To avoid the worst effects of climate change, many billions of metric tons of industrially generated carbon dioxide will have to be captured and stored away by the end of this century. One place to store such an enormous amount of greenhouse gas is in the Earth itself. If carbon dioxide were pumped into the cracks and crevices of certain underground rocks, the fluid would react with the rocks and solidify carbon into minerals. In this way, carbon dioxide could potentially be locked in the rocks in stable form for millions of years without escaping back into the atmosphere.
Some pilot projects are already underway to demonstrate such "carbon mineralization." These efforts have shown promising results in terms of successfully mineralizing a large fraction of injected CO2. However, it's less clear how the rocks will evolve in response. As carbonate minerals build up, could they clog up cracks and crevices, and ultimately limit the amount of CO2 that can be stored there?
In a new study appearing today in the journal AGU Advances, MIT geophysicists explored this question by injecting fluid into rocks and using X-ray imaging to reveal how the rocks' pores and cracks changed as the fluid mineralized over time.
Their experiments showed that as fluid was pumped into a rock, the rock's permeability (the ability of fluid to flow through the rock) dropped sharply. Meanwhile, the rock's porosity (its total amount of empty space, in the form of pores, cracks, and crevices) remained relatively the same.
The researchers found that the minerals were precipitating out of the fluid in the narrower tunnels connecting larger pores, preventing the fluid from flowing into larger pore spaces. Even so, the fluid did keep flowing through the rock, albeit at a lower rate, and minerals continued to form in some cracks and crevices.
"This study gives you information about what the rock does during this complex mineralization process, which could give you ideas of how to engineer it in your favor," says study co-author Matėj Peč, an associate professor of geophysics at MIT.
"If you were injecting CO2 into the Earth and saw a massive drop in permeability, some operators might think they clogged up the well," adds co-author Jonathan Simpson, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS). "But as this study shows, in some cases, it might not matter that much. As long as you maintain some flow rate, you could still form minerals and sequester carbon."
The study's co-authors include EAPS Research Scientist Hoagy O'Ghaffari as well as Sharath Mahavadi and Jean Elkhoury of the Schlumberger-Doll Research Center.
Drilling down
Basalt is a type of erupted volcanic rock that is found in places such as Hawaii and Iceland. When fresh, it's highly porous, with many pores, cracks, and fractures running through the rock. The material also is highly concentrated in iron, calcium, and magnesium. When these elements come in contact with fluid that is rich in carbon dioxide, they can dissolve and mix with CO2, and eventually form a new carbon-based mineral such as calcite or dolomite.
A project based in Iceland and piloted by the company CarbFix is currently injecting CO2-rich water into the region's underground basalt to see how much of the gas can be converted and stored as minerals in the rock. The company's runs have shown that more than 95 percent of the CO2 injected into the ground turns into minerals within two years. The project is proving that the chemistry works: CO2 can be stored as stone.
But the MIT team wondered how this mineralization process would change the basalt itself and its capacity to store carbon over time.
"Most studies investigating carbon mineralization have focused on optimizing the geochemistry, but we wanted to know how mineralization would affect real reservoir rocks," Peč says.
Rocky X-rays
The team set out to study how the permeability and porosity of basalt changes as carbonate-rich fluid is pumped into and mineralized throughout the rock.
"Porosity refers to the total amount of open space in the rock, which could be in the form of vesicles, or fractures that connect vesicles, or even areas between sand grains," Simpson explains. "Because there is so much variability in porosity patterns, there is no one-to-one relationship between porosity and permeability. You could have a lot of pores that are not necessarily connected. So, even if 20 percent of the rock is porous, if they're not connected, then permeability would be zero."
"The details of that are important to understand for all these problems of injecting fluids into the subsurface," Peč emphasizes.
For their experiments, the team used samples of basalt that Peč and others collected during a trip to Iceland in 2023. They placed small samples of basalt in a custom-built holder that they connected to two tubes, through which they flowed two different fluids, each containing a solution that, when mixed, quickly forms carbonate minerals. The team chose this combination of fluids in order to speed up the mineralization process.
In the actual process of injecting CO2 into the ground, CO2 is mixed with water. When it is pumped through rock, the fluid first goes through a "dissolution" phase, in which it draws elements such as iron, calcium, and magnesium out from the basalt and into the CO2-rich fluid. This dissolution process can take some time, before the mineralization process, in which CO2 mixes with the drawn-out elements, can proceed.
The researchers used two different fluids that quickly mineralize when combined, in order to skip over the dissolution phase and efficiently study the effects of the mineralization process. The team was able to see the mineralization process occurring within the rock, at an unprecedented level of detail, by performing experiments inside an X-ray CT scanner. The team set up their experiment in a CT scanner (similar to the ones used for medical imaging in hospitals) and took frequent, high-resolution, three-dimensional snapshots of the basalt periodically over several days to weeks as they flowed the fluids through.
Their imaging revealed how the pores, cracks, and crevices in the rock evolved, and filled in with minerals as the fluid flowed through over time. Over multiple experiments, they found that the rock's permeability quickly dropped within a day, by an order of magnitude. The rock's porosity, however, decreased at a much slower rate. At the end of the longest-duration experiments, only about 5 percent of the original pore space was filled with new minerals.
"Our findings tell us that the minerals are initially forming in really small microcracks that connect the bigger pore spaces, and clogging up those spaces," Simpson says. "You don't need much to clog up the tiny microfractures. But when you do clog them up, that really drops the permeability."
Even after the initial drop in permeability, however, the team could continue to flow fluid through, and minerals continued to form in tight spaces within the rock. This suggests that even when it seems like an underground reservoir is full, it might still be able to store more carbon.
The researchers also monitored the rock with ultrasonic sensors during each experiment and found that the sensor could track even small changes in the rock's porosity. The less porous, or more filled in the rock was with minerals, the faster sound waves traveled through the material. These results suggest that acoustic sensors could be a reliable way to monitor the porosity of underground rocks and ultimately their capacity to store carbon.
"Overall, we think that carbon mineralization seems like a promising avenue to permanently store large volumes of CO2," Peč concludes. "There are plenty of reservoirs and they should be injectable over extended periods of time if our results can be extrapolated."
This work was supported by MIT's Advanced Carbon Mineralization Initiative funded by Beth Siegelman SM '84 and Russ Siegelman '84, with additional funding from the Chan-Zuckerberg Foundation.
