Potential environmental issues of CO2 storage in deep saline aquifers: geochemical results from the Frio-I brine pilot test, Texas, USA

Sedimentary basins in general and deep saline aquifers in particular, are being investigated as possible repositories for large volumes of anthropogenic CO2 that must be sequestered to mitigate global warming and related climate changes. To investigate the potential for the long-term storage of CO2 in such saline aquifers, 1600 t of CO2 were injected at 1500 m depth into a 24-m-thick C sandstone section of the Frio Formation, a regional aquifer in the U.S. Gulf Coast. Fluid samples obtained before CO2 injection from the injection well and an observation well 30 m up dip showed a Na-Ca-Cl type brine with ~93,000 mg/L TDS at saturation with CH4 at reservoir conditions; gas analyses show CH4 comprised ~95% of dissolved gas, but CO2 was low at 0.3%. Following CO2 breakthrough, 51 h after injection, samples showed sharp drops in pH (6.5 to 5.7), pronounced increases in alkalinity (100 to 3000 mg/L as HCO3) and in Fe (30 to 1100 mg/L), a slug of very high DOC values, and significant shifts in the isotopic compositions of H2O, DIC, and CH4. These data coupled with geochemical modeling indicate rapid dissolution of minerals, especially calcite and iron oxyhydroxides caused by lowered pH (initially ~3.0 at subsurface conditions) of the brine in contact with supercritical CO2.