Resolved CFD-DEM simulation of irregularly shaped cuttings removal in deep hard-rock drilling

Deep geothermal energy is a promising renewable energy source that harnesses the Earth's natural heat from deep beneath the surface. However, drilling in deep geothermal reservoirs, especially those exceeding 5 km in depth, poses significant challenges to conventional rotary mechanical drilling due to the presence of hard crystalline rocks. To address this, Plasma-Pulse Geo-Drilling (PPGD) has been proposed, which uses high-voltage pulses to remove rocks. Nevertheless, the significant size of the cuttings generated during the drilling, reaching up to 2 cm in diameter, poses a difficult hurdle for effective removal through injection of pure water.

In this study, we examine the cutting removal efficiency of PPGD by employing a Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) approach. A highly turbulent flow regime necessitates highly refined grid cells, which are significantly smaller than particle sizes. Therefore, a resolved CFD-DEM approach based on a fictitious domain method is used in the open-source C++ package CFDEM, which integrates the URANS solver in OpenFOAM with the DEM solver in LIGGGHTS. The arbitrary shape of the cuttings is represented using a multi-sphere method. The simulation enables the exploration of critical design considerations, including feasibility assessments, determination of the optimal power required for efficient particle removal under diverse operating conditions, and geometrical modification for improving the removal process.