Mechanism And Application Study On CO₂ Replacement And Displacement CH₄ In Low Permeability Coal Seam

With the increase of mining depth in China,the pressure and gas content of coal seam increase,the ground stress increases,the permeability of coal decreases,and the efficiency of gas extraction decreases.This dissertation is based on the theories of adsorption thermodynamics,hydrodynamics and coal petrology,the replacement and displacement mechanism of CH4 after injecting CO2 into coal seam is studied by combining laboratory experiment,physical simulation,numerical simulation and underground industrial test.Industrial tests are carried out in 2302 auxiliary transportation lanes of Changcun coal mine.The main research results are as follows:(1)Study on structure characteristics of cracks in coal samples,gas adsorption mechanism and thermodynamic properties of coal adsorption.The results of pore volume and specific surface area measured by the comprehensive analysis method are between the mercury injection method and the liquid nitrogen adsorption method.Pore volume and specific surface area of Zhaozhuang coal samples are greater than those of Changcun coal.The gas adsorption capacity decreases with increasing water content and temperature.The amount of CH4 and CO2 adsorbed by Zhaozhuang coal sample is greater than that of Changcun coal sample.As the temperature increases,(?)and 1/T increase linearly,and the adsorption constant decreased linearly.The adsorption of CO2 surface free energy,isosteric heat of adsorption and adsorption potential are greater than the adsorption of CH4.Adsorption capacity and adsorption volume are in accordance with the cubic function.(2)Study on microcosmic action mechanism of coal adsorbed CH4 and CO2.The CO2 molecules adsorption capacity and the equivalent adsorption heat are greater than CH4 molecules,and the CO2 energy peak value is smaller than CH4 energy peak value,which is obtained by simulating coal adsorbed CH4 and CO2 with MS software.The NMR experimental study shows that the higher the gas injection pressure of CO2,the better the effect of CO2 displacing CH4.The electron microscope scanning experiment shows that the increase of CO2 injection temperature and injection pressure promotes the development of pore structure in coal.(3)Physical simulation experiment of CO2 displacement CH4 with different gas injection pressures,moisture contents and coal body temperatures.Using self developed CO2 replacement and displacement CH4 experimental platform,it is concluded that CH4 gas permeability is greater than CO2 gas permeability,and the permeability and the injection pressure have a negative exponential relationship.The increase of CO2 gas injection pressure and water content leads to the decrease of replacement volume ratio,the increase of replacement efficiency and the increase of CO2 content in unit coal.The increase of coal temperature leads to the decrease of replacement volume ratio,the increase of replacement efficiency and the reduction of CO2 content in unit coal.The CO2 breakthrough time is shortened under high gas injection pressure,high water content,and low coal temperature.The variation law of CO2 gas concentration and CH4 gas concentration is opposite.The greater the CO2 injection pressure,the lower the moisture content and the lower the coal temperature,the greater the cumulative discharge volume of CH4,the greater the initial gas production rate.(4)Multi-field coupling model and evolution law of CO2 replacement and displacement CH4.The coupled mathematical model for CO2 replacement and displacement CH4 gas-water two-phase flow is constructed,and COMSOL software is used to conduct CO2 replacement and displacement CH4 multi-field coupling simulation.The results show that the higher the pressure of CO2 gas injection,the lower the permeability of coal body,the greater the gas drainage volume,and the effective radius of extraction is approximately consistent with the exponential relation of extraction time.The greater the initial permeability,the negative pressure at the vent and the diameter of the drill hole.With the increase of time,the smaller the gas pressure and gas content,the greater the amount of gas extraction.After the application of the 2302 auxiliary transportation lane in Changcun Coal Mine,the gas concentration increased by 2.02 times,the extraction purity increased by 3 times.The study on CO2 replacement and displacement CH4 technology in low permeability coal seam is of great significance for improving gas extraction efficiency,enhancing gas drainage technology and CO2 storage.