Capturing and storing carbon dioxide (CO2), at large depths, is one of the options for reducing the amount of CO2 in our atmosphere. Although there are places on earth that have a long history with this, knowledge is lacking about how the stored gas, as a ‘supercritical fluid’, behaves at depths of two to three kilometer. How does it mix with other fluids, what is the influence of the soil structure? Researcher Marco De Paoli starts a research project on this, at the University of Twente and the Technische Universität Wien. A so-called ‘Erwin Schrödinger Fellowship’ of the Austrian Science Fund enables him to do this.
Known as CCS (Carbon Capture and Storage), it is one of the techniques for getting CO2 out of our atmosphere. The recent IPCC report emphasizes the need for these technologies within the ‘mix’ of measures for reaching the climate goals. CCS is under discussion, opponents say that it diverts the attention from other options that truly reduce the emitted CO2. At the same time, CCS technology is developing as well, an example is the new ORCA project in Iceland. Still, there are gaps in our knowledge: is storage safe for hundreds of years? What exactly happens at those depths? How does soil structure influence the flows and storage? Marco de Paoli already did extensive simulations on this, using large supercomputers. Doing experiments at those depths is not really feasible, so at the University of Twente he will build a lab set-up to compare simulations and experiments.
Mix and diverge
At two to three kilometers, at the high pressures and temperatures involved, CO2 is no longer a gas but a ‘supercritical’ fluid: that is a state that is between gas and fluid. This sinks into salty water, ‘brine’, as a sort of ‘tears’. This is a complicated mixing process, influenced by the soil granules that make it diverge and spread over a larger area. In the experimental setting, Marco plans to mimic this using a water reservoir, with potassium permanganate flowing in from the top. The way it sinks and mixes, is comparable to supercritical CO2 and water. With a camera, he will track the mixing process, thus obtaining new knowledge next to the simulation outcomes. This will help optimizing CCS technology and make it safer and more reliable.
About the header image: Flow cells formed during the mixing process of CO2 in brine. The blue-to-red colour gradient indicates the CO2 concentration. The density difference existing between CO2 (blue) and the liquid brine present in some permeable rocks (red) drives the flow. The solution exhibits what is known as Rayleigh-Darcy convection.