Potential environmental issues of CO 2 storage in deep saline aquifers: Geochemical results from the Frio-I Brine Pilot test, Texas, USA

Abstract

Sedimentary basins in general, and deep saline aquifers in particular, are being investigated as possible repositories for large volumes of anthropogenic CO 2 that must be sequestered to mitigate global warming and related climate changes. To investigate the potential for the long-term storage of CO 2 in such aquifers, 1600 t of CO 2 were injected at 1500 m depth into a 24-m-thick "C" sandstone unit of the Frio Formation, a regional aquifer in the US Gulf Coast. Fluid samples obtained before CO 2 injection from the injection well and an observation well 30 m updip showed a Na–Ca–Cl type brine with ∼93,000 mg/L TDS at saturation with CH 4 at reservoir conditions; gas analyses showed that CH 4 comprised ∼95% of dissolved gas, but CO 2 was low at 0.3%. Following CO 2 breakthrough, 51 h after injection, samples showed sharp drops in pH (6.5–5.7), pronounced increases in alkalinity (100–3000 mg/L as HCO 3 ) and in Fe (30–1100 mg/L), a slug of very high DOC values, and significant shifts in the isotopic compositions of H 2 O, DIC, and CH 4 . These data, coupled with geochemical modeling, indicate corrosion of pipe and well casing as well as rapid dissolution of minerals, especially calcite and iron oxyhydroxides, both caused by lowered pH (initially ∼3.0 at subsurface conditions) of the brine in contact with supercritical CO 2 . These geochemical parameters, together with perfluorocarbon tracer gases (PFTs), were used to monitor migration of the injected CO 2 into the overlying Frio “B”, composed of a 4-m-thick sandstone and separated from the “C” by ∼15 m of shale and siltstone beds. Results obtained from the Frio “B” 6 months after injection gave chemical and isotopic markers that show significant CO 2 (2.9% compared with 0.3% CO 2 in dissolved gas) migration into the “B” sandstone. Results of samples collected 15 months after injection, however, are ambiguous, and can be interpreted to show no additional injected CO 2 in the “B” sandstone. The presence of injected CO 2 may indicate migration from “C” to “B” through the intervening beds or, more likely, a short-term leakage through the remedial cement around the casing of a 50-year old well. Results obtained to date from four shallow monitoring groundwater wells show no brine or CO 2 leakage through the Anahuac Formation, the regional cap rock.