Monday, 30 June, 2025/ Edition 65

Last week, we looked at some recent discoveries around geologic hydrogen. This edition of Core Elements continues diving into this topic with new reports on specific aspects of hydrogen energy science: fairy circles, serpentinization, and underground hydrogen storage.

Rasoul Sorkhabi

Editor, Core Elements

How Fairy Circles Form

Photo Source: NASA Earth Observatory

Fairy circles are sub-circular features with distinctive vegetation growth related to natural hydrogen seepage.

Definition: These features are subtle depressions with depth-to-diameter ratios as low as 1 to 100.

• These sub-circular structures were first reported from the East European Craton in Russia by Larin and colleagues in 2015.

• Similar structures have been discovered in the United States, Brazil, Mali, Namibia, Australia, and Colombia.

How do they form? A recent article in Geology attempts to answer this question using a geomechanical model.

Geomechanical model: The researchers used Itasca Consulting Group’s software Fast Lagrangian Analysis of Continua (FLAC).

• The researchers simulated a horizontal layer of permeable sedimentary layer with thickness H atop an impermeable basement layer in a tectonically quiescent setting.

• The geometry was discretized into ten-meter-square grids.

• The sedimentary layer behaved as a poro-elastic-plastic medium.

• Terzaghi’s effective stress in soil mechanics was applied to the coupled fluid-mechanical model.

• The flow of immiscible water and gas from a point source through the sedimentary porous medium was then modeled.

• The model included an increase in gas pressure at the point source for 100 days, followed by 200 days of constant gas injection, and finally no-gas entry into the system for 100 days.

Results:

• The researchers found that a decrease in pore fluid pressure and a consequent increase in effective mean stress results in mechanical compaction in the sedimentary layer.

  • These conditions arise when gas inflow from a point-source at depth ceases.

• The model also indicated that depression diameter and depth increase with rising gas pressure and sediment thickness (point-source depth).

• Depth-to-diameter ratios conform with published data on natural fairy circles.

Go deeper: Read the full article.

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Hydrogen Generation via Serpentinization

Haneburger/Wikimedia Commons 

Water reacting with ultramafic rocks produces serpentinite and releases hydrogen. In a recent study, Professor Reza Rezaee of Curtin University in Australia offers an empirical model to quantity the generation rates and volumes of hydrogen from serpentinization.

Hydrogen generation factors:

1. Temperature: The higher the temperature (greater than 200 degrees Celsius) the more chemical reaction.

2. Pressure: The higher the pressure the more chemical reaction.

3. Mineralogy: Olivine with an iron content of 10–35 percent and least stable against weathering is ranked one. Clinopyroxene with two to 25 percent iron and more stable than olivine is ranked 0.5.

4. Water availability: A higher water-rock ratio is better.

Model design: The model involves five steps:

1. Gross rock volume calculation

2. Gross rock mass calculation

3. Incorporating the mineral composition factor with olivine set as one

4. Calculating the rate of hydrogen generation, based on temperature and pressure conditions

5. Calculating total mass of generated hydrogen

Case study: The model was applied to the Giles Complex in Australia.

• The Giles Complex consists of 20 layered mafic-ultramafic intrusions that formed 1070 Ma. It consists of wehrlite, harzburgite, websterite, and olivine-rich orthopyroxene.

• The total volume of the Giles Complex was estimated to be 32,000 square kilometers, with 50 percent olivine, 25 percent orthopyroxene, and 25 percent clinopyroxene.

• Considering a geothermal gradient of 30 degrees per kilometer, the temperature at 20 kilometers is about 623 degrees Celsius.

• The article reports the total amount of hydrogen generate through serpentinization in the Giles Complex to be 2.24 x 10¹⁵ kilograms.  Even 0.01 percent of this number (22.4 trillion kilograms of hydrogen) would still be significant.

Go deeper: Read the full article here.

Hydrogen Storage in a Salt Cavern in the Norwegian North Sea

Richard Nortel/ Wikimedia Commons

A study published in the Journal of Energy Storage highlights the importance of salt formation in the North Sea as potential underground hydrogen storage.

Background: Underground Hydrogen Storage (UHS) sites are critical components of the hydrogen energy revolution. They are considered “subsurface batteries.”

UHS sites are categorized into:

• Deep saline aquifers

• Depleted gas reservoirs

• Salt caverns created by solution mining

Salt caverns are the least risky storage because of their low permeability and low chemical reactivity—although they are the most costly.

Hydrogen in Norway:

• The National Climate Act of Norway has identified hydrogen as a research priority.

• The Norwegian North Sea is estimated to have UHS potential of 7,500 terawatt-hours.

The new study: The researchers selected the Zechstein salt for the case study.

• The Zechstein Group in Northern Europe includes kilometer-thick layered evaporitic sediments deposited during the Late Permian from 258–251 Ma.

The researchers used the following workflow:

• Seismic interpretation, well log, and core analysis

• Time-to-depth conversion of interpreted salt structures

• GIS database construction

• Estimated low, base, and high scenarios for hydrogen storage in Zechstein salt

What they found:

• Their base estimation is about 11,164 billion standard cubic feet of hydrogen (with a capacity of 900 terawatt-hours) in 8,700 salt caverns created within 143 salt domes.

• After factoring 22 percent non-salt content in the salt domes based on well logs, the researchers estimate 699 terawatt-hours, which is about three times Norway’s annual energy consumption in 2023.

• Well data indicate more than 400 kilometers of salt in the subsurface.

Go deeper: Read the full article here.

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✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.