Experimental Study of the Structural Damage to Coal Treated by a High-Voltage Electric Pulse Discharge in Water

High-voltage electric pulse (HVEP) technology has been gradually adopted as an effective method in the drainage and utilization of coalbed methane (CBM) in low-permeability coal reservoirs. An experimental device was established to study the structural variation characteristics of bituminous coal samples treated by an HVEP. The structure parameters of coal samples before and after the HVEP treatment were measured using a scanning electron microscope and low-temperature nitrogen adsorption. Additionally, the current waveforms in the process of HVEP discharge were analyzed by a digital oscilloscope. The results showed that the pressure of the shockwave produced by an HVEP increased with an increase in discharge energy, which intensifies the damage effect on the coal body. Some newly generated pores and cracks appeared in the bituminous coal samples after the HVEP treatment. As the discharge energy increased, the number of pores and fractures increased, and the diameters of the pores and the widths of the fractures also gradually increased. The average pore diameter, specific surface area, and most probable pore diameter of bituminous coal samples treated by an HVEP were greater than those of the untreated coal. Furthermore, after the HVEP treatment, the fractal dimension D1 was larger than that of raw coal, while the fractal dimension D2 was smaller than that of raw coal. This indicates that although the pore structure in coal became more complex after an HVEP, the surfaces of the pores became smoother. These changes were unfavorable to the adsorption of gas in the coal but conducive to the diffusion and flow of gas. The combined action of the shockwave cracking effect, the cavitation vibration effect, and the effect of thermal expansion promoted the expansion and connection of micro- and macropores and fractures in the coal, improving the permeability of coal seams. The research results demonstrate that the HVEP technology can significantly alter the structural characteristics of coal deposits in a way favorable for the development of CBM.