Model of thermal fracturing induced permeability enhancement in coal matrix block

Temperature variation has important influence on the damage and fracturing of coal block. When coal tem- perature changes,the anisotropic stress caused by thermal expansion brakes the critical stress balance of mi-cro-crack in coal matrix,leading to a thermal fracturing that increases the matrix permeability. In this paper,the thermal fractu- ring model of a single micro-pore source in coal matrix was established based on the fracture me-chanics,and the criti- cal sizes of thermal fracturing induced pores in coal matrix were derived. The model shows that the larger temperature change corresponds to the smaller critical crack size,which indicates that the smaller crack tends to be developed. By introducing the fractal dimension of pore size and the characteristic size of micro-pore of the rock mass,a fractal char- acterization method to describe the spatial crack cloud in coal matrix was established. Taking a spherical micro-pore as an example,this paper defined a thermal fracturing caused permeability enhancement factor to establish the relation- ship between the thermal fracturing behavior of coal matrix and the permeability evolution. The research results show that the permeability enhancement factor increases with the increasing temperature gradient,which indicates that the permeability of coal matrix increases significantly. Finally,this paper carried out a series of mercury injection experi- ments with coal samples from Pingdingshan and Jinjia coal mines after different treatments under temperatures of 25, 50,75 and 100 ℃ ,respectively. The experiment results show that the thermal fracturing and permeability enhancement enlarge with the increasing temperature gradient.