2/5-7

General

Wildcat well 2/5-7 is located ca 9 km North of the Tor Field in the southern Norwegian North Sea. The primary objective was to test Late Jurassic and Middle Jurassic/Triassic sandstones on top of a gentle salt-induced dome structure. Secondary objectives were Late Cretaceous chalk and Paleocene sandstones.

Operations and results

Well was spudded with the jack-up installation Neddrill Trigon on 11 August 1983 and drilled to TD at 4531 m in the Triassic Skagerrak Formation. During drilling of the 17 1/2" hole several tight spots and problems with cavings were experienced. While tripping out of the hole at 3956 m, the drill string parted and 193 m of the bottom hole assembly was left in the hole. The fish was recovered after some problems due to tight hole conditions. When pressure testing the 9 5/8" casing the float collar sheared out causing the casing to jump. After re-landing, the casing was found to have dropped a couple of inches. Repair of the casing took 35 days extra rig time. A drilling break occurred at 4161 m where a 1.5 m thick marl was encountered. Maximum gas reading at bottoms-up was 30 %, and slight oil shows were observed in the mud. A kick was taken at 4519 m with an influx of 2.5 m3 water. The well was drilled with seawater and gel down to 862 m, with KCl/polymer mud from 862 m to 2254 m, with lignosulphonate mud from 2254 m to 3529 m, and with gel/lignosulphonate mud from 3529 m to TD.

Only traces of sandstone were encountered in the Late Paleocene, and there were no indications of hydrocarbons. Porous chalk was encountered in the Ekofisk Formation at 3204 m and in the Tor Formation at 3289 m. Both formations tested oil, but the oil flow in the Ekofisk test was interpreted to originate from the Tor Formation (see below). The matrix permeability in these zones is low, but the results from the production tests indicated that fracture permeability is significant. No hydrocarbons were found in the L