What happens when CO2 is stored underground

Study on CO2 storage : Can carbon dioxide be stored in the North Sea?

The greenhouse gas carbon dioxide can be stored in the seabed of the North Sea - even if boreholes make the subsurface leak. This is the conclusion reached by a team of scientists led by the Geomar Helmholtz Center for Ocean Research in Kiel.

According to this, the gas does not enter the atmosphere when it comes out of leaks, but remains in the sea. When dissolved in the water, the CO2 is then quickly distributed by tidal currents in the North Sea. This is what the researchers write in the "International Journal of Greenhouse Gas Control".

10,000 holes make the seabed porous

Despite global warming, man-made CO2 emissions have not yet been reduced. Scientists assume that global warming can only be limited to below 1.5 or two degrees if CO2 is removed from the atmosphere. That is why they are looking for ways to store the greenhouse gas. One possibility is to capture CO2 from the exhaust gases of power plants and then permanently store it in an underground storage facility. The process is called "Carbon Capture and Storage" (CCS).

The greatest storage potential in Europe is located off the North Sea coast in deep geological formations below the sea floor. However, around 10,000 boreholes in the past few decades for the search for gas and oil deposits have made the sea floor porous, as the sediments were mechanically disturbed and weakened during drilling. There methane gas is already escaping from gas leaks. Carbon dioxide stored in the vicinity of such boreholes could theoretically also leave the sea floor this way and then return to the atmosphere. This would mean that the climate-friendly effect would be gone.

A robot pumped CO2 into the seawater

In order to control what would happen to the CO2 in such a case, the scientists started a field test. They lowered a diving robot to the bottom in the middle of the North Sea between the northern tip of Scotland and the southern tip of Norway. At a depth of 82 meters, it blew CO2 into the water in a controlled manner at a rate of 31 tons per year. According to Geomar, this corresponds roughly to the upper end of the methane emissions range.

The result: the CO2 gas bubbles dissolved in the water within two meters above the sea floor. The climate-damaging carbon dioxide was not released into the atmosphere, but remained in the North Sea. However, when carbon dioxide dissolves, the pH of the water changes - it becomes more acidic. "This acidification of the bottom water has a detrimental effect on the organisms living on the sea floor," explained project manager Klaus Wallmann. "But the strong bottom currents that exist there distribute the dissolved CO2 quickly, so that the area on the seabed where potentially harmful effects can occur is small."

"We therefore come to the preliminary conclusion that it is possible to store CO2 safely in formations under the sea floor if the storage location is in an area with few leaky boreholes," was Wallmann's conclusion.

What about the technology in Germany?

Kiel scientists are currently in the North Sea for a second release experiment, as Geomar spokesman Andreas Villwock said. Highly sensitive sensors are used to track the released CO2 and examine the effects on the environment.

As far as the use of CCS is concerned, the Kiel state government has long since spoken out against the storage of CO2 in Schleswig-Holstein, as announced by Patrick Tiede from the Ministry of the Environment. According to the law, this also applies to the coastal waters of the North and Baltic Seas.

However, momentum is emerging again in the debate. Chancellor Angela Merkel recently spoke about the goal of greenhouse gas neutrality by 2050 in an interview with the "Süddeutsche Zeitung". In her opinion, this can only be achieved if one is prepared to store carbon dioxide. In an interview with the "Frankfurter Rundschau", Federal Environment Minister Svenja also spoke out in favor of reassessing the chances of CO2 storage for climate protection. (fsch, dpa)

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