Evolution of pore structure in coal matrix under acidification based on molecular simulation

Coalbed methane (CBM) development is strongly constrained by the inherently low porosity and permeability of coal reservoirs, making permeability enhancement essential for efficient gas production. Acidification is considered an effective stimulation method for improving permeability through pore reconstruction and connectivity enhancement. In this study, a multiscale analysis combining molecular simulation and full-scale pore characterization was established to investigate pore evolution during acidification. Electrostatic potential (ESP) and adsorption energy analyses were employed to evaluate the interaction strength and acidification potential of different acid solutions, followed by low-temperature liquid nitrogen adsorption, mercury intrusion porosimetry (MIP), and calibrated low-temperature nitrogen adsorption (LTNA)-MIP full-scale pore characterization to quantify pore structure evolution. The results indicate that adsorption energies follow the order HF ≥ HCl > CH3COOH > CA, suggesting stronger acid–coal interactions for inorganic acids. Acidification, particularly with 12% HCl, significantly promotes meso-macropore development and pore connectivity while reducing micropore-specific surface area. The reconstructed pore network facilitates the transition of the coal pore system from an adsorption-dominated structure toward a seepage-dominated structure. In addition, the calibrated LTNA-MIP method improves the reliability of full-scale pore characterization. Overall, acidification-induced pore evolution is considered favorable for seepage enhancement in coal reservoirs, providing theoretical and experimental support for CBM reservoir stimulation.