Perspective directions of development in the field of geological hydrogen production.
The extraction of geological hydrogen is a sure way to cheaper hydrogen energy which will allow us to achieve the goal of sustainable development and truly a carbon-free future.
Autor: Kseniia Feodoridi
Place: HYDITEX CORPORATION, London
Date: September 2024
Abstract: Perspective directions of development in the field of geological hydrogen production.
Key Words: White hydrogen, geological hydrogen, natural hydrogen, underground hydrogen.
DOI: 10.13140/RG.2.2.18021.00482
Geological hydrogen. Different sources and mechanisms for the production of natural hydrogen.
Introduction
Hydrogen is of great interest as an energy source, as it is seen as the clean fuel of the future. Currently, most hydrogen is produced by steam conversion of methane, which converts methane into hydrogen. However, this method generates 10 tonnes of carbon dioxide (CO2) for every tonne of hydrogen produced (2).
Hydrogen can also be produced by pyrolysis, which is the splitting of methane to black carbon and hydrogen. This technology requires heat or electricity, often through electrolysis, which utilises electricity to split water into hydrogen and oxygen (3). In the case of electrolysis, a large amount of fresh water is required; however, this resource has already been vastly depleted worldwide (2).
An alternative to hydrogen production is the extraction of natural hydrogen which is also called ‘white hydrogen’ or ‘geological hydrogen’. This hydrogen is produced by natural processes in the Earth's interior and seeps through various geological layers to the surface.
In this paper, we will summarise the existing knowledge about geological hydrogen, how it migrates and why it accumulates near the Earth's surface. We give an overview of the future global demand for geological hydrogen and the prospects for its extraction. Also, we will consider methods that can help to solve problems related to the search, exploration, storage and transport of hydrogen as an energy resource.
Migration of geological hydrogen
Hydrogen is the single most plentiful element in our solar system, it is found almost everywhere. To date, at least 14 sources and mechanisms of natural hydrogen are known (4).
Hydrogen migrates from the core of the Earth to the surface and atmosphere by diffusion and advection (1, 5, 6). These processes depend on a large number of factors related to the origin of hydrogen, the mechanisms of its migration and its interaction with the materials through which it travels (1).
Diffusion through microcrystalline rocks such as volcanic glasses can be instantaneous and this is important for degassing mantle hydrogen in hydrothermal and marine environments. In contrast, in the case of aggregate crystalline rocks, such as granites, diffusion occurs over long periods of time. This allows hydrogen concentrations in fluid inclusions to remain high for billions of years, thus creating an important resource within continents (1, 7, 8).
Hydrogen advection along faults and fractures is also an important method for both long-lived and short-lived hydrogen migration to the Earth's surface (1, 9). Continuous longterm hydrogen flux to the Earth's surface is usually associated with deep faults in the Earth's crust that traverse rocks rich in hydrogen, e.g., serpentinised crystalline basements. Alternatively, periodic hydrogen pulses are linked to seismically induced groundwater movement along faults and neotectonics and also to the migration of CO2 (10). Although both large and small faults can influence fluid migration pathways in sedimentary basins, long-range hydrogen migration is determined by advection along large faults (1).
Despite the migration of natural hydrogen from the Earth's interior, the potential of its extraction has not been evaluated and has not attracted much attention of researchers until now. This is due to the fact that hydrogen diffuses very quickly and combines easily with other elements and molecules, making it challenging to keep it in a gaseous state below the surface (4). However, thanks to qualitative scientific research conducted in Australia, it became possible to determine which rocks are hydrogen-bearing, i.e. do not allow hydrogen to pass through. There are only three such rocks: volcanic rocks, salt domes and fine-grained clay shales. This suggests that there are seals under our feet that favour hydrogen accumulation (3).
World demand for geological hydrogen and prospects for its production
To date, society has limited knowledge of natural hydrogen as very few wells have been identified for exploration; therefore, it is difficult to estimate the total number of global reserves and thus the scale of potential commercialisation. However, there is data that gives us an idea of what we should expect (3, 11). Let us look at some examples:
A well was discovered accidentally in Mali while drilling for water, it was a shallow, unproductive well, and was very cheap to drill. It is estimated that hydrogen could be produced from this well at $1 per kg (the price at which hydrogen competes with fossil fuels). However, other companies involved in the production of natural hydrogen believe that they can produce it at a price close to $0.5 per kg (11).
The Monzon field, discovered in the 1960s in the foothills of the Spanish Pyrenees, recorded high gas emissions of pure hydrogen at a depth of 3,600 metres in a reservoir of high quality sandstone overlaid with a thick layer of salt. This was noted as a geological curiosity at the time and the well, named Monzon-1, was abandoned. In 2022, an extensive geochemical surface survey was carried out. Following the survey, it was decided that another well would be drilled in the second half of 2024, which would be the first well drilled in Europe to produce natural hydrogen (12). This well was named Monzon-2.
The likely recoverable volume of hydrogen in the Monzon field is 1.1 million tonnes. There are several similar structures in the Monzon field that are also expected to contain natural hydrogen, the total recoverable volume is tentatively estimated at 5-10 million tonnes and possibly over 100 million tonnes in the province of Aragon (12).
From the beginning of 2029, hydrogen reserves at the Monzon-2 well are expected to be produced, over a 20 to 30 year period, at a rate sufficient to meet local industrial demand, which is between 55,000 and 70,000 tonnes per year per plateau. The cost of hydrogen under the project is less than 1 euro per kg (12).
It is expected that by 2050, the global demand for hydrogen as a raw material, which can be utilised as fuel for transport, heating buildings and generating electricity, will increase 8-fold to more than 550 million tonnes (3).
Promising methods of searching for geological hydrogen
The search for natural hydrogen flow generation and release sites is a promising direction in geological exploration. Currently, various laboratory and experimental methods are used and being developed. One of the promising methods for hydrogen detection is the biogeochemical method which is based not on the search for hydrogen itself, but on the search for the production of biomass microorganisms which oxidise hydrogen.
Hydrogen-oxidising microorganisms, or hydrogen bacteria, are a group of microorganisms, which include bacteria and archaea, that use the oxidation of molecular hydrogen to produce energy. Hydrogen is an attractive substrate for their growth because hydrogen oxidation belongs to chemosynthesis reactions and is an important inorganic energy source capable of releasing substantial amounts of energy (237 kJ/mol-H2) (13, 14, 15). Along with molecular hydrogen from the Earth's interior, it is also one of the most important gaseous micro-components of soil, where it is formed as a result of anaerobic decomposition of various organic substances by microorganisms.
In normal microbial communities, hydrogen concentrations are very low due to interspecies transfer where one species gains energy in a reaction leading to hydrogen production and the other species oxidises it using an inaccessible enzyme called hydrogenase. This is a well-organised trophic system in which anaerobic organisms produce gas from decomposing organic matter, and the aerobic organisms oxidising hydrogen prevent it from escaping from the soil into the atmosphere. This situation allows us to consider the soil as a kind of bacterial filter.
The rate at which microbial communities slow the flow of hydrogen through geological media depends largely on environmental factors such as the quantity of air, temperature, salinity and acidity of pore and groundwater, and the availability of iron and nutrients. For shallow rocks (below 100 m), which are subject to diurnal and seasonal environmental changes, hydrogen flux is constrained on time scales ranging from hours to months (16). In contrast, for deep rocks (greater than 1 km, such as deep saline aquifers) with sufficient nutrients, microbial responses are sustained over longer time scales and are affected by long-term environmental changes of thousands of years or longer (1).
A large diverse group of hydrogen bacteria is present in soils. Experiments show that hydrogen flow stimulates the microbial community which causes the growth rate of microorganisms to increase (17, 18). Thus, the amount of microbial biomass production is much higher due to the fact that the hydrogen flowing through the medium is much higher than its production by other microflora. This indicates that the biomass production of hydrogen-producing bacteria is a good reference point for finding and determining the size of the hydrogen resource underlying them. This discovery was very promising and led to the biogeochemical method of searching for places of generation and separation of hydrogen from deep natural accumulations, allowing for subsequent extraction. The principle of the biogeochemical method for searching hydrogen generation and release sites is shown in Figure 1.
Figure 1 - Biogeochemical method for searching for places of hydrogen generation and release (18)
The most suitable geological environments where natural hydrogen is most likely to be present are magmatic basic and ultrabasic rocks, coal seams, and deep basement faults. These environments are used for prospecting purposes where soil, silt and rocks, etc, are sampled at the above locations and the biomass production present in them is estimated. The methods of biomass estimation depend on the environment where the microorganisms live. The application of these biogeochemical methods allows us to promptly investigate large areas of prospecting and to identify promising areas where there are natural hydrogen streams. This may also lead to the discovery of hidden objects.
Extraction of natural hydrogen poses the primary task of searching for promising evidence of its deposits in the bowels of the earth. Obviously, the existing exploration methods based on the search for hydrocarbon, groundwater and precious metal deposits cannot be applied to hydrogen because this gas, being the smallest element, rapidly diffuses and combines easily with other elements and molecules, making it challenging to keep it in a gaseous state below the surface (19, 20, 21, 22, 23). In other words, geological environments suitable for hydrocarbon exploration may not be promising sources of natural hydrogen and could in fact be general hydrogen sinks. In which, any free hydrogen is likely to be chemically bound and stored permanently in hydrocarbons or other hydrogen-containing compounds (23).
A critical component for finding promising sources of natural hydrogen in the earth's subsurface and assessing its underground storage resources is modelling the passage of hydrogen from the subsoil to subsurface accumulations (1). Despite the fact that point locations where hydrogen emissions from surface seeps are registered more and more frequently, there is still no standardised method for predicting the paths and time frames of the subsurface hydrogen flow. In geology, modelling of hydrocarbon accumulation reservoirs and groundwater reservoirs are mostly well developed, the former belonging to the field of petroleum geology and the latter to hydrology; however, they are not applicable to hydrogen migration modelling. The main reason for this is that hydrogen can be produced by both fossil, such as mantle, and generative processes, e.g. biogenic activity and serpentinisation. Consequently, new methods and modelling approaches are needed to predict flow pathways and hydrogen migration rates in the subsurface (1).
Discussion
Understanding the migration of geological hydrogen through the Earth's interior and its accumulation beneath its surface is critical for hydrogen prospecting, exploration, production and storage. Understanding the dynamics of the subsurface hydrogen cycle requires a holistic view of hydrogen inputs, releases and intermediate processes, because hydrogen migration pathways are affected by a variety of processes, such as microbial reactions that slow subsurface hydrogen flux and environmental conditions. Modelling of subsurface hydrogen migration should address gaps in existing knowledge and focus on further investigate to address the following questions:
• What long term impact does hydrogen have on microbial community composition?
• What impact does an increase in microbial community abundance and activity have on the concentration and cycling of hydrogen in the subsurface?
• What is the rate of microbial community growth in a natural environment?
• What impact does hydrogen and hydrogen-oxidising microorganisms have on engineered industrial subsurface facilities?
Conclusion
The extraction of geological hydrogen is a sure way to cheaper hydrogen energy which will allow us to achieve the goal of sustainable development and truly a carbon-free future. Further research into the sources and mechanisms of natural hydrogen production is a very important step needed in solving problems related to the exploration, storage and transport of natural hydrogen as a promising energy resource.
As of now, the most promising directions of development in the field of geological hydrogen production have been identified, namely, the development of methods for modelling the prediction of hydrogen flow paths and migration rates in the subsurface and also a biogeochemical method for searching for places of hydrogen generation and separation from deep natural accumulations.
The application of these methods will make it possible to quickly survey the most extensive areas of prospecting and help us to identify promising areas with probable output of geological hydrogen flows, including objects not known to date.
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