Relationship of Various Interfacial Tensions of Surfactants/Brine/Oil Formulations to Oil Recovery Efficiency

Abstract

Chemical enhanced oil recovery aims to increase the oil recovery of mature oil fields. The interfacial tensions (IFTs) between the aqueous solution, the oil phase, and other possible phases are important for designing and estimating a chemical formulation effectiveness. Ultralow (<10–² mN·m–¹) IFTs are needed to increase the capillary number and help mobilize trapped oil droplets. Five types of equilibrium IFTs (EIFTs) have been defined as follows: (I) the un-pre-equilibrated equilibrium IFT; (II) the un-pre-equilibrated EIFTs in the presence of rock; (III) the pre-equilibrated EIFTs in the presence of oil; (IV) the pre-equilibrated EIFT in the presence of rock and oil; and (V) the effluent EIFT. Among the five types of EIFTs, the fourth and fifth were found to be the most important for the highest oil recovery performance in core flood tests, because they capture the effects of surfactant partitioning into the oil phase and the adsorption losses on the rock surface. For three surfactant formulations tested with Berea core flood experiments, the one with the lowest type-IV EIFT (∼0.01 mN·m–¹) had the highest oil recovery ratio (78%), and the one with the highest type IV EIFT (∼0.2 mN·m–¹) had the lowest oil recovery ratio (55%). The other EIFTs correlated less well with the oil recovery performance. Identifying surfactant formulations that have low or ultralow EIFTs, especially of type IV or V, is critical for screening formulations appropriate for core flood tests and target field applications, and for predicting the oil recovery performance.