Development in modeling approaches to mining-induced permeability of coals

The classical permeability models of coal were briefly introduced. Based on the mechanical mechanism of coal and controlling factors of permeability in mining,the key problems in the process of constructing permeability model of coal under mining were put forward. The key problems include the anisotropic characteristic of coal,the dam- age and fracture of coal under mining,and the treatment of sorption and desorption. Meanwhile,the development of each key problem was summarized and analyzed. Among the key problems,the anisotropic coal permeability models can be represented as the variety of effective stress or geometric parameters. The models based on the variety of effective stress are exponential function,but the models based on the variety of geometric parameters are almost the power function of the third power. The damaged coal permeability models were classified into two categories. One was damage variables in a constitutive equation,the other was damage variables in permeability expressions. The models whose damage variables are in constitutive equation have a wider scope of application. And the models whose damage varia- bles are in permeability expressions can more intuitively express the quantitative relationship between permeability and influencing factors. In the treatment of sorption and desorption,the expressions of adsorption strain derived from ad- sorption thermodynamics were summarized. The expression of adsorption strain in the form of Langmuir equation was most widely used in the construction of coal permeability model. Moreover,the established permeability models of coal under mining were introduced. The permeability models of coal under mining were classified into effective stress type, geometric parameter type and coefficient fitting type. Then the expressions and applications of representative model in each type were introduced. The suggestions for further research on establishing the permeability models of coal under mining were proposed.