STUDY ON FULL-SCALE PORE STRUCTURE AND MULTIFRACTAL CHARACTERISTICS OF DIFFERENT RANK COALS COMBINING GREY CORRELATION, GAS ADSORPTION EXPERIMENT, AND MERCURY INTRUSION POROSIMETRY

The pore structure in coals plays a crucial role in the storage and transportation of coalbed methane (CBM). This study selected six samples of different rank coals to investigate the pore structure and fractal properties using mercury intrusion porosimetry (MIP), micro-computed tomography, low-temperature N<sub>2</sub> adsorption, and CO<sub>2</sub> adsorption techniques. The full-size pore structure and fractal characteristics of coal were obtained using multiple methods, and the factors influencing fractal characteristics were analyzed utilizing the grayscale correlation method. The findings indicate that the overall pore volume of the coal samples under investigation is determined primarily by micropores (0.3 to 1 nm) and macropores (50 nm to 1 μm). Low-rank coals exhibit a dominant pore size range in macropores, while medium- to high-rank coals show a dominant pore size range. Moreover, as the degree of coalification increases, the pore volume demonstrates a U-shaped trend, decreasing initially and then increasing. The specific surface area of pores is mainly determined by micropores, with medium-rank coals showing an average contribution of 10.5% from mesopores and micropores contributing up to 99% of the specific surface area in high-rank coals. The fractal dimension calculation results indicate that the pore structure becomes more complex as pore size increases. The overall fractal dimension decreases with increasing coal rank, suggesting reduced pore structure heterogeneity with deeper coalification. Grey correlation analysis ranks the influencing factors of fractal dimensions in different rank coals, with fixed carbon content (FCad) exerting the most significant influence on multiscale comprehensive fractal dimensions. Maximum vitrinite reflectance (R<sub>o,max</sub>), micropore volume V<sub>1</sub>, and micropore specific surface area S<sub>1</sub> are substantial factors affecting multiscale comprehensive fractal dimensions. The findings can provide an experimental basis for the efficient extraction of CBM in the study area.