Effect of crooked drill pipe rotation on cuttings transport in horizontal directional drilling using kinetic theory of granular flow – A numerical perspective

Effective cuttings transportation is essential for trenchless horizontal directional drilling (HDD). Inadequate borehole cleaning is one of the major reasons for drilling problems. Due to complicated mechanical effects accompanying the drilling process, the rotating drill pipe inevitably subjects bending deformation, which has a significant influence on the cuttings-drilling mud two-phase flow in the horizontal borehole, especially in the pilot hole drilling stage when the drill pipe is in a state of full rotation. However, there is a lack of research on the cuttings transport in the borehole where the rotating drill pipe is crooked. In this work, considering the bending deformation of the rotating drill pipe, the cuttings transport has been investigated in horizontal directional drilling based on the kinetic theory of granular flow. Using the dynamic mesh method, the crooked drill pipe is modeled as a sinusoidal form and rotates around the borehole axis. Flow behaviors of cuttings and drilling mud are predicted in pilot holes with a concentrically straight pipe, an eccentrically straight pipe, and a crooked pipe. Modeling results display the complex two-phase flow pattern for the horizontal directional drilling. The rotation of the crooked drill pipe enhances the cuttings spiral flow, where the axial velocity core zones of both cuttings and drilling fluid rotate with the drill pipe. The tangential velocities of the two phases originate from the rotating drill pipe wall, and the secondary flow is generated in the region that closes to the borehole inner wall within the annulus narrow portion. The cuttings transport mechanism is revealed by kinetic theory of granular flow. Predicted granular temperatures confirm more intense collisions between cuttings in the moving bed region due to the high shear rates caused by the pipe rotation. The suspension region contributes most of the cuttings transport velocity, and the fixed bed is formed by cuttings deposition. More uniform distributions of cuttings concentration and axial velocity are found in the borehole with a crooked drill pipe, however, it is found that the average torque of the crooked drill pipe is three times higher than that of the straight drill pipe exerted by the liquid–solid mixture.