Study on the migration law of pressure relief gas based on dynamic evolution model of goaf porosity

In order to obtain the migration law of pressure relief gas in goaf during working face advancing, 40118 working face of Dafosi Coal Mine was taken as the research object. Based on the physical and mechanical parameters of coal strata, a dynamic evolution model of goaf porosity based on the stress distribution characteristics of goaf was established. By the numerical method, the gas migration in goaf under the conditions of various ventilation air volumes, different negative extraction pressure of high-level boreholes and different gas emission volume in the working face were obtained. The results show that the dynamic evolution model of goaf porosity can adapt well to the strata conditions of the coal mine, and it changes dynamically with the advance of the working face. Under the condition of high-level borehole extraction, the gas concentration (methane volume fraction) in goaf gradually decreases with the increase of ventilation volume and negative extraction pressure. However, there is an increase in the concentration within the blast limit, which increases the scope of gas explosion risk to some extent. With the increase of gas emission in goaf, the volume fraction of methane in goaf increases, but the risk of gas explosion decreases.