Numerical Simulation Study Of THM Coupling For Gas Seepage Through Pre-draining Boreholes In Electric Heating High Temperature Field

There are many low permeability gas seams in coal mines in China,there are difficulties in gas extraction,according to the principle of heating to promote coal seam gas desorption and increase the permeability of coal seam,a new idea of using heating cable to heat coal seam to improve gas permeability is put forward.Based on the theory of elastic mechanics,seepage mechanics and thermodynamics of porous media,the variation Law of gas migration in coal seam under the influence of electrothermal high temperature field is studied systematically by means of theoretical analysis and numerical simulation,and a mathematical model of Thermal-HydrologicalMechanical-Damage multi-field coupling including coal seam deformation,gas seepage,gas adsorption and temperature effect is established.And the model is applied to the practice of heating and strengthening pre-pumping gas engineering.Taking the coal seam mining face in Guizhou Province as the research goal,the numerical simulation of coal body damage area and the numerical simulation of gas extraction effect in closed mining face under the condition of different heating hole temperature(200?,400?,600? and 800?)were selected in K15 coal seam of Song he mine.The main research results are as follows:(1)According to the fracture-pore structure characteristics of coal,the dynamic variation equation of permeability containing temperature effect is established.On this basis,the deformation stress field equation,gas migration seepage field equation,temperature field control equation and thermal damage equation of coal body are established respectively.The above equations obtain a multi-field coupling mathematical model of thermal-flow-solid-damage including coal body deformation,gas seepage,temperature field and thermal damage effect.(2)Based on the background of heating pre-pumped coal seam gas to increase coal seam permeability,the mathematical model of heat-flow-solid-damage coupling is numerically solved by using COMSOL Multiphysics software.The results show that the influence of temperature field on thermal damage,elastic modulus and permeability of coal body is consistent,and when the temperature is greater than 600?,the higher the temperature,the higher the influence degree of thermal damage,elastic modulus and permeability of coal seam;the temperature is less than 400?,the temperature has little effect on coal seam thermal damage,elastic modulus and permeability,Large area heat damage occurred inside the coal body at more than 600?.(3)Based on the engineering background of Song he mine in Guizhou Province,the numerical simulation of gas extraction in closed mining face under electrothermal high temperature field is carried out by using the established mathematical model of Thermal-Hydrological-MechanicalDamage coupling.The results show that under the action of pumping hole and heating hole,the gas pressure in coal seam decreases continuously with the increase of pumping time,and the longer the residual gas pressure in coal seam,the lower the time.The gas pressure of coal seam is expanded outward along the pumping hole and heating hole by the influence range of pumping hole and heating hole,and gradually expands to the whole coal seam.When the temperature of heating hole is less than 400?,the effect of gas extraction in coal seam is not obvious,the effect of 600? on coal seam gas extraction gain is remarkable,and the temperature of heating hole is in the transition section of 400?~600?.