Evolution of Pore and Fracture Structure in Coal Under Mining-Induced Unloading Conditions

Understanding the evolution of pore and fracture structures (PFSs) in coal under mining-induced unloading is essential for the prevention and control of gas disasters in coal mines. In this study, coal specimens from the Dongqu Mine, Taiyuan, Shanxi, were subjected to online triaxial nuclear magnetic resonance (NMR) tests under constant axial compression and stepwise confining pressure unloading conditions. Based on the measured T2 spectra, the evolution of PFSs, permeability, and pore space complexity during unloading was investigated, and fractal theory was used to quantify the structural complexity of the pore system. The results show that large pores and fractures (LPFs) exhibit the most pronounced volume variation during unloading and are most sensitive to stress change. Small pores (SPs) contribute negligibly to permeability, whereas permeability is controlled primarily by medium pores (MPs) and LPFs. During unloading, neither SPs nor the overall pore system exhibits clear fractal characteristics, whereas MPs and LPFs display distinct fractal behavior. In addition, pore-volume evolution is inconsistent with fractal dimension variation, indicating that pore-volume change alone cannot adequately characterize PFS complexity. The complexity of the pore system is governed mainly by new pore generation, the expansion of existing pores and fractures, and the interaction between competing processes such as compaction and expansion.