(308a) Roles of Alcohol As a Cosurfactant at Brine-Oil Interface Under a Typical Reservoir Condition

Cosurfactant is a chemical used along with surfactant to enrich the properties of the primary surfactant formulation. Specifically, in petroleum industry, cosurfactants have been deployed in chemical flooding for more than 40 years. As an important component in chemical flooding, understanding the roles of cosurfactants is of great significance. Many studies including experimental measurements, theoretical modeling and computer simulations have been carried out to clarify their roles at brine-oil interfaces. The reported major roles of the cosurfactants differ from case to case. Some reported that the cosurfactants decrease interfacial tension by forming hydrogen bonds with water. While some suggested that the main role of cosurfactants is increasing the solubility of surfactant. Another interesting study demonstrated that the cosurfactants help increase the local fluidity thereby preventing the unfavorable liquid crystal structure of surfactants. Though many suggestive studies have been reported previously, to our best knowledge, the roles of alcohol at the oil-water interface under reservoir conditions are still less clear.

Herein, we report a molecular dynamics simulation study to explore the role of alcohol (propanol) as a cosurfactant at brine-oil interface in chemical flooding under a typical reservoir condition (353 K and 200 bar). We demonstrate that propanol, as a cosurfactant, can transport through the oil and brine phases; such a dislocation of propanol in the system is a dynamic process. The head groups of surfactants are closer to the interfaces than those of propanol. The brine-oil interfacial tension decreases as propanol concentration increases. This is because propanol can form hydrogen bonds with water molecules, while decreasing the number of hydrogen bonds between surfactant and water. The introduction of propanol does not necessarily increase the local fluidity of surfactants at the interfaces. A local maximum fluidity is observed when surfactants are more perpendicular to the interfaces. Our work should provide important insights into the design and optimization of the surfactant formulas for chemical flooding process.