Impact of Brine Composition on Calcite Wettability: A Sensitivity Study

Abstract

Summary Brine composition change has a significant demonstrated impact on the recovery of oil in laboratory corefloods. Although low-salinity waterflood in clastics has been studied extensively, the impact of brine composition on the wettability and recovery in carbonates is relatively less understood and studies are more recent. Wettability measurements by use of contact angles can reflect the surface energy changes caused by adsorption of compounds present in crude oil and the electrostatic, structural components caused by the formation of the double layer. In this study, the impact of ion composition on the contact angles and interfacial tensions (IFTs) between crude oil, brine, and restored/aged calcite-mineral surface is studied by use of a drop-profile-analysis method. A 13-factor experimental design is used to quantify the impact of six salts and seven interactions. This experimental method design, following the Taguchi method (Roy 1990), is used to identify the combination of different salts that leads to the lowest oil-wetness and highest water-wetness as measured through the contact angles. In addition, the impact of the total dissolved solids (TDS) on both the wettability and the IFT is studied for the selected crude oil/brine system. The analysis of variance of the measurements shows that the Alkali/Alkaline Earth metal chloride concentration has a significant impact on the wettability measured with static contact angles. The interactions between sodium chloride and sodium sulfate concentration; between sodium chloride and magnesium chloride salt concentration; and between sodium chloride and calcium chloride concentration are significant. The most-favorable interaction response is obtained at the highest sodium sulfate concentration along with lower sodium chloride, calcium chloride, and magnesium chloride concentrations. Therefore, a combination of higher concentration of sulfate anions with lower cation concentration and reduced salinity can lead to more-water-wet conditions. This composition is found to lead to the lowest contact angle or the most-water-wet condition. It is interesting to note that the lowest contact angle of approximately 29 ° (highly water-wet) is obtained with a relatively high TDS content of 134.5 g/L. This observation is in line with some of the recent studies reported in the literature on carbonate-rock corefloods and offers a fundamental explanation.