The paper will address the specific issues associated with underbalance horizontal drilling using two phase flow. Firstly, the challenges that were overcome in devising a ";safe"; method of deploying a directional drilling BHA into a live well will be presented together with a review of the deployment performance results from over a 100 different horizontal drilling operations.

Secondly, BHA performance issues associated with two-phase flow will be reviewed and results presented. This will include such issues as un-constrained vibration (encountered when the hole is not liquid full) and such effects as explosive decompression and the consequent reduction of elastomer performance.

Perhaps most important will be a discussion of the issues associated with cuttings transport in two phase flow and the methodology devised to reduce (and in many cases eliminate) wiper trips by varying the circulating conditions using a circulating valve incorporated into the BHA. Data from actual jobs will be presented to demonstrate how effective the process has become in regular field operations.

Lost time statistics for coiled tubing underbalanced drilling operations will be summarised and performance improvements highlighted.

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