Characterization of pore evolution in coal based on thermal simulation experiments

With the gradual depletion of shallow Coal Bed Methane (CBM) resources, the development and utilization of deep CBM resources have become increasingly important. Pore structure is an important parameter and indicator in the deep CBM exploration process. To study the pore characteristics of deep CBM reservoirs, coal sample with vitrinite reflectance (R_o) of 0.7 % was selected, and thermal simulation experiments were conducted to artificially advance the rank. Scanning Electron Microscopy (SEM), low-temperature carbon dioxide adsorption, low-temperature liquid nitrogen adsorption, Small Angle X-ray Scattering (SAXS), and Mercury Intrusion Porosimetry (MIP) were used to characterize the pore structure and morphology of the coal sample before and after the thermal simulation experiments at multiple scales. The results showed that intergranular pores are predominant in coal samples with lower maturity, whereas in higher-rank coals, fragmented pores and gas pores are predominant. With increasing maturity, the pore size distribution, Specific Surface Area (SSA), Pore Volume (PV), and fractal dimension of micropores exhibit a trend of initial decrease followed by increase. Micropores dominate in coal samples of varying maturity levels and contribute the majority of the total SSA. The peak of mesopore size distribution gradually shifts toward larger diameters with increasing maturity. The SSA and PV of mesopores are predominantly contributed by smaller mesopores within the range of 2∼20 nm. The surface fractal dimension of mesopores in the range of 4∼7 nm shows a gradual decreasing trend, and the fractal dimension of pores in the range of 4∼50 nm shows a roughly increasing trend. The variation of macropores shows no significant trend with increasing maturity. While macropores provide substantial PV, their fractal dimension reaches a maximum with the range of 400∼450 °C. The total SSA exhibits a U-shaped trend, reaching its minimum at a vitrinite reflectance (R_o) of 1.4 %, while the total PV demonstrates an overall increasing trend.