Spatial evolution of pore and fracture structures in coal under unloading confining pressure: A stratified nuclear magnetic resonance approach

The inherent spatial complexity and strong heterogeneity characterise the pore and fracture structures (PFS) in coal. Understanding the spatial evolution of PFS in coal under unloading confining pressure is crucial for ensuring the safety of coal mining operations. In this study, the stratified nuclear magnetic resonance (NMR) technique and a triaxial mechanical loading system were combined to realise real-time observation of the spatial evolution of PFS in coal during the unloading confining pressure process. A conceptual model depicting the spatial evolution of PFS in coal under unloading confining pressure was formulated. Based on our experimental findings, the mesoscopic mechanical behaviour of coal subjected to unloading confining pressure was simulated using PFC 2D. Our research reveals that throughout the unloading confining pressure process, the PFS within coal samples simultaneously exhibits characteristics of both compacted and fractured states, with mutual transformation occurring in the adsorption and seepage pores. A reduction in confining pressure results in a notable escalation of observed damage within the coal samples and intensified development of PFS. Furthermore, our numerical simulation results closely align with NMR test results, providing additional validation to our findings.