This paper resolve the salinity-dependent interactions of polar components of crude oil at calcite-brine interface in atomic resolution. Molecular dynamics simulations carried out on the present study showed that ordered water monolayers develop immediate to a calcite substrate in contact with a saline solution. Carboxylic compounds, herein represented by benzoic acid (BA), penetrate into those hydration layers and directly linking to the calcite surface. Through a mechanism termed screening effect, development of hydrogen bonding between –COOH functional groups of BA and carbonate groups is inhibited by formation of a positively-charged Na+ layer over CaCO3 surface. Contrary to the common perception, a sodium-depleted solution potentially intensifies surface adsorption of polar hydrocarbons onto carbonate substrates; thus, shifting wetting characteristic to hydrophobic condition. In the context of enhanced oil recovery, an ion-engineered waterflooding would be more effective than injecting a solely diluted saltwater.

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Inorganic scale deposition has been found to affect many industrial processes, including water injection into the oil reservoirs. The incompatibility of high sulfate ion content of seawater with formation water containing calcium ions results in formation damage and production decline. In this study, several simultaneous techniques are utilized for qualitative and quantitative analyses of calcium sulfate scale to get more insight into the formation damage during smart water flooding at micro and nanoscales. In the experimental section, calcium sulfate deposition due to the mixing of the formation water and seawater samples was investigated using the dynamic quartz crystal microbalance technique. The effect of sulfate and magnesium ions existing in the seawater on the amount of calcium sulfate deposition was studied, individually. The results showed that the sulfate concentration of seawater could significantly change the mass deposition in a specific range. Also, at an optimal concentration of the magnesium ions, the total amount of calcium sulfate deposition decreased by 60 percent. However, magnesium ions could decrease the time of the initial stage of deposition significantly. The results revealed the amount of deposition and the time of initial stage beyond 5 times dilution of seawater are not noticeable. In addition, the linear slope of the second stage of deposition for the mixture of formation water and 5-fold diluted seawater decreased by 92 percent compared to the original seawater. To verify the results for the magnesium effect, the molecular dynamics simulation method was used to compare the simulation results with the experimental data. Likewise, the results obtained from the simulation model showed that at an optimal concentration of the magnesium ions in the seawater, the amount of calcium sulfate deposition was noticeably decreased.

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