Acid-induced pore evolution in coal: Multiscale fractal analysis and implications for gas transport dynamics

Accurate characterization of coal pore structure is critical for coalbed methane (CBM) development and gas disaster prevention, while traditional methods suffer from scale gaps and model fragmentation. This study employed a multi-scale fractal approach to quantify pore structure evolution and establish a cross-scale correlation model, using mixed acid-etched typical lignite as the research object.​ Experimental results showed that acid etching selectively dissolved carbonate and silicate minerals, which significantly optimized the pore structure. Scanning electron microscopy (SEM) observations revealed increased crack width (5-20 μm) and proliferated submicron pores; low-temperature nitrogen adsorption confirmed enlarged mesopore volume (prominent 2-10 nm peak), enhanced adsorption capacity, and transformation of micropores to mesopores.​ Fractal analysis indicated that acid-etched coal exhibited higher fractal dimensions than raw coal: the box-counting method (micron scale) reflected increased surface complexity; the island method and low-temperature nitrogen adsorption (nanometer scale) indicated enhanced pore dispersion and nano-scale pore roughness/volume complexity. A micron-nanometer scale fractal dimension correlation equation was constructed with normalized scale parameter λ, clarifying dominant methods for different scales: box-counting method for micron scale (λ > 1000) and island method/low-temperature nitrogen adsorption for nanometer scale (λ < 1000).​ This study fills the gap in full-scale fractal characterization of coal pores and confirms that acid etching synergistically improves pore connectivity and discreteness via mineral dissolution, providing theoretical support for enhancing CBM adsorption/desorption efficiency.