How to get rid of carbon dioxide for good

We have to stop emitting carbon dioxide (CO2) if we want to save the climate – there is no doubt about that. But that alone will not be enough. In addition, it will also be necessary to capture CO2 that is already present in the atmosphere, and store it permanently, for example by pumping it deep into the ground. This naturally raises the question of what happens to this CO2 in the long term. Is it guaranteed to remain in the ground, or is it possible that it could escape over decades or centuries?

Highly sophisticated numerical simulations on supercomputers are now showing for the first time exactly what happens when CO2 mixes with groundwater: in a complex interplay between CO2-richer and CO2-poorer areas, the CO2-richer water slowly sinks downwards, allowing the CO2 to be permanently stored underground.

CO2 rises – but CO2 dissolved in water sinks

Deep underground, the pressure is so high that carbon dioxide remains liquid, but with a much lower density than water. One might therefore think that CO2 would immediately drift upwards when pumped into the groundwater. But the matter is somewhat more complicated.

“Pure CO2 has a lower density than water, but the situation changes when CO2 is dissolved in water. When the two are mixed, the total volume decreases, creating a denser liquid,” explains Marco De Paoli, head of the research project. Water with a high CO2 content has a higher density than water with a lower CO2 content and therefore sinks.

Marco De Paoli is currently working at the University of Twente in Enschede, Netherlands, and at the Institute of Fluid Mechanics and Heat Transfer at TU Wien and is currently in the process of relocating to Vienna. In 2024, he was awarded an ERC grant by the European Research Council, and he will implement this project at the Institute of Fluid Mechanics and Heat Transfer at TU Wien from autumn 2025.

Irregular structures that sink

“Because water with a higher CO2 content has a higher density than water with a lower CO2 content, the dynamics in the porous rock are highly interesting,” says Marco De Paoli. “Where the CO2 concentration is highest, the mixture sinks faster, which in turn ensures even better mixing.” This results in a network-like pattern of areas with higher and lower CO2 concentrations.

Overall, the team was able to show with their computer simulations that the CO2 sinks downwards and remains there – for unlimited periods of time. From the calculations, the team was able to derive simple models that can now be used by engineers to predict the CO2 flow in the ground and design injections strategies without having to carry out complex and massive computer simulations for every situation.

Suitable geological conditions

Of course, this does not work everywhere. First of all, you need a rock layer that is as impermeable as possible, under which the CO2 can initially collect until it has dissolved in water. The rock below should be as porous as possible so that the CO2-containing water can easily sink downwards. Once this has happened, the impermeable rock layer above no longer plays a role. Even geological changes, such as an earthquake or anthropogenic activities, would no longer affect the situation. The CO2 is safely stored in the ground.

“Such geological conditions are not that rare,” says Marco De Paoli. “You could use depleted oil reservoirs. There are also large areas called saline aquifers, located under the seabed or inland, where CO2 storage would be possible according to this scheme. At least six saline aquifers are also present in Austria.”

In the next few years, Marco De Paoli plans to answer further important questions in his ERC research project at the TU Wien. For example, it should also be clarified how the rock changes when CO2-containing water flows through it. Certain chemical reactions can cause rock minerals to dissolve, which would allow an even greater flow of CO2 downwards. “All these questions must be answered in detail if we want to mitigate the effects of climate change on a large scale by capturing CO2,” says Marco De Paoli.

This work, funded by the Horizon Europe research and innovation programme and by the EuroHPC Joint Undertaking, is the outcome of an international collaboration involving scientists from the University of Twente (Enschede, the Netherlands), Sapienza University (Rome, Italy), Newcastle University (Newcastle upon Tyne, United Kingdom) and the PhD students Lea Enzenberger and Eliza Coliban from TU Wien.