Hydrogen Reactivity with (1) a Well Cement - PHREEQC Geochemical Thermodynamics Calculations

Summary TEREGA has been using an aquifer of southwestern France to store natural gas since 1957. The storage and the surface facilities are connected by cement-completed wells. The PHREEQC geochemical thermodynamics calculations presented herein aim at evaluating if hydrogen reactivity with a class G oil-well cement would have an impact on its porosity, in a context of hydrogen co-storage in the aquifer. The reductive dissolutions of the model cement minerals ettringite and hematite were driven by the sulphate and ferric iron reductions by hydrogen. The so-produced sulphides and ferrous iron precipitated as iron sulphide and oxide minerals. Nevertheless these dissolution-precipitation reactions did not affected significantly the cement porosity, as the involved minerals constitute a minor part of the material. The strong hypothesis of this study resides in the fact that the redox reactions reached the chemical thermodynamics equilibrium. But it is known that their extent is kineticallylimited; they need to be catalysed through e.g. the metabolism of hydrogenotrophic microbes. Our study could be thus improved by including such microbial kinetical equations, as well as kinetics for mineral dissolution/precipitation reactions. Also, we could consider reactive transport simulations in which the diffusive transport of hydrogen within cement is taken into account.