Mechanism of Enhanced Permeability in Coal Samples Caused by Electro-Chemical Composite Energy Pulses Fracturing

Background Electrochemical composite energy pulse fracturing technology (based on the electrical explosion of a metal wire to initiate energetic materials) can effectively modify the pore structure of coal reservoirs and enhance coal seam permeability. Elucidating its influence on the modification scale of coal pores and cracks, as well as on gas desorption behavior, can provide a theoretical basis for the wider application of this technology in coal seam permeability enhancement.

Methods Using coal samples from Zhangcun Mine as the research object, pulse-induced fracturing experiments on coal were conducted based on a self-developed Electrochemical composite energy pulse fracturing experimental platform for coal and rock. High-pressure mercury intrusion porosimetry (MIP), nitrogen adsorption, and CO₂ adsorption methods were employed to quantitatively characterize the pore structure of coal before and after fracturing. Specifically, high-pressure mercury intrusion porosimetry was employed to characterize macropores (>10,000 nm), nitrogen adsorption was used to characterize mesopores (2−50 nm), and CO₂ adsorption was applied to characterize micropores (<2 nm). Furthermore, gas adsorption−desorption experiments were conducted to systematically investigate the variations in gas desorption behavior of coal before and after pulse-induced fracturing, and scanning electron microscopy (SEM) was employed to comparatively analyze the morphology and spatial distribution of cracks within the coal matrix before and after fracturing.

Results The experimental results indicate that after fracturing, the macropore volume exhibits a pronounced increase within specific size ranges, with the pore volume of macropores at approximately 140,000 nm increasing by 29.7%. The average pore diameter of mesopores shows an overall increasing trend, whereas the mesopore volume decreases by 34.8% and the specific surface area is reduced by 16.94%, the micropore volume also exhibits a slight decline. These changes are mainly attributed to the enlargement of mesopores and their transformation into macropores after pulse-induced fracturing. Under gas adsorption pressures of 0.5, 1.5, and 2.5 MPa, both the cumulative methane desorption amount and the desorption rate of the coal samples within 120 min are significantly enhanced after pulse-induced fracturing. Scanning electron microscopy observations further reveal typical erosional damage characteristics of coal after pulse-induced fracturing, manifested by the stripping of the coal matrix and the development of pores and cracks in adjacent regions, accompanied by the formation of newly generated serrated cracks and a pronounced enhancement in pore−crack connectivity.

Conclusions It is concluded that after the application of electrochemical composite energy pulse fracturing to the coal reservoir, a progressive transformation process occurs within the coal matrix, evolving from micropores to mesopores, from mesopores to macropores, and ultimately to cracks, which significantly enhances the connectivity of the pore−cracks system. This structural reorganization provides effective pathways for gas desorption, diffusion, and migration, thereby markedly enhancing the gas desorption rate and cumulative desorption capacity. These findings elucidate the permeability-enhancement mechanism of electrochemical composite energy pulse fracturing from the perspectives of pore structure evolution and gas desorption−migration behavior, and provide a robust theoretical basis for the engineering application and wider promotion of this technology in coal seam permeability enhancement.