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

Abstract

The present study is carried out within the context of the ongoing research project ORCHYD where a novel hybrid drilling technology is being developed and customised for deep hard rock geothermal reservoirs access. The hybrid development, combining High Pressure Water-Jetting (HPWJ) and percussive drilling technologies, requires slotting circumferential stress relieving grooves on the bottom of the hole (using HPWJ) to improve the efficiency of rock breaking during impact. A three-dimensional finite element bit–rock interaction model is therefore built to investigate the influence of bit design and bottom-hole geometry conditions on the rock breaking efficiency for an impact from percussive drilling. A pressure and rate dependent elastoplastic damage model is calibrated using a full-scale laboratory drilling test under high confining pressure and applied on a granite rock. The role of geometric conditions of the bit-rock interface is assessed considering various slot dimensions (i.e. depth, width) at the bottom-hole wall-rock and various bit profiles (flat, parabolic, and concave). Parametric percussive impact simulations are performed and an optimal bit rock interface configuration is proposed.