Effect of Surfactants on Oil/CO2 Minimum Miscibility Pressure of Unconventional Liquid Reservoirs-Molecular Dynamics Simulation Study

Abstract CO2 flooding has been identified as an effective method for enhancing the recovery rate of unconventional oil and gas. However, in many oilfields, the miscibility pressure of CO2 with crude oil exceeds the oilfield pressure, preventing them from achieving miscibility. This impedes the desired recovery outcomes. Surfactants present a solution to this challenge, as they can reduce the miscibility pressure between CO2 and crude oil, thereby elevating the recovery rate. Yet, the microscopic dynamics of how surfactants modulate the MMP in CO2 flooding within ULRs remain under-explored. This research aims to delve into this gap, using molecular dynamics to elucidate the underlying mechanisms and potential benefits of surfactant inclusion in CO2 flooding applications for unconventional reservoirs. Our study, rooted in molecular dynamics, seeks to demystify these dynamics and understand surfactants’ role more profoundly. Delving into the CO2-n-decane system, we discovered that C12PO6 and SW320 significantly alter the interfacial width by forming a molecular film, which enhances CO2's solubility in crude oil. Notably, SW320 emerged as more potent than C12PO6 in this regard. Both C12PO6 and SW320 managed to reduce the MMP of the CO2-n-decane system by more than 15%. However, when it comes to cost-effectiveness, C12PO6 shows a compelling balance between performance and affordability. Further insights revealed that the structure of the C12PO6 surfactant plays a crucial role in determining its MMP reduction capacity. Intriguingly, the addition of low carbon alcohols, especially n-pentanol, enhances the C12PO6 surfactant's surface activity, underscoring its superiority over ethanol in reducing interfacial tension. In essence, this research offers a microscopic lens to view the intricate dance of surfactants in CO2 flooding within ULRs. Our findings provide a robust framework for refining recovery strategies in unconventional reservoirs, potentially transforming the landscape of CO2 flooding methodologies to ensure more sustainable and efficient oil and gas extraction.