Estimating minimum miscibility pressure of shale oil/CO2 in organic nanopores using CO2 huff-n-puff

CO2-enhanced oil recovery (EOR) is a widely recognized method for tertiary oil recovery worldwide, particularly in exploiting shale oil and gas resources. This study proposes an approach to estimate the minimum miscibility pressure (MMP) of shale oil/CO2 systems inside a nanopore through huff-n-puff molecular dynamics (MD) simulations. We used graphene and octane molecules to model organic slit and shale oil. We observed that oil recovery increases until reaching the near miscible pressure, after which it plateaus. The reservoir temperature and slit height directly influence oil recovery, as higher temperatures and larger slit heights increase recovery rates. Using an oil recovery graph, we estimated an MMP inside the nanopore. Notably, the confinement effect of the nanopore affects the predicted MMP, resulting in a lower MMP within the nanopore compared to the bulk system. This effect diminishes as the slit height surpasses a specific size, causing the MMP inside the nanopore to rise due to enhanced adsorption along the nanopore walls. These findings suggest that CO2 flooding exhibits greater efficiency in shale oil reservoirs characterized by prominent pores and throats. Our MD-based simulation approach provides a cost-effective and expeditious supplement to CO2-EOR experiments, thereby contributing to advancing shale oil reservoir development.