New insights into methane storage through coal pore opening and closure mechanisms during transient supercritical CO2 fracturing

This study focuses on the often-overlooked closed pores in coal, which play a crucial role in isolating and storing significant amounts of methane, thereby directly impacting the efficiency of methane extraction. Using low-temperature nitrogen adsorption (LP-N2A) and small-angle x-ray scattering (SAXS) combined with multifractal theory, we examined the dynamics of pore opening and closure during supercritical CO2 (SC-CO2) fracturing at various pressures. Initially, chemical dissolution and the extraction of small organic molecules increased the surface area and volume of open pores. Stress-induced pore opening reduced closed pore volume, potentially increasing methane release. Enhanced fractal dimensions indicated greater pore heterogeneity. As fracturing progressed, pore interconnectivity improved, facilitating methane migration. Matrix contraction slightly expanded closed pores, increasing closed porosity. Fractal parameter decreases reflected changes in pore-scale correlation and reduced density. The isolation effect of closed pores delayed stress transmission, leading to asynchronous responses between total and open pores. Later, larger open pores collapsed, fragmenting the coal and increasing pore volume and surface area, while new closed pores raised closed porosity. These findings offer insights into how pore structure evolution during fracturing regulates methane at the micropore level.