High-voltage electric pulse(HVEP)drilling has become a new and efficient rock breaking method, which is also the research focus in the field of drilling speed increase. To probe into rock breaking mechanism of high voltage electric pulse, this study establishes a two-dimensional numerical model of multi-physical field coupling electric breakdown of single pair of electrodes. The model reproduces the generation of plasma channels in homogeneous red sandstone from the coupling of current field, electric breakdown field and circuit field. This paper analyzes the effect of electrode pair angle, voltage and electrode spacing on rock electrical breakdown(that is, the formation of plasma channel in rock). The model includes the circuit structure parameters of pulse tool, the occurrence and development of electrical breakdown, and the relationship between electrical breakdown intensity and time. Results indicate that the plasma channel begins to sprout from the partial area near the top of the discharge electrode and develops towards the partial weak dielectric strength. With the voltage value of loading pulse increasing gradually, the time of electrical breakdown decreases gradually; comparatively, the equivalent failure volume of rock model increases gradually, and there is a positive correlation between them. On the precondition that the rock can be electrically broken, increasing the electrode spacing can improve the rock breaking efficiency of high voltage electric pulse. The equivalent failure volume of rock shows significant fluctuations during the gradual increase of electrode inclination angle of discharge electrode, and its extreme value mostly appears in the range of electrode inclination angle of 35°~ 55°. To further promote the industrial application of high-voltage electric pulse rock breaking, this paper proposes a three-dimensional numerical model of multi-physical field coupling dynamic electric breakdown of red sandstone based on two-dimensional model, reproducing the appearance of the fracture crater in the rock during the rock breaking process with electrode bit. At the same time, the self-designed coaxial electrode bit is selected for experiments of electric breakdown, and the laboratory experimental results of electric breakdown confirm the simulation experimental results.

The conventional rotary rock breaking method faces a technical bottleneck in improving the rate of penetration (ROP) in deep hard formations. Percussive drilling is the most potential approach to increase rock-breaking efficiency and ROP. However, the rock-breaking mechanism of percussive drilling is still unclear enough, especially the micro-fracture mechanism of rock under confining pressure (under lateral pressure and hydraulic pressure). In this paper, the impact rock breaking experiments by four kinds of Polycrystalline Diamond Compact (PDC) cutters are carried out using a drop-weight impact testing machine and an acoustic emission (AE) recording system, the influence of parameters such as cutter shape, rake angle, and impact energy on rock-breaking are systematically analyzed. This study includes a numerical simulation to examine the process of crack initiation, propagation, and cuttings formation during the impact process with the consideration of confining pressure. The results show the conical-shaped cutter is the most aggressive with high breaking efficiency. The penetration depth of the cutter is mainly influenced by the impact energy and cutter shape than the rake angle of the cutter. There exists critical impact energy makes the rock breaking efficiency the highest. The critical impact energy is about 40 J when using the conical-shaped cutter with a rake angle of 15°. The rock mainly failed in tensile mode, and the inter-grain crack is the main crack. Hydraulic pressure can inhibit the formation of horizontal cracks, while lateral pressure can inhibit the formation of vertical cracks and reduce the proportion of tensile cracks. The research results can provide some reference and basis for improving the rock-breaking efficiency in deep hard formations.