Numerical Simulation Study On Dynamic Distribution Of Gas Occurrence Under Drainage Condition

Underground gas extraction is the main means to reduce the risk of coal mine gas disaster and make comprehensive use of gas resources.The dynamic change of gas distribution in the process of gas extraction is the key to evaluate the effect of gas drainage,which provides guidance for the safe production of coal mines.This paper was based on the project of National Natural Science Foundation of China-"the Law of Gas seepage and residual Gas occurrence in the process of drainage"(41472134).The paper takes the coal seam of Hudi coal mine in Jincheng mining area of Shanxi Province as the object of study,and analyzed the physical properties of coal and rock by experiments of mercury injection,isothermal adsorption and permeability,and the main controlling factors of gas distribution are discussed.At last the paper used COMSOL Multiphysics simulation software to simulate and analyze the geological control factors of gas occurrence and the dynamic change of gas content in the process of drainage numerically.The fracture of coal and rock in the study area is developed,the permeability is more sensitive to the stress,the gas content in situ is high,the gas pressure is large and the distribution is uneven.All kinds of geological factors have different influence on the gas occurrence.The development of the buried depth of the coal seam and the collapse column is the main controlling factor of the difference of gas content distribution.This paper deduced the equation of deformation field based on the effective stress principle of Terzaghi,and calculated the dynamic equation of permeability with Kozeny-Carman equation.According to Darcy's law,the governing equation of seepage field was established,and a mathematical model of fluid solid coupling for gas migration is established.Through the simulation of gas extraction in the 1303 working field of main coal seam in the study area,the results show that the gas pressure in different positions is the same in terms of the overall changing trend of drainage,which shows the law that the gas pressure increases rapidly first and then slows down.The difference of each point lies in the depth,the difference of distance will result in the difference of the speed and speed of gas pressure,and the accuracy of the model is verified by the measured data.On the basis of the established mathematical model,combined with the gas extraction conditions of the main coal seam in the study area,the influence of different conditions on the gas drainage effect is simulated by using COMSOL software.It is considered that the extraction conditions are relative to the extraction time.The diameter and arrangement,the depth of burying,the initial porosity,the permeability,the initial gas content and so on have more influence on the effect of gas extraction.In the areas with high gas content in situ,the extraction effect is relatively poor,resulting in a higher residual gas content after drainage.