The evolution of aquifer properties in ATES (aquifer thermal energy storage) systems is modeled based on data from the SPEOS (Lausanne, Switzerland) test site where 50-70°C - water has been injected and withdrawn over 3-6-month cycles for several years into a glacio-fluvial aquifer. The geochemical model is based on a reactive transport code (MPATH, Lichtner, 1992) that includes a kinetically formulated reaction module for mineral-fluid equilibria. The code has been modified to allow for transient, cyclic temperature profiles.
Numerical simulations were carried out for a number of aquifer and water compositions, e.g., accounting for mineralogical variability, and for addition of hydrochloric acid to avoid clogging of surface piping and heat exchangers. Results are best
represented in time slices of distance vs. reaction rate (or mineral volume fraction). The processes are dominated by carbonate reactions, modified to some extent by Fe-hydroxides and Al-silicates, resulting in a complex pattern of reaction fronts and
associated changes in porosity and pH. Results also indicate that dolomite precipitation may occur in aquifers with relatively little calcite and large clay fractions.
Lichtner, P. C., J. Geophys. Res. 28, 3135-3155 (1992).