Evaluating the feasibility of stimulating natural hydrogen production from the Lizard Ophiolite Complex, UK

Natural hydrogen produced by fluid-rock interactions such as serpentinisation has recently been gaining traction as a potential source of carbon-free, green energy, that could go a long way towards mitigating the ongoing climate crisis¹. This has led to accelerated efforts globally to identify geological sites at which hydrogen production can be stimulated, and the pressures, temperatures and fluid compositions at which hydrogen production can be optimised. Hydrogen production through serpentinisation involves a coupled redox transformation of Fe²⁺ to Fe³⁺ and H₂O to H₂, owing to which ultramafic lithologies are promising targets for stimulated hydrogen production, owing to their substantial Fe content².

In this contribution we present the results of modelled fluid-rock interactions between a serpentinised peridotite from the Lizard Ophiolite Complex, United Kingdom and an engineered brine of a composition similar to ones used for CO₂ sequestration experiments. Models were constructed by utilising the PHREEQC suite of codes³ using the carbfix.dat database⁴, at pressures of 50, 100 and 200 bars, temperatures of 100⁰, 200⁰ and 300⁰C and mass of H₂O in the solution varying from 0.05-200kg. 1 kg of an almost completely serpentinised peridotite, consisting of chlorite, serpentine and magnetite was chosen as the starting material and fluid injection models were simulated by reacting increasingly dilute solutions with the host rock in successive steps. The models predict hydrogen production to peak at 200 bar and 300⁰C, at which 5.73 mole/kgw hydrogen is produced at low water/rock ratios. The amount of hydrogen produced appears to have a positive correlation with temperature and increases rapidly with increasing temperature. On the other hand, hydrogen production is inversely correlatable with the mass of H₂O in the solution and decreases with increasing amounts of H₂O as the simulations proceed. The effect of temperature appears to be much more pronounced on the amount of hydrogen produced, compared to the effect of fluid pressure. Only minor increases are observed in the amount of hydrogen produced with increasing fluid pressure (5.68 mole/kgw at 50 bar and 300⁰C increasing to 5.73 mole/kgw at 200 bar and 300⁰C). Our results, although preliminary, highlight the potential of ultramafic lithologies such as the Lizard Ophiolite Complex to play an important role in natural hydrogen stimulation endeavours.

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