Effect of Crack Orientation on Gas Permeability of Coal and Gradient of Gas Pressure. Study on Gas Permeability of Coal by Crack Network Model (3rd Report).

The purpose of the present study is the effect of crack orientation on the gas permeability and gradient of gas pressure in coal. Practically, the model of anisotropic crack network was applied to the computer simulations under the state of an uniaxial compressive stress. Additionally, the simulations of gas permeation and gas pressure were investigated by setting the normal distribution as the frequency distribution of crack orientation. The results of these simulations were as follows.1) The anisotropy of crack orientation in coal is very effective on gas permeability, and the gas permeability in the orientation of maximum principal stress is higher than that of minimum principal stress.2) In the anisotropy crack network, the gas permeable cracks of the direction of minimum principal stress close at their neck. Consequently, gas flow is intercepted by the difference stress between an external stress and gas pressure in the cracks of the direction of minimum principal stress become high.3) The occurrence mechanism of a gas outburst is explained by applying the bending theory of circular plates to the anisotropic crack network saturated with high pressures gas.