The purpose of this study is to develop the mathematical model on the limit deposit velocity for hole cleaning in coiled tubing drilling. The model was developed for coarse cuttings by using the two-layer model. It was found from the calculated results by using the developed model that the minimum flow rate to convey the coarse cuttings, that is critical flow rate, increases with increasing ROP (rate of penetration) in the range of low ROP. However, the critical flow rate showed a maximum at certain ROP and then the critical flow rate decreases gradually with increasing ROP. Furthermore, it was confirmed that the effect of well inclination angle and cuttings diameter on the critical flow rate was significant. This result indicates that the behavior of cuttings in the well should be monitored carefully by measuring the supplied mud flow rate and cuttings delivered to the ground surface.
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