Investigation of Affecting Factors on the Wettability and Wettability Alteration of Low-permeable Carbonate Reservoir Rocks in Presence of Mixed Ionic Surfactants

Altering the wettability of the carbonate reservoir rocks from oil-wet to water-wet has been figured prominently as one of the enhanced oil recovery methods in recent years. Putting the idea into practice requires a basic understanding of the wettability and the controlling factors. In this work, the influence of the factors such as; presence of polar components, electrical surface charges and the initial aqueous phase on formation of oil-wet carbonate surfaces have been investigated by measuring the contact angles in n-hexane (or n-decane)/aqueous phase/calcite systems at different compositional conditions. A very low permeable carbonate rock from one of the Iranian oil reservoirs has been selected for this study. The rock has an absolute permeability less than 1md and a measured porosity of 16.5 %. The effect of different ionic surfactants on wettability alteration of the rock, saturated with crude oil, has been measured and the consequent improvement in oil production is investigated by spontaneous imbibitions of solutions containing cationic, anionic and cationic-anionic surfactant mixtures. The results of imbibition tests are furthermore supported by measuring contact angles; taking pictures of producing oil drops from different exterior core plug surfaces and also observing the distribution of the remaining oil in cores at the end of experiments. The results show that cationic surfactants at concentrations higher than CMC can cause a remarkable oil production through alteration of wettability and, moreover, the extent of oil production increases with temperature. In contrast, it is observed that the anionic surfactants are not able to change the wettability of an oil-wet surface and their combination with the cationic type would reduce the oil production potential of cationic surfactants. Study of the sulfate ion concentration in water on wetting characteristics of carbonate surfaces at two different temperatures, 20 and 80°C, demonstrated that the sulfate ions could improve the wettability towards water wetness at higher temperature, and the contact angle would further decrease by increasing the concentration.