A fractal model of thermal–hydrological–mechanical interaction on coal seam

Abstract

Although the microstructure of matrix has a significant impact on coal thermal–hydrological–mechanical interaction, this effect has not been included in the permeability analysis of the coal bed methane (CBM) extraction. Previous studies have typically investigated the relationship between coal porosity and permeability through classical cubic permeability model, neglecting the contribution of the coal seam microstructure to permeability. In this paper, we proposed a new fractal model by defining the permeability of the coal as a function of temperature and effective stress, and characterized the permeability by two microstructural parameters of coal with a thermal variable: (1) fractal dimension of the fracture; (2) maximum fracture length; (3) coal seam temperature. And this fractal model is applied to couple the gas flow, thermal conduction and deformation of the coal. The results show that the fractal permeability model is more effective in characterizing the thermal conduction and seepage processes in coal seam than the classical cubic permeability model. Compared with the cubic-law permeability model, the permeability changes about 19.62% with the different fractal dimension, and about 95.01% with the different maximum fracture length. As the fractal dimension increases from 1.25894 to 1.25926, the gas pressure decreases by 185,117.5 Pa. Furthermore, permeability decreases with the increase of coal seam temperature, and different coal parameters have various contributions to the structural parameters. However, the classical cubic permeability model cannot capture these conclusions.