Western Australia basalt-CO2-brine wettability at geo-storage conditions.

CO2 geo-storage is a technique, where millions of tonnes of CO2 are stored in underground formations every year for permanent immobilization to reduce greenhouse gas emissions. Among promising geo-storage formations, basalt is attracting keen interest from researchers and industry. However, the literature severely lacks information on the wetting behaviour of basaltic rocks at geo-storage conditions. To enable a more general statement of basalt-scCO2-brine contact angles, the wettability of a basalt from Western Australia was compared with a similar rock type from Iceland. This study reports the advancing and receding contact angles for a basalt-scCO2-brine system at pressures ranging from 0.1 to 20 MPa and temperatures of 298 and 323 K, respectively. Based on the experimental data, the amount of CO2, expressed by the column height, which could be safely trapped beneath the basalt was then calculated. The basalt was initially water-wet but with increasing pressure, it was converted sequentially from a water-wet to an intermediate-wet and then finally into a completely CO2-wet template at pressures exceeding 15 MPa and 323 K. Under those experimental conditions, found in the field at depths below 1500 m, injected supercritical CO2 into a porous basalt reservoir is assumed to flow freely in lateral and vertical directions and is less impeded by capillary/residual trapping, potentially leading to CO2 leakage. It is suggested that the injection depth should not be chosen too deep to avoid increased free CO2 plume mobility. It is found from CO2 column height calculations that at 800 m depth (a minimum requirement to keep CO2 supercritical), the height of the CO2 column that can be safely trapped below the cap rock, was still 100 m but shrank to nil at ≥1500 m.