Fractal characteristics of the microstructure of loaded coal based on NMR and μ-CT

The change in the fracture microstructure of loaded coal affects the gas seepage characteristics of the coal. Using micron CT scanning (mu-CT) and nuclear magnetic resonance (NMR) experiments, the fractal characteristics of coal fracture structure under triaxial compression are studied from two aspects of theoretical and experimental fractal dimension for the first time. VG Studio MAX software was used to reconstruct the three-dimensional (3D) fracture structure of the coal samples, and the 3D fracture structure was modified by the porosity measured by NMR. Through the analysis of the reconstruction model and the T-2 relaxation spectrum, the spatial distribution of internal cracks in coal was obtained. The fractal geometry theory, NMR experimental fractal dimension algorithm and 3D box counting method were used to calculate the theoretical and experimental fractal dimensions and then comprehensively analyze the fractal characteristics of cracks in loaded coal. The results show that an external load promotes the development of pores/cracks in coal. With increasing theoretical fractal dimension, CT porosity increases from 0.0630% to 0.101%, and NMR porosity increases from 0.0655% to 0.105%. The relationship between porosity and theoretical fractal dimension can be expressed by y = ax(2)+ bx+c (fractal dimension is an independent variable). With increasing porosity, the experimental fractal dimension of NMR decreases linearly from 2.975 to 2.951, and the experimental fractal dimension of mu-CT increases linearly from 2.191 to 2.324. The different changing trends of theoretical and experimental fractal dimensions verify the differences in their representational meanings.