This paper reviews an offshore underbalanced drilling project in the Danish sector of the North Sea, undertaken during the summerof 2010. Two oil wells were drilled from a jack-up drilling rig, with produced oil and gas transferred to the accompanying production platform. A closed-loop circulating system was employed, to mitigate against the potential for release of gas vapours, completely bypassing the rig’s pits and shakers. Crude oil was initially selected as the drilling fluid for the reservoir section to reduce the baseline coefficient of friction and minimise the potential for emulsions. A bespoke disc-stack centrifuge was utilised to control the composition of drilling fluid injected into the standpipe by removing fine chalk solids and separating the oil from any produced water.
Underbalanced drilling was identified as an enabling technology to combat conventional drilling challenges due to variations in pressure profile along lengthy horizontal sections.