Natural hydrogen from deep underground could be an important building block for the sustainable energy system of the future, but it is currently still difficult to predict where and at what depth elevated concentrations are located. New study results from the Department of Geology at the University of Vienna could make such predictions easier in the future. So-called "fairy circles" – round patches with vegetation damage – could be helpful indicators. This is because these "fairy circles" subside due to the seepage of natural hydrogen. According to the study, the deeper the hydrogen source underground and the higher its pressure, the larger the "fairy circle" on the Earth's surface. The study was recently published in the journal Geology.

In many parts of the world – from the East European Craton in Russia to the USA, Brazil, Namibia and Australia – mysterious circular depressions with little or no vegetation can be found on the Earth's surface. These so-called "fairy circles" are typically hundreds of metres wide and a few metres deep. It has only been clear for around ten years that fairy circles emit natural hydrogen and thus indicate underground hydrogen sources. However, it has not yet been possible to explain why they subside and whether their size could also provide clues about the depth or yield of the hydrogen source.

But this is precisely important information for the energy industry: With its almost negligible carbon footprint, natural hydrogen is considered a promising sustainable energy source for the future. "But before expensive drilling can be carried out, we need to understand how fairy circles form, how large the deposits might be and how deep we need to drill," explains Martin Schöpfer from the Department of Geology at the University of Vienna and employee of NiMBUC Geoscience.

A study subsidised by OMV and led by Schöpfer has now been able to explain, with the help of geomechanical computer simulations, why the Earth's surface subsides in hydrogen-emitting fairy circles. According to the study, the reason for this is the interaction between gas and water flow and the soil (sediment) in a two-phase process.

Soufflé rises and collapses

The assumption was made that loose sediment, like sand or clay, with its interstices (pores) saturated with groundwater is overlying solid rock. If gas – hydrogen – enters this sedimentary layer at a point source, it partially displaces the water, which then seeps at the surface. Hydrogen also seeps through the ground, and the altered gas mixture could damage vegetation. In addition, the Earth's surface is slightly uplifted. "You could say that the sediment rises like a soufflé, but here geomechanical processes are at work, whereas with a soufflé it is chemical processes," says Schöpfer. When the hydrogen flow then ceases in a second phase, the pressure of the gas-water mixture in the interstices of the sediment decreases, leading to compaction. "The soil compresses and subsides, similar to a collapsing soufflé," describes the geologist.

With the help of these computer simulations, the study investigated the interactions between the depth and gas pressure of the source with the deformation of the sediment. The results were astonishing: the diameters and subsidence depths of the simulated fairy circles correspond almost perfectly to the natural structures discovered in regions such as Russia, Brazil and Australia.

The larger the fairy circle, the deeper the hydrogen source

According to this, the diameter and depth of the fairy circles are likely to be directly related to the pressure and the depth of the gas source. "These findings are a real breakthrough," emphasises Bernhard Grasemann, deputy head of the Department of Geology. "Fairy circles could thus serve as natural signposts in the future for finding underground hydrogen sources – a potentially inexhaustible and environmentally friendly energy source".

Interest from the energy sector and the colours of colourless hydrogen

Gabor Tari, chief geologist at OMV and co-author of the study, explains: "The energy sector's interest in natural hydrogen as a potential new energy source with a negligible carbon footprint is growing, especially in comparison to all other types of artificially produced hydrogen. White – or golden, i.e. natural – and orange hydrogen in particular are the focus of intensive international research, as they have the potential to become profitable and probably significantly cheaper alternatives to the black, grey, blue, pink and green types of hydrogen currently in use, all of which have a significant carbon footprint. That is why OMV Energy supports basic research such as this to better understand the future potential of this green energy source as part of the energy transition."

However, Schöpfer emphasises that numerous further studies on natural fairy circles are still needed before the results can be applied, "for example, simulations with different soil types or with pulsating gas emissions, but also field studies of the subsurface, which could show that chemical reactions may also dissolve solid material and thus be partly responsible for the subsidence".