Study on the effect of multiple non-breathable gangue occurrence on gas migration in coal seams

Abstract As the mining environment gradually moves toward deep, the form and occurrence mode of gangue are more complex. The occurrence of non-breathable gangue seriously worsens gas migration conditions, which greatly reduces the gas extraction efficiency and increases the possibility of coal and gas outburst. Therefore, in order to explore the effect of multiple non-breathable gangue occurrence on gas migration in coal seams, the self-developed " the constant pressure coal and rock mass gas seepage test system " is used to systematically study the characteristics of coal seam gas migration under different layers of gangue and the differentiated occurrence of multiple gangue. The equivalent characterization of coal seam gas migration under the condition of multiple inclusions is proposed through theoretical analysis and numerical simulation. Research shows: the permeability factor of the whole coal sample, sample containing a single layer gangue and sample containing double layers gangue exhibits a “V"-shaped variation law with the increase of the gas adsorption equilibrium pressure. There is a critical gas pressure point when the permeability factor is the lowest, and with the increase in the number of gangue layers, the greater the pressure value of the critical gas pressure point. According to the characteristics of gas pressure distribution, the sample containing gangue can be roughly divided into four influence areas: gas expansion affected area, gas stable seepage area and upper and lower effective seepage area. The more the number of gangue layers and the larger the area of gangue layers, the upper and lower effective seepage area is smaller, the closer the positions of gangue are to the gas source, gas expansion affected area is smaller, both of them will cause more difficult in gas migration and the lower of the sample permeability factor. When different sizes of gangue are used at two positions of the gangue in the samples, with the increase of gas adsorption equilibrium pressure, the attenuation rate of permeability factor in the early stage of the sample is significantly higher than that of the sample with the same size of gangues in the front and at the back. The results of the paper have certain actual guiding significance for improving gas extraction efficiency and preventing mine gas disasters.