Natural hydrogen generation in granitic geothermal reservoirs

MURRAY, JESICA; WALLENTIN ANNA; FRITZ BERTRAND; LEMARCHAND DAMIEN; LUCAS YANN; SCHMITTBUHL JEAN

Lyon

Congreso; Goldschmidt 2023; 2023

European Association of Geochemistry and Geochemical Society

Non-ultramafic basements in deep continental hydro-thermal systems are potential zones for natural H2 generation. In particular, the generation of H2 from granitic rocks [1,2] could represent a process of global magnitude because they are major components of the continental crust. Our research is focused on the hydro-thermal context of the Upper Rhine Graben (URG). In this continental environment, geothermal brines of up to 200 °C circulate through granitic reservoirs at > 2000 m depth. In previous research, we performed geochemical modeling and experimental work to simulate the hydrothermal alteration of the granitic geothermal reservoir at the Soultz-sous-Forêts geothermal site [1]. The results provided indications for H2 generation by the alteration of ferrous iron present in some minerals of the granite such as biotite (1).Fe2+ + H2O → Fe3+ + OH- + 1/2H2 (1)To better understand the H2 generation process in the granitic geothermal reservoirs of the URG, a new set of water-rock-gas interaction experiments and geochemical modeling are being developed. For that, batch reactions using specimens of Fe2+-rich biotite, amphibole, and magnetite (present in the URG granites) are performed to simulate the hydrothermal alteration at different temperatures of the reservoirs (130 to 200 °C). In addition, some experiments simulate the presence of CO2 to address its effects on the Fe2+/Fe3+ redox equilibrium. The geochemical simulations performed in parallel, provide a better understanding of the mineral evolution during the reactions.This research will improve the understanding of abiotic H2 generation in deep granitic environments, which had received less attention in comparison to the serpentinization of ultramafic rocks. In the context of the energy transition, the generation of H2 in geothermal sites could be seen as an additional decarbonized source of energy together with the current production of electricity, heat, and critical elements.[1] Murray, et al., (2020), Applied Geochemistry 119, 104631.[2] Truche, et al., (2021), Geochimica et Cosmochimica Acta 308, 42–59.