Structural Evolution Characteristics of Middle-High Rank Coal Samples Subjected to High-Voltage Electrical Pulse

High-voltage electrical pulse (HVEP) technology has been proposed to increase the gas production of low-permeability coal reservoirs in recent years. In this study, we investigated the variation characteristics of the pore structure of coal samples by combining scanning electrical microscopy with mercury intrusion porosimetry analysis, to better understand the structural evolution characteristics of middle-high rank coal subjected to HVEP. Furthermore, changes in the chemical structure of the coal samples before and after HVEP treatment were investigated by Fourier transform infrared spectroscopy analysis. The results show that, under the action of HVEP, both anthracite and bituminous coal samples can be crushed into many small pieces. Because the conductivity of anthracite coal samples is better than that of bituminous coal samples, the average breakdown voltage of anthracite coal samples is lower than that of bituminous coal samples. It was found that the greater the breakdown voltage, the more the number of particles formed after the coal is broken and the smaller the particle diameter. At the same time, many pores and cracks were generated in the middle-high rank coal subjected to HVEP. In particular, the meso- and macropores of coal samples subjected to HVEP are clearly higher than those of raw coal samples. The fractal dimensions of seepage pores of coal samples subjected to HVEP are bigger than those of raw coal samples. The increased pores and cracks are conducive to the release and migration of methane. In addition, oxidation reactions occur on the surface of coal subjected to HVEP. Following HVEP treatment, the surface chemical structure of both anthracite and bituminous coal showed significant changes; importantly, the concentration of oxygen-containing functional groups increased, which benefits methane release.