Possible Coupling of Abiotic Hydrogen and Heat Generation in the EGS Soultz-sous-Forêts Site (France): a Hydrogeochemical Modeling Approach
Jesica MURRAY, Alain CLEMENT, Bertrand FRITZ, Jean SCHMITTBUHL, Vincent BORDMANN, Jean Marc FLEURY
For solving the problem of energy transition for the future, the research for alternatives to fossil fuels sources of energy such as H2 is necessary. In this general context, investigations on geochemical processes responsible for native H2 generation in natural reservoirs are of major interest. In most of the recent studies related to natural abiotic H2 generation the focus is on serpentinization of mafic and ultra-mafic rocks. In these natural systems the source of H2 is linked to the oxidation of Fe(II) bearing minerals and the so called reduction of H2O. In this study, we investigate a parallel route of abiotic H2 generation from a rich biotite granite, an acid rock that constitutes the basement of the Soultz-sous-Forêts EGS site. We used a geochemical and reactive transport modeling, and the existing data base of the Soultz-sous-Forêts project, to simulate the hydrothermal alteration of the granite for the in situ range of temperatures (130 to 200 ˚C), and a Redox potential of around -250 mV. The simulation showed that the generation of abiotic H2 is possible by hydrothermal alteration of biotite as a source of Fe(II) oxidized into Fe(III) and precipitated as ferric iron minerals, while H+ is reduced into H2. The amounts of H2 which can be produced depend on the type of Fe(II/III) secondary minerals precipitating as a function of temperature and redox conditions of the reservoir. As observed in the serpentinization process, optimal conditions for H2 generation are related to magnetite precipitation. On the contrary, when Fe(II) secondary minerals (e.g. chamosite) precipitate, the iron oxidation process and H2 generation is inhibited. This research has implications for possible coupling between heat extraction and H2 production.
|        Topic: Geochemistry||Paper Number: 14019|