Wettability alteration of carbonate reservoirs from oil-wet to water-wet is an important method to increase the efficiency of oil recovery. Interaction between surfactants and polymers can enhance the effectiveness of surfactants in EOR applications. In this study, the interaction of polyethylene glycol (PEG) with an ionic surfactant, sodium dodecyl sulphate (SDS), is evaluated on an oil-wet carbonate rock surface by using contact angle measurements. The results reveal that wettability alteration of carbonate rocks is achieved through PEG/SDS interaction on the rock surface above a critical aggregation concentration (CAC). The behaviour of PEG/SDS aqueous solutions is evaluated using surface and interfacial tension measurements. Furthermore, the effect of PEG and SDS concentrations and impact of electrolyte addition on PEG/SDS interaction are investigated. It is shown that electrolyte (NaCl) can effectively decrease the CAC values and accordingly initiate the wettability alteration of rocks. Moreover, in a constant SDS concentration, the addition of NaCl leads to a reduction in the contact angle, which can also be obtained by increasing the aging time, temperature and pre-adsorption of PEG on the rock surface.

For a low-permeable water-saturated cap-rock, interfacial tension between CO₂ and connate water and interfacial interactions among reservoir rock, connate water and CO₂ system are major parameters to evaluate the capillary sealing characteristics of the cap-rock. Moreover, the amount of CO_2, which is trapped by capillary forces, depends on the wetting properties of the reservoir rock. In this work, contact angles of a water/shale system with CO₂ were determined at high pressures and a constant temperature of 318 K and at a salinity of pure water and 1 M NaCl. To prevent neglecting the di-electrical and petrophysical properties of natural rocks, all experiments have been performed using natural shale surfaces without any chemical treatment. In addition, to describe the wetting properties of the shale at different temperatures, surface-free-energy analysis based on the modified equation of state (EOS) is used. The results show that the contact angle of a CO₂/water/shale system increases with decreasing temperature. However, the change of contact angle with temperature is less significant at higher pressures. According to these results, it can be concluded that, for the CO₂/silty shale cap-rock systems, the possibility of capillary breakthrough of CO₂ phase is low even at very high pressures and low temperatures.