Pore-fracture and seepage characteristics of coal during low-temperature oxidation

Low-temperature oxidation of coal is a critical trigger for spontaneous combustion, during which the pore-fracture structures continuously evolve, significantly influencing seepage channels. However, current studies on the evolution of pore-fracture and seepage characteristics during low-temperature oxidation lack clear stage division and sufficient understanding of seepage mechanism. This study focused on bituminous coal from the Datong coalfield in Shanxi Province. Based on various experiments including scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP), and micro-CT, as well as seepage numerical simulation, the evolution law of pore-fracture characteristics during low-temperature oxidation (25 °C, 50 °C, 100 °C, 150 °C, and 200 °C) and its influence mechanism on seepage characteristics were explored. The results show that based on the pore-fracture characteristics obtained from NMR and MIP, the low-temperature oxidation process can be divided into three stages: Initial Oxidation Stage (25–50 °C), Stable Expansion Stage (50–150 °C), and Accelerated Expansion Stage (150–200 °C). The pore network model (PNM) constructed from micro-CT reveals that with increasing temperature, the mean equivalent pore radius increases. The maximum coordination number continuously grows, indicating enhanced connectivity of hub pores, with connectivity increasingly concentrated in a few dominant pores. Both the equivalent throat radius and throat channel length increase with temperature. From 100 °C to 150 °C, a "selective throat remodeling" is observed, where throat expansion precedes pore growth. The seepage simulation results show that the main seepage pathways are through fracture channels. Flow bottlenecks are clearly present at 150 °C, leading to pronounced differences in seepage velocity. At all temperatures, the pore pressure decreases along the flow direction. At 25 °C and 50 °C, the pressure drop is concentrated in the rear region, whereas at 100 °C, 150 °C, and 200 °C, the pressure drop is more uniform, resulting in a more stable seepage process. Further validation shows that nitrogen injection effectively suppresses the development of coal permeability and increases tortuosity. The strongest inhibitory effect is observed at 150 °C, suggesting it as the critical temperature for fire prevention measures in goaf areas in this region. This study reveals the evolution law of pore-fracture and seepage characteristics of coal during the low-temperature oxidation, providing a theoretical foundation for implementing measures to prevent spontaneous combustion.