A hybrid CO2-surfactant thermal system can efficiently and eco-friendly improve the oil recovery rate for post-steam heavy oil reservoirs by modifying heavy oil characteristics. However, the complex conditions of high-temperature and high-pressure reservoirs hinder experimental investigations into the microscopic mechanisms of this system. In this study, Molecular Dynamics (MD) simulations reveal the molecular interaction mechanism between a hybrid thermal system, heavy oil, and pore surfaces. The hybrid thermal systems comprise CO2 and a cost-effective SDS (sodium dodecyl sulfate) surfactant. The effects of surface wettability, external temperature, and CO2 concentration on the transition of heavy oil microstructure are investigated.
Results show that the hybrid CO2-surfactant systems can effectively improve the microstructure of heavy oil by promoting the thermal expansion process. Moreover, surface wettability and CO2 concentration significantly affect the microstructure of heavy oil. Specifically, the thermal expansion of heavy oil is suppressed on hydrophobic surfaces compared to hydrophilic surfaces. This is because an oil-surface interaction promotes the formation of dense clusters of asphaltenes within a heavy oil layer, making the heavy oil on hydrophobic surfaces more challenging to recover. Meanwhile, the concentration of CO2 determines the state of a hybrid thermal system and ultimately affects the distribution of heavy oil. The hybrid thermal system with moderate dynamic activity and a high effective distribution ratio of CO2 can efficiently improve the microstructure of heavy oil for recovery and demonstrate the potential of CO2 storage. Furthermore, an optimum CO2 concentration of 10 wt.% is recommended for designing the hybrid thermal system.
This study provides insights into the molecular transition mechanism of heavy oil in various hybrid thermal systems. It offers valuable theoretical guidance for designing efficient and eco-friendly heavy oil recovery operations to support the transition towards carbon neutrality.
Keywords molecular transition mechanism; hybrid CO2-surfactant thermal system; heavy oil; molecular dynamics simulation, CCUS