Experimental study on microscopic mechanisms of coal deformation and methane adsorption under liquid nitrogen cold soaking.

Abstract

To explore the influence mechanism of liquid nitrogen treatment on the methane adsorption characteristics and swelling deformation behavior of coal, this study adopts coal samples with different degrees of metamorphism as research objects and conducts isothermal methane adsorption experiments and swelling deformation tests before and after liquid nitrogen cold soaking (LN₂-CS). Combining Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and low-temperature nitrogen adsorption (BET), the research shows that compared with the saturated adsorption capacities of the raw lignite, bituminous, and anthracite, the saturated adsorption capacities of methane in coals with different metamorphic degrees after a single liquid nitrogen treatment all decrease. The decline rates are 28.82%, 10.68%, and 11.60%, respectively. The swelling deformation caused by methane adsorption in coals with different metamorphic degrees after liquid nitrogen treatment is all smaller than that of the raw coal. The volume strain of lignite changes the most before and after liquid nitrogen treatment, with the maximum decrease reaching 13.66%. Through peak-fitting calculations of the Fourier transform infrared spectroscopy of each coal sample, it is found that liquid nitrogen-treated does not change the types of functional groups on the coal surface, but the contents of hydroxyl and carboxyl groups increase significantly. Hydroxyl and carboxyl groups weaken the coal's methane adsorption process by reducing the coupling energy between coal and methane. Combining the results of BET and NMR experiments, the liquid nitrogen-treated promotes the transformation of coal pore sizes towards larger pores; it enhances pore connectivity but reduces the effective adsorption space. Pearson correlation analysis further reveals that for coals of different metamorphic grades, the adsorption pore specific surface area and BET specific surface area exhibit a strong positive correlation with saturated adsorption capacity (Pearson correlation coefficient > 0.71), whereas BJH total pore volume and average pore diameter show a strong negative correlation with saturated adsorption capacity (Pearson correlation coefficient > 0.73). The enhanced connectivity accelerates initial gas migration, but the reduced adsorption space slows the subsequent deformation rate during methane uptake. These findings reveal the microscopic influence law of LN₂-CS on coal methane adsorption deformation and supply fundamental microscopic data for related coal reservoir research.