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

: Drilling cost is one of the main obstacles hindering the development of deep geothermal energy, especially in hard granite formations at great depths. In ORCHYD, an H2020 project, we develop an innovative drilling technology that merges percussion drilling and high-pressure water jetting for drilling hard crystalline rocks. In laboratory, we have demonstrated a fourfold improvement in drilling performance when contrasted with conventional techniques. This article delves into the study of impact of realistic operational conditions, characterized by high in-situ confining stress and hydrostatic pressure, on the drilling technique. We study variables affecting the mud hammer's performance. Experimentally, our drilling test laboratory houses a set-up designed to dissect the mechanisms of rock fracture during indentation under realistic downhole conditions. Numerically we harnessed our in-house rock fracture software, SOLDITY that leverages a hybrid FDEM method to simulate the intricate interactions of rocks under confining pressures. Our analysis of experimental and numerical data centered on rock fractures. Our findings show that breaking rock can be 2-3 times more challenging under higher confinement compared to lower confinement conditions. This insight has significant implications for optimizing ORCHYD techniques, improving the efficiency of rock destruction. 1. INTRODUCTION Geothermal energy is positioned as one important component in the drive to achieve Net Zero Emissions (NZE) by 2050, offering a clean, sustainable, and reliable source of renewable energy. Nevertheless, the high costs associated with geothermal drilling pose a significant obstacle to its widespread adoption and scalability (Angelone, 2014). The primary cost driver in deep geothermal drilling stems from the necessity to penetrate deep and dense crystalline rock formations, such as granites, typically located at depths exceeding 4 km. Deeper drilling not only escalates operational expenses but also demands a substantial increase in the energy required to break rocks under extreme confining stresses and hydrodynamic pressures. Conventional rotary drilling, such as roller cone and polycrystalline diamond carbide (PDC) bits, encounter difficulties penetrating hard formations like granite, resulting in sluggish rate of penetration (ROP) (Cardoe et al., 2021; Baujard et al., 2017). Percussion drilling, characterized by its effectiveness in penetrating hard formations, has garnered considerable attention in deep geothermal industry, specifically for basement drilling. Despite its advantages, such as enhanced rock penetration and reduced drilling costs, percussion drilling encounters limitations, notably in achieving higher ROP under high confining conditions dictated by geological stresses and mud pressure in the wellbore (Gnirk and Cheatham, 1965; Han et al., 2006).