CO2, CH4, and N2 Desorption Characteristics in a Low-Rank Coal Reservoir

Abstract

With increasing emphasis on low-carbon environmental protection, CO₂ enhanced coalbed methane production and methane reuse in abandoned mines (rich in N₂) have gradually become one of the future development directions. These scenarios involve the coordinated migration of different gases such as CO₂, CH₄, and N₂, and the differences in properties of different gases that affect the flow process. Previous studies often focused on the adsorption differences between gases, neglecting the differences during desorption process. In view of this, the current work conducted experiments and finite element numerical analysis on the desorption process of CO₂, CH₄, and N₂, clarified the differences and influencing factors of desorption among the gases, and analyzed the flow change rules under different permeability and diffusion capabilities. The results indicated that the main differences among CO₂, CH₄, and N₂ during desorption are reflected in the parameters of Langmuir volume, permeability, and diffusion coefficient. These parameters showed that CO₂ has the highest value during desorption, while N₂ has the lowest. The factors affecting the magnitude of differences between CO₂, CH₄, and N₂ are mainly their compositions. Specifically, ash content significantly affects the difference in adsorption capacity, while moisture content influences permeability and diffusion coefficient. During desorption, permeability plays a continuous role throughout the whole process, while diffusion coefficient is exhibited mainly in the initial stage of desorption. Different gases have varying sensitivities to permeability and diffusion coefficients during desorption. Changes in permeability and diffusion coefficient significantly affect the CO₂ desorption process. N₂, on the other hand, is the least sensitive, especially to changes in diffusion coefficient. During gas flow, when reservoir permeability is less than 0.01 mD (= 9.869233 × 10⁻¹⁸ m²), permeability becomes the main factor that affects flow. When the diffusion coefficient is less than 5 × 10⁻⁹ m²/s, increasing the diffusion coefficient is necessary to effectively promote gas outflow. To effectively increase gas production, it is necessary to comprehensively consider the magnitudes of permeability and diffusion coefficient.