Rocks play a vital yet often unrecognized role in the carbon cycle. Through a natural process that has helped regulate Earth’s temperature for millennia, chemical reactions transform atmospheric carbon dioxide into solid carbonate minerals and lock it away underground, practically forever.

That carbon mineralization process, if enhanced and quickened, could be used to store atmospheric carbon, and be especially useful in dealing with legacy emissions and those from hard-to-abate sources.

“There is strong consensus among leading scientists that we do need [CO₂ removal and storage] alongside deep emissions reductions” to avoid the impacts of catastrophic climate change, says Katie Lebling, an associate at the World Resources Institute, a global nonprofit.

Today, scientists are tapping into the natural mineralization process and working out how to more rapidly store atmospheric carbon dioxide underground.

This carbon storage technology, dubbed subsurface or in-situ carbon mineralization, involves injecting carbon dioxide (and often water) directly into certain rock types to speed up the chemical reactions that transform the gas into a solid. When paired with a carbon capture method (such as direct air capture) it is a type of carbon capture and storage (CCS).

The most suitable rocks within which mineralization can occur are those high in magnesium and calcium, as they react quickly with CO₂ and form stable carbonate minerals — rocks like igneous basalts. These rocks are plentiful in many places around the world.

This method has already been demonstrated at a number of sites, first and most notably in Iceland, and interest is growing globally. Proponents say the technique could offer a relatively safe, scalable method of permanently storing CO₂.

The trick is figuring out how to make the reactions happen quickly, economically and at scale.

First steps in Iceland

In the early 2000s, geochemist Eric Oelkers, who was then working for CNRS Toulouse, a French government research institution, teamed up with Sigurður Gíslason, a geochemistry professor at the University of Iceland. Together, they were studying how weathering plays into the long carbon cycle in Iceland.

Oelkers and Gíslason analyzed a weathering process well known to science: Basically, when igneous basalt is worn away by physical and chemical weathering, calcium and magnesium ions are released into the ocean. There they react and combine with dissolved CO₂ to form stable carbonate minerals, which eventually get sequestered deep in the Earth’s crust.

When, in the wake of the Kyoto Protocol climate agreement, the president of Iceland asked a group of scientists (including Oelkers, Gíslason, colleagues from Columbia University in the U.S., and Reykjavik Energy) to come up with ways to reduce the island nation’s greenhouse gas emissions, the team wondered if they could somehow mimic and speed up the natural mineralization process to store CO₂ underground.

“We looked at it, and said, ‘Well, it’s the same process’ [as we’ve been studying],” Oelkers recalls. But, “instead of bringing the basaltic material — ground by weathering and in Iceland’s case, by glaciers — [to the ocean,] we’ll just put the CO₂-rich water [directly] into the rocks.”

Reykjavik Energy, a geothermal company, was already drilling boreholes and disposing of water underground. So all the research team needed to do was add gaseous CO₂, which could be captured from the geothermal plant. It seemed a plausible hypothesis worth testing.

“From everything we knew, [it] should make carbonate minerals,” Oelkers says, and provide a means for rapidly and permanently storing atmospheric carbon.

The Iceland team, named CarbFix, began running modeling exercises, lab experiments and field investigations in 2006. They looked at the chemistry of groundwater, the composition and reactivity of basaltic rocks, and subsurface flow paths. This helped them understand environmental risks, predict where the CO₂ would go, and design a way to measure the amount of mineralization.

By March 2012, they were ready. Using an apparatus reminiscent of a giant soda machine, the team piped carbon dioxide and water into a 500-meter-deep (1,640-foot) drill hole at the Hellisheiði geothermal power plant, east of Reykjavík. About 200 metric tons of CO₂ were injected in total (due to equipment failure, most of the CO₂ in the initial pilot was purchased, with only a portion captured from the power plant).

Because researchers couldn’t see underground, they took water samples from a nearby well to determine what was happening. Those samples showed that while tracer gas concentrations continued increasing, CO₂ levels soon plateaued, then dropped off. They knew that if the CO₂ wasn’t in the water, it must have been left behind in the rock, as calcite, magnetite and siderite. Within just two years, more than 95% of the CO₂ had been locked away.

“Some people in the literature say we were surprised,” Oelkers says. “The fact is, we just copied nature.”

New projects in Iceland, U.S. and Oman

Since that groundbreaking demonstration, the CarbFix project has progressively scaled up.

In 2014, it began commercial operation at the Hellisheiði power plant, capturing and mineralizing about a third of the CO₂ and two-thirds of the hydrogen sulfide produced by the facility (both gases are a byproduct of power generation). In 2017, CarbFix partnered with the Swiss direct air capture company Climeworks. Their Orca terminal, built in 2021, was the first ever to offer commercial-scale direct air carbon capture coupled with mineral storage.

To date, CarbFix has mineralized more than 100,000 metric tons of atmospheric CO₂. Though originally formed as an industry-academic collaboration, CarbFix is now a subsidiary of Reykjavik Energy. Planned projects include the Coda terminal, a CO₂ transport and storage hub that will receive liquified CO₂ by ship and is anticipated to have the capacity to inject 3 million metric tons of CO₂ underground per year by 2032.

Another early demonstration of subsurface carbonate mineralization occurred in the U.S. In 2012, researchers from the Pacific Northwest National Laboratory (PNNL) injected 1,000 metric tons of liquified supercritical CO₂ into Columbia River flood basalts at Wallula in Washington state. Two years later, testing found 60% of the CO₂ had been mineralized.

That project ended in 2015, but now, buoyed in part by new U.S. funding opportunities, the research is picking up. This includes the HERO CarbonSAFE Phase II project, in Oregon, where researchers from PNNL and the University of Wyoming hope to soon drill a well to test carbon storage potential.

Researchers are also exploring the carbon storage potential of peridotite, another igneous rock. Peridotite is usually found deep within Earth’s mantle and is inaccessible, but in some places it’s been pushed to the surface. Oman has some of the world’s most impressive outcrops, but surface-level peridotite is also found in New Caledonia, Papua New Guinea, western Canada, and elsewhere.

In 2020, the Omani startup 44.01 successfully demonstrated mineralization in peridotite, injecting 1.2 metric tons of CO₂ underground, with 80% of it mineralized in just 45 days. In 2022, the company won the coveted Earthshot Prize in the Fix Our Climate category, and in May its Project Hajar, in partnership with the U.S. direct air capture company Aircapture, became one of the top 20 contenders for the XPRIZE for Carbon Removal.

A handful of other efforts are underway. In mid-2023, for example, researchers, including Oelkers, injected 130 metric tons of CO₂ into volcanic rock on the Saudi Arabian coast, as part of oil giant Saudi Aramco’s corporate decarbonization strategy. Tests indicated more than half the CO₂ had mineralized within six months. In Canada, the startup DeepSky recently announced a partnership with CarbFix; they plan to drill their first basalt hole in the province of Quebec in 2025.

Advantages and challenges

Proponents of subsurface mineralization point to its advantages. First, nearly every continent has rock suitable for CO₂ storage. That’s especially important for hard-to-abate industries like cement production, which aren’t necessarily located near the sedimentary rocks typically used in conventional geologic CCS, says Chris Consoli, principal of storage at the Global Carbon Capture and Storage Institute.

Second, the subsurface mineralization process is fast, secure and permanent. Once the CO₂ is mineralized, it can never escape back into the atmosphere. That’s an advantage over conventional geologic storage, where CO₂ is pumped into sedimentary basins (such as those under old oil and gas fields), where there’s a risk of leaks back to the atmosphere.

In subsurface mineralization, “you can actually verify how the carbon is changing state, and going from a gas phase to a solid phase … in less than two years,” says Catalina Sánchez-Roa, head of carbon mineralization at DeepSky. “It’s a really big leap from what’s going on right now in conventional storage, where you have to monitor your fields [for leaks] for 50 plus years.”

And while nature-based solutions like reforestation bring many co-benefits (including for biodiversity), they don’t offer the same level of permanence in terms of carbon storage; forests that store carbon can burn, but rocks are forever.

“We can guarantee that the actual carbon credits are going to stay there for millions of years,” Sánchez-Roa says.

Lastly, the chemical reactions themselves don’t require energy inputs, and the process seems to carry few environmental risks. Induced seismic activity and groundwater contamination have been flagged as potential risks, but are avoidable with proper planning and monitoring, say proponents.

However, there are still challenges to overcome before subsurface mineralization can be scaled up. One is identifying ideal sites, says Sánchez-Roa. Subsurface mineralization requires what’s called mafic and ultramafic rocks, rich in magnesium and iron, while permeability or fracture networks are also important. Some regional geologic data already exists, but the industry needs to gain the tools and expertise to find, map and characterize promising sites, Sánchez-Roa says, and that research takes time.

There’s also ongoing challenges around water. The methods used in Iceland and Oman are water-intensive. Projects have made progress using seawater, and in Oman, treated wastewater, instead of freshwater, is being used to bring down costs and environmental impacts. Researchers are also exploring ways to recycle water in a continuous loop. There are also questions about how injecting far larger amounts of CO₂ will influence reaction rates and pathways.

But many experts say these technical unknowns don’t pose significant barriers to scaling up. “The technology is there,” Oelkers says. “It’s not difficult to do. We just need to have more pilots and we’ll learn as we go along.”

The need for financing and regulatory support

With sufficient backing, carbon mineralization as a whole — including technologies like enhanced rock weathering and other types of ex-situ mineralization — could store a billion metric tons, or 1 gigaton (Gt), of CO₂ per year by 2035, and 10 Gt annually by 2050, according to the Innovation for Cool Earth Forum’s Carbon Mineralization Roadmap published in 2021.

However, that’s far short of the 350 Gt to 1,200 Gt of CO₂ needing to be captured and stored this century, using all CCS methods, to hold the global temperature rise at 1.5° Celsius (2.7° Fahrenheit), according to the IPCC. Obviously, say experts, such methods can’t replace drastic reductions in fossil fuel production and burning. Currently, humans continue to release 40 Gt of carbon dioxide a year into the atmosphere. Slashing those emissions as quickly as possible is the first and most important step.

A hurdle common to all CCS projects: How to create demand. Currently, CCS is a product no-one needs to buy, and unlike electric cars or solar energy, market forces alone aren’t enough to generate demand, says Lebling.

Thus far, initial buyers have been big tech companies, those that have ample capital, or have reputations to uphold, Lebling says. Microsoft is by far the biggest purchaser of carbon dioxide removal credits, with Airbus, Amazon and Google also in the top five. This year, direct air capture company Climeworks, which partners with CarbFix in Iceland, entered carbon removal agreements with U.S. bank Morgan Stanley and British Airways. But unless there are compliance policies along with procurement incentives — sticks as well as carrots — voluntary purchases won’t be enough to get mineralization projects up to scale, say experts.

Also needed is a robust regulatory framework to drive mineralization forward. “We’ve seen that where there’s long-term stable policy supportive of CCS domestically — [as with] a tax credit, like in the U.S.A., or a target such as in Europe — that really helps drive CCS,” Consoli says.

As devastating climate change impacts escalate, there isn’t any time to waste in pursuing carbon mineralization as part of a suite of workable climate solutions, alongside deep emissions cuts.

Banner image: Snow and cold at the original CarbFix injection well in Iceland during late 2011 made working conditions at this field site more challenging. Image courtesy of S.R. Gíslason.

Calls for caution as enhanced rock weathering shows carbon capture promise

Citations:

Gíslason, S. R., Sigurðardóttir, H., Aradóttir, E. S., & Oelkers, E. H. (2018). A brief history of CarbFix: Challenges and victories of the project’s pilot phase. Energy Procedia, 146, 103-114. doi:10.1016/j.egypro.2018.07.014

Matter, J. M., Stute, M., Snæbjörnsdottir, S. Ó., Oelkers, E. H., Gíslason, S. R., Aradottir, E. S., … Broecker, W. S. (2016). Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions. Science, 352(6291), 1312-1314. doi:10.1126/science.aad8132

Oelkers, E., & Gíslason, S. (2023). Carbon capture and storage: From global cycles to global solutions. Geochemical Perspectives, 12(2), 179-349. doi:10.7185/geochempersp.12.2

Seifritz, W. (1990). CO2 disposal by means of silicates. Nature, 345(6275), 486-486. doi:10.1038/345486b0

White, S. K., Spane, F. A., Schaef, H. T., Miller, Q. R., White, M. D., Horner, J. A., & McGrail, B. P. (2020). Quantification of CO2 mineralization at the Wallula basalt pilot project. Environmental Science & Technology, 54(22), 14609-14616. doi:10.1021/acs.est.0c05142

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