A Possible Snag in Burying CO2
By Richard A. Kerr
Science, June 30, 2006
Scientists testing the deep geologic disposal of the greenhouse gas carbon dioxide are finding that it's staying where they put it, but it's chewing up minerals. The reactions have produced a nasty mix of metals and organic substances in a layer of sandstone 1550 meters down, researchers report this week in Geology. At the same time, the CO2 is dissolving a surprising amount of the mineral that helps keep the gas where it's put. Nothing is leaking out so far, but the phenomenon will need a closer look before such carbon sequestration can help ameliorate the greenhouse problem, say the researchers.
Drillers often inject CO2 into the ground to drive more oil out, but researchers conducting the U.S. Department of Energy-sponsored Frio Brine Pilot Experiment northeast of Houston, Texas, pumped 1600 tons of CO2 into the Frio Formation to see where the gas went and what it did. "We're the first looking in this huge detail so that we can see what's going on," says geochemist and lead study author Yousif Kharaka of the U.S. Geological Survey in Menlo Park, California. He and colleagues found that the CO2 dropped the pH of the formation's brine from a near-neutral 6.5 to 3.0, about as acid as vinegar. That change in turn dissolved "many, many minerals," says Kharaka, releasing metals such as iron and manganese. Organic
matter entered solution as well, and relatively large amounts of carbonate minerals dissolved.
The loss of carbonates worries Kharaka particularly. These naturally occurring chemicals seal pores and fractures in the rock that, if opened, could release CO2 as well as fouled brine into overlying aquifers that supply drinking and irrigation water. Perhaps more troubling, says Kharaka, is that the acid mix could attack carbonate in the cement seals plugging abandoned oil or gas wells, 2.5 million of which pepper the United States. The lesson is that "whatever we do [with CO2], there are environmental implications that we have to deal with," he says.
Geologist Julio Friedmann of Lawrence Livermore National Laboratory is less concerned about corrosion eating away the seals on a sequestration site. "The crust of Earth is well configured to contain CO2," he says. He points to 80 U.S. oil fields injected with CO2 for up to 30 years. "We've seen no catastrophic failures." Nevertheless, the Frio results do "suggest an aspect of risk we hadn't considered before," says Friedmann. There is a "new potential risk should CO2 leak into shallow aquifers."
Gas-water-rock interactions in Frio Formation following CO2 injection: Implications for the storage of greenhouse gases in sedimentary basins
Issn: 0091-7613 Journal: Geology Volume: 34 Issue: 7 Pages: 577-580
Authors: Kharaka, Y.K., Cole, D.R., Hovorka, S.D., Gunter, W.D., Knauss, K.G., Freifeld, B.M.
Article ID: 10.1130/G22357A.1
To investigate the potential for the geologic storage of CO2 in saline sedimentary aquifers, 1600 t of CO2 were injected at 1500 m depth into a 24-m-thick sandstone section of the Frio Formation, a regional brine and oil reservoir in the U.S. Gulf Coast. Fluid samples obtained from the injection and observation wells before CO2 injection showed a Na-Ca-Cltype brine with 93,000 mg/L total dissolved solids (TDS) at near saturation with CH4 at reservoir conditions.
Following CO2 breakthrough, samples showed sharp drops in pH (6.55.7), pronounced increases in alkalinity (1003000 mg/L as HCO3) and Fe (301100 mg/L), and significant shifts in the isotopic compositions of H2O, dissolved inorganic carbon (DIC), and CH4.
Geochemical modeling indicates that brine pH would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides. This rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for CO2 and brine leakage. Dissolution of minerals, especially iron oxyhydroxides, could mobilize toxic trace metals and, where residual oil or suitable organics are present, the injected CO2 could also mobilize toxic organic compounds.
Environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater. The ´18O values for brine and CO2 samples indicate that supercritical CO2 comprises