25/7-1 S

General

Well 25/7-1 S was drilled in the southern part of the Viking Graben, close to the UK border. The primary objective was test Late Jurassic sands in a down thrown fault block located northwest of the Balder Field. Secondary objectives were Middle Jurassic sandstones and the Paleocene Heimdal Formation sands, if developed. Prognosed TD was 4525 m.

Operations and results

Wildcat well 25/7 1S was spudded with the semi-submersible installation Nortrym on 14 April 1986 and drilled to TD at 3592 m in Silurian basement. While tripping from 1525 m the pipe became stuck and this resulted in a 38 m fish being left in the hole from 1067 to 1105 m. The kick off to bypass the fish was at 1024 m. The hole was then drilled to 2094 m before becoming stuck at 1930 m during a trip. The drill string was freed again after 5 days. Drilling then proceeded to 2477 m at which depth the pipe became stuck for a further 17 hours. The well was drilled with seawater down to 1024 and with KCl/polymer mud from 1024 m to TD. Two % diesel was added to the mud below 3241 m to alleviate high torque.

The original geological model for the prospect was wrong. Jurassic - Devonian rocks were not found. A possible explanation is that drilling took place along a fault zone. No good hydrocarbon indications or fluorescence were observed in the well. One sample at 2084 m (Balder Formation) was described as having an orange/brown medium fluorescence, occasionally bright, with a fast, strong, cream/white fluorescent cut.

Onecore was cut at 3547.0 - 3554.8 m across an indeterminate, possibly Triassic section overlying basement, and a second at 3589.0 - 3592.0 m in basement roc