Nearly 40 years ago during a water drilling operation in Mali, West Africa, workers ran into a stream of gas that ignited before them. It was discovered to be pure hydrogen coming from stacked reservoirs in the subsurface. Later drilled by an oil and gas company, the well has been producing hydrogen for more than 12 years and is used to power a nearby village with electricity.
Geoffrey Ellis, a research geologist and petroleum geochemist with the U.S. Geological Survey, learned about this well five years ago.
“This piqued my curiosity because I was well aware of the paradigm that accumulations of hydrogen cannot exist due to its reactivity and diffusivity,” he said. “Research into this topic led me to conclude that this widely held notion is not well-founded.”
As the world strives for net-zero emissions, there is a growing interest in exploring for natural hydrogen – sometimes called “white hydrogen” for its clean-burning nature – and it has spread to every continent on the planet.
Spearheaded by Ellis, the USGS has begun developing hydrogen system models and determining global resource potential for this natural resource. The effort is backed by the strong belief that geologic hydrogen has the potential to become a clean fuel source for energy-intense sectors, such as aviation, steel manufacturing, industrial heating and mining – none of which have economic clean energy solutions to date.
Hydrogen is currently used for petroleum upgrading, fertilizer and petrochemicals and it is manufactured as a byproduct of the oil and gas industry from methane steam reforming and coal gasification. Yet, for every kilogram of this “grey hydrogen” produced, 10 kilograms of CO₂ are produced and released into the atmosphere.
There are efforts to produce “blue hydrogen” by combining the production of grey hydrogen with carbon capture utilization and storage, as well as efforts to manufacture “green hydrogen” by producing the gas as a byproduct of renewable energy. However, both are likely to be costly and mineral intensive, given the limitations of current technologies, Ellis said.
“Hydrogen is viewed exclusively as a medium for energy storage and transport, and not a primary energy resource,” he said. “But what is the resource potential of natural hydrogen?”
Modeling and Mapping
Energy policy analysts predict an expanded role for hydrogen in the future, accounting for as much as 30 percent of the energy supply in some sectors, with demand increasing more than five-fold by 2050, Ellis said. Annual global demand is projected to be approximately 430 megatons (or million metric tons) by 2050, according to the International Energy Agency. If clean energy is the ultimate goal, natural hydrogen resources must be seriously considered.
“Although the presence of natural hydrogen in the subsurface of the Earth is well documented in a variety of geologic environments, economic accumulations of natural hydrogen have generally been assumed to be non-existent,” Ellis said. But the discovery in Mali and other places, for example, have prompted some geoscientists to acknowledge that they have not looked for native hydrogen in the right places with the right tools.
Understanding hydrogen systems requires multidisciplinary teams, and the knowledge needed will tap practically every subdiscipline within the Earth sciences, including sedimentology, igneous petrology, aqueous geochemistry, hydrology, seismic interpretation, potential fields geophysics, geomicrobiology and many others, Ellis said.
For Ellis and his team at the USGS, the petroleum system served as a sound analog for building a hydrogen system model. Much like oil and gas, natural hydrogen as a resource depends on sources, migration pathways, reservoirs, traps and seals, preservation and timing.
Yet, there are differences to consider as well. In terms of source rocks, hydrogen generally requires water-rock interaction, so hydrothermal mineralization expertise is needed. And, to predict hydrogen generation potential, knowledge in geothermal resource exploration is required to understand the hydrology of hydrothermal systems.
While the fields of petroleum geology, minerals and geothermal energy cannot be directly applied to geologic hydrogen exploration, it is necessary to adopt and adapt certain tools and approaches from these disciplines to develop an optimized strategy, Ellis said.
A lot is already known about the occurrence of subsurface hydrogen, such as its generation mechanisms and consumptive processes. (Hydrogen is consumed in the subsurface through both biotic and abiotic processes.)
By looking at similar molecules, such as helium and CO₂, geoscientists can estimate the amount of hydrogen that might be trapped in a reservoir with the right seals. By studying rates of consumption during migration and in a reservoir, they can reasonably estimate the volume of hydrogen remaining.
The USGS has determined that the most probable amount of global, in-place hydrogen resources is approximately 5 million megatons. However, because the model represents the entire Earth’s crust, the majority of the gas is likely inaccessible because accumulations would be too deep, too far offshore or in small supplies, Ellis said.
However, he remains far from discouraged.
“If there are 5 million megatons in the Earth’s subsurface and we could find and recover 2 percent of that, that would represent 100,000 megatons, which would supply that projected hydrogen demand of 430 megatons per year for more than 200 years,” he said.
The global proven natural gas reserves are thought to be on the order of 7,257 trillion cubic feet, which is half as much energy as might be stored in a recoverable amount of hydrogen. “This really could be a tremendous energy resource,” Ellis said.
Possible Plays
As optimistic as it might sound, a major caveat lies in the lack of understanding of the processes and settings that are most conducive to the formation of significant accumulations of hydrogen – accumulations that could be economically found and produced.
Ellis believes that the petroleum system could further this understanding as well. While hydrocarbons are generated from organic-rich rocks heated over millions of years, hydrogen can be generated from many natural processes, although only a few appear conducive to generating sizeable accumulations. These include the reduction of water by iron-rich rocks; radiolysis by the radioactive decay of radiogenic minerals; and deep-sourced hydrogen that can be generated within the Earth’s crust or mantle, or primordial hydrogen migrating to the surface from great depths.
“It is important to recognize that, just like conventional petroleum systems, the rocks that are the source of hydrogen may not be suitable as reservoirs or seals,” Ellis noted. “Hydrogen is very mobile and is expected to migrate away from the source and potentially charge porous reservoirs elsewhere.”
Taking this into consideration, Ellis surmised that there are likely three types of natural hydrogen plays:
A natural accumulation play is most like the petroleum system in that an external source, such as an iron-rich rock, generates hydrogen that migrates into a reservoir with an effective seal and is preserved over time.
A natural generation play would involve drilling into the rock that is generating the hydrogen and producing the gas in real time. This could include drilling directly into migration pathways as well. Ellis believes this might be possible because hydrogen is generated more quickly than petroleum.
A stimulated generation play could involve a variety of different techniques that enhance the generation of hydrogen and could be as simple as injecting water into subsurface iron-rich rocks.
Ellis noted that the USGS’s global model estimating 5 million megatons of in-place hydrogen only accounts for the accumulation scenario.
“If the other two scenarios were feasible, it could significantly add to the resource potential,” he said.
Furthermore, the second two scenarios might also constitute a renewable resource.
Of course, the seals for a reservoir are a critical element for forming and preserving hydrogen accumulations.
“The kinetic diameter of the diatomic hydrogen molecule is similar to the monoatomic helium atom,” Ellis said. “Therefore, we can look to the trapping of helium accumulations as an analog for the types of lithologies that can form effective reservoirs, traps and seals for hydrogen.”
The hydrogen accumulation in Mali is sealed by dolerite. Yet, the seal capacity of other lithologies is not yet well understood.
In terms of migration and preservation, hydrogen is more soluble in water than methane at greater depths. The abiotic consumption of hydrogen is therefore more likely to occur at higher temperatures, exceeding 200 degrees Celsius, whereas microbial consumption will be more likely to occur at temperatures of 120 degrees Celsius or lower. So, somewhere between 100 and 200 degrees Celsius there exists a window for hydrogen accumulations. Again, more research is needed.
Foundation for Hydrogen
The USGS published its global hydrogen system model in the peer-reviewed journal Science Advances in December. It has also developed a methodology for mapping prospectivity of geologic hydrogen resources and applied it to the contiguous United States. This includes identifying known locations for iron-rich rocks, such as the Atlantic Coast and Midcontinent areas. This work is currently under review and is expected to be released in the near future.
The USGS is now working on mapping geologic hydrogen across the state of Alaska, which has “interesting geology and a high degree of prospectivity,” Ellis said. That work is expected to be completed in 2025.
“The potential for natural hydrogen to be a primary energy resource is a concept that has only very recently gained widespread acceptance,” Ellis said. “Now that we recognize that large quantities of hydrogen could be in the subsurface, the key is to understand the processes that could lead to accumulations and develop tools that can effectively guide exploration. Our hydrogen system model and the associated prospectivity methodology will provide a framework and workflow for identifying locations of interest for detailed studies to determine geologic hydrogen resource potential.”
More research across the board is needed to further understand the potential for geologic hydrogen as a resource. The hydrogen system model will need ongoing refinement in terms of geology and geochemistry. The development of surface exploration techniques is needed to identify hydrogen flux, and this will require expertise in geochemistry and remote sensing. The development of subsurface exploration tools to detect hydrogen system elements is essential and will require expertise in geophysics and numerical modeling.
“If commercial scale production of geologic hydrogen can be realized, this could provide a new low-cost, low-carbon, primary energy resource,” Ellis said. “This would substantially help society meet the growing demand for energy while mitigating the impact on the climate.”