Effect of sulfate ions on oil detachment from calcite surface: experiments and molecular dynamics simulations

To investigate the influence of sulfate ions in low-salinity waterflooding of carbonate reservoirs, we combined the physical experiments and molecular dynamic simulations. Contact angle experiments results showed that there is an optional salinity during the low-salinity waterflooding of carbonate reservoirs. Moreover, the sulfate ions have a significant effect on core wettability. Based on the experiments results, we conducted molecular dynamic simulations to explore the reaction mechanism in molecular scale. Results showed that under the dispersion force and the electrostatic interaction, the oil molecules migrate to the calcite surface and array in two layers, and form the oil-wet surface. The detachment of oil from calcite surface can be divided into two steps: the formation of water channel under electrostatic interaction; the formation of water film under H-bonding interaction. As a result, the calcite surface will alter from oil-wet to water-wet. Results shows that the sulfate ions participate in the electrostatic interaction and H-bonding interaction, accelerate the formation of water channel and water film, and accelerate the oil detachment from calcite surface.