Pore-Fracture Evolution and Fractal Characteristics of Deep Coal Under Coupled Seepage and Mining-Induced Stress

Clarifying the evolution of pore-fracture structure (PFS) and the associated seepage mechanisms of deep coal under coupled seepage and mining-induced stress is important for the safe and efficient in situ fluidized mining of deep coal resources. In this study, a nuclear magnetic resonance (NMR) online triaxial testing system was used to conduct coupled seepage–mining-induced stress tests under different seepage pressures, following a mining-induced stress path characterized by increasing axial stress and decreasing confining pressure. Transverse relaxation time (T2) spectra and nuclear magnetic resonance imaging (NMRI) were combined to characterize the dynamic evolution of PFS in terms of its spatial distribution, pore volume, mean pore size, pore compressibility, and fractal characteristics. The results show that, based on the NMRI characteristics, the deformation and failure process of coal can be divided into three stages: compaction and elastic deformation, PFS propagation, and post-peak failure. Across the tested specimens, higher seepage pressure was associated with an earlier onset of PFS propagation, a lower PFS damage threshold, and enhanced PFS propagation and connectivity. The volumes of adsorption pores (APs), seepage pores and fractures (SPFs), and total pores (TPs) generally increase initially and then decrease during the compaction and elastic deformation stage, increase slowly or remain relatively stable during the PFS propagation stage, and increase sharply at the peak-strength point, with the magnitude of the increase differing among the three tested specimens. The mean pore size, represented by T2g, initially increases and then gradually stabilizes with increasing strain, followed by a rapid increase at the peak-strength point. For specimen M2, SPF exhibited a substantially stronger compressibility response than AP and TP at the peak-strength point. The fractal dimension of SPF remains relatively stable before the peak strain but decreases sharply at the peak strain, indicating reduced structural complexity and enhanced connectivity of SPF. These observations suggest that, in the tested specimens, SPF expansion and coalescence under coupled seepage and mining-induced stress were associated with the reorganization of the internal seepage pathways of coal. These findings provide an experimental basis for evaluating stress-dependent permeability, gas transport pathways, and seepage-related failure risks in deep coal under mining-induced stress.