Simply reducing greenhouse gas emissions will not be sufficient to reach the 2°C target established in 2010 at the United Nations Climate Change Conference – let alone the most recent version calling for a maximum of 1.5°C of warming relative to pre-industrial times. According to a special report by the Intergovernmental Panel on Climate Change (IPCC) in 2018, meeting this target will also require CO ₂ to be removed from the atmosphere and stored underground. However, these negative emissions can only be achieved if the separated CO ₂ can be stored reliably over hundreds of years.

Whether this will work depends not least on the nature of the bedrock. CO ₂ pumped into the ground can escape not only through the borehole but also through existing fractures in the cap rock. “Suitable cap rock should ideally be impermeable and act as a reliable seal for the reservoir containing the CO ₂ . We’re interested in how effectively CO ₂ can be retained by cap rock with fractures, known as fault zones,” explains Professor Stefan Wiemer, Head Swiss Seismological Service, who is leading the project. The physical and chemical processes that affect whether and how CO ₂ can escape through such fault zones are still poorly understood. It is also unclear what influence CO ₂ injections have on rock deformations and chemical interactions that can trigger earthquakes. In addition, little is currently known about the prevailing conditions in Switzerland, making it hard to assess whether underground CO ₂ storage is even an option in this country.

Do rock fractures impair CO ₂ storage?

For this reason, scientists from the Swiss Seismological Service (SED) at ETH Zurich and the Swiss Competence Center for Energy Research – Supply of Electricity (SCCER-SoE) are conducting an experiment in close collaboration with the Department of Mechanical and Process Engineering and the Institute of Geophysics at ETH Zurich, as well as Swisstopo and EPFL. They are investigating how well fractured rock can store CO ₂ , the conditions under which induced seismicity occurs, and how best to monitor a reservoir of this kind. “Theoretically, Switzerland has the potential to store several gigatonnes of CO ₂ underground. With our experiment, we’re contributing to a more comprehensive understanding of the relevant geological processes that influence storage potential. We’re also laying the foundation for better-informed decisions on possible CO ₂ storage projects in Switzerland,” says Wiemer.

No reservoirs planned yet

The experiment conducted in the Mont Terri rock laboratory, deep within the Jura mountains, forms part of the Elegancy project , which is funded by the European Commission and the Swiss Federal Office of Energy (SFOE). The scientists plan to inject small quantities of CO ₂ -enriched salt water into a borehole passing through a small fault zone. To find out how the fractured rock in this fault zone reacts to the CO ₂ , they will monitor the stability of the rock and study the relationship between shear displacement, pore pressure and flow paths. Active and passive seismic sensors will monitor changes in seismic velocities near the injection site and detect any potential micro-earthquakes.

Unlike a real CO ₂ storage project, this experiment analyses potential leaks on a very small scale. Nevertheless, the findings will contribute to a better understanding of the relevant processes that influence the movement of CO ₂ through fault zones. The experiment will therefore also help to improve site characterisation. Worldwide, a handful of CO ₂ storage projects are already in operation, each with up to 3 million tonnes of CO ₂ captured and stored per year. Further such projects are in the pipeline. There are currently no plans for a CO ₂ storage project in Switzerland.