Mechanisms of CO2–CH4 displacement in shale reservoirs: Insights from triple-porosity structural controls

CO2 displacement technology plays a crucial role in unconventional natural gas recovery; however, binary gas interactions induce microstructural alterations in shale reservoirs. This study proposes a fluid–solid coupling numerical model that integrates CO2–CH4 competitive adsorption theory with shale triple-porosity theory. The model enables comprehensive analysis of CO2–CH4 migration dynamics and associated shale structural evolution during the displacement process. Finally, taking Zhaotong shallow shale as an example, the effect of CO2 injection on CH4 extraction efficiency and the storage of CO2 in shale were studied. The main conclusions of this paper are as follows. (1) The injection of CO2 slows down the deformation of shale under in situ stress. (2) The effect of CO2 displacement on CH4 production efficiency is good. (3) The displacement effect will be affected by gas injection pressure and shale geometric parameters.