Caractérisation hydrogéothermique d'un aquifère fracturé accueillant un système géothermique avec puits à colonne permanente

Standing column wells are particularly efficient ground heat exchangers open to the aquifer and employ groundwater as the heat transfer fluid. There are currently a limited num- ber of publications investigating the impact of hydrostratigraphic conditions on the thermal response of a well that have involved experimental tests. To date, the available work is primarily based on modelling and most often involves bleed. This document presents the hydrogeothermal characterization of a site that allowed the study of a standing column well operated without bleed in a fractured aquifer. Following a review of the geological and hy- drogeological context at the regional scale, the Clé-des-Champs School located in Mirabel (Quebec) was selected to host a standing column well system. The hydrogeothermal characterization of the aquifer involved two exploratory boreholes, a thermal response test, a pumping test, geochemical analyses of the groundwater and ve- locimetry profiles. This allowed to establish a detailed picture of the site and to confirm the significant geothermal potential to implement standing column wells. Indeed, the thermal response test revealed a significant thermal performance, even for a standing column well operated without bleed. The first-order infinite line source approximation did not yield a re- alistic thermal conductivity value for the geologic materials around the well. It was deduced that conduction alone could not explain the thermal response obtained. To explore this hy- pothesis, a more advanced study was required to understand the heat transfer mechanisms that occurred during the thermal response test. Thus, a 3D numerical finite element model coupling heat transfer and groundwater flow was developed based on the field observations to interpret the thermal response test. The model was also developed to study the influence of the hydrostratigraphic context on the thermal response of a standing column well operated without bleeding. Indeed, the aquifer located at the Clé-des-Champs School site is characterized by a fractured and transmissive zone, observed in the lower part of the boreholes, and which corresponds to the Cairnside Forma- tion. Modelling works has shown that in these geological conditions, advection, could be the dominant heat transfer mechanism, even without bleed. In addition, the work highlighted the influence of hydrostratigraphic conditions on the interpretation of the thermal response test, i.e., the average value as well as the hydraulic conductivity profile influence the thermal response of the well. This showed that the first-order approximation of the infinite source line cannot always be used to interpret thermal response tests performed on standing column wells. The results also support the fact that a fracture at the base of the well, under the conditions encountered at Mirabel, may slightly affect the thermal performance of the well. An erroneous assessment of the hydrogeological parameters could lead to an inappropriate design of the system. This conclusion calls for caution and encourages a hydrogeological study to properly define the hydrostratigraphic context in order to consider its effect when interpreting thermal response tests.