Three-dimensional numerical study of DTH bit-rock interaction with HPWJ downhole slotting: Influence of bit design and bottom hole geometric conditions on rock breaking efficiency in percussive drilling

The EU H2020 ORCHYD project seeks to enhance drilling efficiency in hard rock environments, particularly for deep geothermal wells, by integrating innovative rock weakening techniques. In this context, 3D finite element simulations of bit-rock interactions were performed to assess how combining high pressure water jetting (HPWJ)-induced groove and bottom-hole geometry can contribute to improve the down-hole percussive drilling performance. A Red Bohus granite rock was modelled using a continuum, elasto-visco-plastic, and damage-based model calibrated using Brazilian, uniaxial compression, and triaxial material tests as well as single insert impact tests. Bit-rock interaction with an HPWJ groove was studied through modelling of twelve different groove depths and bottom-hole configurations. Results demonstrate that deeper grooves significantly reduce impact loads by up to 35% and increase penetration up to 40%, leading to higher material removal (up to 240%). Groove depth also influences damage propagation between adjacent indents, with grooves facilitating a broader zone of fractured rock, particularly near the groove itself. Notably, the drilling efficiency benefits from HPWJ slotting are highly dependent on bit design: flat and concave bit profiles exhibit 70% greater improvement in drilling performance compared to other profiles.