Where Does the Energy Go in Percussion Drilling? FDEM's Answer

: A drilling system incorporating percussive action is generally preferred for very hard rock to propagate significant cracking and fragmentation. Therefore, improving energy utilization efficiency becomes crucial for increasing the drilling rate. In this study, 3D FDEM simulations were employed to model the piston-bit-rock interaction in percussive drilling, revealing how the impact energy is partitioned and consumed during the impact process. To ensure the accuracy of energy analysis, rigorous validation against experiments was conducted, comparing the bit indentation and rebound velocity, fragment mass, and the range of rock cracking after impact. The results revealed that, for both St Anne limestone and Rhune sandstone, during a single impact, only 2.4-2.6% of the energy was utilized for crack propagation, while 30-70% of the energy was consumed by friction between fragments. As the piston impact energy increases, there is no significant change in the proportion of fracture energy, while the energy lost to friction gradually increases, then remains unchanged. 1. INTRODUCTION With the increasing demand for underground mineral resources and green energy (i.e., geothermal energy), efficient drilling technologies are crucial for reducing the cost of drilling (Zappa et al., 2019). When drilling into hard brittle rock formations, impact-assisted drilling methods achieve faster drilling rates compared to purely cutting drilling technologies. Therefore, for percussion drilling, achieving faster drilling rates to reduce drilling costs has become a focus of many engineers and researchers (Aldannawy et al., 2022; Kahraman et al., 2003). It also becomes crucial to fully understand the cracking mechanism and energy transition during the multi-body interaction (i.e., piston-bit-rock interaction), to improve the drilling rate and the energy utilization efficiency. Several experimental studies have been conducted at various scales to study the rock fragmentation efficiency (i.e., fragment mass per impact) of percussion drilling. Factors such as insert geometry (Qin et al., 2014), impact energy (Saksala et al., 2014), "weight on bit" (Aldannawy et al., 2022) and rock confinement pressure (Li et al., 2021) have been studied for their impact on the rock fragmentation process.