Wettability alteration of carbonates—Effects of potential determining ions (Ca2+ and SO42−) and temperature

The wettability of carbonates is dictated by the stability of the water film between the rock and the oil phase, which is related to the Zeta-potentials of the oil–water and water–rock interfaces. At high temperatures, it is shown that sulfate present in the injection fluid can change the wetting state of chalk from preferential oil-wet to preferential water-wet conditions. The effect of sulfate increased as the temperature increased. The objective of this paper is to obtain information about the chemical mechanism behind this wettability alteration process. Artificial seawater, SW, where [SO42−]≈2[Ca2+], was used as the reference injection fluid, and in the modified seawater used, the relative concentration of the potential determining ions Ca2+ and SO42− appeared to have a crucial effect on the surface chemistry. The adsorption of sulfate onto chalk cores was studied at different concentration of Ca2+ and temperatures by means of a chromatographic technique previously used to determine the wettability of carbonates. The adsorption of sulfate onto chalk increased as the temperature and concentration of Ca2+ increased. The mechanism of the wettability alteration process is discussed in terms of the results from the chromatographic adsorption and the imbibition studies. Zeta potential measurements of the rock–water interface also confirmed that the relative concentration of Ca2+ and SO42− dictated the charge properties at the chalk surface at pH 8.4.