Atomic-scale theory of relative wettability of surfaces for enhanced oil recovery

Extraction of oil from wells is hampered by the fact that oil sticks to rock surfaces and water does not pry it loose easily. Technically, this is an issue caused by the relative wettability of rock surfaces. Experiments have shown that Na ions that are present in sea water have a negative effect on oil extraction, while Ca, Mg, and other ions enhance oil extraction. However, only limited understanding of the pertinent mechanisms has been achieved. Atomic-scale modeling of wettability is usually pursued using classical molecular dynamics based on empirical potentials. Only limited research based on quantum mechanical calculations has been reported so far. Here we describe the development and implementation of parameter-free, quantummechanical approaches, at different levels of approximation, that can provide detailed understanding of relative wettability and have predictive capabilities. At the lowest level of approximation, we calculate the binding energies of water and prototype oil molecules to calcite surfaces in vacuum as indicators of relative wettability. At the next level, we calculate binding energies in the presence of liquid water using quantum molecular dynamics. We find that the binding energy of Na acetate is larger than the binding energy of acetic acid, a prototype oil molecule, which suggests that, upon reacting with Na ions, a layer of oil becomes stickier on calcite rocks. On the other hand, Ca and Mg acetate desorb easier than acetic acid, facilitating oil extraction, as observed. At a much more sophisticated level of approximation, we calculate the wetting angle, a measurable quantity that serves as a measure of relative wettability. We applied this method to water on graphene and graphitic surfaces, which has been studied extensively and for which we have obtained new experimental data.