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 CO 2 is a promising technique that combines shale gas extraction and CO 2 capture and storage (CCS) perfectly. In this study, a kerogen-based slit-shaped pore with a width of ∼21 Å 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 CH 4 and CO 2 in the kerogen matrix and slit nanopores and explore the displacement efficiency of the residual CH 4 by CO 2 in kerogen slit nanopores. The adsorption energy of CH 4 and CO 2 on the kerogen fragment surface and the isosteric heat of CH 4 and CO 2 in kerogen slit nanopores were examined to demonstrate the competitive adsorption of CO 2 over CH 4 in kerogen slit nanopores, and the different intensity of interactions between the CH 4 and CO 2 molecules with the pore surface plays a key role. An effective displacement process of the residual adsorbed CH 4 by CO 2 in kerogen slit nanopores was performed. The efficiency of displacement was enhanced with the increasing bulk pressure, and the sequestration amount of CO 2 in kerogen slit nanopores increased at the same time. Moreover, it was found that part of CH 4 adsorbed firmly inside the intrinsic pores of the kerogenmatrix was very hard to be displaced by the CO 2 injection. This work demonstrates the microbehaviors of CH 4 and CO 2 in kerogen slit nanopores and the microscopic mechanism of the displacement of CH 4 by CO 2, for the purpose of providing useful guidance for enhancing shale gas extraction by injecting CO 2 .