Experimental investigation on the rock breaking mechanism of electrode bit by high-voltage electric pulses

Because of the low rate of penetration (ROP) and high cost of traditional rotary rock breaking, it is important to explore some unconventional new rock breaking methods. High-voltage electrical pulse (HVEP) drilling has attracted much attention because of its advantages such as environmental protection, good borehole wall quality, and high rock breaking efficiency. This paper independently designs and builds a complete indoor experimental platform of HVEP electric breakdown, and selects the self-designed classic coaxial type and cross-type electrode bits to carry out laboratory experiments of HVEP rock-breaking, to truly reveal the rock breaking mechanism of HVEP drilling and further promote its industrial application. The influence of the anode structure of electrode bit, liquid insulation medium, drilling fluid circulation condition, and pulse voltage on rock breaking mechanism are investigated. The findings indicate that the bottom-hole morphology of the rock sample is related to the electrode bit structure. The S-type electrode bit has a certain discharge blind area, which is not conducive to the rock breaking of HVEP. With the increase of pre-charging voltage, the effective discharge rate (EDR) of both electrode bits increases gradually, trending towards 100% as the pre-charging voltage grows. Under non-circulating working conditions, greater conductivity of liquid medium results in a lower EDR of electrode bits. In all kinds of liquid media, under low voltage (pre-charging voltage <15 kV), the single pulse penetration depth (SPPD) under the non-circulating working condition is larger than that of the circulating condition, in contrast, at a higher voltage (pre-charging voltage >15 kV), the SPPD under the circulating condition is larger than that the non-circulating condition. To further observe the generation of plasma channel and the rock damage characterization of HVEP, this paper also establishes a three-dimensional numerical model of dynamic electrical breakdown of red sandstone to reproduce the generation of plasma channel, which can be mutually confirmed by indoor experimental results. The research results are essential for a deeper understanding of the rock breaking mechanism by electric pulse and for providing some theoretical guidance for the industrial application of electric pulse drilling technology.