A Possible Snag in Burying C02

By Richard A. Kerr
ScienceNOW Daily News
28 June 2006

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."