Experimental research on different metamorphic grades of coal bodies with macro–mesoscopic structure fractal characteristics

Abstract

This paper studies the Daliuta mine, Xinglongzhuang mine, Qingdong mine and Qincheng mine. Here, nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were performed on different metamorphic grades of coal samples from 4 main mining areas in China. The coal body fissure–pore structure parameters were measured in the range of 100 nm∼5000 nm, and the fractal characteristics were obtained for the different metamorphic grades of coal bodies at macro–meso-structure scales, revealing the influence of reservoir formation on the complexity of the coal body structure. The results show that with the increase in metamorphic degree, the ratio of total pore volume occupied by pores and macropores in coal samples gradually decreases, while the volume ratio of transition pores and micropores increases; in addition, the influence of coal reservoir formation processes on fissure–pore structure parameters of coal is obtained. Based on computer graphics theory and fractal geometry theory, the relationship between the 3D volume fractal dimension of the coal bodies, as measured by NMR, MIP and other fluid intrusion methods, and the box-dimension statistical method of SEM to measure the surface fractal dimension is analyzed. The 3D volume fractal dimension calculated by NMR and MIP is consistent with the change in metamorphism degree; that is, with the increase in metamorphism degree of the coal sample, the 3D volume fractal dimension increases. In addition, the 3D volume fractal dimension measured by NMR is larger than that measured by the MIP; the difference between the 3D volume fractal dimension measured by NMR and 2D surface fractal dimension measured by SEM is approximately 1. Here, the influence of reservoir formation on the characteristics of coal structure fractal is obtained. The research results provide important scientific guidance for enhancing coalbed methane recovery and mine disaster prevention.