Systematic analysis of coal deformation and permeability evolution during multi-condition gas injection: implications for CO 2 -ECBM

In the pilot test of gas injection for enhanced coalbed methane (ECBM) recovery, the phenomenon of gas injection failure was observed. Using different types of gases, physical simulation experiments under various conditions were conducted to systematically study the variations in coal deformation and permeability. The implications of the experimental results for ECBM were analyzed. The study found that pore pressure governs gas adsorption and desorption, thereby influencing coal deformation, whereas the pore pressure gradient controls the gas migration velocity. Under constant gas injection pressure, a higher backpressure leads to greater pore pressure and a smaller pore pressure gradient, resulting in slower gas migration. This prolongs the interaction between the gas and the coal matrix, making the adsorptive swelling more pronounced. Consequently, an excessively long gas breakthrough time can lead to localized blockage of flow channels near the injection zone due to adsorption-induced coal swelling. Based on this, a gas injection strategy of “rapid breakthrough - pressure buildup - depressurization drainage” was proposed. This strategy effectively displaces methane while maintaining unobstructed migration pathways in the coal, thereby resolving localized blockage near the injection zone. It ensures that the gas can be effectively injected, adequately dispersed, and smoothly produced.