RIASSUNTO
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ABSTRACT
Drilling Top-Hole in the Northern North Sea has been a regular problem foroperators since the 1970s.Loose sand sections below the seabed are easilydrilled but appear to become unstable and subsequently collapse before surfacecasing can be set.This can result in stuck drill-string or casing; in the worstcase, the hole is lost and has to be re-spudded.
CNR experienced this challenge during 2006-7 and had to re-spud severalwells following a spate of top-hole problems that included parted drill-stringsand stuck casing.In all, eight spuds delivered only four wells and thesituation was made worse by prolonged bad weather and high rig rates. Variousconventional techniques were tried to combat the problem, but with littlerepeatable success.
CNR deployed the Riser-less Mud Recovery (RMR) system from asemi-submersible rig in winter and was able to maintain a closed circulationsystem with treated mud whilst drilling the problem zone.
It was soon noted that the root problem was not so much the formationcollapsing, but massive losses reversed the annulus flow to unload drilledcuttings back down the hole. By curing the losses and drilling with lower pumprates, the hole was drilled and cased off successfully.
INTRODUCTION
Canadian Natural Resources Ltd. (CNR) is the operator of the Ninian fieldlocated in Quadrant three of the UK North Sea (Fig. 1, Location).As part of the greater field development, CNR is drilling a number of sub-seawells to be tied back to the host platforms. The water depth isapproximately 485 ft and the wells are drilled from a floating semi-submersibledrilling unit. The shallow formations comprise mainly clay and sandsections and the well design called for 20 in surface casing to be set at2000-2200 ft. (Table 1, Shallow Formations)
Drilling the first batch of three wells started in spring 2006. The 30in conductor was set without incident but the first problem appeared shortlyafterwards while drilling 26 in hole at 1200 ft. With no warning signals,the drillstring became instantaneously stuck whilst circulating and could notbe freed.