Learning from nature: trapping and storing carbon dioxide underground

Carbon dioxide levels in the atmosphere are higher than ever, mainly due to the burning of fossil fuels and other anthropogenic activities. This, in turn, increases global temperatures and impacts sea levels and the ocean ecosystems.

A potential solution to this crisis would be to trap and store CO₂ underground as solid minerals, which is a natural process that occurs over long periods thanks to the reaction of CO₂ with rocks in the Earth’s crust and mantle.

Scientists have been studying the injection of CO₂ in the subsurface for years. For example, in Sleipner, in the North Sea, millions of tons of carbon dioxide have been injected into a sandstone geological reservoir in the past fifteen years, where CO₂ is stored in liquid form. CO₂ can also be stored in solid form through mineralization processes, minimising the risk of leakage. Small-scale ongoing projects, such as Carbfix in Iceland, show promising results but questions of efficiency remain.

“The natural process is very effective but too slow, so we wonder whether we could somehow accelerate it so that large quantities of CO₂ could be injected underground, without leakage”, explains François Renard, director of the Njord Centre at the University of Oslo and ESRF user.

The natural process

Atmospheric CO₂ and water from precipitations naturally react with rocks present at the Earth’s surface - this process is called weathering. Some of these rocks have been produced by volcanic activity (basalts in Iceland) or were exhumed to the Earth’s surface from the mantle (peridotites). When reacting with CO₂ and water, they may dissolve partially over geological time scales, liberating magnesium, iron, and calcium ions that can bind with carbon dioxide, in a process called mineral carbonation, which converts CO₂ into minerals. The end product is a calcium, iron or magnesium carbonate, which are stable minerals that effectively trap carbon dioxide into a solid form.

Renard and his team are focusing on storing CO₂ in basaltic and peridotite rocks, rich in magnesium and calcium, as they are the most efficient environments for it due to their high reactivity. They make up about 70% of the Earth’s surface and are responsible for 1/3 of the trapping of CO₂ from the atmosphere through weathering. Estimates suggest that mid-ocean ridges worldwide can store up to 100,000 Gt of CO₂. This is more than 2000 times the annual global emissions of CO₂.

Once in the basaltic or peridotite rocks, the CO₂ quickly reacts with the divalent cations (Ca²⁺, Mg²⁺, and Fe²⁺) from dissolving minerals in the rock and form carbonate minerals. In comparison, it might take several tens of thousands of years for significant amounts of CO₂ to mineralize in a sandstone reservoir. After it becomes a mineral, the carbon will not move over geological timescales.

Carbonation at the ESRF

The team is focused on studying how basalts and peridotites can host large quantities of flows of carbon dioxide mixed with water, which will react with the rock to produce carbonate minerals.

They study these across different scales, from kilometers to millimeters to fracture propagation at the nanoscale, the latter at the ESRF. They reproduce the process in the Hades deformation rig set-up at beamline BM18 of the ESRF. “We reproduce the pressure and temperature (around 200 C) required for reactions to take place, equivalent to up to 5 km underground”, explains Benoît Cordonnier, scientist at the ESRF and the University of Oslo. “In two days, we can see some microstructural evolution inside the sample using dynamic X-ray microtomography at the ESRF” says Wenlu Zhu, professor at the University of Maryland.

François Renard and Wenlu Zhu during the experiment. Credits: ESRF/M. Thierry.

The first experiments have just taken place. Researchers have used a rock analogue, a magnesium-rich ceramics, and injected water and CO₂ to let the reaction progress. The main challenge is to find the optimal conditions so that the reaction can occur within few days, and direct observations can be made under in situ conditions.

The Extremely Brilliant Source at the ESRF enables such a combination of conditions. Preliminary results show that mineral carbonation occurs through a complex process where the magnesium-rich minerals start to react, then fracture, and mineral precipitation occurs in these fractures. “These results are promising, as they represent a first step to reproduce natural carbonation in basalt and peridotite rocks”, concludes Renard.

Text by Montserrat Capellas Espuny

Top image: The Carbfix injection site in Iceland. Credits: Siljaye, CC BY-SA 4.0 , via Wikimedia Commons