Gas transport behavior of tectonic and intact coal reservoirs during CO2-enhanced coalbed methane recovery

Gas transport behaviors during CO2-enhanced coalbed methane recovery (CO2-ECBM) critically govern CO2 sequestration efficiency and CH4 recovery rates. While existing research predominantly focuses on intact coal reservoirs, the migration mechanisms in China's prevalent tectonic coal formations under CO2 injection remain insufficiently understood. This study establishes a modified thermo-hydro-mechanical multi-field coupling model for binary gas migration in both intact and tectonic coal seams, implemented through COMSOL multiphysics simulations. The evolution of permeability and the difference in CO2 influence range for intact and tectonic coal at various CO2 injection pressures, initial coal seam pressures, and CO2 injection temperatures were analyzed systematically. The results of this study demonstrated that under the influence of different parameters, both intact coal permeability and tectonic coal permeability presents a three-stage distribution characteristic of “decrease–stable–increase.” The permeability of tectonic coal in the vicinity of the injection well is greater than that of intact coal. Furthermore, the permeability of intact coal is greater in proximity to the extraction well than that of tectonic coal. As injection pressure and initial pressure increase, the CO2 influence range for intact coal and tectonic coal increases, and the increase in the CO2 influence range in intact coal is more pronounced. The injection temperature has been shown to be inversely proportional to the CO2 influence range for both intact coal and tectonic coal. However, its influence on the CO2 influence range is relatively negligible for both intact and tectonic coal. Finally, the study revealed that the reconstruction of seepage channels caused by structural damage to tectonic coal is the fundamental cause of the formation of differential migration behavior, and the selection of target coal seams for the CO2-ECBM project was discussed with reference to the results in this paper. These findings provide critical theoretical support and practical guidelines for reservoir selection in CO2-ECBM projects.