A Possible Snag in Burying CO2

By Richard A. Kerr

ScienceNOW (subscription needed)
28 June 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.
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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."

Frio Brine Project
Web abstract of results at 20 days.

Cross posted to Kiashus blog