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| Fig. 1: Radiolysis of water for hydrogen production. (Image Source: H. Moise) |
Naturally occurring hydrogen, whose advocates are still deciding whether they prefer the gold or white label, has recently taken center stage in the world of hydrogen. Aside from a new color being added to the hydrogen rainbow, the emergence of this new form of hydrogen is unlike other forms as it is not a derivative of fossil fuels. Headlines of articles are already comparing it to the next oil rush and describe scientists and investors alike franticly trying to find the next Titusville, PA. It is far too early to gauge the feasibility of extracting and utilizing this energy source for many reasons, one of the main reasons being that we still do not have a good explanation of how its produced in the subsurface which makes it challenging to purposefully identify and exploit these source rocks. The most widely accepted theory is that it results from the reaction of water with ultrabasic rocks and serpentization. Other theories include but are not limited to degassing of deep-seated hydrogen from the earth's core and mantle, decomposition of hydroxyls and organic matter, biological activity, and radiolysis of water. [1] It may be that each of these hypotheses hold some truth in varying degrees, depending on the location of generation, or that each work in tandem with one another in the same system. Radiolysis is a particularly intriguing phenomenon to explain the production of natural hydrogen as this is process is not something we can likely replicate safely up here on the surface.
Radiolysis and its production rates have been extensively studied at our surface to ensure safe operation of nuclear processes such as nuclear reactors that implement water-cooling systems and nuclear waste storage whose cementitious materials can retain water. [2] The role of radiolysis in the production of subsurface hydrogen has garnered somewhat less investigation.
Water is the most common fluid inclusion trapped in subsurface minerals, making it the most abundant liquid in the upper crust. This same crust consists of radioactive elements including uranium, thorium, and potassium which decay over time and as they do, they emit radiation in the form of α, β, and γ particles which are capable of breaking the chemical bonds of water molecules in these inclusions. The final products of the radiolysis of water can include hydrogen, oxygen, peroxides, and hydroxyls (see Fig. 1). [3] Typically, modeling the radiation chemical yield for hydrogen production through radiolysis of water finds that 0.1-2 molecules of hydrogen can be produced per 100 eV. This value can depend on many factors such as the emission type (i.e. α versus β particles), emission source (i.e. thorium versus uranium), and the permeability and porosity of the geological formation. [4] A system that produces 0.5 molecules of hydrogen per 100 eV means that the energy required to produce one mole of Hydrogen gas is
| 200 eV molecule-1 × 1.602 × 10-19 J eV--1 × 6.022 × 1023 molecules mole-1 | |
| = | 1.93 × 107 J mole-1 |
Comparing this to the 492 kJ/mole covalent bond strength of O-H, it becomes apparent that most of the energy emitted from these radiation sources is lost as heat to the surrounding rock system.
Assuming that radiolysis is a considerable source of natural hydrogen, one obvious way to potentially identify hydrogen reservoirs produced through radiolysis would be to find geological formations that contain these irradiating materials. It is challenging to confirm the presence of these materials in the subsurface without direct sampling from drilling boreholes. Radiometric surveys can measure gamma radiation from the surface and while this tool can be used to identify potential sites for uranium mining today, it is limited to a depth of less than a few meters. While this tool cannot confirm the presence of irradiating materials at the depths required for natural hydrogen production, its purpose is to pinpoint locations suitable for more comprehensive methods like chemical sampling. Chemical sampling is required at multiple depths to properly de-risk a potential asset, but this form of drilling is minor compared to the size and costs associated with drilling for excavation. [5] Measuring hydrogen concentrations may be an obvious marker during chemical sampling, but hydrogen concentrations below ppm can be hard to measure properly in the field and may also result in false negatives as the correct source rock without the water fuel cannot produce hydrogen. Unless one finds high concentrations of hydrogen, it is more valuable to identify the source rock itself as well as the physical properties of the surrounding environment. The porosity and permeability of the rock formation ultimately set the rate of radiolysis. Porosity determines the volume of water that can be exposed to these irradiating materials and permeability determines a rock formations capacity to transmit a fluid through these pores. If we look at a fixed unit volume of this rock formation, we would see that a decrease in porosity would decrease the production of hydrogen as there is less fuel to consume. We would also see an increase in the amount of irradiating materials (density is inversely related to porosity), which would increase the total energy flux being emitted. This would suggest that there is an optimal porosity to achieve maximal hydrogen production, and this optimal porosity value can be used to identify potential sites for reservoirs.
While there are certainly high hopes for natural hydrogen, it may take some time for us to verify its potential and even further, how to extract and utilize it economically. The first step in exploiting this resource is to understand how it is made, what geological formations allow it to be sealed and trapped, and how can identify these reservoirs efficiently. Understanding these questions will allow us to develop methods to extract it and even stimulate its production to increase the viable footprint of potential sites, similar to how hydraulic fracturing unlocked unconventional natural gas.
© Henry Moise. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] V. Zgonnik, "The Occurrence and Geoscience of Natural Hydrogen: A Comprehensive Review," Earth-Sci. Rev. 203, 103140 (2020).
[2] S. Le Caër, "Water Radiolysis: Influence of Oxide Surfaces on H2 Production Under Ionizing Radiation," Water 3, 235 (2011).
[3] A. Bouquet et al., "Alternative Energy: Production of H2 by Radiolysis of Water in the Rocky Cores of Icy Bodies," Astrophys. J. Lett. 840, L8 (2017).
[4] L.-H. Lin et al., "Radiolytic H2 in Continental Crust: Nuclear Power For Deep Subsurface Microbial Communities," Geochem. Geophys. Geosyst. 6, Q07003 (2005).
[5] "Trends in U.S. Oil and Natural Gas Upstream Costs," U.S. Energy Information Administration, March 2016, p. IHS-7.