Investigation of rock-cutting behaviour in high-temperature and high-pressure conditions

Geothermal drilling efficiency is limited by the challenging high-temperature and high-pressure (HTHP) underground conditions. To enhance polycrystalline diamond compact (PDC) drill bit performance in geothermal drilling, it is essential to delve into the cutting and fragmentation mechanisms of rocks within HTHP environments. In this study, a two-dimensional discrete-element method heterogeneous granite model with realistic grain boundaries is developed to investigate thermal damage characteristics under HTHP conditions, and simulations are conducted based on this model to analyse granite cutting processes using a single PDC cutter. The results show that the quantity of thermal cracks ranks as follows under the same temperature but different stress conditions: atmospheric pressure > lateral pressure > hydrostatic pressure > overlying rock pressure > bidirectional pressure > confining pressure. Regarding rock failure modes, under thermal stress, rocks primarily experience tensile failure. However, under rapid cooling of drilling fluid, shear failure predominates, with lateral pressure promoting the transition from shear to tensile failure and hydraulic pressure facilitating the reverse. The balance between lateral and hydraulic pressures also influences rock fragmentation. These findings offer valuable insights into the fragmentation mechanisms of HTHP granite.