Study on the adsorption characteristics and pore-fissure response mechanism of meagre coal and anthracite under different methane pressures

Methane emissions significantly exacerbate the greenhouse effect, and green exploitation of coalbed methane (CBM) can effectively mitigate such emissions. This necessitates a systematic investigation into coal's methane adsorption characteristics and pore-fissure response mechanisms. Using a low-field nuclear magnetic resonance (LF-NMR) system, transverse relaxation time T 2 spectra of meagre coal and anthracite under varied methane pressures were acquired via Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences. Key findings include: Both coal types exhibited four distinct T 2 peaks, with micropores (>72 % area proportion) dominating the adsorption capacity . Micropore area followed Langmuir-type growth with increasing pressure. Expansion of non-microporous regions (T 2 = 2–2000 ms) and recovery of their area ratio indicate coal matrix swelling under high pressure, altering pore connectivity. Shift in peak position further reveal adsorption-phase transitions under elevated pressures. A linear model correlating T 2 spectral area with porosity, calibrated using porosity standard samples, enables quantitative characterization of methane-occupied porosity. Anthracite exhibits significantly higher porosity occupancy at adsorption saturation compared to meagre coal. This study validates LF-NMR as a non-invasive tool for resolving multiscale methane adsorption behaviors in coal, providing a theoretical foundation for sustainable CBM extraction. • The expansion of non-microporous regions and the rebound in their area ratio indicate that high-pressure methane adsorption induces coal matrix swelling, resulting in altered pore connectivity. • Peak positions correlated with methane densification and surface relaxation rate alterations, proposing a novel indicator for in-situ gas-phase monitoring. • The area ratio of 1# peak provides a robust quantitative indicator for microporosity characterization, offering critical guidance for identifying high-gas-content coal reservoirs.