Molecular Insights into the Enhanced Shale Gas Recovery by Carbon Dioxide in Kerogen Slit Nanopores

Full exploitation and utilization of the unconventional reservoirs of shale gas have become a central issue due to the increasing worldwide energy demand. Enhancing shale gas recovery by injecting CO2 is a promising technique that combines shale gas extraction and CO2 capture and storage (CCS) perfectly. In this study, a kerogen-based slit-shaped pore with a width of similar to 21 angstrom was constructed by two kerogen matrices, and the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to investigate the adsorption and diffusion properties of CH4 and CO2 in the kerogen matrix and slit nanopores and explore the displacement efficiency of the residual CH4 by CO2 in kerogen slit nanopores. The adsorption energy of CH4 and CO2 on the kerogen fragment surface and the isosteric heat of CH4 and CO2 in kerogen slit nanopores were examined to demonstrate the competitive adsorption of CO2 over CH4 in kerogen slit nanopores, and the different intensity of interactions between the CH4 and CO2 molecules with the pore surface plays a key role. An effective displacement process of the residual adsorbed CH4 by CO2 in kerogen slit nanopores was performed. The efficiency of displacement was enhanced with the increasing bulk pressure, and the sequestration amount of CO2 in kerogen slit nanopores increased at the same time. Moreover, it was found that part of CH4 adsorbed firmly inside the intrinsic pores of the kerogenmatrix was very hard to be displaced by the CO2 injection. This work demonstrates the microbehaviors of CH4 and CO2 in kerogen slit nanopores and the microscopic mechanism of the displacement of CH4 by CO2, for the purpose of providing useful guidance for enhancing shale gas extraction by injecting CO2.