FROM THE AIR OR FACTORY INTO UNDERGROUND ENERGY PRODUCTION
CO₂ Plume Geothermal (CPG)
The illustration shows a landscape with industrial plants, pipelines, compressors, a turbine/generator, a cooling tower, power lines, and a village. Below, a geological cross‑section depicts porous reservoir rock overlain by an impermeable caprock, with a yellow‑red CO2 plume. A numbered cycle explains the process: 1) CO2 is captured from industry or air and liquefied. 2) It is transported by truck, rail, ship, or pipeline to a facility and injected at depths of at least two kilometers where temperatures exceed 100 °C. 3) The CO2 is pressed into sandstone with many pores. 4) Geothermal heat makes it gaseous; it rises and accumulates beneath the caprock as a CO2 plume. 5) Under the caprock the CO2 is stored in four ways: structural trapping beneath the caprock, residual trapping in rock pores, dissolution into porewater forming carbonic acid/carbonate, and long‑term mineral binding. 6) A second well produces part of the heated CO2. 7) At the surface it drives a turbine to generate electricity; residual heat is used for district heating. 8) In a cooling tower the gas cools and becomes liquid again. 9) The liquefied CO2 is reinjected while additional “new” CO2 is also injected, progressively turning the subsurface into a long‑term carbon sink. A magnified inset shows porous grains with water and minerals to illustrate the storage processes.
CPG combines CO₂ storage with the use of geothermal heat: CO₂ is pumped via a borehole into deep rock layers, where it heats up and is then returned to the surface. There, it is used to generate electricity and heat before being pumped back underground. Over time, more and more CO₂ remains stored underground, while additional CO₂ is continuously fed in.
CPG can reduce greenhouse gas emissions and remove CO₂ from the atmosphere. On the one hand, the CO2 savings are achieved by using geothermal heat to produce renewable, CO₂-neutral energy. On the other hand, greenhouse gases are prevented from entering the atmosphere because they are captured during industrial processes and permanently stored underground. This is referred to as carbon capture, utilisation and storage (CCUS). In addition, CO₂ is removed from the atmosphere (carbon dioxide removal, CDR) by filtering it directly from the air. Another advantage is that geothermal heat produces electricity and heat much more efficiently with CO₂ than with water, which is normally used for this purpose.
CPG is currently in transition from the pure research phase to practical implementation – a decisive step for which the CPG consortium was established at ETH Zurich in 2023.
Before CPG can be tested in nature, complex experiments and modelling in the laboratory are required to better understand processes and minimise risks. Photo: Ulrike Kastrup (focusTerra)
Interactive exhibit
Photo: Nicola Pitaro
Back: sandstone (Ergolz-Member); Schleitheim, Seebi
Front: shale (Ardesia Nera); Italy
A BUBBLING ROCK
To store CO₂ underground, you need porous, permeable rock, among other things. This means rock that not only needs to have as many pores as possible, but the pores also need to be interconnected so they can be filled with CO₂. Test the permeability of two types of rock in this experiment.
- Pull the handle up and push it down again. This pumps air into the centre of the porous sandstone in the water basin. Please pump carefully to protect your back.
- What happens to the air after you have pumped a few times?
- Can you see a difference between the left and right halves of the sandstone? What effect does the dark shale covering the sandstone have?
EXPLANATION
While air flows easily through the sandstone’s pores and escapes at the open sides, dark, dense shale does not allow it to pass through. This makes shale a good cap rock: it ensures that air from the porous rock below cannot escape upwards.
The same principle is used when storing CO₂ in the geological subsurface: the CO₂ is injected into porous rock and remains trapped under a dense cap rock for thousands or millions of years. In nature, the cap rock usually covers a large area, so that the CO₂ cannot escape sideways. Over time,
the CO₂ dissolves in the surrounding pore water.
Porous rock layers with overlying cap rocks are also a natural storage site for oil and gas. This is why depleted oil and gas fields are particularly well suited for CO₂ storage.