Low-temperature hydrogen production and consumption in partially-hydrated peridotites in Oman: implications for stimulated geological hydrogen production

Abstract

The Samail Ophiolite in Oman, the largest exposed body of ultramafic rocks at the Earth’s surface, produces a continuous flux of hydrogen through low-temperature water/rock reactions. In turn, the scale of the subsurface microbial biosphere is sufficient to consume much of this hydrogen, except where H2 is delivered to surface seeps via faults. By integrating data from recent investigations into the alteration history of the peridotites, groundwater dynamics, and the serpentinite-hosted microbial communities, we identify feasible subsurface conditions for a pilot demonstration of stimulated geological hydrogen production. A simple technoeconomic analysis shows that the stimulation methods to be used must increase the rate of net hydrogen production at least 10,000-fold compared to the estimated natural rate to economically produce hydrogen from engineered water/rock reactions in the peridotite formations. It may be possible to meet this challenge within the upper 1–2 km, given the projected availability of reactive Fe(II)-bearing phases and the lower drilling costs associated with shallower operations. Achieving ≥10,000-fold increases in the H2 production rate will require a combination of stimuli. It will likely be necessary to increase the density of fracturing in the reaction volume by at least two orders of magnitude. Then, the H2-production rates must also be increased by another two orders of magnitude by increasing the water/rock ratio and modifying the chemistry of the injected fluids to optimize formation of Fe(III)-bearing secondary phases. These fluid modifications must be designed to simultaneously minimize microbial consumption of H2 within the stimulation volume. In contrast, preserving the high potentials for biological H2 consumption in the shallow groundwaters replete with oxidants such as nitrate, sulfate and dissolved inorganic carbon will reduce the potential for any inadvertent leaks of hydrogen to the atmosphere, where it acts as an indirect greenhouse gas.

A Review of the Migration of Hydrogen From the Planetary to Basin Scale

“…More recently, Templeton et al (2024) discusses factors that influence the timescales of fluid migration through the Semail Ophiolite, Oman, such as the hydraulic conductivity of partially-hydrated peridotites, the extent of fracturing, and the geochemical dynamics of the subsurface environment. Fluid flow in the most shallow and fissured rocks occurs in transmissive zones located within 50 m of the surface, indicating that fluid dynamics are highly heterogeneous.…”

“…Fluid flow in the most shallow and fissured rocks occurs in transmissive zones located within 50 m of the surface, indicating that fluid dynamics are highly heterogeneous. Templeton et al (2024) also describes that some zones are most sensitive to conductive channels, such as partially mineralized fractures, whereas other zones are supplied from rocks above and below. This complexity, including the presence of fractures partially filled with secondary minerals produced from both modern and ancient water/rock reactions, suggests that fluid migration rates and, by extension, hydrogen migration rates through the crust are highly variable and dependent on local geological conditions.…”

“…The assimilation of biogenic hydrogen into shallow groundwater is essential as this will be saturated with oxidizing agents such as nitrate, sulfate, and dissolved inorganic carbon. The recommendations of Templeton et al (2024) serves to mitigate the risk of unintentional hydrogen emissions into the atmosphere, where it contributes as an indirect greenhouse gas.…”

“…These results may be interpreted as meaning the degree to which subsurface microbial activity moderates hydrogen flow may vary on timescales relevant to groundwater flow through the host rock or sediment, for example, the acidity and salinity of pore fluids in top soils may vary on timescales of hours-days, whereas deeper rocks and aquifers may attenuate environmental signals over thousands of years or longer. Interestingly, Templeton et al (2024) demonstrate that the alteration of the chemical composition of fluids introduced into geological formations during stimulated hydrogen generation is pivotal for the optimal generation of Fe(III)-enriched secondary mineral phases. Templeton et al (2024) argue that modifications in fluid chemistry should be strategically engineered to concurrently diminish the microbial uptake of H 2 within the stimulated region, whilst maintaining elevated capacities for biogenic hydrogen assimilation in the shallow groundwater systems.…”

“…Interestingly, Templeton et al (2024) demonstrate that the alteration of the chemical composition of fluids introduced into geological formations during stimulated hydrogen generation is pivotal for the optimal generation of Fe(III)-enriched secondary mineral phases. Templeton et al (2024) argue that modifications in fluid chemistry should be strategically engineered to concurrently diminish the microbial uptake of H 2 within the stimulated region, whilst maintaining elevated capacities for biogenic hydrogen assimilation in the shallow groundwater systems. The assimilation of biogenic hydrogen into shallow groundwater is essential as this will be saturated with oxidizing agents such as nitrate, sulfate, and dissolved inorganic carbon.…”