Is natural hydrogen of the "fairy circles” the new eldorado?

In  some  countries,  strange  circular  areas can be seen where, compared to their surroundings, vegetation is much less dense or even non-existent. Even today  no  one  has  come  up  with  a  convincing explanation for their origin and so these so-called “fairy circles” are often associated   with   stories   and   legends.   Surprisingly, a lot of these circles are also the site  of  hydrogen  gas  emissions,  which  means  that this gas that we manufacture using various processes  as  part  of  the  energy  transition  is  actually naturally present below ground.

A “fairy circle” in Brazil from which natural hydrogen leaks.

For a long time, the existence of these sources of hydrogen was purely anecdotal, however it is gradually coming into the limelight with the deve-lopment of new projects that are trying to unders-tand how natural hydrogen is formed. And what if  this  natural  resource  was  much  more  wides-pread than we had imagined and above all exploi-table?  After  all,  this  is  only  the  start,  and  our  investigations are at the same point today as they were 160 years ago for oil and gas. Once they had got over the surprise of seeing hydrogen  leaking  out  of  the  ground,  geologists  began to take a closer look, in particular along the Mid-Ocean Ridge where the oceanic crust is for-med. The first assessment of the quantity emitted is stupefying: several tens of millions of tonnes of hydrogen per year! The observation is the same on land: measurements (often taken in the vici-nity  of  fairy  circles)  confirm  that  hydrogen  is  released in considerable quantities. To find out more and quantify these emis-sions, ENGIE has developed a permanent moni-toring  system,  PARHyS  (Permanent  Analyses  of Renewable Hydrogen with Sensors). Around 100 of these detectors were recently deployed for a several month period in the São Francisco basin  in  Brazil  (see  box  below).  They  revealed  flows in the range of 1,000 m3 per day, in other words around 10 tonnes per year.

THE KEY ROLE OF WATER

If certain hypotheses as to the exact mecha-nisms  at  play  are  still  the  subject  of  debate,  certain clues suggest that water plays a major role in the natural hydrogen cycle. This can be observed  at  underwater  faults  where  water  contributes to the oxidization of ferromagne-sian  minerals  (in  the  newly  created  and  still  extremely hot rocks) and the resulting produc-tion of hydrogen. This rapid chemical reaction takes place at a relatively shallow depth.

But what exactly is happening on land? The main indications of the presence of hydrogen are often  found  in  cratonic  sedimentary  basins  -  a  craton is an extremely old and stable part of the Earth’s continental crust that has remained more or less unchanged for at least 500 million years. In the geological history of some of these basins, a certain amount of activity has led to ruptures in the  underlying  continental  crust.    Along  these  deep  fractures,  ferromagnesian  mineral-contai-ning mafic and ultramafic rocks from the upper mantle  have  sometimes  been  injected  into  the  sedimentary layers. One possible origin of natural hydrogen in sedimentary basins would therefore be the result of the oxidization of these minerals by water in nearby aquifers. This is the most likely hypothesis to explain the presence of hydrogen in Bourakébougou (Mali). In  other  sites  in  these  same  mountains,  hydrogen would seem to be produced by the radio-lysis  of  water  seeping  in  through  faults  –  water  radiolysis  is  the  dissociation  of  water  molecules  under the effect of ionising radiation from radioac-tive minerals naturally present in the Earth’s crust. Elsewhere,  for  example  in  Oman  or  in  New  Caledonia,  hydrogen  is  released  in  zones  where  tectonic  uplift  resulting  from  plate  tectonics  has  brought  ferromagnesian  rock  (peridotite)  from  the oceanic crust closer to the surface. Faults allow water from aquifers to access these minerals with which  it  reacts  to  produce  hydrogen.  One  last  example of the association between hydrogen and water seen in Iceland is the presence of hydrogen in the steam component of geothermal fluids. All of  these  examples  seem  to  show  that  water  is  at  the heart of the hydrogen cycle.

WHAT IS THE POTENTIAL OF HYDROGEN?

If the tools we have at our disposition are able to measure the hydrogen that escapes to the sur-face, the actual subsurface quantity is much har-der to estimate, however as only a fraction of the hydrogen  produced  actually  reaches  the  surface,  it  is  cer-tainly much higher. The expla-nation   is   that   subsurface   dihydrogen molecules (H2) are a source of energy used in both chemical  reactions  and  by  microorganisms.  As  a  result,  most of this hydrogen probably never  reaches  the  Earth’s  sur-face. In order to try and unders-tand  how  it  can  be  preserved,  ENGIE  has  created  an  indus-trial  chair  in  partnership  with  Pau  University  and  Ifpen  to  focus on the behaviour of sub-surface hydrogen. Although   natural   gas   obviously  follows  a  very  dif-ferent  cycle  to  hydrogen,  we  can nevertheless draw an ana-logy. Every year, an estimated 52 megatonnes of methane naturally rises to the surface, in other words the same order of magnitude as natural hydrogen.  The  quantities  of  methane  below  ground  are  however  much  higher  (at  least  200  gigatonnes)  and  surface  emissions  are  no  more than the tip of the iceberg. As  H2  molecules  are  much  smaller  than  methane  (CH4)  molecules,  the  former  are  pro-bably more easily released to the surface; in addi-tion,  as  hydrogen  is  very  reactive,  its  subsurface  consumption is certainly higher. Even taking these factors  into  account,  it  is  still  possible  that  large  quantities  of  hydrogen  are  either  trapped  in  or  transiting through the ground. In fact, drilling ope-rations for water or hydrocarbons - for example in Kansas, Mali and Brazil - have revealed accumula-tions of hydrogen rich gases completely by chance. This hydrogen had probably been trapped in reser-voir rocks in the same way as natural gas.But how long can this hydrogen stay trapped? Was it formed like hydrocarbons on a geological time scale - in which case it would have been pre-served in these reservoirs for millions of years - or on  the  contrary  has  it  been  there  for  a  short  period, whilst being rapidly replenished? Oil and gas exploration have largely contri-buted to our understanding of the lithosphere. The tools that have been developed and the data collected can now help us to understand what a lot of people are calling the “hydrogen system”. Exploration and exploitation technologies from the gas sector will probably be able to be adapted to  this  new  resource.  Production  costs  will  depend on the depth of the well and the produc-tion rate, but are expected to be competitive, i.e. less than one euro for 1kg of H2. If that is indeed the case, the gas industry will have found an ave-nue for its large-scale reconversion, whilst faci-litating  the  green  transition.  Perhaps  it  has  indeed found its fairy godmother! 

REFERENCES

I. Moretti et al., Long term monitoring of natural hydrogen superficial emissions in a Brazilian cratonic environment. Sporadic large pulses versus daily periodic emissions, InternationalJournal of HydrogenEnergy, vol. 46(5),pp. 3615-3628, 2021.

S. Worman et al., Abiotic hydrogen (H2) sources and sinks near the Mid-Ocean Ridge (MOR) with implications for the subseafloor biosphere, PNAS, vol. 117(24), pp. 13283-13293, 2020.

V. Zgonnik, The occurrence and geoscience of natural hydrogen : A comprehensive review, Earth-Science Reviews, vol. 203, art. 103140, 2020.

This article was writen by :

Olivier Lhote - Special Adviser Hydrogen - ENGIE Research
Jan Mertens -  Scientific Director - ENGIE Research
Maria Rosanne - Manager of innovative R&D projects - ENGIE Research
Louis Gorintin - Nanotech Laboratory Director, Sensors and Connectivity - ENGIE Research

Tiphaine Fargetton - Senior Geological Engineer, Project Manager - STORENGY
Laurent Jeannin - Senior Ingenior - STORENGY