In drilling horizontal wells and extended reach wells effective transport of drill cuttings is critical. Cuttings deposition and accumulation could lead to formation of deposit bed(s) inside the annulus, causing stuck drillpipe, eccentric borehole, poor penetration rate, etc. Experimental investigations reported in the literature previously focused on evaluation of cuttings transport performance in directional wells, as functions of mud flow rate, mud rheology, angle of inclination, pipe rotation, flow regime, etc. Empirical correlations were also developed for critical flow rate for different flow conditions.

In this study, both experimental investigation and numerical simulation were performed to determine the critical cuttings transport velocity in inclined annulus, for different flow conditions. Experiments were carried out in a flow-loop, using a novel data-acquisition-and-analysis system. The behavior of drill cuttings at both steady-state and unsteady-state flow conditions were recorded by a CCD video camera system. The captured images were then analyzed to obtain the velocity profile, the cross-sectional distribution, and average velocity of cuttings in annulus. Results from this experimental investigation were compared against those predicted by an unsteady-state cuttings transport numerical model developed in-house. The numerical model developed for the study allows for the interaction between the drill cuttings and drilling fluid, and follows broadly the two-layer model reported in the literature. It enabled the determination of pressure, fluid and cuttings velocity, fluid and cuttings concentration distributions inside the annulus at different times. The model was run using parameters similar to experimental conditions. Its results agree closely with experimental data for the critical flow rate for cuttings transport in inclined borehole, for the majority of cases.

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