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Geological processes and renewability assessment of a fault-controlled geothermal system: the case study of Euganean Geothermal System (NE Italy)

The Euganean Geothermal System (EuGS), located in the central part of the Vento Region (NE Italy), represents one of the most important low-enthalpy geothermal field in the southern Europe (Fabbri et al. 2017). This thermal water is of meteoric origin and infiltrates in the norther part of the system (Veneto Pre-Alps) an elevation of about 1500 m a.s.l. A particular structural setting allows the existence of the EuGS. As a matter of fact, the groundwater flow from the recharge area to the exploitation area is favor by a system of high-angle NE-dipping faults. Moreover, the rising of the deep thermal fluids in the exploitation area is favor by a pattern of local fractures that increase the permeability of the bedrock (Pola et al 2015). Approximately 15x10⁶ m³ per year of thermal waters with a temperature ranging from 63 to 87 °C are exploited from approximately 200 wells. This water is mainly used for balneotherapy and recreational purposes, feeding about 195 SPA, and the related tourism industry produces an income of about 300 million euros per year (Fabbri et al. 2017). These data testify the important role played by this natural resource on the economy of the Veneto Region. As a consequence, a strategy for its sustainable exploitation is required in order to preserve it for the future generations. Numerical simulations can be used as a tool to assess the processes driving the geothermal resource and its renewable component and to define the sustainability of its utilization (Monterrosa and Montalvo Lopez 2010). However, the reliability of the results is strictly related to the dataset that is implemented into the numerical model (Axelsson 2010). Typically, the geological features characterizing a system are reproduced in a very schematic way using some assumptions that reduce the necessary numerical effort. Consequently, the results can be partially useful to perform a reliable utilization strategy. In the EuGS case study, a detailed geological reconstruction of the analyzed geothermal system was carried out through MOVE software. Different type of data (geological and seismic section, wells data, gravimetric and structural maps) were implemented in the geological model to reproduce the main geological features allowing the existence of the geothermal system. This approach was adopted to increase the reliability of the numerical model, reducing the simplification of the conceptual model. To preserve the detail obtained through the geological reconstruction into the numerical model an unstructured grid approach was employed. The unstructured mesh allows to reproduce the high heterogeneity that characterize the geological setting of the analyzed geothermal system (Blӧcher et al. 2010 ; Passadore et al. 2012). As a matter of fact, these meshes can be deformed and distorted in zones characterizing by high variations (i.e. fault and fold zones). This procedure was possible using the open source MeshIt software (Cacace and Blocher 2015). The obtained unstructured mesh was implemented in the FeFlow code to perform coupled simulations of fluid flow and heat transport. The results allowed to define the interplay of geological features ensuring the existence of the geothermal system and its renewability. Starting from this point, particular attention will be paid to analyze the sustainability of the geothermal resource related to the current exploitation of the Euganean geothermal resource.

3D geological modeling ; numerical modeling ; unstructured mesh ; Euganean Geothermal System

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