CO2 repository: What happens in case of a leak?

Experiment tests consequences of leaky CO2 storage under the seabed

Sleipner gas field off the Norwegian coast: Since 1996, carbon dioxide has been disposed of below the seabed. © Bair175 (CC BY-SA 3.0)
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Where to go with the carbon dioxide? The controversial CCS technology aims to squeeze the greenhouse gas into suitable rock strata beneath the seabed to reduce emissions to the atmosphere. The safety aspects of this method have now been studied by British scientists in field trials. Because what happens when such a memory gets a leak? And how can this be recognized as quickly as possible?

For years, there have been calls to curb the immense emissions of climate-damaging carbon dioxide (CO2) - most recently in the run-up to the world climate summit in New York. But humanity is far from independent of fossil energies. Until that happens, we will continue to blow carbon dioxide into the atmosphere for years to come. The idea of ​​"Carbon Capture and Storage" (CCS), for example, "CO2 capture and storage" sounds so tantalizing: To slow down the greenhouse effect, CO2 can be separated already at the origin. Instead of entering the atmosphere, the greenhouse gas is then pressed, for example, into porous sandstone or saltwater-bearing rock under the seabed.

Controlled leak as a test case

The process is not new. The Norwegian oil company Statoil, for example, began in 1996 to initiate carbon dioxide into a sandstone formation under the North Sea over several years. The consequences of a leak in such a reservoir for the flora and fauna in the affected marine area, however, is completely unclear. Studies on this topic have so far been limited to experiments in the laboratory or observations of natural CO2 sources.

But that's not realistic enough, as Jerry Blackford of the Plymouth Marine Laboratory and his colleagues explain. The geologists and marine scientists have therefore carried out a practical test: they themselves put a small, eleven meters below the seabed CO2 storage off the coast of the Scottish village Benderloch. Here they initiated a total of 4.2 tonnes of carbon dioxide over a period of 37 days. Then they controlled a small leak and watched what happened. Their goal: To analyze the effects on the surrounding ecosystem and to investigate suitable methods to detect such leaks.

Acoustic and chemical evidence

In fact, after only a few hours, the first visual and audible signs of the leak appeared: Bubbling gas bubbles formed on the seabed. Using seismic imaging, the researchers were also able to see that the carbon dioxide first made its way through the sediment in a vertical gas vent. The pressure increases so much that breaks in the rock occur. display


The team was also able to identify chemical changes: After 30 days of CO2 leakage, the proportion of dissolved, inorganic carbon in the water trapped in the rock spores had increased tenfold. The concentration of calcium ions increased and the water became more alkaline - an indication that the calcium carbonate in the sediment dissolves due to the released CO2.

Biological consequences significant, but reversible

The sea-based community also responded to the leak: after the cessation of CO2 release, the community of bacteria, algae, crustaceans, clams and fish was composed quite differently than in unaffected areas, the researchers report. However, these consequences were limited to a radius of a few meters.

The good news: both chemical and biological changes were not definitive. A few weeks after the artificial leak was closed again, everything was back to normal. "Although a leak has consequences for the surrounding ecosystem, " the researchers write. "However, these are not catastrophic and recovery is possible within a few weeks." They warn, however, that larger and longer lasting leaks could cause more massive consequences.

Monitoring with underwater vehicles

It seems all the more important to develop a monitoring strategy to find leaks as quickly as possible - a complex challenge given the large volumes of water that need to be monitored for this purpose. The main problem is that clear signs such as gas bubbles are not always generated and may have other causes. So it is not enough to pay attention only to a specific warning signal. Instead, effective monitoring systems need to raise a variety of factors.

The scientists suggest using mobile underwater vehicles that move independently on the seabed and are equipped with a variety of sensors. For example, acoustic sensors could detect the sounds of gas bubbles. Chemical sensors, in turn, would register the changes caused by the dissolution of carbon dioxide. "Monitoring would certainly be demanding, but quite feasible with a multivariate approach, " the researchers conclude. In order to be able to develop a reliable method, however, above all further basic studies are indispensable.

(Nature Climate Change, 2014; doi: 10.1038 / NCLIMATE2381)

(Blackford et al., Nature Climate Change, 29.09.2014 - DAL / AKR)