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In remote northern regions, lacking deep geothermal exploratory boreholes, a theoretical approach to provide a first-order estimate of the stress regime seems a useful tool. Literature data was used in this context to evaluate the orientation of the stress components and empirical relationships were applied to calculate their magnitude in a community of Nunavik, northern Quebec. A Monte Carlo-based sensitivity analysis was carried out due to the uncertainty of the input parameters. Mohr–Coulomb friction and slip tendency analyses were additionally undertaken to assess the stress state and potential reactivation of existing fractures. The results highlight how the poor knowledge of the stress field has an important impact on the design and development of engineered geothermal energy systems in the Canadian off-grid community of Kuujjuaq.

Geothermal energy systems, such as heat pumps relying on aquifers, use renewable sources of energy that are accessible in urban areas. It is necessary to characterize the subsurface hydraulic properties prior to the installation of such systems. In this context, a heat‐tracing experiment is a typical field test that can help with the characterization of the subsurface. During a heat‐tracing experiment, monitoring with downhole temperature sensors, water‐level pressure transducers and electrical resistivity tomography (ERT) can be used to help characterize the hydrogeological properties. Previous monitoring tools have shortcomings, such as low‐resolution data and over‐smoothing; thus, they fail to reproduce the heterogeneity of hydrogeological properties. Ensemble Kalman filter (EnKF) is a promising tool that can overcome the over‐smoothing problem to replicate the hydrogeological property heterogeneity. In this work, we proposed a new procedure to assimilate time‐lapse cross‐borehole ERT data into a numerical model of groundwater flow and heat transfer, where the groundwater is extracted and heated water is reinjected into an unconfined sandy‐gravel aquifer. The finite element model (FEFLOW 7.3) of groundwater flow and heat transfer is integrated with petrophysical relationship and electrical forward modelling (ResIPy) to estimate cross‐borehole ERT measurements. Then, the estimated apparent resistivity is assimilated to update the hydraulic conductivity model using EnKF. The results of the application of the proposed approach to an experimental site located in Quebec City (Canada) demonstrate that the heterogeneity of K is correctly reproduced as the updated K model is reasonably consistent with the lithological log. In addition, the proposed approach was able to replicate the cross‐borehole ERT field and temperature measurements. The comparison between prior and posterior distribution of K with slug test results shows that the EnKF made the final (assimilated) distribution of K move towards K values inferred with slug tests. [ABSTRACT FROM AUTHOR]

The construction of 'green' buildings using geothermal energy requires knowledge of the ground thermal conductivity, assessed when designing the heating and cooling system of commercial buildings with ground-coupled heat pumps. The most commonly used method for active field assessment is the thermal response test (TRT), which consists of circulating heated water in a pilot ground heat exchanger (GHE) where temperature and flow rate are monitored. The transient thermal perturbation is analyzed to evaluate the subsurface thermal conductivity. Heat injection can also be performed with a heating cable in the GHE to conduct a TRT without water circulation, which can be affected by surface temperature variations. Passive methods, such as the interpretation of geophysical well logs and the analysis of temperature profiles measured in exploration wells, are emerging as alternatives to TRTs. Steady-state and transient laboratory measurements performed on samples collected in surface outcrops or drill cores can also be achieved. Methods to characterize the subsurface in the context of geothermal system design have evolved significantly since the original TRT concept proposed during the 1980s with different techniques inspired from the Earth science sector. Key words: geothermal, geothermics, thermal geophysics, heat pump, thermal conductivity, thermal response test, ground Heat exchanger. La construction des batiments << verts >> qui utilisent l'energie geothermique necessite une connaissance de la conductivite thermique du sous-sol, evaluee lors de la conception d'un systeme de chauffage et de climatisation d'un batiment commercial munis de pompes a chaleur couplees au sol. La methode la plus utilisee pour une evaluation active sur le terrain est celle du test de reponse thermique (TRT), laquelle consiste a faire circuler dans un echangeur de chaleur au sol (ECS) pilote de l'eau rechauffee ou sont mesures la temperature et le debit de circulation. La perturbation thermique transitoire est analysee afin d'evaluer la conductivite thermique de la sous-surface. L'injection de chaleur peut egalement etre effectuee a l'aide d'un cable chauffant insere dans l'ECS pour realiser le TRT sans circulation d'eau, celle-ci pouvant etre affectee par les variations de temperature en surface. Des methodes passives, comme l'interpretation des diagraphies de forage et l'analyse d'un profil de temperature mesure dans un forage exploratoire, emergent maintenant en guise d'alternative aux TRT. Des mesures de laboratoire, en regime permanent ou transitoire, effectuees sur des echantillons, provenant d'affleurements ou de carottes de forage, peuvent finalement etre envisagees. Les methodes de caracterisation de la sous-surface utilisees dans un contexte de conception des systemes geothermiques ont grandement evoluees depuis le concept original du TRT propose durant les annees 1980 avec differentes techniques inspirees du secteur des sciences de la Terre. Mots-cles : geothermie, geothermique, geophysique thermique, pompe a chaleur, conductivite thermique, test de reponse thermique, echangeur de chaleur au sol., Introduction Geothermal heat pump systems, also named ground source heat pumps (Omer 2008), have become an important component of the living environment, tying buildings to the subsurface. The thermodynamic cycle [...]