In Situ Nuclear Magnetic Resonance Observation of Pore Fractures and Permeability Evolution in Rock and Coal under Triaxial Compression

This study presents a novel approach that incorporates nuclear magnetic resonance (NMR) measurement and triaxial loading chamber to accomplish the laboratory in situ continuous observation of the pore-fracture evolution in the coal-rock samples during triaxial compression. Triaxial compression tests with in situ NMR observation are conducted on coal and sandstone samples. The evolution of pore-fractures and permeability in a complete stress-strain process is detected in situ. The pore-fractures are quantitatively featured by fractal theory to characterize the pore-fracture evolution. The results show that the NMR signal intensity could directly reflect the pore-fracture evolution. The NMR signal intensity first continues to decrease gradually during the loading, and then increases significantly when approaching peak strength. During the deformation, the microscopic pore-fractures develop gradually, while the mesoscopic and macroscopic pore-fractures first decrease gradually and then increase significantly when approaching peak strength. The heterogeneity of mesoscopic and macroscopic pore-fractures first decreases gradually and then increases significantly when approaching peak strength, indicating the development and propagation of the mesoscopic and macroscopic pore-fractures from disorder to order. The permeability presents a negative correlation with fractal dimension of seepage (mesoscopic and macroscopic) pore-fractures.