Mechanisms of Surfactant Adsorption and Interfacial Tension Lowering for Enhanced Oil Recovery (EOR) Applications*

The robust evaluation of aqueous surfactant formulations depend on their ability to lower the interfacial tensions (IFTs) of oil/water interfaces for mobilizing oil. For a formulation to be most effective, the IFT of an oil/water interface should be ultralow (<0.01 mN/m). We have discovered that there are two types of relevant equilibrium IFTs (EIFTs). The un-pre-equilibrated EIFT (EIFT_up) is established when primarily the surfactants have equilibrated across the interface, and applies to the early stages of the EOR process. The pre-equilibrated EIFT (EIFT_p) is established when the oil and aqueous components are also equilibrated, and is applied to the later stages. The two EIFTs can be different due to the preferential partitioning of the various surfactant components usually present in commercial surfactants. Such partitioning can change drastically the chemical potentials and the partition coefficients of these surfactant components, and causes an IFT shift to a more stable value as the oil and aqueous phases components equilibrate also at longer times. The results have significant implications for understanding the effect of the surfactant concentration and composition on the EIFT.