16/8-2

General

Exploration well 16/8-2 is located in the Ling Depression south of the Utsira High and North of the Danish Norwegian Basin. The primary target was Late Jurassic sandstones; secondary target was the Danian/Late Cretaceous limestones and Rotliegendes/Devonian sandstones.

Operations and results

Well 16/8-2 was spudded from the semi-submersible installation Sedco H on 3 April 1980 and drilled to TD at 3585 m in Late Permian Zechstein evaporites. The well was drilled with bentonite and seawater down to 542 m, with a Spersene lignosulphonate/gypsum/CMC mud from 542 m to 2275 m, and with a salt saturated Drispac polymer/XC polymer/Polysal starch mud from 2275 m to TD.

Down to the setting of the 13 3/8" casing, the well progressed as programmed. However, the absence of the Triassic and the appearance of the Zechstein evaporites much shallower than expected caused the 9 5/8" casing to be set 474 metres higher than programmed. The drilling of the 8 1/2" hole commenced with a 1.45 SG salt saturated mud as programmed. Two runs with a turbine/Stratapac bit were made. However, on pulling out of the hole from 3519 m, tight hole was encountered and while attempting to work through this section, the drill string parted leaving 32.81 m of BHA at a depth of 3462 m. On running in with an overshot, the well was observed to be flowing. It was shut in but pressure continued to increase even after the appropriate mud weight increases had been effected. The mud weight was eventually raised to 2.03 SG creating a fine balance between sufficient fluid density and exceeding fracture pressure. The influx (thought to be from a Carnallite zone at approximately 3513 m) had an adverse effect on the mud properties causing the barite to settle out and reducing the pH to an acidic level. The magnesium and calcium sensitive Drispac polymer was replaced with the more tolerant 'XC Polymor1 to maintain the barite in suspension, and an inhibitor was