2/8-6

General

Well 2/8-6 was drilled to test the chalk approximately on the crest of the Valhall structure in the southern North Sea. The Valhall structure was mapped as a large northwest-southeast trending anticline at the Tertiary level covering approximately 65 km2 with a vertical relief of ca 490 m. The crestal portion of the Valhall structure, ca 26 km2 in area, was blanketed by a low velocity "bright spot" zone in the overlying Tertiary (Miocene). Such "bright spot" had been observed also over the Hod discovery, the Ekofisk and the Eldfisk. The 2/8-6 and 2/11-2 wells had confirmed that these "bright spot" anomalies were caused by oil and gas present in the younger Tertiary beds, creating extremely low velocities.

Operations and results

Wildcat well 2/8-6 was spudded with the semi-submersible installation Waage Drill I on 7 April 1975 and drilled to TD at 2669 m in the Early Cretaceous Sola Formation. The well was drilled in 38 days. Of these a total of 9 days were lost due to drilling problems; 2 days cementing between the 20 and 13 3/8-inch casing to stop gas entry into the annulus, 1.5 days to repair BOP stack connector, and 5.5 days to combat gas kick and lost circulation while drilling at 2438 m. The well was drilled with seawater and hi-vis mud down to 155 m, with shale-Trol gel mud from 155 m to 1890 m, and with lignosulphonate mud from 1890 m to TD.

The chalk was encountered at 2463 m. Good oil shows were encountered in the intervals 2465 - 2472 m and 2526 - 2551 m with minor oil saturations occurring between the above intervals and down to approximately 2560 m resulting in a gross oil column thickness of 110 m. Log derived porosity in the oil column ranged between 27 and 43%, averaging 33 %. Cor