Study On Displacement Of Methane By Injecting Mixed Gas Into Coal Under Stress

China is a big coal producing country,rich coal resources is an important guarantee for China’s social and economic development.However,the mining of coal mines not only provides fuel for Industry and civil use,but also brings many mine disasters.Coal and gas outburst is one of the most serious disasters.The main reason of coal and gas outburst is that there is a large amount of gas in coal seam.In recent years,scholars at home and abroad put forward the method of gas injection displacement displacement of coal seam gas,which can not only effectively reduce the coal seam gas content,reduce the disaster of coal and gas outburst,but also extract and utilize the coal seam gas.Due to the complexity of coal properties and occurrence environment,the technology of gas injection displacement for coal seam gas is not mature,and the practical engineering application is less.At present,the most widely used two kinds of gas are CO2 and N2.The two kinds of gas displacement for methane have their own advantages and disadvantages,and the mixed gas of the two can achieve better effect by combining the advantages and disadvantages The law and mechanism of methane are of great significance.In this paper,the same confining pressure(1.5 MPa)and different axial pressures(6 MPa,9 MPa,12 MPa,15 MPa)were tested by the triaxial loading seepage displacement tester The results show that the higher the stress is,the worse the gas adsorption capacity is.Although the volume expansion and specific surface area increase slightly when the stress reaches the yield point(15 MPa),the inhibition effect of stress on the gas adsorption capacity is far less than that of stress,In the stage before coal yield,the competitive adsorption capacity of N2 relative to CO2 becomes worse with the increase of stress,and when the yield point is reached,The competitive adsorption capacity of N2 increased to 1.16 times of that at 12 MPa;in addition,the competitive adsorption capacity of N2 relative to CO2 increases with the increase of adsorption equilibrium pressure.Therefore,under the condition of low stress and high gas injection pressure,the displacement effect of mixed gas is the best.The test gas is made of pure CO2,80%CO2 and 20%N2,60%CO2 and 40%N2,40%CO2 and 60%N2,20%CO2 and 80%N2 and pure N2 as test gases.When the axial pressure and confining pressure are both 1.5 MPa,the methane displacement test is carried out with mixed gas.The results show that the higher the CO2 component in the test gas,the higher the methane displacement rate,the higher the N2 component,the higher the coal seam permeability.Among them,N2 component and coal seam permeability basically conform to the regular of quadratic equation.When the CO2 component in the mixed gas is higher(>60%),the displacement methane capacity is close to the pure CO2,while the concentration of N2 is higher(>60%),the better the effect of improving the permeability of coal seam is,and it is applicable to the coal seam The equation of gas component,methane displacement rate and coal seam permeability fitted according to the test data can be used to calculate the displacement rate and permeability of specific component mixture gas at 30 min,so as to measure the displacement displacement effect.The permeability model of coal rock based on the influence of effective stress on coal adsorption performance is derived by modifying the permeability model of coal rock studied by predecessors.On the basis of the test results,the displacement mechanism of methane by mixed gas under stress is discussed and analyzed.The displacement mechanism of CO2 and N2 is different,and CO2 mainly relies on strong adsorption performance to compete for adsorption N2 can promote methane desorption and permeability through gas partial pressure,and promote gas carrying to displace coalbed methane.The increase of stress will inhibit the displacement effect.After the adsorption and desorption reach the dynamic equilibrium,even if the methane is replaced,it is difficult to move out of the coal seam.Therefore,displacement and carryover play a dominant role in the extraction of coal seam methane by gas injection.