Monitoring CO2 leakage sites on the ocean floor

The goals of the Paris climate protection agreement can hardly be achieved with the reduction of greenhouse gas emissions alone. The Intergovernmental Panel on Climate Change (IPCC) is therefore discussing additional measures to reduce the concentration of greenhouse gases in the atmosphere. For example, carbon dioxide produced during the combustion of fossil fuels could be captured or removed directly from the atmosphere and subsequently stored in deep geological formations. The Norwegian company Equinor (formerly Statoil) has been operating such a carbon capture and storage facility since 1996. About one megaton of CO2 is injected yearly into a layer of sandstone beneath the North Sea. However, reliable ways of effectively monitoring gas leaks from such undersea storages are still being discussed.

In this context, researchers at GEOMAR Helmholtz Centre for Ocean Research Kiel investigated natural volcanic carbon dioxide seeps off the coast of Italy. Based on the results of these investigations, they have developed models that can be used to monitor the safety of future submarine CO2 deposits. The study has been published now in the international journal Environmental Science & Technology of the American Chemical Society (ACS).

The storage of carbon dioxide in layers of rock beneath the seabed poses a lower risk to humans than storage below land. If gas escapes from the seabed, it dissolves in the seawater. However, it can lower the pH there and damage the local marine ecosystem. "At present, there is no established method to locate such carbon dioxide leaks and determine the total amount of gas released when the leaks extend over several hundred square meters of the seabed," says Dr. Jonas Gros of GEOMAR, lead author of the new study. He and his colleagues therefore investigated changes in the pH value in the vicinity of natural carbon dioxide seeps near Panarea, a small island off the coast of Northern Sicily.
 
During diving operations and with the help of ship-based instruments, they collected gas and water samples close to the seeps. The team used these data to test a computer model they had developed to predict pH changes in seawater due to carbon dioxide leakage. Simulations showed that over 79% of the released carbon dioxide already dissolves within four meters from the seabed.

The team also found that the model was able to predict a pattern of pH variation in the waters around the gas seeps that was comparable to the measured sensor data. "The new model can thus serve as a guideline for strategies for routine monitoring of CO2 storage in the seabed and for estimating the effects of carbon dioxide emissions on the local marine environment," stresses Dr. Gros.

Reference:
Gros, J., M. Schmidt, A. W. Dale, P. Linke, L. Vielstädte, N. Bigalke, M. Haeckel, K. Wallmann, S. Sommer (2019): Simulating and quantifying multiple natural subsea CO2 seeps at Panarea Island (Aeolian Islands, Italy) as a proxy for potential leakage from sub-seabed carbon storage sites. Environmental Science & Technology. https://doi.org/10.1021/acs.est.9b02131

Please note:
The study was funded by the ROBEX - Robotic Exploration of Extreme Environments Alliance, the ECO2 project, the Kiel Cluster of Excellence "The Future Ocean" and the STEMM-CCS project as part of the European Union's Horizon 2020 programme.

Images:
 A research diver investigates gas bubbles off the italian island of Panarea. Photo: Christian Howe, http://www.h2owe.de/