Microscopic fracturing and unfrozen water hysteresis effects analysis of lignite and anthracite.

This article focuses on the effects of liquid nitrogen freeze–thaw cycles on the pore structure and unfrozen water behavior in low-rank lignite and high-rank anthracite coal samples from China, using nuclear magnetic resonance (NMR) technology. The study finds that lignite, with its more porous and hydrophilic structure, exhibits greater increases in porosity (0.9%) and pore throat count (0.9%) compared to anthracite (0.1% and 0.13%, respectively) after freeze–thaw treatment, with significant expansion of seepage pore diameters in both coals. A pronounced hysteresis effect in unfrozen water content during freezing and thawing is observed, attributed to supercooling and phase transition dynamics, peaking at −5 °C for lignite and 5 °C for anthracite. The combined effects of thermal stress from rapid cooling and freeze expansion forces optimize coal pore connectivity and permeability, offering insights for enhancing coalbed methane extraction through liquid nitrogen fracturing tailored to coal rank.