The effects of cuttings bed at intermediate deviation of wellbore (30 – 60°) from vertical are quite complicated in cuttings transport because the movement of cuttings bed with its direction has not been well understood. The effect of cuttings bed movement in this region has also been neglected in most other mathematical models. The reason for this might be the instability of cuttings bed. This study shows that neglecting the cuttings bed characteristics may result in improper selection of coiled tubing drilling parameters, which may frequently lead to costly problems such as stuck pipe. However, the existing models, which can handle from vertical to horizontal wells with only a function of the wellbore deviation, cannot properly characterize the effects of cuttings bed movement in cuttings transport mechanism for the intermediate deviation of wellbores.

A mathematical model is proposed based on momentum and force balances in this region. All parameters, which involve cuttings bed movement and its direction, build up and erosion of cuttings bed, and wellbore deviation, are analyzed quantitatively. The analysis of the data obtained from the simulation allows interpretation of the relationship between major parameters affecting characterization of the cuttings bed in this region. The concept of minimum anti-sliding velocity of cuttings bed (MASV) is developed based on the interrelationship between parameters, which involves fluid rheology, wellbore deviation angle, interfacial friction between suspension layer and cuttings bed, and in-situ fluid velocity in suspension layer. This study also provides the selection guide of drilling parameters affecting wellbore cleaning by a compromise process for optimum combination of the parameters.

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