2/8-9

General

Well 2/8-9 was drilled as an appraisal to help establish the commerciality of the southern North Sea Valhall Field, which had earlier been discovered by 2/8-6 and confirmed by 2/8-8. The primary objective of 2/8-9 was an evaluation of the Chalk reservoirs of Maastrichtian (Tor Formation) and pre-Maastrichtian (Hod Formation) age. The existence of hydrocarbons in the Early-Middle Miocene section creates a low velocity "bright spot" on the seismic, blanketing the crest of the Valhall structure and causing "dimming" or total loss of the top chalk section. The Valhall crest is therefore an area where the estimated thickness of the Tor section and the fault patterns are questionable. Only one previous well, 2/8-4, had been drilled in the vicinity some 2.5 km to the west. Well 2/8-4 was essentially a dry hole with only 1 m of Tor and tight but oil bearing Hod Formation.

Operations and results

Appraisal well 2/8-9 was spudded with the semi-submersible installation Sedco 135 G on 9 April 1976 and drilled to TD at 2703 m in the Early Cretaceous Rødby Formation. The well was drilled in a total of 30 days, without any major drilling problems. It was drilled with sea water and viscous mud down to 183 m, with seawater and gel from 183 m to 392 m, and with Drispac/lime/seawater mud from 392 m to TD.

Well 2/8-9 penetrated a normal of Quarternary -Tertiary section from surface to top of the Late Cretaceous. The Danian was found to have been removed by erosion. The well found the Chalk hydrocarbon bearing at 2524 m correlating 53.3 m high and 76.8 m low to wells 2/8-4 and 2/8-8, respectively. The Tor Formation was 5.5 m thick with high reservoir quality; porosity of up to 48% and a water saturation close to zero. The Ho