Elucidation of evolutionary characteristics of pore structure in anthracite through microwave-LN2 freeze–thaw cycling

Coalbed methane (CBM) extraction has been restricted by low permeability. Liquid nitrogen fracturing has been proven to be a valid method for ameliorating the condition of coal reservoirs. Microwave-liquid nitrogen (LN2) cyclic processing (MLCP) may be a potentially viable technology to increase the coal reservoir permeability. Therefore, this study conducted an analysis of the pore structure and surface morphology of single-mode processing coal (SPC) and MLCPC based on LT-N2GA and SEM. The results indicate that maximum N2 adsorption capacity and the frequency of cycles for both meso- and macropores volume of the cycles coal samples are positively correlated, and the effects are more significant than that of the SPC. Moreover, the fractal dimension D2 tended to increase and then decrease as the cycles deepened. This means that MLCP destroys the pore structure at the beginning of the cycle. When the frequency is exceeded the cycle threshold (n > 20), it will promote the development and fusion of micro- and mesopores to simplify the pore structure. After 30 freeze–thaw cycles, the TPV increased by 242.2%, and isolated pores were penetrated. The study provides essential theoretical support and a scientific basis for the engineering application of MLCP.