Natural Flow and Vertical Heterogeneities in a Sedimentary Geothermal Reservoir (Paris Basin, France): Geochemical Investigations

Abstract

Three geothermal wells tapping the Dogger aquifer were studied in detail for their variations in chemical composition with time or conditions of exploitation. Analytical improvements for the determination of C1, SO4, Ca, Mg, Na and K make i t possible to detect variation s respectively of 0.15, 0.8, 0.6, 1.8, 1.8 and 1.4 X. Despite the fact that the natural flow may be important in some parts of the basin aquifer, we conclude that this factor is not responsible for the small variations noticed in mineralization within the one year survey period. The results concerning reactive and nonreactive species are best explained if a vertical heterogeneity of the chemistry of the fluid is assumed. A number of calcareous sub-layers, already demonstrated by geological studies, contribute to varying degrees to the production of the hot water. The changes in pumping rates, which are fixed according to external requirements, play a major role in the hydrodynamic and chemical disequilibrium of the wells. The consequences for the geothermal exploitation s are emphasized.