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 T2 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 T2 peaks, with micropores (>72 % area proportion) dominating the adsorption capacity. Micropore area followed Langmuir-type growth with increasing pressure. Expansion of non-microporous regions (T2 = 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 T2 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.