Key design considerations in coiled tubing drilling are associated with the wellbore hydraulics, which includes drilling fluid types, liquid and gas injection rates, injection method (gas injection down coiled tubing or through parasite string), circulation, downhole and wellhead pressures, cuttings transport, and rate of penetration. This paper presents a design methodology to achieve efficient hole cleaning and maintain a specified under-(or over-) balance, based on the use of a suite of computer programs. Two design programs are used to determine fluid types and injection rates, and a pump schedule is generated. The pump schedule is then used in a transient computer wellbore simulator to verify that all operational requirements are satisfied for the entire drilling operation. During the design stage, the effects of the coiled tubing size, drilling fluid types, injection method, overbalance or underbalance are investigated using the computer programs. Examples are given to illustrate the design process.

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