The study led by the Institute of Environmental Assessment and Water Research (IDAEA) and the Mediterranean Institute for Advanced Studies (IMEDEA CSIC-UIB), both belonging to the Spanish National Research Council (CSIC), is reported to show that injecting billions of tonnes of atmospheric CO₂ underground has a ‘low risk of leakage back to the surface’. According to the simulations conducted as part of the study, the CO₂ would remain deep in the subsurface for millions of years, even if the overlying low-permeability rocks were fractured. The findings suggest that ‘geological CO₂ storage can be safely undertaken to mitigate climate change’, state the researchers.

The study, published in the journal Geophysical Research Letters, was carried out in collaboration with the Lawrence Berkeley National Laboratory and the University of Illinois at Urbana-Champaign. The interdisciplinary research team developed what they claim is a novel methodology to calculate the probability of CO₂ leakage when considering billions of tonnes of CO₂ injected underground over a time scale of millions of years, much larger than what has been investigated until now.

‘The objective of CO₂ storage is to take this greenhouse gas from the hard-to-abate industry and inject it deep underground. For the gas to remain at depth, it must be injected into rocks with high permeability and porosity, such as sandstones. However, there is a risk of CO₂ leakage, as CO₂ is less dense than the saline water that fills the pores at great depth, so it can float upwards and leak back to the surface,’ explains IDAEA-CSIC researcher Iman Rahimzadeh Kivi.

To calculate the risk of CO₂ leakage, the research team predicted the gas flow to the surface after its injection at 1,550 metres deep (the common depth to store the gas underground) using numerical transport models in two different scenarios.

‘Our predictions show that, in the best-case scenario, when the underground rock properties remain intact, the CO₂ would only rise 200 metres upwards after one million years. In our worst-case scenario, when the rocks present a large number of fractures, CO₂ would rise 300 metres upwards,’ continues Victor Vilarrasa, researcher at IMEDEA-CSIC-UIB and principal author of the study.

‘This means that even in the worst possible scenario, the CO₂ would be indefinitely contained in the subsurface at 1,250 metres depth for millions of years,’ notes Kivi.

The researchers believe their study will help increase confidence in the security of underground CO₂ storage to achieve carbon neutrality and mitigate the effects of the climate emergency. ‘The scenarios proposed by the Intergovernmental Panel on Climate Change (IPCC) to achieve zero emissions, and even net-carbon removal from the atmosphere, require geological CO₂ storage. And this study shows that permanent CO₂ storage can be safely achieved,’ concludes Vilarrasa.

Denmark awards first offshore CO₂ storage licences
Meanwhile, the Danish Ministry of Climate, Energy and Utilities has awarded its first three licences to explore CO₂ storage potential in the Danish North Sea – two to TotalEnergies and the third to a consortium consisting of Ineos E&P and Wintershall DEA.

Danish national oil and gas company Nordsøfonden will hold a 20% stake in the licences, the award of which marks an important step towards Denmark reaching its goal of carbon neutrality by 2050.

Ineos and Wintershall DEA estimate they will be able to store 1.5 t/y of CO₂ before the end of 2025, and upwards of 8mn t/y in 2030. TotalEnergies expects to be able to store more than 5mn t/y from 2030. Denmark is estimated to have some 22bn tonnes of offshore storage potential.

CCS and the UK’s road to net zero
In related news, the North Sea Transition Authority (NSTA) has responded to the rapid growth of the UK’s carbon capture and storage (CCS) industry by setting up a dedicated team that will oversee the delivery of offshore carbon transportation and storage developments following successful exploration and appraisal.

The move comes as it considers applications for the UK’s first ever CCS licensing round. Launched in June last year, the round offered 13 areas, for which 26 bids were made. Licences are due to be offered for award in the coming weeks. The round is expected to be the first of many, with estimates that up to 100 separate stores could be required for the UK to meet its domestic storage requirements, according to the NSTA.

The UK government has set a target to capture 20–30mn t/y of CO₂ by 2030 as part of its net zero by 2050 goal.