Multiple fractal characterization of medium-high rank coal integrating mercury intrusion porosimetry, N₂ and CO₂ adsorption experiments

Multifractal features of the pore structures of coal reservoirs control the transport and sustainable production of coalbed methane (CBM), which directly determines the efficiency of CBM extraction and is of great importance for CBM extraction. In order to investigate the multifractal characteristics of the pore structures of medium-high rank coals and their evolutionary trends during coalification, the medium-high rank coal samples from production mines in the Qinshui coal field were collected to investigate the evolution of the multiple fractal characteristics of medium-high rank coals macropores (> 50 nm), mesopores (2−50 nm) and micropores (< 2 nm) during coalification and their influencing factors by using multifractal theories and integrating the mercury intrusion porosimetry, low temperature nitrogen gas adsorption and low temperature carbon dioxide adsorption experiments, respectively. The results show that the generalized dimensional spectrum (D_q−q) and the multifractal singular spectrum (f(α)−α) of macro-, meso- and micropores of medium-high rank coals both satisfy the multifractal characteristics, which implies that the macro-, meso- and micropores of medium-high rank coals all exhibit multifractal behavior. Compared with macropores and mesopores, micropores have stronger non-homogeneity and lower pore connectivity exhibiting larger α₀ and ∆D values and smaller H values. Coalification promotes the aggregation of macromolecules in coal, which changes the coal reservoir type from macropores dominant reservoir and macropores-micropores coexisting reservoir to micropores dominant reservoir, and the pore size distribution of different scales in coal tends to be homogenized, leading to the improvement of pore structure homogeneity and pore connectivity in medium-high rank coal reservoirs. Macropore and micropore volume fractions have positive and negative effects on pore structure heterogeneity in their respective pore size ranges, while mesopore volume fraction is not an effective constraint on mesopore pore size distribution heterogeneity. The vitrinite and inertinite groups show opposite effects on the heterogeneity of pore size distribution, with a positive correlation between V/I and H values and a negative correlation with α₀. Vitrinite-rich coals develop more micropores and thus showing stronger pore structure homogeneity and better pore connectivity.