To achieve its net zero climate target by 2050, Switzerland must press forward with the energy transition—whether in electricity, heating or mobility. The permanent storage of CO₂ is another important challenge. In particular, Switzerland must find a permanent solution for emissions that are difficult or impossible to avoid, such as those produced by waste incinerators.

Researchers at ETH Zurich have conducted the first ever study to investigate whether CO₂ can be permanently stored underground in Switzerland in the form of carbonate rock, and what criteria would need to be met for this to happen. They present their findings in a study that was recently published in the Swiss Journal of Geosciences.

How rock can be used to store CO₂

To begin with, ETH researchers want to find out whether there are any zones in Switzerland where CO₂ can be permanently stored in the rocky underground. Permanent storage of CO₂ underground requires that the rock be rich in calcium, magnesium and iron, while at the same time containing as little silicon dioxide as possible. Potential candidates include basalt, peridotite and serpentinite.

For ideal storage capacity, the rock in the subsurface must also be of a certain volume and located at a depth of at least 350 meters in order for the pressure to be high enough to keep the CO₂ in the water. An optimal temperature of between 90°C and 185°C, plus the age, alteration condition, porosity and permeability of the rock all play an important role as well.

“These are some of the criteria that have to be met before an area can even be considered as a reservoir,” says Adrian Martin, whose master’s thesis forms the basis for this study. He has since expanded on his work at the Energy Science Center at ETH Zurich under Marco Mazzotti, Professor Emeritus of Process Engineering.

To store the CO₂ underground, it is dissolved in water and pumped underground as carbonic acid. The water used is initially acidic, i.e., it has a low pH value. It penetrates and dissolves the porous rock, releasing iron, magnesium and calcium ions.

This causes the pH of the injected water to increase, and at a certain point a reverse reaction occurs: the CO₂ combines with calcium and magnesium to form white carbonate rock, e.g., limestone. “This process is called in-situ mineralization,” says Martin.

Potential recognized, but feasibility questionable

Thanushika Gunatilake, a former postdoc with Stefan Wiemer, a professor in the Department of Earth and Planetary Sciences and head of the Swiss Seismological Service, also worked on the study. She is now an assistant professor at the Vrije Universiteit Amsterdam and emphasizes that this nationwide search for suitable rock types is the first of its kind in Switzerland.

Martin has not only analyzed numerous scientific studies; he has also examined geological maps area by area and identified those locations that meet the criteria and could therefore be suitable for in-situ CO₂ mineralization. These areas include the Zermatt-Saas zone and the Tsaté nappe in Valais, as well as the Arosa zone in Graubünden.

The areas identified by Martin are not currently suitable for permanent underground storage of CO₂. “Although we do have suitable rock types in Switzerland, we face major technical challenges,” says Martin.

The geological structure is very complex due to the heavily folded rock strata and tectonic faults. At the Tsaté nappe in Valais, the layers in suitable rocks in areas such as the one between Gouille and Mont des Ritses can have a thickness of over 500 meters, while at Les Diablons, thickness is only around 150 meters.

Other factors compound the issue: the rocks in the Zermatt-Saas zone, for example, were transformed in the past by high pressures and temperatures and now already contain many carbonate minerals, indicating that natural CO₂ absorption (i.e., previous mineralization) has already occurred. Furthermore, the Zermatt rocks are packed very close together underground and contain few open cavities or cracks into which the CO₂ could penetrate.

Additionally, the volume of water required for in-situ mineralization is enormous—close to 25 tons of water would be needed to store 1 ton of CO₂. Martin adds, “On top of that, there are economic and societal hurdles: Who will bear the costs? How do you overcome the skepticism of local residents who are concerned about water pollution, for example? What would be the legal regulations?”

Alternative methods of CO₂ storage

The researchers conclude that permanent storage of CO₂ through in-situ mineralization in Switzerland is not feasible in the short term and appears unsuitable in the long term. They therefore recommend investigating alternative storage methods.

Gunatilake has recently published another study, in Carbon Capture Science & Technology, this time focusing on storing CO₂ in saline aquifers. For this project, researchers used numerical simulations to analyze data from the area around Triemli in Zurich.

They succeeded in injecting CO₂ into the geological unit, the lower Muschelkalk, to a depth of more than 2,000 meters without water. “This method of CO₂ storage is promising,” emphasizes Gunatilake.

There are also projects that successfully demonstrate permanent storage of CO₂ underground. “One example is the DemoUpCARMA project, where CO₂ from Switzerland was transported to Iceland where it is now stored underground in the form of carbonate rock,” says Martin.

It is important to raise public awareness of the issue and to dispel myths and rumors. “Many people think we’re creating a bubble underground and that it could even explode at some point,” explains Martin. “But the risk to the public from underground CO₂ storage is minimal and the methods are scientifically well proven.”