Geochemical Parameters as Precursors to Predict the Decline of Temperature in the Dogger Aquifer (Paris Basin, France)

Abstract : Geothermal energy of the carbonate Dogger aquifer of the Paris Basin is exploited for heating since 1970s using geothermal well doublet technology where the warm produced waters (57-85°C) is extracted by a production well and re-injected after used as cooled waters (about 40°C) in the same aquifer via a different well. The injection of the cooled waters into the Dogger reservoir disturbs the initial thermodynamic equilibrium between the Dogger aquifer phases. This creates a cold and growing bubble around the injection well which risks propagating toward the production wells. This study, based on geochemical monitoring and field data analyses, aims to identify chemical precursor(s) of the thermal decrease in the production well field. Ca and HCO3 concentrations do not appear to be relevant precursors, because they are linked to fast mineral dissolution/precipitation reactions (carbonates). However Si concentration could be an appropriate precursor of the temperature drop, because it depends only on slow mineral reactions (silicates dissolution/precipitation). The results of this analysis are presented and discussed in this paper.