The migration law of cuttings in the case of low-velocity laminar flow during large-diameter horizontal directional drilling

The hole diameter of large-diameter horizontal directional drilling can be more than 10 times the diameter of drilling pipe. The mud in the annulus of drilling hole is not only in the laminar state which is not conducive to the migration of cuttings, but also at very low flowing velocity. The migration efficiency of cuttings in the annulus is very low and a stable cuttings bed is formed at the bottom of the hole, so accidents tend to happen easily in the hole. In order to solve this problem, the differential equations on the motion of annular mud in cylindrical coordinate system were firstly derived. Then, several simulation experiments were carried out by using the comprehensive simulation experimental device for the cuttings migration in the annulus. Finally, the distribution of annular mud flow field and the migration law of cuttings were revealed by means of numerical simulation. It is shown that the migration efficiency of large diameter cuttings isn't improved significantly by increasing the PAM content of mud. And the increasing of annular mud velocity can improve the migration efficiency of single cuttings to a certain extent while increasing the back flow area at the end of the cuttings bed, and consequently the pressure difference inside the bed is increased. The research results provide the theoretical analysis and experimental conclusion for the construction of horizontal directional drilling and have the practical significance for engineering guidance.