A Planetary Fix?

By Amanda Mascarelli

Underground carbon storage is one of the most viable solutions for funneling heat-trapping gases out of the atmosphere. Volcanic rock lying beneath Iceland and elsewhere around the globe could provide a vast and permanent geological burial ground for CO2, stored as rock.  The biggest hurdle is not a technological one.

In southwestern Iceland, steam rises up from the gritty, volcanic landscape like miniature smokestacks on the horizon. The steam and gases emanate from geothermal pools that bubble up from deep within the island country’s underbelly. In this region, some 15 miles east of Reykjavik, researchers have engineered a scheme to funnel carbon dioxide from the atmosphere back underground into porous basalt reservoirs that were formed by lava flow. If all goes as planned, the CO2 will stay there, turned into rock.

The Iceland experiment will help determine whether geologic carbon storage has promise to help arrest the momentum of global warming. With global greenhouse gas emissions rising at a runaway pace and irreversible changes to the planet already set in motion, according to the latest report by the Intergovernmental Panel on Climate Change, options for reining in these heat-trapping gases are sorely needed. Underground carbon storage, often called “carbon, capture and storage” (CCS), is considered a viable technology, and various iterations have been explored over the past few decades. But scaling the technology up to a level that could put a sizable dent in greenhouse gas emissions would involve massive investments by governments and industry—and policies to back them up.

Traditionally, carbon sequestration projects have been set up to capture CO2 at coal-fired power plants. But Iceland benefits from an abundance of geothermal energy. There, Hellisheiði—the world’s largest geothermal power station—provides a perfect test bed for the basaltic CO2 storage experiment. The power station is run by Reykjavik Energy, a utility company that supplies energy and water to more than two-thirds of the country’s 323,000 residents. When CO2 captured from the power plant combines with wastewater at shallow depths, the reaction resembles the jet of bubbles delivered by a seltzer machine. But when the gas and water mixture reaches the injection point 1,650 feet underground, the pressurized gas is fully dissolved—much like the contents inside an unopened soda can.

After injection, this carbonated cocktail seeps into cracks and fissures within the basalt layer. The acidic solution begins to dissolve ions from the porous rock, which is rich in calcium magnesium. Slowly, the reaction neutralizes the acid and produces calcium carbonate, a white, crystalline material that coats the surrounding rock. The scheme mimics a natural weathering process that has helped to keep atmospheric concentrations of CO2 at bay over geologic timescales. But the researchers hope to speed up the process.

The Icelandic project, known as CarbFix, run by The Earth Institute at Columbia University’s Lamont-Doherty Earth Observatory in New York, is still in the experimental stage. During its pilot phase from 2012 to 2013, the researchers injected 175 tons of CO2 into the underground basalt formation. (That’s roughly equivalent to the annual emissions of about 25 American cars.) To understand what is happening after CO2 is injected, the team uses a drill with a steel pipe to bring 9-foot-long core samples back to the surface. To their surprise, they found that between 80 to 90 percent of the CO2 injected during the pilot phase was converted to rock in less than a year. The researchers discussed the results in a perspectives piece in April in the journal Science. “It was really, really fast,” says Juerg Matter, a geochemist at the University of Southampton, in the UK (previously based at Columbia University’s Earth Institute) and a team leader on the CarbFix project. “That’s the amazing part—it doesn’t take hundreds of thousands of years to convert that CO2.”

Set in Stone

How CO2 is injected and stored underground could matter—a lot. CarbFix is one of only two underground carbon storage experiments in the world that are exploring the possibility of capturing CO2 from the atmosphere and turning it into stone within basaltic formations. The other basalt project, run by the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL), is based in southeastern Washington, near the town of Wallula. Unlike the CarbFix project, which dissolves CO2 gas into water and injects the mixture underground, the Wallula project uses the conventional approach of injecting ‘supercritical’ fluid CO2 into basalt. This means the CO2 is slightly less dense than water, like an ice cube that floats in a glass of water. Impermeable basalt layers above the injection zone keep the CO2 trapped, allowing time for mineralization to occur. The team is still analyzing its results from fluid samples collected following injection but are seeing elevated concentrations of elements such as calcium and magnesium that would indicate that minerals are being formed, says Pete McGrail, a laboratory fellow from PNNL who leads the Wallula basalt project.

Most carbon storage projects inject CO2 into sedimentary rock basins. The oil and gas industry has a long history of experience with sedimentary reservoirs for energy extraction. The first CCS project, the Sleipner CCS program off the coast of Norway, which began in 1996, has injected more than 11 million tons of CO2 into an aquifer formation beneath the North Sea. Still, all of the CCS projects worldwide lock away only a relative smidgen of CO2, given that humans release some 30 billion tons of CO2 each year.

An important difference between CO2 storage in basalt versus sedimentary basins is that in sedimentary systems, the mineralization process would likely take thousands of years, allowing the gas to remain buoyant. Some experts fear that CO2 could escape back into the atmosphere due to earthquakes or incomplete seals. Turning CO2 into stone within basaltic bedrock “could provide the only permanent storage solution, because rocks do not leak out of the ground,” says Matter.

Since basalt exists across wide swathes of the Earth, including the Indian sub-continent and most of the ocean floor, basaltic reservoirs could substantially increase CO2 storage capacity. “There are enough basaltic rocks on earth to take care of all our anthropogenic CO2 emissions,” says Matter. “That’s a theoretical statement,” he adds. What is the realistic capacity of basaltic reservoirs? “That question we still have to answer.”

Sally Benson, a hydrogeologist and director of the Global Climate and Energy at Stanford University, who is not involved with the CarbFix project, agrees that basalt storage could be an effective option. “The really big benefit of the basalt work is that it provides the opportunity to store CO2 in locations that would otherwise not have been thought to have resources that would be available for CO2 storage,” says Benson.

This year the CarbFix researchers began operating at industrial scale. During the pilot phase in 2012 and 2013, the researchers injected pure CO2 into water, which was then pumped underground, capturing about 2 percent of the plant’s CO2 output. Now, the team is capturing and injecting a combination of CO2 and hydrogen sulfide (H2S) gas, which creates the rotten egg odor common to hot springs, amounting to between 20 and 30 percent of the power plant’s emissions, or some 8,000 tons per year of the gases. It is too early to tell whether the rock conversion is happening as quickly this time around because the analysis is still underway. But one very promising sign, Matter says, is that while they were extracting rock cores recently, a key piece of equipment, a pump that was submerged underground, broke mid-experiment. That may sound like a bad thing, but it represents a telltale sign of rock formation from the freshly injected CO2. Specifically, the pump was encrusted in carbonate scales carrying the team’s radiocarbon ‘tracer’ to identify the source of the carbon.

No rock-solid future

While the technology for carbon, capture, and storage is ready for wide-scale commercial use, the economics do not yet work in its favor in most cases. First, there is currently no global agreement to limit greenhouse gas emissions, which would motivate governments and businesses to reduce emissions. Secondly, says Matter, “because there is no price on carbon emissions, there is no economic incentive for companies to invest” in CCS.

The CarbFix project benefits from the fact that new limitations on H2S emissions went into effect in Iceland this year. “It was just a bonus that we could use this method that we had already developed” to also limit H2S, says Edda Sif Aradóttir, a chemist and reservoir engineer with Reykjavik Energy and project manager for CarbFix. “If you look at all the ongoing CCS projects in the world, most of those have some sort of added value, because you don’t get your money back otherwise.” On a global scale, says Aradóttir, “If the economics make sense then I think [CCS] can definitely be a part of the solution.”

Capturing CO2 from power plant emissions is by far the most expensive piece of the puzzle. Stripping CO2 from smokestack gases requires massive equipment to handle the huge volumes of gases emitted. A typical coal-fired power plant might emit 8 million tons of CO2 per year (roughly equivalent to the emissions of 2 million cars), and CO2 represents only about one-tenth of the total gases emitted from a power plant, explains Benson.

Benson predicts that to make CCS projects economically feasible, the cost of emitting CO2 into the atmosphere would need to run on the order of $40 per ton. For reference, in California, which has adopted a cap-and-trade system, the value of carbon hovers around $11. “I’m an optimist. I think the day will come when the idea that we put carbon dioxide into the atmosphere is just as abhorrent as throwing trash on the street or not having a sewage system,” says Benson. “Hopefully it will be sooner rather than later.”

Edited by Susan Moran