Molecular Dynamic Simulation of CO2 Flooding Into Mineral Nanopores in the Presence of Residual Oil

Greenfield residual oil zones (ROZ) are considerably thick columns of oil at residual or near-residual oil saturation. In these zones, oil can only be recovered through the application of unconventional methods. Therefore, greenfield ROZ has emerged as an attractive target for enhanced oil recovery (EOR) by CO2 flooding. The geochemistry of the solid surface, reservoir conditions, as well as oil composition are parameters known for having a significant impact on the wettability of the rock to reservoir fluids. Although extensive research has been dedicated to studying EOR by CO2 flooding, the impacts of the aforementioned factor on CO2 flooding efficiency are still not clear. In this work, we use molecular dynamics (MD) simulations to investigate, at an atomistic scale, how the effectiveness of oil recovery is affected by (i) the geochemistry of different mineral types, (ii) temperature, and (iii) oil composition. In this work, we investigate interfacial interactions of systems composed of CO2/oil/water/minerals. The minerals included in this work are illite, kaolinite, and calcite. Oil phases of different compositions are analyzed. First, we evaluate an oil phase composed of only decane molecules. Subsequently, sodium decanoate molecules are mixed with decane molecules to represent an oil phase of different compositions. To represent a mineral nanopore at residual oil conditions, the surface of these minerals was randomly dispersed with the organic components. Then, MD simulations were performed to allow the organic molecules to equilibrate at the mineral surface. Finally, the system was flooded with CO2, and MD simulations were performed again. To investigate the effects of temperature on the CO2/oil/mineral interactions, simulations were performed at 350, 375, and 400 K. From the outputs of the simulations, density profiles and diffusion coefficients of the investigated molecules were quantified. Results indicated a higher decane adsorption on calcite surfaces compared to illite and kaolinite. We also observed that decanoate anions do not adsorb to the kaolinite silicate surface. Instead, they tend to interact with the hydroxyl surface. The results of the simulations performed on systems containing illite and sodium decanoate molecules suggested that these molecules will remain adsorbed to a clay surface when a bridging cation is present. When carbon dioxide was flooded into the mineral nanopores at a temperature of 350 K, a few oil molecules diffused into the bulk solution. An increase in temperature to 375 K and subsequently to 400 K led to an increase in the detachment of more oil molecules from the mineral into the bulk carbon dioxide solution. The outcomes of this work elucidate the CO2/oil/rock interfacial interactions and clarify the effects of oil composition, temperature, and mineral type on these interactions. Moreover, the results presented here have a direct contribution to the improvement of the efficiency of the displacement process of oil by CO2 injection in residual oil zones.