TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe major differences between conventional rotary drilling and Coiled Tubing drilling are the absence of pipe rotation and the continuous circulation of drilling fluids.• In rotary drilling the circulated fluid must be selected to support the cuttings while making a connection. In slide drilling, the circulated fluids can be continuously circulated. This capability greatly aids the use of gasified fluids in underbalanced drilling. • In slide drilling, the pipe does not rotate; while in rotary drilling, the pipe rotation enhances the cuttings transport by the agitation effect due to the pipe rotation and vibration. For a typical Coiled Tubing drilling job, the challenge is achieving the circulation rate required to keep the hole clean. Current field practice is to perform frequent wiper trips.The economics of hole cleaning are crucial to the industry and therefore we seek answers to questions such as:• What is the change in the cuttings bed height when the fluids flow rates are altered? • How long does it take to clean out an existing cuttings bed?• Is there a need to remove the cuttings bed fully before resuming drilling? In this study, 600 tests were conducted and a new computer program was developed for the prediction of cuttings transport in the multi-phase system (gas+liquid+cuttings). The sensitivity of cuttings bed height with respect to liquid/gas volume flow rate ratio, in-situ liquid velocity, ROP, inclination angle and circulation fluid properties was conducted. The hole cleaning time with both circulation mode and wiper trip is also discussed. The results from the sensitivity analysis presented in this paper indicate that: i) the fraction of the circulation liquid has a significant impact on the cuttings transport in underbalanced drilling with gasified fluids, ii) among different variables, the in-situ liquid velocity is the most important variable for cuttings transport, iii) the hole cleaning time decreases non-linearly with increasing fluid circulation rate.