Abstract: Coal is a kind of porous media, in which the pore structure is very complex. Thus, the accurate evaluation of the pore characteristics of coal mass has great significance to research the adsorption and desorption of gas and the efficient gas drainage. In this paper, based on fractal theory, a kind of random fractal model which can be use to describe the pore space distribution of porous was researched, and on the basis of its construction method, the relationship between porous media porosity and fractal dimension was established. In addition, by mercury injection experiment, the correctness of fractal model was verified, and the fractal description of porosity for porous media of coal mass was carried out. The results show that porosity is determined by fractal dimension, and the porosity decreases with the increase of fractal dimension. Meanwhile, the smaller the observing scale, the bigger the porosity, the smaller the average diameter of the same order pore, the smaller the porosity, and the less the amount of the same order pore, the smaller the porosity. Moreover, the pore size distribution can be different with the same porosity, which leads to different gas flow, and adsorption and desorption laws. Therefore, the fractal dimension can reflect the characteristics of pore size distribution in coal mass, which has great significance to the study of pore types of coal mass and gas flow in the coal seam.

Key words: porous media, porosity, fractal dimension, mercury injection experiment