The downhole friction is a response to mechanical and hydraulic forces. The mechanical friction takes place when the drillpipe is moving in the wellbore, either in translational or rotational mode. The hydraulic friction occurs due to the circulation of fluids flowing downwards in the pipe, and upwards in the annulus. The additional factors that affect the hydraulic friction are drillpipe rotation, surface roughness, wellbore geometry, etc.

This paper experimentally examines hydraulic friction and studies its variations with adjustable operational parameters (flow rate and rotational speed). The experiments were performed using two pipes, where one was located inside the other. The outside pipe with an inner diameter of 0.04 m simulated the wellbore walls, and the inside one simulated the drillpipe with an outer diameter of 0.025 m. The pipes’ inclination was 5° from the horizontal axis. The drillpipe could be rotated by an electric motor with speeds up to 1000 RPM (revolutions per minute). Spherical glass beads were used to simulate the cuttings behaviour in the wellbore. The pressure gradient variations associated with fluid flow, pipe rotation as well as particles movement was measured over the test section (length of 1.52 m), using a pressure transducer. In addition, a high-speed camera, installed outside the outer pipe, was used to take images of the particles distribution and pipe dynamic oscillations.

The combined effect of fluid flow rate, drillpipe rotation and presence of particles on the hydraulic friction between the outer and inner pipes is investigated in this study. The data analysis for the single-phase liquid flow showed that the pressure loss increased with RPM. It was seen that the secondary flow, created by the drillpipe rotation, was not the only reason to affect the pressure loss, but the drillpipe's eccentricity variations with RPM and flow rate also had a considerable effect. A higher drillpipe eccentricity provided lower friction factor, in the case single-phase liquid flow. Finally, empirical relations to describe the drillpipe eccentricity and hydraulic friction factor as functions of RPM and flow rate were proposed. When the particles were present, the friction factor decreased with RPM.

The above-mentioned conclusions and relations are valid for a Newtonian fluid in the turbulent regime.

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