Computer simulations suggest that CO2 can be stored underground indefinitely

If you want to save the climate, you must stop the release of carbon dioxide (CO2). There is no doubt about that. But that’s not enough. Additionally, it needs to be stored permanently by capturing CO2 already present in the atmosphere and sending it deep into the ground, for example.

This naturally raises the question of what will happen to this CO2 over the long term. Is it guaranteed to stay on the ground, or could it escape for decades or centuries?

A very sophisticated numerical simulation of the supercomputer shows exactly what happens when CO2 mixes with groundwater. In the complex interaction of the CO2 Richer and the CO2 poison area, the CO2 Richer water slowly sinks below, allowing CO2 to be stored underground forever.

CO2 rises, but CO2 dissolves in the water sink

Deep underground, the pressure is very high, and carbon dioxide remains liquid, but has a much lower density than water. Therefore, you might think that CO2 will soon float upward when pumped into groundwater. But the problem is somewhat complicated.

“Pure CO2 is less dense than water, but the situation changes when CO2 dissolves in water. When the two are mixed, the total volume decreases and the liquid is produced with a more dense liquid,” explains Marco de Paris, head of the research project. Water with a high CO2 content will sink because it is more dense than water with a low CO2 content.

Irregular structure sinking

“The dynamics of porous rocks are very interesting because water with a high CO2 content is more dense than water with a low CO2 content,” says De Paoli. “If the CO2 concentration is highest, the mixture sinks faster, which guarantees better mixing.” This results in a network-like pattern with high CO2 concentrations and low regions.

Overall, the team was able to show in computer simulation that CO2 had sunk down and remained there. From the calculations, the team was able to derive a simple model that engineers could use to predict CO2 flow in the ground and design design injection strategies without performing complex, large-scale computer simulations in all situations. This research has been published in the journal Geophysical Research Letters.

Appropriate geological conditions

Of course, this doesn’t work anywhere. First, a rock formation that is as impermeable as possible is required, and a rock formation that can be collected first until CO2 is dissolved in water. The rock below should be as porous as possible so that CO2-containing water can easily sink downwards. When this occurs, the above impermeable rock formations no longer play a role. Even geological changes such as earthquakes and anthropogenic activities no longer affect the situation. CO2 is safely stored on the ground.

“That geological condition is not particularly rare,” says De Paris. “Depleted oil reservoirs can be used. There is also a large area called saline aquifers located below the seafloor or inland, allowing CO2 storage according to this scheme. At least six salines are also present in Austria.”

Over the next few years, De Paoli will answer even more important questions in his Tu Wien research project. For example, it is necessary to clarify how the rock changes as water containing CO2 passes through it. Certain chemical reactions can cause rock minerals to dissolve, which lead to even greater flows of CO2 downwards.

“If you want to massively mitigate the impact of climate change by capturing CO2, all of these questions need to be answered in detail,” says De Paoli.