Development of a Thermo-Hydro-Chemical (THC) Model for Studying the Hydrogeochemical Behavior of Standing Column Wells

English Abstract:

Standing column wells are ground heat exchangers that provide cooling or heating energy to buildings. In certain cases, they might allow to realize energy savings compared to other low temperature systems. Nonetheless, as they use groundwater directly as heat carrier fluid, they favor scaling and precipitation of minerals in the heat exchanger and well, therefore reduce the generated energy savings and increase the operation costs. With the aim to study the hydrogeochemical behavior of a standing column well and identify potential mitigation measures, a coupled thermo-hydro-chemical (THC) model is presented. In this work, geochemical reactions are expressed as slow chemical reactions for precipitation and dissolution while fast reactions are used to model local equilibrium reactions. The resulting model involves nine solute chemical species and three slow elementary reactions involving calcite. Reaction rate constants of these reactions are obtained through empirical relations taking into account groundwater temperature. The geochemical reactions are integrated within a heat and transport standing column well model that couple temperature, flow and chemical reactions. The transport equations are solved for three total concentrations under the constraint that a local equilibrium of the solute species is established. In order to validate the approach used, a simplified thermo-hydro-chemical model was developed and compared with reference solutions provided by PHREEQC. Both approaches show consistent results in terms of concentration of the aqueous species with a relative error lower than 10 %. Current simulation results confirm that dissolution in the rock around the well is possible and that a significant mass of calcite could precipitate in the well and within the mechanical equipment. Therefore, this study provides the first evidence that maintaining water quality and system’s integrity in a standing column well could require water treatment systems.