Experimental Study on the Fractal Pattern of a Coal Body Pore Structure Around a Water Injection Bore

In order to enhance the disaster prevention effect of coal seam water injection technology, in this paper, the structural characteristics of the coal sample under the true mechanical environment of coal seam water injection are measured via nuclear magnetic resonance technology, and the quantitative relation between the theoretical and the experimental pore volume fractal dimension is analyzed based on fractal geometrical theory. The results show that there is a large difference between the porosity of seepage pores and absorption pores, 1.345-2.818% and 6.840-7.940%, respectively, indicating obvious inhomogeneity of the internal structure development. However, their porosities' overall change with pore water pressure and confining pressure is consistent, that is, increasing confining pressure decreases porosity, while for increasing pore water pressure it is the opposite, and confining pressure and pore water pressure have a greater impact on the seepage pores' porosity; meanwhile, based on the pore size distribution curves, it can be found that pore water pressure can enlarge pore volume, and confining pressure can reduce pore volume. In addition, seepage pores' experimental and theoretical fractal dimension values are between 2.920-2.968 and 2.0737-2.2327, respectively, and adsorption pores' experimental and theoretical fractal dimensions are between 2.296-2.343 and 2.4146-2.4471 respectively. The quantitative relation between theoretical and experimental fractal dimensions is established to achieve a common characterization of the pore structure of a coal body under load via both the theoretical and experimental fractal dimensions.