Toward the Rational Design of Chemical Formulations for EOR from Carbonates: Molecular–Level Understanding of Carbonate Wettability and its Reversal by Surfactants and Ions

Wettability is a key factor influencing oil production, particularly from the oil-wet carbonate reservoirs where the recoveries are often low. This is a serious problem for the oil industry as significant portion of the world's hydrocarbon reserves resides in carbonate formations. Since the wettability has its roots in the inter-molecular interactions between the oil and the mineral, our objectives are, first, to provide the molecular-level understanding of the carbonate wettability and, second, to apply this understanding to devise effective approaches for wettability alteration. Specifically, we focused on chemical additives such as surfactants and ions, which have demonstrated potential as wettability reversal agents. Molecular dynamics (MD) simulations were used as the primary method to study the wettability properties on newly-developed model calcite and dolomite surfaces that mimic experimentally-known mineral properties. Wettability reversal by cationic, anionic, and non-ionic surfactants, as well as by divalent ions (Ca2+, Mg2+, and SO42-) were investigated. A systematic approach for maximizing the surfactant efficiency by tuning the cationic surfactant head-group chemistry was proposed. To validate the MD simulation results, experimental contact angle measurements on dolomite chips were conducted. The MD simulation results demonstrated that, in the absence of asphaltenes, the oil-wetness of the carbonate minerals arises from the electrostatic attraction between the (negatively charged) oil carboxylates and the (positive) surfaces. Due to this electrostatic nature, the wettability could be reversed only by the cationic (positive) surfactants, which screen the oil-surface attraction. Other surfactant types had negligible effect, in agreement with the experimental contact angle measurements. Moreover, the wettability alteration efficiency of the cationic surfactants was directly related to their molecular charge distributions, offering guidelines for the practical design of the most potent wettability-reversing molecules. The simulations of the wettability alteration by Mg2+, Ca2+, and SO42- ions were likewise consistent with the contact angle measurements. The roles of individual ions in the multiple ion exchange (MIE) mechanism were deduced, and the known strong temperature dependence of their wettability alteration effect explained by the stability of the ion hydration shells. Finally, the simulations also exposed differences between the wettability reversal mechanisms on calcite and dolomite minerals, which may have important practical impact. Our results offer a novel perspective on the carbonate wettability and its reversal from the standpoint of atomic-level interactions and molecular mechanisms. New models for the carbonate surfaces were developed for reliable simulations of the wetting properties, which led to new insights into the origins of carbonate oil-wetness and the mechanisms of its reversal in two types of minerals. Lastly, the MD simulations demonstrated their utility as a powerful tool for the practical design and evaluation of potential chemical agents for EOR from carbonate reservoirs.