Multifractal Analysis of Pore Structure in Middle- and High-Rank Coal by Mercury Intrusion Porosimetry and Low-Pressure N2 Adsorption

It is of great significance to study the pore heterogeneity of coal because it affects coalbed methane exploration and exploitation. In this study, mercury intrusion porosimetry (MIP) and low-pressure N2 adsorption methods were adopted to analyze the multifractal characteristics of coal pore structures with different coalification degrees. Next, a comparison was made between the two methods regarding pore structure heterogeneity. The relationships among multifractal parameters and coal ranks, proximate analysis, coal compositions, and pore structures were discussed further. The results show that the multifractal parameters of pore structures determined using the two methods had no obvious correlations. However, they were all found to be influenced by coalification degree. As coalification degree increased, the heterogeneity and complexity of the pore structures increased. Micropores measured by the MIP method had dominantly negative effects on pore connectivity and homogeneity. Meanwhile, the influencing effects of transitional pores, mesopores, and macropores were observed to be the opposite. In addition, increases in vitrinite content could potentially decrease pore connectivity and increase pore complexity, while increases in inertinite content have the opposite effects.