Abstract:In order to reveal the main influencing factors of gas migration in surface wells under the influence of mining and realize efficient gas extraction in surface wells, the stress and seepage equations of coal body were deduced, based on the multi-physical field coupling mathematical model of coal body containing gas, and then the relationship between the porosity and permeability and the parameters such as strain and gas pressure was obtained. Finally, with a mine in Shanxi Province as the engineering background, COMSOL Multiphysics software was used to simulate the surface well gas extraction practice,the initial and boundary conditions of numerical simulation are established, and the influences of different in-situ stress conditions, different initial permeability and different initial gas pressure on drilling production and efficiency were analyzed. The result shows: The gas extraction rate and quantity of surface drilling are obviously affected by ground stress, gas pressure and permeability, among which permeability is the decisive parameter, but permeability is mainly affected by ground stress. Under certain in-situ stress conditions, porosity and permeability decrease first and then increase with the increase of extraction time, and there is a critical residual gas pressure point; Initial permeability has an obvious influence on surface well gas extraction. The larger the initial value is, the more significant the gas pressure drops, and the higher the extraction rate and quantity simultaneously. Therefore, increasing the initial permeability of coal seam can be regarded as a way to realize efficient extraction in surface wells. The field test of mining surface well shows that the extraction concentration and pure quantity increase step by step under the influence of full mining pressure relief and permeability increase, which greatly improves the extraction efficiency of surface well.
