MODELING ABIOTIC HYDROGEN GENERATION FROM THE GRANITE BASEMENT IN THE SOULTZ-SOUS-FORÊTS GEOTHERMAL SYSTEM, RHINE GRABEN, FRANCE

MURRAY JESICA; SCHMITTBUHL JEAN; CLÉMENT ALAIN; BORDMANN VINCENT; FRITZ BERTRAND; JEAN-MARC FLEURY

La Haya

Congreso; European Geothermal Congress 2019; 2019

Investigations on geochemical processes for H2 generation in natural reservoirs are important for the present energy transition where alternatives to fossil fuel sources of energy such as H2 are necessary. Most of the studies related to natural abiotic H2 generation are focused on the serpentinization of mafic and ultra-mafic rocks, where 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 route for abiotic H2 generation from a rich biotite acid rock like granite that constitutes the basement of the Soultz-sous-Forêts Enhanced Geothermal System. Using geochemical and reactive transport modeling and the existing data base of the Soultz-sous-Forêts project, we simulated the hydrothermal alteration of the granite for the in situ temperature of 165 ˚C and the redox potential, of -245 mV. The simulations showed that generation of abiotic H2 is possible by hydrothermal alteration of biotite as a source of Fe(II), which oxidizes to Fe(III) leading to the precipitation of ferric iron minerals and reduction of protons (H+) under such low redox conditions. Most likely, optimal conditions for H2 generation are related to magnetite precipitation, as observed in the serpentinization process. This study has implications for possible coupling of heat extraction with H2 production.