Experimental Investigation of Wettability Alteration in Oil-Wet Reservoirs Containing Heavy Oil

Solvent injection is an effective way to lower the viscosity of heavy oil and is considered when thermal techniques are not practically applicable. However, for the economic success of the method, large fractions of the injected solvent must be recovered. This requires further treatments of the reservoir, including water injection with chemicals to penetrate into the oil-wet matrix by changing the wettability. In this paper, we investigate the effect of wettability alteration of the oil-wet rock on solvent as well as oil recovery. Different wettability-alteration agents were tested, including cationic and anionic surfactants, ionic liquids, nanofluids, high-pH solutions, and low-salinity water. The potential of these materials to modify the wettability of aged sandstone and limestone samples was evaluated by use of imbibition tests. After conducting a total of 35 experiments, the most-promising wettability- alteration agents were identified to be anionic surfactants and high-pH solutions in addition to low-salinity water for the sandstone cases. Ion-pair interaction in sandstone and the dissolution of mixed-wet clay particles in carbonate are the main mechanisms of wettability alteration by those chemicals. Cationic surfactants and high-pH solutions were identified as the best wettability modifiers for the limestone samples. Although cationic surfactant changes the wettability by the ion-pair-interaction mechanism, the pH solution is believed to restore the water-wetness by decreasing the attraction forces between calcite and organic components. Ionic liquid at low concentration is able to alter the wettability of carbonate better than other conventional wettability modifiers. One important finding of this work is that solvent injection in heavy-oil-containing reservoirs is essential to condition the reservoir (i. e., to dilute the heavy oil before any wettability-alteration treatment can take place).