Imbibition of sulfate and magnesium ions into carbonate rocks at elevated temperatures and their influence on wettability alteration and oil recovery

Oil recovery by an imbibition process at elevated temperatures depends not only on fluid composition but also on the history of the process, that is, the composition of the initial fluid in contact with the carbonate rock before the drainage process. In this work, extreme cases have been investigated with ion-free water and water containing Mg2+ or SO42-. Modified carbonate rocks with stearic acid that was initially saturated with ion-free water followed by an imbibition process with fluids containing Mg2+ or SO42-, at the same concentration as the injected seawater, shows the highest oil recovery when Mg2+ is present in the imbibing fluids. Whereas the initially saturated fluid and imbibing fluids contain SO42-, an inconsiderable difference between the oil recovery factor with SO42- and that for the ion-free water was observed. This is in contrast to Mg2+-containing imbibing fluid, which showed the lowest recovery. Computation of the disjoining pressure for the three systems (ion-free water, SO42-, and Mg2+) indicates that the presence of magnesium ions gives a more stable water film and requires increasing capillary pressure to rupture the water film as the temperature increases, as shown by the maximum in the repulsive energy. In addition, the calculation by Derjaguin, Landau, Verwey, and Overbeek theory and experimental observations may suggest that fine detachment is one of the mechanisms that alters the wettability of the rock by increasing the temperature.