Long-term evolution mechanisms of permeability during gas injection for enhanced coalbed methane recovery

Abstract

Understanding the long-term evolution of permeability during gas (N₂ or CO₂) injection for enhanced CBM recovery (G-ECBM) is of great significance for improving CBM recovery efficiency and achieving effective CO₂ sequestration. This study develops a thermo-hydro-mechanical (THM) coupled model to comprehensively capture the mechanical response, fluid transport, and thermal variations during G-ECBM. The model is validated and subsequently employed to elucidate the long-term evolution mechanisms of permeability under N₂ and CO₂ injection from the standpoint of coal strain. N₂-ECBM significantly improves CH₄ recovery rate, but the rapid breakthrough of N₂ reduces CH₄ concentration. In contrast, the recovery rate of CO₂-ECBM is lower than that of N₂-ECBM, but it maintains a relatively high CH₄ concentration consistently. At early stage of N₂-ECBM, the permeability decreases near production well (PW) as effective stress increases, while the permeability increases near injection well (IW) as effective stress decreases. Later, coal seam permeability increases as effective stress decreases and CH₄ desorption. For CO₂-ECBM, early permeability near PW shows minimal change as effective stress increases offsets CH₄ desorption, while later stages see reduces driven by CO₂ adsorption. Near IW, early permeability increases due to CH₄ desorption outweighs CO₂ adsorption, but later declines as CO₂ adsorption becomes stronger. The offer provides a basis for optimizing injection strategies and improving recovery rate.