Enhanced Oil Recovery Mechanism Of Supercritical CO₂ Flooding In Silica Nanopore By Molecular Dynamics Simulations

Recent years,with the promotion of oil production technology for unconventional reservoirs,the role of unconventional oil&gas increases in today’s energy supply.Unconventional oil,generally light oil with low viscosity,usually occurs in adsorption state and aggregates in nanoscale channels system.Those result in oil molecules being really difficult to transport in nanoscale channels.Researches demonstrate that the injection of carbon dioxide（CO₂）into unconventional reservoirs not only swells oil but also declines the oil/water interfacial tension,thus resulting in enhanced oil detachment and transport in nanopore.Here,molecular dynamic simulation was used to study microscopic behavior and mechanism of CO₂ injection detaching adsorption oil and promoting oil transport through pore-throat in silica nanopore.Our work would provide a comprehensive interpretation of EOR mechanism of CO₂ flooding,and the results could provide insights into the optimization of the engineering process in unconventional reservoir.To the oil detachment with CO_2WAG injection,simulation results indicate that the CO₂and water play different roles in CO_2WAG:the CO₂ slug could selectively dissolve apolar oil components,and the water slug provides stable water/scCO₂ promoting interface,which compels the miscible oil/CO₂ slug out of the nanopore.Furthermore,the synergistic effects of scCO₂ slug and water slug were found to account for a decreased injecting pressure and an improved dissolution capability ofscCO₂ slug,which endows the CO₂ WAG higher oil recovery efficiency than that of both scCO₂ injection and water injection.To CO₂ promoting oil transport through pore-throat,results show that oil transport is hampered by nanopore throat.The energy barrier of the transport process mainly results from deformation of oil and interaction between oil molecule and nanopore.Furthermore,in the present of CO₂,oil transport is shown to be kinetically activated,i.e.energy barrier and threshold pulling force decline,which was ascribed to the increase of oil diffusion coefficient and decrease of interfacial tension caused by the CO₂ perturbation.