Subsurface monitoring of anthropogenic CO 2 injected in sedimentary basins: Results from the Frio-I brine test, Texas, USA

To investigate the potential for the long-term storage of CO 2 in deep saline aquifers, 1600 t of CO 2 were injected at 1500m depth into a 24-m-thick "C" sandstone of the Frio Formation near Houston, Texas. Fluid samples obtained before CO 2 injection from the injection well and an observation well 30m updip showed a Na-Ca-Cl type brine with ∼93,000 mg/L TDS at saturation with CH 4, but only 0.3% CO 2. Following CO 2 breakthrough, samples showed sharp drops in pH, pronounced increases in alkalinity and Fe, and significant shifts in the isotopic compositions of H 2O, and DIC. These parameters, together with perfluorocarbon tracer gases were used for monitoring migration of injected CO 2 into the overlying Frio "B", a 4-m-thick sandstone, separated from the "C" by ∼15m of shale and siltstone beds. Results from "B" 6 mo after injection show significant CO 2 (2.9% vs. 0.3% CO 2) migration into the "B" sandstone. Results of samples collected 15 mo after injection, however, show no indications of additional CO 2 in the "B" sandstone.