Macro-Meso-Micro Evolution Characteristics of Coal Under Acid Corrosion and Its Implication to ECBM Recovery

Coalbed methane (CBM), as an associated resource of coal, has important strategic energy strategic significance. However, most reservoirs in China have low permeability, which poses challenges for CBM mining. Based on the background of acidification to increase CBM production, the macroscopic mechanics seepage, mesoscopic morphology observation, and microscopic pore measurement experiments of coal under acid treatment were carried out, and the influence of acid on macro-meso-micro characteristics of coal and insights into enhancing CBM were revealed. It is found that acid corrosion can weaken the strength and energy storage capacity of coal, and obvious toughness characteristics appear under postpeak stress loading. The effect of acidification on the permeability of coal is time-dependent. In the early stage of acidification, the sensitivity index is larger, rendering the permeability more susceptible to variations over time. However, as the process advances to later stages, blindly increasing acidification time gradually weakens the effect of permeability enhancement and even decreases. Evidence of acid etching of mineral components in fracture peaks and valleys was found under the microscope, with originally blocked pores achieving unblocking and connecting to primary pores, and it was also surprising to find that acidification reaction increased the surface roughness of samples. Furthermore, following acid treatment, the samples exhibited an increase in the average pore diameter, pore volume, porosity, and Knudsen number. The pore fractal dimension and tortuosity decreased, which made it easy for gas diffusion and seepage activities. The research results have guiding significance for CBM energy development and underground disaster prevention.