Comparative Study of Variable-Flow Gas Injection Patterns on CH4 Diffusion Dynamics: Experimental Insights into Enhanced Coalbed Methane Recovery

Variable-flow displacement has been effectively used to enhance oil recovery; however, it has rarely been investigated for coalbed methane production, and the CH4 diffusion laws in this process are not clear. In this paper, we carried out a study on the CH4 diffusion law in the bidirectional diffusion process displaced by variable-flow gas injection. The emission and desorption quantity of CH4 under variable-flow gas injection, the displacement effect under the principle of equal time and quantity, and the applicability of the CH4 diffusion model for the bidirectional diffusion process were analyzed. The results indicate that the variable-flow injection modes emit more CH4 compared to constant flow injection. The CH4 emission and desorption quantities for each injection mode are as follows: step-changed > sinusoidal-changed > constant flow. Secondly, the order of CH4 emission and desorption quantity in each gas injection mode is as follows: step-changed > sinusoidal-changed > constant flow. When CO2 is the injection gas source, the outlet CH4 emission and desorption quantity are larger than N2 injection. Thirdly, through the analysis of the principle of equal time and equal quantity, the variable-flow injection modes consume less gas for each volume of emitted CH4, resulting in a more effective displacement. Finally, the diffusion fitting effect of the bidisperse model for CH4 in the bidirectional diffusion process is better than that of the unipore model, and the bidisperse diffusion model can better fit the mt/m∞ curve of CH4 diffusion during the variable-flow gas injection replacement process.