CO2驱替煤层CH4中混合气体渗流规律的研究

CO2驱替开采煤层气过程中，由于CO2和CH4的竞争吸附，CO2/CH4混合气体在运移时CH4体积分数会不断发生改变，进而影响煤体变形和渗透特性。利用自主研发的三轴渗流系统，采用稳态渗流法对焦煤样进行单一组分气体(He,CH4和CO2)和不同配比的CH4/CO2混合气渗流试验。渗流过程中保持温度和体积应力(30 ℃、33 MPa)恒定，并利用LVDT测量煤体的轴向变形。结果表明:① He和不同配比CH4/CO2混合气的渗流过程均受滑脱效应的影响，气体渗透率随入口压力增大呈先减小后缓慢增大的变化;对于非吸附He，入口压力Symbol|@@2 MPa时滑脱效应对气测渗透率的影响要远远大于有效应力效应;② 在一定的体积应力条件下，不同配比CH4/CO2混合气体吸附引起的煤体膨胀应变随入口压力增加而增大，变化规律符合Langmiur方程，且在相同入口压力条件下，混合气体中CO2浓度越高，煤体膨胀应变越大;③ 在考虑有效应力效应、吸附膨胀应变对渗透率的动态影响以及滑脱因子b随煤体渗透率变化的基础上，建立了煤体气测渗透率理论模型，该模型能够描述不同配比CH4/CO2混合气体以及He渗透率随入口压力的变化;④ 随着煤储层CH4/CO2混合气体压力增大或者CO2体积分数升高，基质膨胀应变对煤体渗透率的影响逐渐减小。煤体中靠近孔裂隙的基质吸附膨胀对渗透率的影响(β)随入口压力的增加逐渐减小;CH4/CO2混合气体中CO2体积分数越高，β减小速率越大。

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 condition,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 swelling strain of coal increases with the increase of CO2 concentration in the CH4-CO2 mixture.③ On the basis of considering 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 matrix 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 concentration in the CH4-CO2 mixture,the larger the β reduction rate.