Numerical Insights into Water Saturation Effects on CO2 Sequestration in Enhanced Coalbed Methane Recovery

The injection of carbon dioxide (CO2) into coal seams serves a dual objective, namely augmenting coalbed methane (CH4) production and providing a geological CO2 storage solution, thereby offering both economic and environmental advantages. Given the typically water-rich nature of underground porous media, the impact of water on CO2-enhanced coalbed methane (CO2-ECBM) becomes a crucial factor that cannot be ignored. In order to assess the influence of varying reservoir initial water saturation and the physical parameters of coal rock on the CO2-ECBM process, we have developed a comprehensive Thermal-Hydro-Mechanical-Chemical (THMC) model utilizing COMSOL software. The simulation results indicate a substantial enhancement in CH4 recovery with increased CO2 injection pressures, demonstrating a positive correlation between higher CO2 injection pressures, augmented CO2 storage rates, and enhanced CH4 production efficiency. With the increase of displacement time and injection volume, the effective displacement radius increases, but the influence radius increment decreases gradually. The reason is that the competitive adsorption of CO2 in the matrix has a higher adsorption capacity than CH4, resulting in a decrease in reservoir permeability. In addition, the higher the water saturation, the lower the influence radius and the displacement effect. The reason is that the existence of reservoir water limits the migration speed of gas in the reservoir to a certain extent. In the mechanical field, the permeability of coal decreases slowly, and the output of methane gas in the production well is affected. The mechanical properties of coal reservoir are closely related to the evolution of permeability. The mechanical parameters of coal in this paper are mainly Young 's modulus and Poisson 's ratio. The results show that low Young 's modulus and high Poisson 's ratio show higher deformation resistance, and the output effect of CH4 is better.