Carbon Sequestration: Speed Bump or Wall?

No, that's not the head of a fly. It's a carbon dioxide (CO2) molecule. Looks harmless, but now it's causing some headaches among the scientists who are developing carbon sequestration methods (basically, capturing CO2 from the combustion of fossil fuels and burying it under ground to keep it from contributing to global warming). Richard A. Kerr writes in Science: "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." It's not leaking so far, but it will require a second look before carbon sequestration can be used on a large scale.

A pilot experiment in Houston, Texas, 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 says that "The crust of Earth is well configured to contain CO2," and that there has been no catastrophic failure at the 80 US oil wells that have been injected with CO2 (to help get the oil out) for 30 years, but one can wonder if such a failure might have gone unnoticed since scientists have just learned about the problem.

Carbon sequestration doesn't look quite as good if it causes a toxic mix of metals and organic substances to leak into shallow aquifers. We'll keep an eye open for more developments about this story. ::A Possible Snag in Burying CO2, via ::Carbon Sequestration: An Idea Whose Time Has Gone?. See also: ::Congress May Insure Against Coal-Induced Flatulence