Natural Hydrogen: A New Source of Carbon-Free and Renewable Energy That Can Compete With Hydrocarbons

Emanations of natural hydrogen are observed on the surface of the Earth at multiple points, on the five continents and on the mid-ocean ridges. When the geological conditions are favourable, this gas can accumulate at shallow depths and thus be of economic interest in contributing to the decarbonation of the energy mix. The first deposit of natural hydrogen was accidentally discovered in 1987 in Mali and is currently in the industrial development phase. In the last two years, a dazzling multiplication of exploration projects dedicated to natural hydrogen have been launched and the first successes have been announced. At the same time, few countries have adapted their mining codes to facilitate permit submission. The year has also been marked by the second H-Nat international congress, which brought together scientists and industrialists on the issue of exploring and producing natural hydrogen in the short term. At the same time the European EARTH2 club was founded on the initiative of 45−8 Energy, CVA and the Avenia cluster, to overcome competitive tensions and jointly promote underground solutions to the ‘hydrogen revolution’. This article provides an inventory of knowledge of the hydrogen produced naturally by the Earth and presents exploratory guidelines. The hydrogen currently on the market is of manufactured origin, but natural hydrogen can also be exploited from the subsoil 70 Mt of hydrogen are consumed each year worldwide, mainly for industrial purposes. This hydrogen, called ‘grey hydrogen’, is manufactured by steam reforming of hydrocarbons (78%) and coal (18%). ‘Green hydrogen’, produced by electrolysis of water, represents only 4% of this mix. However, hydrogen also exists in the subsoil, in its natural state (Prinzhofer and Deville, 2015), it is called ‘white hydrogen’ or ‘native hydrogen’. Steam reforming is a developed technology but emits a lot of CO2 (more than 10 kg of CO2 per kg of H2). Including CO2 capture and storage, the production cost is around $ 2–4/kg. Water electrolysis uses available but energy-intensive production processes. The cost of production from renewable electricity remains high, between 5 and 8 $/kg. Natural hydrogen is a resource in constant renewal. Its exploitation requires little energy, no fresh water and does not emit CO2. Production costs are estimated at less than 1 $/kg and decrease in a coproduction business model (geothermal energy, helium, high-value brines). Natural hydrogen is therefore cheaper than manufactured hydrogen and does not emit CO2. It would therefore be an ideal complement to hydrogen produced by electrolysis in a carbon-free energy mix. Its lower cost than other renewable and carbon-free energy sources places it, in terms of competitiveness, in a favourable position to challenge fossil hydrocarbons. The small investments needed today to develop it, its possibly local and decentralized use, make it a paradigm changer for our energy future. Even if the presence of natural hydrogen was highlighted in water or hydrocarbon drilling more than a century ago in France and Australia (Ward et al., 1933), the first drilling devoted to this exploration is much more recent. The first large-scale exploratory projects were carried out by the Hydroma company in Mali from 2008 in the Bourakebougou region, 20 years after the accidental discovery of the deposit (Prinzhofer et al., 2018). Today, small companies are focused on hydrogen exploration such as in the USA where NH2E carried out deep drilling in Nebraska in 2019 or Desert Mountain Energy which announced in February 2022 the discovery of a natural hydrogen field in Arizona. In Australia, Santos, after several exploration wells, announced in 2021 the completion of a first natural hydrogen producing well in the Amadeus basin. In Europe companies are also developing this type of activity, such as Hynat in Switzerland, 45−8 Energy and Engie in France or Helios in Spain. To reduce costs, natural hydrogen can also be considered as a co-product of geothermal energy. But hydrogen can also be associated with other gases of economic interest such as methane, CO2 and more particularly helium. A coupled H2-He production for example, would make it possible to optimize this type of operation. This covalorization approach, which has been developed by 45-8 Energy since its creation, has now been widely disseminated in the scientific and industrial community. In most countries, the mining code is not yet adapted to the regulation of hydrogen exploration and production, but updates are in progress The mining codes were drafted and adopted when natural hydrogen was still unknown as a natural resource. It is therefore necessary to adapt it so that natural hydrogen can be classified in one of the categories explicitly mentioned by the mining code. Several countries modified (or are modifying) their mining code to provide industrial initiatives with the necessary regulatory framework, such as Australia, Mali, Morocco, Congo, Ukraine, France and Germany. Natural hydrogen finds its source in the subsoil, at depth, before migrating to the surface and finally dispersing in the atmosphere. However, this source-migration-accumulation-leakage system has elements that distinguish it from the oil system. Natural hydrogen can be produced in the Earth’s crust from different processes. Some even propose a deeper origin in the mantle or the core of the earth which would have preserved primordial hydrogen (Larin et al., 1993). The natural hydrogen produced in the Earth’s crust can be generated by the radiolysis of water due to natural radioactivity, or by the oxidation of ‘ferrous’ iron to ‘ferric’ iron reducing water into hydrogen. In the natural context, this last reaction, such as the serpentinization of mafic and ultramafic rocks, is particularly effective around 300°C in the presence of water, but it can also take place more slowly at lower temperatures, then at a shallower depth, as has been shown in the laboratory. Natural hydrogen can also result from other processes such as pyritization (Arrouvel and Prinzhofer, 2021) and ammonium decomposition (Jacquemet, 2022), mechanical friction of silicates at faults, dark fermentation of matter organic matter, the bio-photolysis of water or the cracking of organic matter. If we define the hydrogen system as the dynamic association source-migration-accumulation-loss, the comparison between the petroleum system is tempting. However, the differences are numerous. First of all, the depths that are at stake. The genesis of hydrogen may be deeper than that of hydrocarbons and the accumulations of hydrogen may, on the contrary, be shallower, as is the case in Mali. The main source of hydrocarbons is organic matter, while hydrogen is formed by mineral chemistry reactions, in rocks which may be sedimentary or plutonic. While for hydrocarbons it is necessary to have traps to capture the fluids, the accumulations of hydrogens can be perceived as more dynamic. Any change in rock properties that would help in slowing the gas on its migration path can promote transient accumulation on human timescales. Consequently, while the resource of a hydrocarbon deposit is measured in volume, the resource of a hydrogen deposit must integrate the notion of feeding flow. From a temporal point of view, the petroleum system is a system that operates on the scale of geological time. Hydrocarbons are therefore considered non-renewable on a human scale. In comparison, the natural hydrogen accumulations are continuously fed by large flows and the hydrogen that reaches the surface oxidizes in the form of water, which makes this new renewable carbon-free energy resource part of the water cycle. Hydrogen fluxes are much larger, both in terms of their genesis and in terms of surface exudations. The inventory of natural hydrogen emissions at the surface shows that the resource is widely distributed on all continents, in various geological contexts. Natural hydrogen is present in the atmosphere but in very low concentrations, around 0.5 ppm. However, it is found in higher concentrations at point sources such as submarine or continental fumaroles, hot springs, ‘fairy circles’ or along fractures and faults. Many boreholes have also found hydrogen at varying depths, from a few metres to more than 1000 m (Guélard, 2016, Prinzhofer et al., 2019, Boreham et al., 2021 and Pélissier et al., 2021). Surface emissions have been mapped globally and show a wide distribution (Prinzhofer and Deville, 2015, Zgonnik, 2020, see Figure 1). They appear along oceanic ridges, on obducted oceanic plates (ophiolites from Oman, New Caledonia, the Philippines, Turkey, etc.) or in mountain ranges (Pyrenees). They are also observed on the edges of graben (Rhine Graben and Rhine Ditch) and in Proterozoic cratons (Russia, USA, Brazil, Australia, Africa, etc.).