Study on fractal characteristics of coal and rock pore fissure structure and permeability model

In order to explore the linkage relationship between the fracture structure and permeability characteristics of coal and rock pores, scanning electron microscopy, polarized light and fractal methods were used to analyze the distribution characteristics of coal and rock pore and fracture structure, and the self-developed outlet end positive pressure triaxial seepage device was used to carry out the seepage test of pore pressure increase under constant effective stress conditions. Based on the fractal theory, and considering the influence mechanism of coal surface pore distribution on coal permeability, the coal permeability model considering the fractal characteristics of pores and fractures was established and the rationality of the test was verified by experiments and the fractal dimension and permeability coupling under the fracture of coal pores were quantitatively analyzed. The results show that: ① The surface of coal in Liupanshui mining area contains a certain number of pores and fissures. The Sijiaotian No. 7 coal seam has the best development of pores and cracks, with two clear and large cracks, accompanied by a large number of intersecting micro-cracks. Pores are developed, and the coal and rock structure is seriously damaged. ② The distribution of pores and cracks in coal rock has obvious fractal characteristics through box dimension method and the porosity of coal rock is positively correlated with fractal dimension. ③ Under the condition of constant effective stress, the permeability of coal rock decreases sharply with the increase of pore pressure then tends to be gentle. Affected by the pore and fissure structure, the permeability of coal rock is obviously different under the same pore pressure. The more complicated the pore and fracture structure of coal and rock surface, the larger the fractal dimension, which is helpful for gas migration and the permeability is on the rise. ④ The calculated values of the coal permeability model considering the fractal characteristics of the pores and fractures are in good agreement with the measured values. Compared with the previous research results, both the applicability of the theoretical mechanism and the matching of the test points are more applicable and it reflects the linkage relationship between pore pressure and permeability.