Impact of effective stress and CH₄-CO₂ counter-diffusion on CO₂ enhanced coalbed methane recovery

This study addresses how the competitive impact between the effective stress and the CH₄-CO₂ counter diffusion-induced coal matrix swelling/shrinking affects the CO₂ enhanced coalbed methane recovery. This issue is important both for the extraction of methane as an energy resource and for the safe sequestration of carbon dioxide in a coal seam. Through this study, a novel fully coupled coal deformation, gas flow, and CH₄-CO₂ counter-diffusion finite element (FE) model was developed and applied to the quantification of the combined net effects on coal permeability among the coal matrix swelling/shrinking due to gas displacement, pore pressure and in-situ stress. These combined effects are quantified through solving a set of coupled field equations which govern the coal deformation, prescribe the transport and interaction of gas flow in' a similar way to poroelastic theory, and define CH₄-CO₂ counter diffusion and flow in a coal seam. Numerical results demonstrate that the evolution of porosity and permeability is controlled by the competing influences between the effective stress and the CH₄-CO₂ counter-diffusion induced volume change. This achievement extends our ability to quantify the opposite impacts of effective stress-induced and CH₄-CO₂ counter diffusion-induced coal matrix deformation on the CO₂ enhanced coalbed methane recovery under field conditions.