The impact of Standing Column Well operation on Carbonate Scaling

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.

 

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.

 

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^oC, 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.

 

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.