Numerical Analyses of the Granite Fragmentation in Rotary-Percussive Drilling with the Consideration of Pre-Existing Cracks

: Rotary-percussive drilling technique is an efficient rock-breaking method and can be applied in geothermal drilling. The rock’s fragmentation mechanism was investigated using the distinct element method with the consideration of pre-existing cracks. We developed a cutter-rock interaction model to simulate the rotary-percussive drilling. In this model, a discrete fracture network (DFN) was used to reflect the rock’s complex behavior. Bonded Block Model (BBM) was used to simulate cracks initiation that can merge and propagate to fracture the rock. As a dynamic problem, the rock’s damping effect was introduced into our model by fitting it to the laboratory tests. Results represented the rock’s fragmentation process under the cutter indentation and the coupling of impact load and lateral movement well. The pre-existing cracks can significantly enhance the drilling speed, which demonstrates the significance of considering the rock’s non-continuous nature. Raising dynamic impact load is good for improving the penetration results in our study. For the back rake angle, 50° is the best choice in the evaluated range. But the cutter with the back rake angle of 20° obtains the highest fragmentation volume and lowest specific energy under the coupling of impact load and lateral velocity. This paper’s research is of great significance to guide the application of rotary-percussive drilling and reduce the cost of developing geothermal resources. 1. INTRODUCTION The geothermal resource is increasingly valued as a clean, secure, alternative energy (Liu et al., 2011; Tester et al., 2007; York, 2012; Zhang et al., 2018). Its development requires drilling a lot of wells deep into the target formations, which accounts for 30% to 50% of the cost of the hydrothermal geothermal electricity project and more than half of the total cost of enhanced geothermal systems (EGS) (Glowka, 1997; Yost et al., 2015). It is necessary to develop an efficient drilling method to reduce the time and cost (Miyazaki et al., 2019). Rotary-percussive drilling (RPD) is an efficient method of making holes in hard rocks and is well-suited for geothermal drilling (Finger and Blankenship, 2010; Lundberg and Okrouhlik, 2006). It can provide economic benefits for reducing drilling time and drilling cost by combing rotary drilling and percussive drilling (Patil and Teodoriu, 2013; Peng et al., 2018; Zhong, 2016). In this drilling method, the impactor and upper string’s impact load is imposed on the conventional rotary bit to penetrate and crush the rock (Chiang and Elias, 2008). A sound understanding of bit-rock interaction and rock fragmentation mechanisms under the impact load and rotary movement is essential to design down-hole tools, such as drill bit and impactor, and optimize the drilling parameters.