Impact of Brine Composition on Carbonate Wettability: A Sensitivity Study

Abstract

Abstract Brine composition has a significant demonstrated impact on the recovery of oil in laboratory corefloods. While low-salinity waterflood in clastics has been studied extensively the impact of brine composition on the recovery in carbonates is relatively less understood and studies are more recent. Wettability measurements using contact angles can reflect the surface energy changes due to adsorption of natural surfactants present in crude oil and the electrostatic, structural components due to the formation of double layer. In this study we measure the impact of ion composition on the contact angles and interfacial tensions between crude-oil, brine and restored/aged calcite mineral surface using a drop profile analysis method. A 9-factor experimental design is used to quantify the impact of six salts and three interactions. This experimental method design (following the Taguchi method) has the objective to identify the combination of different salts that leads to the lowest oil-wetness and highest water-wetness as measured through the contact angles. The salts used in this study are MgSO4, CaSO4, Na2SO4, NaCl, CaCl2 and MgCl2. The interaction parameters studied are between sodium chloride and sodium sulphate, sodium chloride and calcium chloride, sodium chloride and magnesium chloride respectively. Based on the outcomes of the analysis of variance (ANOVA) of the results we conclude that the chlorides have the maximum impact on the contact angle change to water-wetness (22% and 31 % respectively for sodium and calcium salts). Reduction in chlorides has the obvious benefit of changing the contact angle to more water-wet and this is a fairly well established result also. The interaction between chloride and sulphates and the interaction between sodium and magnesium ions was found to be significant. The above results indicate that a combination of salts such as high sulphates with lower cation concentration, can lead to more water-wet conditions. This combination is found to be optimal brine chemistry that leads to the lowest contact angle or the most water-wet condition. It is interesting to note that the lowest contact angle of ~29 degrees (highly water-wet) was obtained with a relatively high total dissolved solid content of 157,000 ppm. This observation is in line with some of the recent studies reported in the literature on carbonate rock corefloods and offers a fundamental explanation.