Seepage law of mixed gases in CO2 displacement of coal seam CH4

During the CO2 flooding process of coalbed methane,due to the competitive adsorption of CO2 and CH4 ,the CH4 concentration of CO2 -CH4 mixture will change continuously during infiltration,which will affect the deformation and permeability characteristics of coal. Using the self-designed triaxial seepage system,the steady-state infiltration method was used to test the coking coal with single component gases (He,CH4 and CO2 ) and different ratios of CO2 -CH4 mixture. The temperature and volume stress (30 ℃ ,33 MPa) were kept constant during the seepage,and the axial deformation of the coal was measured by LVDT. The results show that ① the seepage process of He and different ratios of CH4 -CO2 mixture is affected by the slippage effect,the gas permeability decreases first and then increases slowly with the increase of inlet pressure. For non-adsorbed He,the effect of slippage on the gas permeability is much greater than the effective stress effect when the inlet pressure is less than 2 MPa. ② Under a certain volume stress con- dition,the swelling strain caused by the adsorption of different component gases increases with the increase of inlet pressure,the variation law accords with the Langmiur equation;and under the same inlet pressure condition,the swell- ing strain of coal increases with the increase of CO2 concentration in the CH4 -CO2 mixture. ③ On the basis of consid- ering the effective stress effect,the dynamic influence of the adsorption swelling strain on the permeability and the change of the slip factor b with the permeability of the coal,the gas permeability model is established. This model can describe the variation of He and CH4 -CO2 mixture permeability with inlet pressure. ④ As the pressure of CH4 -CO2 mixture in coal reservoir in-creases or the concentration of CO2 in the CH4 -CO2 mixture increases,the influence of ma- trix swelling strain on coal permeability decreases. The influence of the adsorption swelling of the matrix near the pore fracture in the coal on the permeability (β) decreases with the increase of the inlet pressure;the higher the CO2 con- centration in the CH4 -CO2 mixture,the larger the β reduction rate.