Modeling Acid Oil Component Interactions with Carbonate Reservoirs: A First-Principles View on Low Salinity Recovery Mechanisms

Abstract

Enhanced oil recovery by low salinity water (LSW) injection in carbonate reservoirs constitutes one of the main interests for petroleum industry. A molecular mechanism involving acid oil components and the replacement of surface Ca atom by Mg atom has been proposed. To determine the thermodynamic feasibility of this mechanism, we study the propionic acid adsorption upon calcite (10–14) and Mg-calcite (10–14) surfaces by first-principles calculations based on density functional theory. In vacuum, the propionic acid adsorption energy upon both surfaces is negative, not in favor of the proposed mechanism. In solvent, as a first approach, we include a water monolayer (ML), and we observe a decrease on the adsorption energy due to a change on the surface oxygen atom involved in the acid–surface hydrogen bond, but the acid remains stable upon both surfaces. Additionally, we explore the effect of adding an aqueous media on the adsorption energy. We employ a continuum solvent model without and with explicit water ML. For the first case, we obtain substantial changes on the energy adsorption due to the increasing of the acid–surface hydrogen bond, whereas for the second one, where the effect of explicit water ML is considered, a minor energy variation is observed. In both scenarios the acid becomes unstable upon Mg while remains stable upon Ca, in favor of the proposed mechanism. Furthermore, the effects of salinity, temperature, and pressure are explored in an effective way through the variation of the solvent dielectric constant (50–90). It can be inferred from our results that if Ca–Mg replacement occurs, this mechanism is thermodynamically feasible for the whole dielectric constant range studied.