| Summary: | Wettability is one of the main parameters controlling CO<sub>2</sub> injectivity and the movement of CO<sub>2</sub> plume during geological CO<sub>2</sub> sequestration. Despite significant research efforts, there is still a high uncertainty associated with the wettability of CO<sub>2</sub>/brine/rock systems and how they evolve with CO<sub>2</sub> exposure. This study, therefore, aims to measure the contact angle of sandstone samples with varying clay content before and after laboratory core flooding at different reservoir pressures, of 10 MPa and 15 MPa, and a temperature of 323 K. The samples’ microstructural changes are also assessed to investigate any potential alteration in the samples’ structure due to carbonated water exposure. The results show that the advancing and receding contact angles increased with the increasing pressure for both the Berea and Bandera Gray samples. Moreover, the results indicate that Bandera Gray sandstone has a higher contact angle. The sandstones also turn slightly more hydrophobic after core flooding, indicating that the sandstones become more CO<sub>2</sub>-wet after CO<sub>2</sub> injection. These results suggest that CO<sub>2</sub> flooding leads to an increase in the CO<sub>2</sub>-wettability of sandstone, and thus an increase in vertical CO<sub>2</sub> plume migration and solubility trapping, and a reduction in the residual trapping capacity, especially when extrapolated to more prolonged field-scale injection and exposure times.
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