35/7-1 S

General

Well 35/7-1 S was drilled on the Apollon prospect on the Marflo Spur, west of the Vega Field in the Northern North Sea. The main objective for the well was to test the hydrocarbon potential in Tarbert, Ness and Etive formations sandstones of the Middle Jurassic Brent Group. The secondary objective was to prove hydrocarbons in the Lower Jurassic Cook Formation within the Dunlin Group. The well was designed with an S-shaped path to avoid a possible gas charged sand.

Operations and results

Well 35/7-1 S was spudded with the semi-submersible installation Aker Barents on 13 May 2011. A 9-7/8" pilot hole was drilled below the 30" casing shoe to section TD at 1414 m to check for shallow gas. No shallow gas was recorded. Severe mud losses were experienced when drilling the 17-1/2" section in the transition from Balder to Sele formations at 1855 m. Decision was made to plug back the existing 17-1/2" section and sidetrack the well from below the 20" casing shoe, at 1425 m. When the 12 1/4" section was drilled to just above prognosed section TD, an influx of 400 l was reported and the well shut-in. The well kill with 10 SPM was performed and was successful. This was however an extremely slow rate to kill such a long well and required an extensive amount of time. After the well kill, the 9 7/8" casing was run and cemented and the 35/7-1 ST2 technical sidetrack was drilled on without significant problems to TD at 4825 in the Early Jurassic Amundsen Formation. The well was drilled with seawater and hi-vis pills down to 511 m, with Glydril/KCl mud from 511 m to 1414 m in the primary well and to 1417 m in the sidetrack, with Versatec oil based mud from 1417 m to 3736 m and with Versatherm OBM from 3736 m to TD.

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