reserves in the major northern fault block; confirm the significance of the seismic flatspot as a direct hydrocarbon indicator; further assess the significance of the oil shows found in 31/2-1; and evaluate the influence of earlier Kimmerian fault movements on reservoir characteristics.

Operations and results

Appraisal well 31/2-3 was spudded with the semi-submersible installation Borgny Dolphin on 28 March and drilled to TD at 2601 m in Late Triassic sediments in the Statfjord Formation. Drilling took 115 days. The reason for the long drilling period was safety inspection of the rig as a result of the "Alexander Kielland" accident and a strike amongst the Norwegian rig crew. The well was drilled with bentonite and seawater down to 816 m and with a gypsum/lignosulphonate mud from 816 m to TD.

No potential reservoir zones were encountered above top Jurassic. The well confirmed that the Late Jurassic sandstone reservoir encountered in wells 31/2-1 and 2 was well developed also in this more northerly part of the structure. A gross gas column of 189 m was penetrated with top at 1384 m. The uppermost 120 m was in a good clean sand (Sognefjord Formation) while the lower part of the gas column was in a micaceous and poor reservoir sand (Heather and Fensfjord Formations). Below the gas a 12 m thick oil zone was encountered, the same thickness and at the same level as in well 31/2-2. In this well however, the oil was in a very micaceous and poorly developed reservoir. The reservoir was uncomformably overlain by Palaeocene claystones, which thus act as an effective seal for the reservoir. Below the OWC at 1585 no moveable hydrocarbons were seen in th