Numerical investigation of a double-circulation system for cuttings transport in CBM well drilling using a CFD-DEM coupled model

In this study, a numerical model of a partial wellbore is developed by coupling computational fluid dynamics (CFD) with the discrete element method (DEM) in order to simulate the operation of a double-circulation system (DCS) in coalbed methane (CBM) well drilling, which is a novel technique used for cuttings transport. The values of the flow field are computed by a fluid solver and the motions of the particle phase are calculated by the DEM, for which an additional multi-sphere particle model is integrated. After verification of the model, the analysis centers on the working mechanism, the DCS parameters, and optimization of the angle θ of the injection nozzles, which is the most critical parameter affecting the efficiency of the DCS. Simulations reveal the working details of the DCS and prove its applicability to cuttings transport, especially in the curved and horizontal parts of the well. The effects of viscosity and particle size are investigated, and a low-viscosity drilling fluid is recommended. The optimal range for θ is obtained as 30° to 50°. This work provides a reference for the engineering application of DCSs, and by feedback from drilling sites the reliability of the results can be verified.