A Study of Rock Breakage Under Extreme Submerged Confining Pressure: Can DTH Hammer Drilling Deliver

Abstract Drilling costs are a major barrier to developing deep geothermal energy, particularly in hard granite formations. As part of the EU H2020 ORCHYD project, we are developing an innovative technology that combines percussion drilling with high-pressure water jetting to enhance performance in hard crystalline rocks. Laboratory tests show a fourfold increase in efficiency over conventional methods. This study investigates the impact of realistic operational conditions—characterised by high in situ confining stress and hydrostatic pressure—on the performance of this drilling technique. Key variables influencing the performance of the mud hammer were examined. Experimentally, drilling tests were conducted using a laboratory setup that simulates rock fracture mechanisms under confining pressures up to 30 MPa, reflecting semi-realistic downhole conditions. Numerically, a hybrid finite-discrete element method (FDEM) was employed to model the interactions between the bit insert and rock under confining pressures up to 130 MPa and mud pressures up to 50 MPa, representing extended real-world deep drilling conditions. The results show that rock breakage is 2–3 times more challenging under higher confinement than lower confinement. The effects of impact energy and weight on bit were also studied. This improvement is due to two factors: a global stress release at the bottom hole, increasing crater depth by up to three times, and a local effect at the insert impact point, where crater depth matches the groove depth. These insights guide optimisation of bit design and drilling strategies—where full-scale trials are costly/impractical—de-risking deep geothermal operations, reducing costs, and improving economic viability.