Standing column well constitutes a recent promising solution to provide heating or cooling and to reduce greenhouse gases emissions in urban areas. Nevertheless, scaling issues can emerge in presence of carbonates and impact their efficiency. Even though a thermo-hydro-geochemical model demonstrated the impact of the water temperature on carbonate concentration, this conclusion has not been yet demonstrated by field investigations. To do so, an experimental ground source heat pump system connected to a standing column well was operated under various conditions to collect 50 groundwater samples over a period of 267 days. These field samples were used for mineral analysis and laboratory batch experiments. The results were analyzed with multivariate regression and geochemical simulations and confirmed a clear relationship between the calcium concentrations measured in the well, the temperature and the calcite equilibrium constant. It was also found that operating a ground source heat pump system in conjunction with a small groundwater treatment system allows reduction of calcium concentration in the well, while shutting down the system leads to a quite rapid increase at a level consistent with the regional calcium concentration. Although no major clogging or biofouling problem was observed after two years of operation, mineral scales made of carbonates precipitated on a flowmeter and hindered its operation. The paper provides insight on the impact of standing column well on groundwater quality and suggests some mitigation measures.

<p>The interest towards standing column well (SCW) is increasing due to their higher thermal efficiency and a lower initial construction cost compared to conventional vertical ground heat exchangers. The SCW pumps and reinjects the groundwater inside the same well. They are usually coupled with an injected well to discharge a portion of the pumped groundwater, an operation called “bleed”, to increase punctually the thermal capacity of the system. Since groundwater is the heat carrier fluid, clogging issues can develop if detrimental conditions are locally present. The most common issue for hard water is calcite scaling. The impact of SCW on calcite precipitation had already been studied with a thermohydrochemical model and field experiments. However, it still lacks a reactive thermohydrochemical model calibrated with field acquired data. Once calibrated, this model could help defining the best strategy to avoid calcite precipitation.<br />A full-scale SCW was operated with a geothermal mobile laboratory for 70 consecutive days. A fractured zone intersects the SCW close to the surface. The operation corresponded to a heat injection with different flow rate sequences. In addition, a groundwater treatment unit installed in the laboratory was used to test different treatment sequences. During this experiment, 20 groundwater samples were collected and analyzed. Those analyses focused on the physico-chemical parameters and the major ions. A reactive thermohydrochermical model was developed in the Comsol Multiphysics environment. This model includes a complex geometry, groundwater flow, heat transfer, and reactive solute transport. The reactive solute transport is composed of two parts; the transport and kinetics model for three primary species and the chemical equilibrium of nine secondary species of calcite reaction. The calibration is achieved by imposing operational parameters as input variables for hydraulic and thermal model as well as the initial concentration.<br />The calibration identified the presence of CO<sub>2</sub> degassing. The parameter with more influence on ion calcium concentration is bleed flow. In fact, bleed operation generates a groundwater flow of native groundwater toward the SCW. During this operation, the fracture contributed up to 33 % of the total calcium flux coming to the SCW. This flux is a convective flux. The concentration of total calcium transported by the fracture is closed to the initial concentration. As a consequence, the ion calcium concentration stabilized near the initial state. Thus, the groundwater treatment performance is minimized. The saturated index of the calcite is above zero. On the opposite side, when bleed is not activated, the groundwater is recycled is the SCW. As a results, the treatment unit is responsible of the observed decreases of the ion calcium concentration. The saturated index decreases below zero after five days. Also, when the total calcium concentration decreases in the SCW, a diffusive flux emerged into the fracture.<br />In conclusion, this study highlights the alteration of the solute transport as a function of the bleed operation. The type of flux depends on bleed and the treatment. In addition, the treatment of groundwater is unnecessary when bleed is activated.</p>

<p>Low-temperature geothermal systems have shown great potential to reduce greenhouse gas emissions. One emerging solution, named standing column well, is particularly promising and is characterized by low installation costs and higher thermal efficiency compared to widespread closed-loop wells. In a standing column well, groundwater is continuously recirculated in an uncased well. As the well and the mechanical devices are prone to clogging and scaling, the occurrence of new operational conditions can have an impact on long-term performance and generate significant maintenance costs. Although current literature identifies the main causes of clogging, the impact of the operation strategy of a standing column well operation on clogging development has not yet been extensively studied.</p> <p> </p> <p>The chemical signature of groundwater and the operation parameters of a real-size experimental standing column well were monitored during a two-year period using a geothermal mobile laboratory. This laboratory contains heat pumps, heat exchangers, pumps, monitoring devices and a water treatment unit enabling treatment of a fraction of the total pumping flow. This work highlights how the operation of a standing column well impacts the clogging rate by establishing a direct link with the observed calcium concentrations. Two specific operation schemes were found to be critical for the development of clogging.</p> <p> </p> <p>First, the “on-off” sequences of the pump allowed for water stagnation in the mechanical devices and promoted a temperature rise since the geothermal laboratory is maintained at 20<sup>o</sup>C, thus creating ideal conditions for precipitation. In addition, the calcium concentration in groundwater increased with shutdown duration and with a kinetic similar to the one observed in an independent batch test. This batch test conducted with demineralized water and samples of the local rock was carried out in close atmosphere at 10°C to measure the dissolution kinetics. Both the two-year monitoring and batch test confirm that groundwater slowly dissolves the carbonates in the standing column well that precipitate in the mechanical devices during the off sequences.</p> <p> </p> <p>The second critical operation scheme was observed during cooling mode. As groundwater temperature gradually increases with the operation of the system, the calcium stability index increased, leading to precipitation in some mechanical devices. After two years of operation, some mineral deposits were recovered on the probes of two faulty flow sensors. The deposits were analyzed with a scanning electron microscope, which indicated high concentrations of calcium, oxygen, and carbon, all compatible with calcite precipitates. Further works will focus on the development of new operation strategies to hinder clogging and scaling of the mechanical equipment connected to a standing column well.</p>

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