Application of stable isotopes and fluid chemistry to understanding anthropogenic CO2-brine-rock interactions in sedimentary basins: results from the Frio brine pilot tests, Texas, USA

To investigate the potential for the long-term storage of CO2 in deep saline aquifers in sedimentary basins, 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. Stable isotopes of water, gases and solutes proved powerful tools in mapping the distribution, transport and interactions of the injected CO2, and in tracking its leakage into the local shallow groundwater, and into the overlying Frio 'B' sandstone, separated from the 'C' by ∼15 m of shale and siltstone. Fluid samples obtained from the 'C' before CO2 injection showed a Na-Ca-Cl type brine with ∼93,000 mg/L TDS at saturation with CH4, but only 0.3% CO2. Following CO2 breakthrough, samples showed sharp drops in pH, pronounced increases in alkalinity, Ca, Fe and Mn, and significant shifts in the isotopic compositions of H2O, CH4, DIC, and Sr. Gas and isotopic signatures coupled with perfluorocarbon tracers demonstrated significant CO2 migration into the 'B' sandstone. Results obtained to date from four shallow monitoring groundwater wells show no brine or CO2 leakage through the Anahuac Formation, the regional cap rock. The δ18O values for brine and DIC, used to calculate brine/CO2 ratios in the reservoir, gave results comparable to those obtained by geophysical methods. (author)