A novel method for decoupling thermo-hydraulic processes from chemical reactions to understand the effect of heat on chemical reaction

Abstract

Geochemistry of groundwater is affected by temperature among other things. We propose a novel method that can be used to develop analytical and semi-analytical solutions for calculating reaction rates for non-isothermal cases, to verify numerical models and give a better understanding of thermo-hydro-chemical (THC) processes. Aqueous and mineral reactions are assumed in equilibrium. The method decouples the chemistry from the thermo-hydraulic (TH) processes. The chemical part of the method consists of batch calculations in which minerals dissolve or precipitate and water chemistry varies as a result of changing temperature. The thermo-hydraulic part consists of calculating temperature and spatial and temporal derivatives of temperature. From this, chemical composition of groundwater and precipitation or dissolution rates of minerals can be calculated straightforwardly. We applied the method to a simple 1D steady state case, for which an analytical solution could be obtained, and to a 2D Aquifer Thermal Energy Storage (ATES) system of the Forsthaus pilot project near Bern (Switzerland), for which we developed a semi-analytical solution. The use of the method for the simulation of this ATES system reduced computational costs seven-fold in comparison with a standard numerical code. Moreover, the method has provided understanding on the dominant reactive transport processes (which we have divided into mixing, heat retardation and heat conduction terms), mineral reaction rates and porosity changes of the two cases. At interfaces with abrupt changes in temperature gradients, reaction rates tend to infinity. A comparison of thermodynamic databases reveals that not only the temperature dependencies of chemical properties are important, but also first and second derivatives with respect to temperature.