Saudi Arabian basalt/CO₂/brine wettability: Implications for CO₂ geo-storage

The geological sequestration of carbon dioxide, including mineralization in basaltic formations, has been identified as a promising method of attaining a low-carbon economy. However, successful CO₂ storage depends on both the CO₂ wettability of the basaltic rocks and the basalt rock-fluid interfacial interactions. The contact angles of brine/CO₂ systems for Western Australian (WA) and Iceland basalts have been recently reported in the literature. However, contact angle datasets for evaluating the CO₂ wettability of Saudi Arabian (SA) basalt have not been previously reported. Moreover, there is limited information on the impact of organic acids on the wettability of the basalt/CO₂/brine system. In the present study, the contact angles of supercritical CO₂/brine systems on SA basalt are measured at temperatures of 298 and 323 K, and at various pressures of 0.1–20 MPa in the absence and presence of organic acid (10⁻² mol/L stearic acid). Various analytical methods are used to characterize the SA basalt surface, and the wetting behavior of the SA basalt is compared with that of the WA and Iceland basalts. The quantity of CO₂ that can be safely trapped underneath the SA basalt (in terms of CO₂ column height) is then computed from the experimental data. At the highest tested temperature and pressure (20 MPa and 323 K), the pure SA basalt is found to remain strongly water-wet, with advancing (θ_a) and receding (θ_r) contact angles of 46.7° and 43.2°, respectively, whereas the Iceland basalt becomes moderately water-wet (θ_a = 85.1° and θ_r=81.8°), and the WA basalt becomes CO₂-wet (θ_a = 103.6° and θ_r=96.1°). However, the organic-aged SA basalt attains a CO₂-wet state (θ_a = 106.8° and θ_r = 95.2°). In addition, the CO₂ column height of the pure SA basalt is higher than that reported for the WA and Iceland basalts. Further, at 323 K, the CO₂ column height decreases from 835 m at 5 MPa to −957 m at 20 MPa. These results suggest that there could be both freer plumb and lateral movement of CO₂ into the SA basalt in the presence of organic acid, thus resulting in lower residual and mineral trapping capacities, and fewer eventual leakages of CO₂, across the geological formation.