Carbon-Capturing Rock

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Mineral springs: Calcium carbonate rock that forms as alkaline, calcium-rich water seeps from underground springs in Oman. Credit: Jürg Matter

The researchers propose a carbon-sequestration strategy that would eliminate the need to transport carbon dioxide, as well as the need to heat up the rock. In this scenario, they would access rock formations in shallow ocean waters off the coast of Oman and elsewhere by drilling into them and fracturing the rock using existing oil-industry techniques. The researchers would drill two holes. Into one, they'd pump cool seawater. Rock temperature increases with depth, so as the water is pumped into the holes, it will get hotter, until it reaches nearly 185 °C. Carbon dioxide naturally dissolved in the water would then precipitate out of the solution. The hot water would eventually make its way through the fractured rock to the second drilled hole, where it would rise to the surface via convection. This seawater would quickly absorb more carbon dioxide, since shallow waters and surf mix well with the atmosphere. Because "the atmosphere transports carbon dioxide all over the world for free," Kelemen says, this approach, if deployed on a grand scale, could be used to lower worldwide levels of carbon dioxide.

This scenario would be limited by the concentration of carbon dioxide in seawater, so a cubic kilometer of rock would only sequester about a million tons of carbon dioxide a year. But since it wouldn't be necessary to transport carbon dioxide or pay to heat the rock, Kelemen says, it would be possible to work with much larger areas of rock, and thereby reach a rate of billions of tons of carbon dioxide per year.

"From a conceptual point of view, all they say makes sense," says Mazzotti. Yet questions remain about whether the methods will work in practice. For one thing, the self-sustaining reactions depend on the magnesium carbonate and other precipitates continuing to fracture the rock to expose more of it. The researchers have observed that this has happened in the geology in Oman, but it's not a given that it would continue in the scenarios that they propose. The researchers' concepts should now be complemented with large-scale tests, Mazzotti says.