Study On Numerical Simulation Of Gas Drainage Base On Thermo-hydro-mechanical Coupling

Gas content and gas pressure gradually increased day by day with the depth of coal mining become more and more larger. The influence of ground pressure and temperature underground become more and more obvious. Gas extraction is necessary to solve the problem of outburst in deep mine. Gas extraction in shallow coal seam is different from deep coal seam. The traditional way of gas extraction is not suitable for the new conditions because of the high ground stress and temperature. In order to saving resources, arranging bore of drainage gas reasonably and optimizing design scheme, methane flow regularity should be studied and gas flow equation including various factors affecting the flow should be set in high temperature mine. These equations will solved by numerical simulation software and result showed by picture. Multiphysics coupling software COMSOL Multiphysics is base on partial differential equations which uses the method of finite element analysis. It can solve the problem of gas flow in coal seam quite well.The paper describe porosity’s distribution law and the affecting factors. Porosity’s mathematical expression is built according to the definition of porosity and including affecting factors. The equation of coal’s permeability is derived according to Carman-Kozeny equation. The paper study the combined form of free gas and adsorption gas in coal seam. It also describes the factors which affecting gas content. The relationship between free gas and adsorption gas is discussed in Langmuir equation. Gas flow equation is proposed based on Dary’s Law. Deformation equation which describes coal seam that containing gas is built according to Terzaghi formula. Temperature field equations of gas flow in coal seam is founded from the point of energy. Thermo-hydro-mechanical coupled equations are founded which describes gas flow in coal seam as the reference above.Thermo-hydro-mechanical equations which describe gas flow in coal seam are solved by COMSOL Multiphysics. The paper study on the law of gas migration under the condition that drill holes across and along seam. A comparative study on gas pressure, gas content, porosity and permeability under the circumstances of different temperature, crustal stress, bore space and permeability. At the beginning of gas drainage, gas pressure drops rapidly around the drill holes, so as the gas content drops with time from the simulation result. Porosity and permeability drop rapidly because of the change pressure of free gas. The effect of heat transfer and methane desorption make the temperature of coal seam fall. Many drilling holes which used for gas drainage will strengthen the effect of pressure relief. It will reduce the gas content in coal and short the time for gas drainge. Through study on methane flow regularity under the condition of different earth stress, high earth stress is not good for gas flow in coal seam. Measures should be taken to reduce stress of coal seam for short the time of gas drainage. Through raise the temperature of coal seam, porosity and permeability become larger because of the desorption of adsorption gas. That is good for gas drainage. So it can raise the temperature of coal seam which is low permeability to make sure that there is no gas in coal seam.