Molecular insight into minimum miscibility pressure estimation of shale oil/CO2 in organic nanopores using CO2 huff-n-puff

CO2-enhanced oil recovery (EOR) has been regarded as an essential means of tertiary oil recovery worldwide, which has been gradually applied in exploiting shale oil and gas resources. In this study, we proposed a new method to estimate the MMP (minimum miscibility pressure) of shale oil/CO2 systems using huff-n-puff molecular dynamics (MD) simulations. The bulk MMP of the shale oil/CO2 system obtained agrees reasonably well with the vanishing interfacial tension (VIT) and the available experimental results. The organic nanopore oil reservoir is modeled with graphene as a substrate and octane molecules as shale oil. We found that oil recovery increases up to near miscible pressure and plateaus after that. The oil recovery increases with the rise of the reservoir temperature and slit height. Due to the nanopore's confinement effect, the predicted MMP is lower inside the nanopore than the bulk counterpart. The MMP inside the nanopore decreases with a slit height up to a specific size and then starts to climb up due to the increased adsorption effect of nanopore walls. It means that CO2 flooding will be more efficient in shale oil reservoirs with more prominent pores and throats. Some CO2 molecules remain inside the nanopore after the completion of the huff-n-puff process. Like the oil recovery, the amount of CO2 trapped inside the pore after the huff-n-puff procedure plateaus after some critical pressure. So, CO2 huff-n-puff has two benefits — oil recovery and carbon sequestration. Our method of simulating huff-n-puff using MD can be a quick and economic supplement to the CO2-EOR experiments, which might benefit the development of shale oil reservoirs.