THE IMPACT OF STANDING COLUMN WELL OPERATION ON CARBONATE SCALING


 <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>&#160;</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>&#160;</p><p>First, the &#8220;on-off&#8221; 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&#176;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>&#160;</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>



Experimental Results Conclusions
2/8 o Standing Column Wells (SCW) demonstrate high heat exchange capacity (Orio et al., 2005) and represent a promising solution to reduce the capital cost compared to classic closed loop, by 49% to 78% (O' Neill et al., 2006).
o In a SCW, groundwater is continuously recirculated in an uncased well, bringing the water in direct contact with the rock.
o During peak periods, the SCW can discharge a portion of the outlet water into an injection well.For a fraction of 10% of groundwater flow, it can improve heat transfers by about 2.5 times (Beaudry et al., 2019).
o This technology yields great results in igneous rock, which has limited geochemical reaction (Orio et al., 2005).
o The clogging of a geothermal system impacts groundwater flow rate and heat transfer.Indeed, 0.8 mm of scale on the heat exchanger surface results in 19% higher consumption for the heat pump compared to a clean heat exchanger (Rafferty, 2004).
o Geothermal systems are particularly exposed to clogging events.The pumping rate, the variation of temperature and the mixing of water promote several clogging processes which can be physical (Burté, 2018), microbiological (Kim et al., 2017;Lerm et al., 2013) and chemical (Palmer and Cherry 1984;Possemiers et al., 2014).
o Precipitation of calcite is identified as a potential problem when groundwater temperature increases (i.e. when the geothermal system is operated in cooling mode).

3/8
Objective : To evaluate which operating condition creates the optimal conditions for clogging in standing column well constructed in carbonate aquifer.

Problematic :
The risk of carbonate scaling is known in the literature to be a hazard during the operation of open-loop systems.

Initial rock composition Initial Groundwater composition
Stratigraphy adapted form Beaudry et al, 2019 o For this study, a 215.8-m-deep experimental standing column well was monitored over a oneyear period using a geothermal mobile laboratory located near Montreal.o The aquifer is composed primarily of silicates, and its high concentrations in calcium and magnesium make it reactive, since they are the more reactive elements (Appelo and Postma., 2004).o The initial groundwater composition is also above the recommendations made by Rafferty ( 2004) to avoid calcium precipitation.This limit is fixed at 80 ppm CaCO3 for hardness.o The red boxes represent the periods where the geothermal system was offline.The downtime is used as a variable in the stepwise regression.

Introduction
o The temperatures LWT and EWT are recorded for each sampling.
o A new variable is created to represent the three modes linked to temperature (the "heating mode", the "recirculating mode" and the "cooling mode").
o The regression found 10 operating variables explaining the calcium concentration, with a correlation factor of 0.86.

The most significant operating variables :
o The pumping groundwater flow has the most impact on groundwater calcium concentration.
o The LWT also influences the calcium concentration.This result is in agreement with the simulation performed by Eppner et al. (2016).Moreover, the transition from recirculation to cooling leads to precipitation of calcium.
o The downtime has a strong impact on the calcium concentration.o Nevertheless, the effect of the scaling on the system's operation was limited to the flowmeters.

Introduction Experimental Results Conclusion
8/8 o This study explored the impacts of standing column well operation on the stability of calcite.These impacts are limited to the precipitation of calcite in the flowmeters.
o Future works will focus on the impacts of these deposits on thermal exchanges, on finding a way to avoid their formation through the use of a water treatment system and on strategies to limit the impacts of system downtimes.

oo
In operation since 2016 o This study takes place between January 2018 and October 2018 (1 year) o 51 groundwater samples o 1 advanced monitoring system (flow, temperature) o 4 sample points o 1 solid deposit sample scheduled for the end of This figure presents the operating parameters and the calcium concentrations of water.It presents the major variables used by the stepwise regression performed between the operational parameter and the groundwater calcium concentration (SP-1).oThe monitoring system logs the bleeding water flow (red line), the total flow (blue line) and the treated water flow (green line).

o
This chemical composition and the mineral structure indicate a deposit composed of calcite.The X-ray had a score of 81 for the calcite.