Multifractal Behavior of the Micro- and Mesopore Structures of Brittle Tectonically Deformed Coals and Its Influence on Methane Adsorption Capacity

The well-developed tectonically deformed coals (TDCs) and the high heterogeneity of coalbed methane (CBM) occurrence severely restrict the sustainable development of the CBM industry in structurally complex areas. To better understand micro- and mesopore structures (0-50 nm) of TDCs and their influence on methane adsorption properties, we conducted methane adsorption tests and multifractal characterization (based on N-2 and CO2 gas adsorption tests) on samples with different degrees of brittle deformation. Multifractal analyses indicate that the pore volume (PV) and pore surface area (PSA) distributions of micro- and mesopores have multifractal characteristics. The multifractal dimensions can be used to quantitatively characterize the heterogeneity and continuity of PV and PSA distributions. Correlation analyses demonstrated that the multifractal properties of the PV and PSA distributions of micro- and mesopores are significantly influenced by pore structure changes related to coal deformation. Interestingly, the multifractal dimensions of pore size distributions (both the PV and PSA distributions, particularly 1-10 nm pores) have significant negative correlations with the methane adsorption properties of coal, indicating that a uniform pore size distribution is favorable for a high maximum methane adsorption capacity. The increase in 1-10 nm pore occurrence due to tectonic activity also enhanced the adsorption capacity of strongly deformed coals. The influences of coal deformation on the methane adsorption capacity of coal could be attributed to two aspects: increase in the PV and PSA of 1-10 nm pores and decrease in heterogeneity differences of pore size distributions. These two effects result in a higher gas content and pressure in the distribution areas of strongly deformed coals, which lead to higher risks of gas-related disasters.

Automated summary

The study found that the pore volume and surface area distributions of micro- and mesopores in tectonically deformed coals exhibit multifractal characteristics, with the multifractal dimensions providing quantitative measures of heterogeneity and continuity. Changes in pore structure related to coal deformation significantly influence the multifractal properties of pore volume and surface area distributions. The methane adsorption properties of coal are significantly correlated with the multifractal dimensions of pore size distributions, particularly 1-10 nm pores.