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Potential Environmental Issues of CO2 Storage in Saline Aquifers: Metal Mobilization from the Frio-II Brine Test, Texas, Usa

Yousif K. Kharaka1, Thomas D. Bullen1, Kevin G. Knauss2, David R. Cole3, and Susan D. Hovorka4
1MS/427, U.S.Geological Survey, Menlo Park, CA
2Lawrence Livermore National Lab, Livermore, CA
3Oak Rigde National Lab, Oak Ridge, TN
4BEG, University of Texas, Austin, TX

To investigate the potential for the long-term storage of CO2 in deep saline aquifers, 300 t of CO2 were injected at 1560 m depth into a 17-m thick “Blue” sandstone section of the Frio Formation, a regional aquifer in the northern Gulf of Mexico basin. Fluid samples obtained before CO2 injection from the injection well and an observation well 30 m updip showed a Na-Ca-Cl-type brine with ~102,000 mg/L TDS, a relatively low metal content, and dissolved gas at saturation with CH4, but low (~0.3%) CO2. Following CO2 breakthrough, ~50 h after injection, samples showed sharp drops in pH (6.5 to 3.7) measured with an online probe, pronounced increases in alkalinity (100 to 1200 mg/L as HCO3) and Fe (30 to 1200 mg/L) and other metals, and shifts in the isotopic compositions of H2O, Sr, and DIC.

The origin of metals is being investigated by detailed analyses of Fe, Mn, Pb and other metals from a large number of brine samples and three pieces of well pipe, determination of Fe isotopes of selected brine samples and pipe solutions, and geochemical modeling of data obtained from field and plug-flow laboratory experiments. Results show that metal increases are caused by both the dissolution of the observed Fe-oxyhydroxide minerals, and corrosion of well pipe that contact the low pH brine. Fe isotopes and metal proportions are used to calculate metal fractions obtained from Fe-oxyhydroxides and from the low-carbon steel pipe used in wells.

Environmental impacts could be major if large volumes of CO2 and brine with mobilized metals migrated to the surface or into potable groundwater through paths created by either the dissolution of minerals or corrosion of well pipes. In addition to injection of CO2 into aquifers with adequate barrier seals, it may be necessary to avoid areas with faults and fracture zones, and to recomplete the cementing of some of the nearby abandoned wells.


AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas