Experimental Investigation and Numerical Simulation of Pore Characteristics on Rock Fragmentation Mechanism by High Voltage Electric Pulses
Authors: Liu Weiji , Zhang Youjian , Zhu Xiaohua 
Published: September 2023

ABSTRACT
High voltage electrical pulse (HVEP) is an innovative technique with the low-energy and high efficiency. In this study, we establish a two-dimensional numerical model of multi-physical field coupling the electrical breakdown of porous rock with randomly distributed pores to investigate the effect of pore characteristics (porosity, pore media composition) on the partial electrical breakdown of rock (i.e. the generation of plasma channel inside the rock). The findings indicate that as the porosity of rock increases, so does the intensity of the electric field in the "electrical damage" region—the greater the porosity, the greater the effectiveness of rock breaking. As the fraction of pore fluid (Swater/Sair) declines gradually, the generation time of the plasma channel lowers, and the efficacy of rock-breaking by HVEP increases. In addition, this work conducted an indoor experiment utilizing an electric pulse drill to break down the rock to recreate the growth mode of the plasma channel in the rock. Moreover, the experimental results are consistent with the simulation results; additionally, the development type of partial electric breakdown is verified to be related to electrode polarity and pore characteristics via the experiment of the symmetrical needle-needle electrode arrangement, which further demonstrates the mechanism of partial electric breakdown. This research is significant for comprehending the process of electric impulse rock-breaking and gives theoretical guidance and technological support for advancing electric impulse drilling technology.

INTRODUCTION
The efficiency of drilling and rock breaking determines the economics of project operation from deep geo-energy resources (e.g., geothermal, oil, and gas) because drilling expenses account for the bulk of total project expenditures (Gao., 2003; Tester et al., 2007). In addition, the expense of drilling operations increases exponentially as drilling depth increases (Huet al., 2013; Diaz et al., 2018; Meng et al., 2022). High voltage electric pulse (HVEP) drilling technology stands out due to its advantages, which include high rock-breaking efficiency, good wellbore quality, green environmental protection, directional rock-breaking and easy control of the rock-breaking process, as well as the fact that its drilling cost is not limited by drilling depth. It is considered as a new green rock-breaking technology, having enormous growth potential (Schiegg et al., 2015; Kusaiynov et al., 2017; Volger et al., 2020). As of now, HVEP drilling technology is a new rock-breaking technique that is near to industrialization (Timoshkin et al., 2004; Biela et al., 2009; Zhu et al., 2020), and its viability has been proved by numerous nations (Anders et al., 2015; 2017). However, the complex characteristics of rock strata encountered in the process of electric pulse rock-breaking, particularly the influence of rock pore characteristics on the mechanism of electric pulse rock-breaking, are insufficiently understood. The mechanism of rock-breaking by coupling multiple physical fields involved is difficult to describe precisely, which limits the industrial application of HVEP technology to some degrees.