Wettability evaluation of silty shale caprocks for CO₂ storage

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.