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Could the numerical modelling improve the conceptual one? The example of the Euganean Geothermal System (NE Italy)

1. Introduction The Euganean Geothermal Field (EGF) is the most important thermal field in the northern Italy. The EGF extends on a plain band of approximately 25 km² to the southwest of Padova (Veneto ; Italy), and comprises the towns of Abano and Montegrotto Terme (Fig. 1). Currently about 170 wells are active, and the total average flow rate of exploited thermal fluids is 14*10⁶ m³/year. The surface temperature of thermal waters ranges from 65°C to 86°C, and their TDS is up to 6 g/L (primary Cl- and Na+). ³H and ¹⁴C measurements suggest a fluid residence time greater than 60 years, probably in the order of thousand years. The EGF is located within the NW-trending Schio-Vicenza fault system (SVFS), a group of high-angle faults partly buried under Quaternary alluvial sediments and bordering to the West the foreland of the eastern Southern Alps (Pola et al., 2014). These faults, originated partly in the Mesozoic as normal faults, were reactivated in an extensional regime during the Paleogene. The EGF is located within an inferred subsurface relay zone between the Schio-Vicenza (SV) and the Conselve-Pomposa (CP) fault segments (Pola et al., 2015). Given the present sinistral transtensional regime of the western foreland margin, this structure is submitted to a local tensional regime that opens an ESE-WNW set of fractures maintaining the local conduits for the rising of the thermal fluids. The results of approximately 50 years long studies on the EGF and its thermal waters were used to propose a regional, tectonically-controlled conceptual model of the Euganean Geothermal System (EGS ; Pola et al., 2015). The aim of this research is to evaluate if and how a regional numerical model can improve the proposed EGS conceptual model. 2. EGS conceptual model The Euganean waters are of meteoric origin, as depicted by stable isotope analysis, and infiltrate about 80-100 km to the north of the EGF in the carbonate Sette Comuni-Tonezza plateau and the reliefs facing the area (Fig. 1A). The hydrological mass balance of the area shows that 23% of the infiltration (260 mm/y) is not balanced by the discharge of the springs located at the base of the reliefs representing the possible recharge of the system. The waters infiltrate in depth thanks to the high secondary permeability of the outcropping rocks and flow to the South in a Mesozoic carbonate reservoir (Fig. 1B). The SVFS defines the flowpath of the hot waters. In particular, the associated fracture mesh of the faults damage zones greatly enhance the fluid flow thanks to the high permeability (Faulkner et al., 2010). In the middle part of the EGS, the waters reach a depth of about 3000 m and warm up by the regional normal density heat flux (70-80mW/m²). Near the EGF area, the thermal fluids intercept the transtensional relay zone linked to the SVFS. This structure increases the rock fracturing and permeability, enhancing the migration to the surface of the thermal waters. 3. Methodology The conceptual model was reproduced in a 3D numerical model, simulating the regional fluid flow for the first time. The aim was to physically test, and improve, the EGS conceptual model by the results of the numerical simulations taking into account the regional geological constrains. The simulations are performed using the hybrid approach with the finite element code FEFLOW (Diersch, 2014). The model domain is 80 km long, 26 km width and 5 km deep, and it is discretized in 1.5 M nodes on 24 layers (Fig. 2). The geological setting is implemented by grouping the 24 layers into 5 horizons with different hydro-thermal properties obtained from a literature research. The main regional geological structures driving the fluid flow (i.e., the Schio-Vicenza and the Conselve-Pomposa damage zones, and the relay zone) are reproduced increasing the porosity and permeability of the nodes corresponding to these structures (Fig. 2). In addition, the fluid flow in the EGF is concentred by a network of planar discrete elements comparable with the one deforming the subsurface of the EGF. The simulations were conducted on a plausible geologic time of 2.5 My, and their execution time by a 24-core Intel Xeon CPU at 2.1 GHz was approximately 12 hrs. A calibration was also performed, with the aim to evaluate how different parameters could affect the temperature and the flow in the subsurface of the EGF. Finally, the temperature results of the numerical model were compared with the experimental well thermal logs carried out in the EGF. 3. Results & Conclusion Initially, the conceptual model expected a reservoir temperature of 90°-100 °C in EGF as depicted by SiO2 geothermometer results, and few degree lower than the exploited water temperatures. Conversely, the simulated temperatures are higher in the EGF reservoir. The increase of permeability, and consequently of the fluid circulation, within the relay zone enhances the EGF reservoir temperature despite a normal density heat flux (70-80 mW/m²) in the central Veneto plain. These results are also endorsed by Na-K-Ca geothermometers, suggesting a reservoir temperature of up to 170°C. Such a temperature is modelled into the accommodation zone at a depth of 2 km, while 100°C is simulated at 1 km as experimentally measured by well thermal logs.

Euganean Geothermal System (EGS) ; Euganean Geothermal Field (EGF) ; conceptual and numerical model

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