Gas–water–rock interactions in sedimentary basins: CO2 sequestration in the Frio Formation, Texas, USA

To investigate the potential for the geologic storage of CO2 in saline sedimentary aquifers, 1600 ton of CO2 were injected at ∼1500 m depth into a 24-m sandstone section of the Frio Formation — a regional reservoir in the US Gulf Coast. Fluid samples obtained from the injection and observation wells before, during and after CO2 injection show a Na–Ca–Cl type brine with 93,000 mg/L TDS and near saturation of CH4 at reservoir conditions. As injected CO2 gas reached the observation well, results showed sharp drops in pH (6.5 to 5.7), pronounced increases in alkalinity (100 to 3000 mg/L as HCO3) and Fe (30 to 1100 mg/L), and significant shifts in the isotopic compositions of H2O and DIC. Geochemical modeling indicates that brine pH would have dropped lower, but for buffering by dissolution of calcite and Fe oxyhydroxides. Post-injection results show the brine gradually returning to its pre-injection composition.