Effect of salinity, Mg2+ and SO42- on "smart water"-induced carbonate wettability alteration in a model oil system

Hypothesis: We present a systematic study of the "smart water" induced wettability alteration. This process is believed to he greatly affected by the brine salinity and the presence of Me2+ and SO42- in the brine. Experiments and modelling: To characterize the wettability alteration, we perform spontaneous imbibition measurement using Indiana limestone cores and a model oil with added naphthenic acid. Both singleelectrolyte-based and seawater-based "smart water" are tested to investigate the effect of Me2+, SO42- and salinity on wettability alteration. Rock/brine and oil/brine zeta potentials are measured, and the electrostatic component of disjoining pressure is calculated to understand the role of electrostatics in the wettability alteration. The surface concentration of charged species on the limestone surface is analyzed based on a natural carbonate surface complexation model (SCM). Findings: Both the reduction of Na+ and addition of SO42- are found to contribute to wettability alteration. Mg2+ is found to be unfavorable for wettability alteration. Ca2+ is believed to facilitate SO42- with wettability alteration based on the comparison between the single-electrolyte-based and seawater-based brines. The reduction of the Na+ surface complexation (>CaOH center dot center dot center dot Na+0.25) in low salinity brines is believed to be a critical mechanism responsible for wettability alteration based on the SCM calculations. (C) 2019 Elsevier Inc. All rights reserved.