Underground injections turn carbon dioxide to stone

Carbon dioxide pumped into deep wells in Iceland underwent a surprising chemical transformation.

J. Matter

Underground injections turn carbon dioxide to stone

Researchers working in Iceland say they have discovered a new way to trap the greenhouse gas carbon dioxide (CO2) deep underground: by changing it into rock. Results published this week in Science show that injecting CO2 into volcanic rocks triggers a reaction that rapidly forms new carbonate minerals—potentially locking up the gas forever. The technique has to clear some high hurdles to become commercially viable. But scientists say the project, dubbed CarbFix, offers a ray of hope for beleaguered efforts to fight climate change by capturing and storing CO2 from power plants. “This is a great step forward,” says Sally Benson of Stanford University in Palo Alto, California, a geologist unaffiliated with the project.

Dozens of pilot projects around the world have sought to test carbon capture and storage (CCS) as a way of curbing CO2 emissions from power plants. Very few have been scaled up, owing to prohibitive costs, estimated at $50 to $100 per ton of 
CO2 sequestered.

CCS also faces technical hurdles, and one of the largest is where to store the captured gas. Most researchers favor formations of sedimentary rock, often sandstone harboring briny groundwater or depleted oil wells, because industry has long experience in working with them. But scientists fear that fissures in the rock layers that cap the storage aquifers could let CO2 leak back into 
the atmosphere.

So in 2006, Icelandic, U.S., and French scientists proposed a different approach: injecting CO2 into underground layers of basalt, the dark igneous rock that underlies Earth’s oceans and crops up in parts of continents as well. They knew that unlike sandstone, the basalt contains metals that react with CO2, forming carbonate minerals such as calcite—a process known as carbonation. But they thought the process might take many years. To find out, they launched the CarbFix experiment 25 kilometers east of Reykjavik, intending to dose Iceland’s abundant underground basalt with CO2 that bubbles from cooling magma underground and is collected at a nearby geothermal power plant.

In 2012, the researchers injected 
220 tons of CO2—spiked with heavy carbon for monitoring—into layers of basalt between 
400 and 800 meters below the surface. They also added extra water, which reacted with the gas to form a key driver of mineral reactions, carbonic acid. Then they monitored the pH, geochemistry, and other characteristics of the subsurface by taking samples from nearby wells.

What happened next startled the team. After about a year and a half, the pump inside a monitoring well kept breaking down. Frustrated, engineers hauled up the pump and found that it was coated with white and green scale. Tests identified it as calcite, bearing the heavy carbon tracer that marked it as a product of carbonation.

Measurements of dissolved carbon in the groundwater suggested that more than 95% of the injected carbon had already been converted into calcite and other minerals. “It was a huge surprise that the carbonation happened so fast,” says Juerg Matter, a geologist with CarbFix at the University of Southampton in the United Kingdom. Laboratory tests by Matter’s team and others, along with computer modeling, had previously suggested that carbonation in basalt would take at least a decade. (Sandstone aquifers are so unreactive that carbonation is thought to take centuries at conventional CCS sites.)

The speedy carbonation “means this method could be a viable way to store CO2 underground—permanently, and without risk of leakage,” Matter says. Unpublished data from a similar project in basalt near the Columbia River near Wallula, Washington, point to a similar conclusion. And there is no lack of basalt formations on land or offshore, which could make CCS possible for power plants “not near sedimentary rocks or depleted oil wells,” Matter adds.

Bigger field tests are needed, says geo
logist Peter Kelemen of Columbia University, to confirm that such a high fraction of the injected carbon was mineralized. (Columbia is a CarbFix partner, but Kelemen is not on the project.) Scaled-up demonstrations could also make sure that the speed of the reaction won’t turn into a drawback, Stanford’s Benson says. If carbonation generates minerals that quickly plug the pores in the basalt, she worries, they could trap CO2 near the injection site instead of letting it spread through the rock.

But even CarbFix’s own scientists acknowledge that the biggest obstacle to CCS in basalt is financial: Power companies have little incentive to pursue it. “Without a price on carbon emissions, there’s no business case,” admits Matter, who hopes policymakers will create such an incentive. Otherwise, projects in basalt could suffer the same fate as the dozens of conventional CCS projects around the world that have failed to be commercialized. In the meantime, says Benson, the success in Iceland is a welcome development. “We could all use some positive news in this field,” she says.