STUDY ON FRACTAL CHARACTERISTICS OF FISSURE SPACE STRUCTURE AND TIGHT SOLID STRUCTURE OF COAL

The microstructure of a coal is a kind of fissured fractal tight porous media. The formation and action mechanism between its space and solid structure has a strong fundamental significance for engineering seepage problem. Currently, the research and experimental theory of the quantitative characterization of the fissure space structure is relatively complete. However, the physical meaning of the fractal dimension used to characterize structural features remains unclear. At the same time, the tight feature is attributed to the original complexity, and fractal topography dominates the behavioral complexity. Therefore, it is of great significance to clarify the distribution of tight solid structure from the fractal perspective for clarifying the complexity of coal structure. In this paper, the proportion of space structure and solid structure in coal body is accurately measured by scanning electron microscope (SEM) experiment combined with nuclear magnetic resonance (NMR) experiment, and a theoretical model which can be used to measure the fractal dimension of solid structure is proposed based on Menger sponge theory. In the process, the physical meaning of fractal dimension of the fissure space structure measured by the image analysis method and fluid intrusion method is proposed. The research shows that for the fissure space structure, the fractal dimension of fissure space structure calculated based on the SEM image and the box dimension method is between 1 and 2, showing an obvious positive correlation with the proportion of the fissure space structure area. The fractal dimension based on NMR [Formula: see text] spectrum curve fitting mainly characterizes the complexity of the pore size distribution. The fractal dimension increases with the increasing complexity of pore size distribution. For the tight solid structure, the fractal dimension obtained by theoretical calculation is less than 3, the compound fractal scaling law. The two-dimensional fractal dimension obtained through experiments and related algorithms is greater than 2, which do not conform to the fractal scaling law in general, but it can represent the proportion of solid structure in a limited scale as the theoretical fractal dimension.