Magmatic Thermal Effects on Multifractal Characterization of Pore and Gas Storage/Outburst in Handan–Fengfeng Coalfield

Magma intrusion significantly impacts the circumstance of coal-bearing strata, affecting pore structure and the occurrence and migration of gas, with direct implications for the efficient and safe exploitation of coalbed methane (CBM) and coal mines. To investigate the effects and mechanisms of regional thermal metamorphism on multi-scale pores, gas occurrence, and outburst, seven coal samples were collected from the Handan–Fengfeng coalfield along a south-to-north metamorphic gradient and analyzed using a combination of tests. The results show that the coal rank of the samples increases from south to north, accompanied by higher moisture content, lower volatile matter, and non-monotonic variations in atomic ratios. Multifractal analysis shows that thermal metamorphism and the accompanying increase in coal rank exert contrasting effects across pore scales: macropores decrease in volume fraction and connectivity while becoming more heterogeneous; mesopores exhibit a narrowed size range and reduced heterogeneity; and micropores increase in abundance with a homogenized morphology. Methane adsorption capacity correlates strongly with micropore characteristics. Additionally, the initial gas release velocity rises with increasing Langmuir volume, indicating elevated gas outburst risks in thermally altered regions. Initial gas release velocity and diffusion coefficients are higher in samples from mines with magma intruding into strata, reflecting accelerated gas emission potential. A theoretical framework is proposed to illustrate the chain from magmatic intrusion to gas occurrence and outburst. These findings highlight that magma-induced thermal metamorphism amplifies gas storage in micropores while intensifying release dynamics, posing dual challenges for CBM recovery and gas hazard mitigation. Further validation with expanded sampling will help refine this framework.