Effect of CO₂ Sinusoidal, Step-Flow Injection on Coalbed CH₄ Desorption-Diffusion Mechanisms

Injecting carbon dioxide into coal seams not only enhances coalbed methane recovery and supplements natural gas resources but also sequesters CO2 in the coal seams, thereby reducing greenhouse gas emissions. This has significant implications for improving the efficiency of traditional energy utilization and environmental protection. Traditional constant-rate CO2 injection methods are costly and result in low CH4 desorption and displacement rates. Therefore, this study proposed four injection methods with an average flow rate of 100 mL/min: sinusoidal (Sin-200, Sin-150) and stepwise (SBS-200, SBS-150), and compared them with the 100 mL/min constant flow rate injection method (CF-100). The results showed that, under the same CO2 injection volume, the total CH4 desorption amount with the Sin-200 method doubled compared to the CF-100 method. The most significant increase occurred in the first stage, reaching 1.05 times, primarily due to the Sin method suppressing the closure of coal fractures and macropores during the low-flow gas injection stage, which favored CH4 desorption. The Sin-200 method increased the CH4 displacement rate by 0.86% at T30%, while it decreased by 0.15% at T1%, reducing CO2 injection volume by 2767.18 mL and saving 12 min of injection time. Using the SBS-200 method, the total volume of sequestered CO2 increased by 5.5 times. The Sin-200 method effectively improved coalbed methane recovery, reduced CO2 injection costs, and shortened injection time. The SBS-200 method sequestered more injected CO2 in the coal, significantly reducing greenhouse gas emissions. This was mainly because the SBS-200 method provided ample competitive adsorption time for CO2 and CH4 when the injection flow rate was 0 mL/min. Compared to the CF-100 method, the Sin-200 method increased the effective diffusion coefficient of CH4 by 1.09 times. The Fick-diffusion relaxation model accurately reproduced the diffusion process of CO2 displacing CH4 in coal seams. The diffusion mechanism of CO2 displacing CH4 in coal seams is controlled by the combined effects of Fick diffusion and relaxation diffusion. Ignoring relaxation diffusion leads to a significant deviation between experimental data and the analytical model curve, while ignoring Fick diffusion increases the maximum average relative error of the analytical solution by 10.85 times. This study proposed a novel CO2 injection method for displacing coalbed methane, which can be applied in field engineering.