This paper describes the first few months of operation of a new, purpose built, underbalanced (UB) coiled tubing drilling unit (CTDU).

The unit is designed to drill sidetracks and extensions to existing wells to improve reservoir drainage. The package is fully self-contained, and native crude is used as the primary drilling fluid. Natural gas injection through parasite strings or conventional gas lift mandrels is used to maintain underbalanced conditions.

The paper outlines the drilling and production challenges faced by the operator, including brief overviews of current conventional operations and a trial campaign of wells drilled with coiled tubing. With this background, the expected benefits of underbalanced drilling with coiled tubing (CT) are outlined.

The unit was built during 1998 and mobilised from the USA to Oman at the end of November 1998. Following final construction work and commissioning, the first two wells were drilled during February and March 1999. As part of the commissioning and shake-down process, these wells were drilled overbalanced using a conventional fluids package. The underbalanced drilling process system was integrated into the unit during April 1999 and commissioned in the field prior to initial underbalanced operations on the third well. Details of subsequent operations are included with an overview of drilling performance for the first 5 wells.

The paper concludes that underbalanced drilling technology can be applied safely and successfully. Although the paper deals solely with operations up to the end of June 1999, initial performance indicates that the predicted gains are achievable.

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