31/2-4 R

General

Well 31/2-4 is located some 10 km NNW of the discovery well 31/2-1 in a separate fault compartment near the crest of the Jurassic gas accumulation. This crestal area appeared also to coincide with the culminations of all deeper horizons that could be mapped. The well had two main objectives. The first was to appraise the Jurassic gas accumulation in the 31/2-1 Troll Discovery in a location with nearly maximum gross hydrocarbon column. The well should test lateral variations in this reservoir. The second major objective was to explore deeper Triassic/Palaeozoic Formations. The well would test possible hydrocarbon accumulations under the Late Jurassic Troll reservoir, explore possible pre-Jurassic source rocks, and provide geological ages for deep horizons and thus improve the regional geological setting.

TD of the well was proposed to be 5000 m below sea level or in Palaeozoic formations whatever came first.

Well 31/2-4 was originally drilled with Borgny Dolphin down to 815 m in 1980. The well was however suspended on 13 September 1980 due to a blow out on 34/10-10, where Borgny Dolphin's services were immediately required.

Operations and results

Well 31/2-4 was re-entered (31/2-4 R) with the semi-submersible installation Borgny Dolphin on 9 October 1980 and drilled to TD at 5035 m in Early Triassic sediments of the Hegre Group. The well bore deviation did not exceed 4 deg and at 4693 m, the deepest survey point reported, there was only 2 m difference between drilled and true vertical depth. The top hole down to 815 m had previously been drilled with seawater and hi-vis pills. From 815 m to 1280 m the well was drilled with KC/polymer mud, from 1280 m to 1951 m it was drilled wi