Geological parameters and gas mixture composition on enhanced coalbed methane recovery: A THM modeling approach

The utilization of coalbed methane (CBM) cannot only alleviate the energy crisis, but also reduce greenhouse gas emissions. Gas injection is an effective method to enhance CBM recovery. Compared to single-gas injection, the injection of CO 2 /N 2 mixtures can balance the sharp decline in permeability caused by pure CO 2 and the premature breakthrough by pure N 2 . In this study, a more comprehensive thermo-hydro-mechanical (THM) coupled mathematical model was developed, incorporating processes such as ternary gas non-isothermal adsorption, gas dissolution in water, gas–water two-phase flow, energy exchange, and coal deformation. After experimental validation, the model was applied to simulate the entire process of gas mixtures for enhanced CBM recovery (GM-ECBM). Results indicate that the permeability near the production well (PW) initially decreases due to increased effective stress, then increases as a result of CH 4 desorption. Near the injection well (IW), the permeability first increases due to reduced effective stress and later stabilizes under the combined effects of effective stress and CO 2 /N 2 adsorption. The initial CH 4 pressure and coal seam permeability have the most significant impact on CH 4 production, while the coal seam permeability and temperature significantly affect CO 2 /N 2 injection. As the coal seam permeability increases, the optimal CO 2 /N 2 ratio also increases accordingly. These findings provide important theoretical guidance for improving GM-ECBM efficiency in coal seams with varying permeabilities.