Specificity and Synergy at the Oil–Brine Interface: New Insights from Experiments and Molecular Dynamics Simulations

The interfacial tension (IFT) between oil and brine is a key parameter affecting the enhanced oil recovery process. Despite the several theoretical and experimental investigations on the oil–brine system, the salinity effect on the IFT of oil–brine is still not fully understood. There is a contradiction in the literature rather than consistency. In the present study, we combine molecular dynamics (MD) simulations with the pendant drop method to investigate the molecular interactions at the oil–brine interface to better understand the salinity–IFT relationship. Herein, we are taking into account the complex composition of both crude oil and brine and the pH and total acid number. Different salinity conditions have been considered ranging from deionized water to connate (formation) water. We also consider the effects of individual brines of the main alkali salts (i.e., NaCl, MgCl2, and CaCl2) that are common in carbonate reservoirs. The specificity and synergy of the molecular interactions are observed via the confrontation of the results of the mixed brines (seawater and formation water) with those of the individual brines. We observed a significant impact of the divalent cations on the oil–brine interfacial tension. Due to the specificity of the organic acid–Ca2+ type of interaction and the synergy between the different ions, complete encapsulation of the Ca2+ ions has been observed within the formation water brine. This induces the depletion of the organic acids at the interface and thus increases the IFT. Such ionic encapsulation has not been observed in the individual brines because the cation–anion (Cl–) and the cation–water interactions are strong enough to prevent the cation–acid encapsulation. The interplay between the electrostatic interactions and the cations’ dehydration-free energies is the main parameter that controls their specificity and synergy, affecting the oil–brine interfacial properties. This work provides important details on the ionic interactions influencing the interfacial properties between crude hydrocarbons and brine.