Investigation of coal pore structure evolution under temperature–time coupling using low-field nuclear magnetic resonance

Abstract

To investigate the coupled effects of temperature and time on the evolution of coal pore structure, this study employed low-field nuclear magnetic resonance (LF-NMR) combined with oil bath heat treatment experiments to systematically characterize variations in T₂ relaxation spectra, pore size distribution, throat structure, and porosity in coal samples. The experimental findings demonstrate that the T₂ spectra of all coal samples display a bimodal distribution, wherein micropores represent the predominant pore type, whereas macropores and fractures remain predominantly undeveloped. After heat treatment, the micropore peak intensity decreased and shifted to a lower value, indicating pore contraction or collapse. At 35 °C, the pore-throat volume and porosity exhibit characteristics of a dynamic equilibrium, showing partial recovery after an initial short-term contraction. In contrast, at 65 °C, the thermal stress induced by differential thermal expansion exceeds the strength of the coal matrix, resulting in the irreversible collapse of micropores and a monotonic decrease in porosity. Submicron-sized pore throats (<0.1 µm) exhibited higher sensitivity to the coupled effects of temperature and time, whereas macropore throats remained largely unaffected. Furthermore, this study reveals that the coupled temperature–time effect primarily governs the irreversible damage processes in the micropore-throat system through a thermal stress mechanism. This, in turn, modulates the coal's oxidative activity and spontaneous combustion (SC) propensity, thereby providing a critical theoretical foundation for the early prediction and warning of coal spontaneous combustion (CSC), as well as for the development of targeted inhibition and control strategies.