Experimental Study on Effective Permeability of Supercritical CO₂/CH₄ Mixture Fluid in Coal

For CO₂ enhanced CH₄ recovery in the coal seam, whose effective permeability would change with the different phase CO₂. To evaluate this, a steady-state seepage test method was used to conduct the permeability tests under varying multiphase and multi-component CO₂/CH₄ mixture fluid. The mixture permeability of gaseous or supercritical CO₂ + CH₄ under different fractions decreases with respect to the CO₂ concentration increasing. Meanwhile, the mixture permeability first decreases and then increases with the increasing injection pressure, and there is an obvious slippage effect when the injection pressure is lower than 3 MPa. The established permeability model values are in good agreement with the experimental data. The coal deformation accords with the Langmuir equation in the seepage process of CO₂ + CH₄ mixture under in situ conditions, and the two equation parameters (ε_L, P _L) have a good linear relationship with CO₂ concentration. The CH₄ concentration in the outlet increases about 24.5% in the supercritical CO₂ + CH₄ mixture but less dramatic only about 8.5% in the gaseous CO₂ + CH₄ mixture. At the same time, the gaseous CO₂ (or CH₄) effective permeability increases (decreases) with increasing gaseous CO₂ concentration, while the supercritical CO₂ or CH₄ effective permeability basically remains unchanged with increasing supercritical CO₂ concentration. This study is meaningful for the prediction of mixed fluid permeability in the process of the CO₂ displacement exploitation of CH₄.