Influence mechanisms of dynamic metamorphism on the evolution of micro/nano pore structures in tectonically deformed coals

ObjectiveMicro/nano pore structures in coals act as a significant space for gas occurrence. Exploring the evolutionary mechanisms of micro/nano pore structures in tectonically deformed coals (TDCs) under the influence of dynamic metamorphism assists in gaining a deep understanding of the mechanisms underlying coal and gas outbursts. MethodsUsing CO2 adsorption experiments, Fourier transform infrared (FTIR) spectroscopy, and molecular dynamics simulations, this study investigated the evolutionary characteristics and mechanisms of micro/nano pore structures in different TDCs occurring within the No.72 coal seam of the Qi’nan Coal Mine in Suzhou Province. Results and ConclusionsThe results indicate that with enhanced tectonic deformations, the pore sizes of ultra micropores (0.3 to 0.7 nm) shifted towards small pore sizes generally, and the micro/nano pores exhibited increased fractal dimensions of volumes and specific surface areas, intensified heterogeneity, and elevated complexity of pore structures. The results of molecular structure tests and molecular dynamics simulations reveal that the TDCs subjected to brittle deformations experienced primarily stress-induced degradation. Specifically, the activity of the molecular structures was enhanced by the breaking and dissociation of hydrogen bonds, aliphatic side chains, and oxygen-containing functional groups in the molecular structures of the TDCs, and the macromolecular structures of the TDCs were initially compressed under tectonic stress. In contrast, TDCs subjected to brittle-ductile and ductile deformations underwent significant stress-induced polycondensation. In detail, the aromatic structures in the TDCs experienced chemical changes like rotation, folding, and recombination, forming a more tightly arranged molecular structural configuration. Therefore, the enhanced tectonic deformations can lead to continuously compressed and changed macromolecular structures, as well as partitioned gaps between molecular structures. Consequently, the morphologies and sizes of micro/nano pore structures change, their pore complexity increases, and their pore sizes shift towards small pore sizes generally.