Characteristics of Porosity Distribution and Gas Migration in Different Layers of Comprehensive Working Face Goaf

The fracture field and permeability distribution model of comprehensive working face goaf was integrated upon the theoretical examination to investigate the fracture field distribution law of goaf and gas migration and accumulation characteristics, and this model has been applied to the mathematical model of gas migration and accumulation in goaf. The ANSYS FLUENT numerical simulation software was used to obtain the characteristics of gas migration and accumulation in goaf and its influencing factors and analyze the applicability of solving the features of gas migration and proliferation using the porosity model of layer division in goaf. The results were as follows: the porosity around the caving zone was a little big, whereas the porosity in the middle was a little small. The porosity was almost equal along the inclination and strike in a symmetrical distribution. The porosity occurred at the fracture zone with an "O" shape. As the gob layer height increased, the porosity tended to be small. The maximum value of the porosity of the goaf would shrink to the middle of the goaf with the increase of gob layer height. The gas mass fraction along the goaf inclination showed the growth characteristics of "exponential function", the gas mass fraction along the goaf strike on the air inlet side showed the growth characteristics of "Boltzmann function", and the gas mass fraction along the goaf strike on the air outlet roadway side manifested the growth characteristics of "linear function". The main influencing factors were air leakage speed, negative pressure, and porosity distribution. The distribution model of porosity and permeability of different layers of gob can more accurately simulate the characteristics of gas migration and storage.