Investigating the Effect of the Water–Oil Molar Ratio on CO2-Enhanced Oil Recovery from Nanoporous Shale Using Molecular Dynamics

CO2-enhanced oil recovery (CO2-EOR) can be an effective method for recovering shale oil and for the sequestration of CO2, but the influence of water on the microscopic CO2 displacement behavior of shale nanopores is not well understood. Molecular dynamics (MD) simulations were conducted to analyze the microscopic CO2 displacement behavior of hydrophilic quartz nanopores at a fixed NaCl salinity of 40,000 ppm with a range of molar ratios of water and oil. The distribution of carbon dioxide and petroleum molecules in the pores can be described by introducing two parameters, the directional residence coefficient in pore space (DRCPS) and relative local fluctuation (RLF), which can clearly define the migration dynamics of the two liquid phases in various water saturation conditions. CO2 preferentially accumulates near pore walls, occupies adsorption sites originally occupied by oil molecules, weakens oil–wall interactions, and promotes oil desorption. Meanwhile, dissolved CO2 enhances oil mobility by reducing intermolecular interactions. Pore size significantly affected displacement performance, with 7.5 nm pores exhibiting higher recovery than 4.5 nm pores. For the 2:3 water–oil molar system, CO2 exhibited consistently high DRCPS values, and a moderate water content reduced the transport resistance and enhanced the nanopore displacement efficiency. The RLF of water has a smaller impact on CO2 transport in small pores. However, in large pores, and even at higher MWORs, the presence of water molecules may form a “barrier” to CO2 migration, thus affecting the displacement effect. Adsorbed water layers can weaken oil–wall interactions, while excessive water forms water bridges that hinder CO2 migration and reduce displacement efficiency. These findings demonstrate that an appropriate MWOR can improve CO2-EOR performance in shale nanopores and provide microscopic insights for optimizing CO2 flooding in shale reservoirs.