Undersea Portuguese volcano could store gigatonnes of CO2

Offshore underwater volcanos could be great locations for carbon capture and storage (CSS) projects, researchers have found. 

In order to analyse the possibilities of these sites, the research team looked at how much carbon could be stored in the Fontanelas volcano, located off the coast of Portugal, partially buried around 100 kilometres offshore from Lisbon.

Their findings showed that the extinct volcano could store as much as 1.2-8.6 gigatonnes of carbon dioxide, the equivalent of 24 to 125 years of the country’s industrial emissions. In contrast, 42.6 megatonnes (0.0426Gt) of carbon dioxide was removed from the atmosphere by international carbon capture and storage efforts in 2022, according to the Global CCS Institute.

Therefore, the new study presents offshore underwater volcanos as a promising direction for CSS efforts. 

“We know that most countries, including Portugal, are making efforts to decarbonise the economy and our human activities. This is a message that this may be one of the instruments to solve the problem” said Ricardo Pereira, a geologist at the NOVA School of Science and Technology, and co-author of the study.

Most CCS projects rely on the injection of carbon dioxide into porous sedimentary basins that are sealed to prevent the migration of the gas out of reservoirs. Over time,  the carbon will form minerals. However, this process can take decades and even centuries.

In contrast, carbon dioxide could be stored in an extinct volcano using a process known as ‘in situ mineral carbonation.’ In this process, elements such as calcium, magnesium, and iron present in volcanic rocks combine with carbon dioxide to form the minerals calcite, dolomite, and magnesite, respectively.

The process's much shorter mineralisation time makes the process safer and more effective, and once carbon is stored in minerals, issues like potential leaks are no longer a concern.

“What makes mineral carbonation really interesting is the time," said Davide Gamboa, a geologist at the University of Aveiro and co-author of the study. "The faster it gets into a mineral, the safer it becomes, and once it’s a mineral, it is permanent.”

Although any volcanic surface would be useful for this process, researchers targeted an offshore volcano for their structure and location, which would keep the carbon away from large populations, but also within expert reach. 

The researchers studied the storage potential at the ancient Fontanelas volcano, using 2D and 3D studies produced during offshore oil exploration, to estimate the amount of CO2 it could hold.  

They also relied on data from samples that had been dredged from the area in 2008, which contained naturally formed carbonate minerals, indicating that the chemical reactions required to store carbon were already happening and that intentional efforts to mineralise carbon in these rocks should be successful.

The samples also had up to 40 per cent pore space, meaning there are spaces within the rocks where carbon dioxide could be injected and mineralised. The researchers also indicate that low-permeability layers imaged around the flanks of the volcano could help with containing the carbon dioxide before it is mineralized.

While this study demonstrated a large potential carbon storage capacity at the Fontanelas volcano, the authors highlight that many other places around the world may have similar offshore volcanoes that could be candidates for carbon capture and storage.