The role of fault damage zones in fluid flow: the case study of the Euganean Geothermal System (NE Italy)

The Euganean Geothermal Field (EGF) extends on a plain band to the south of Padova in the central part of Veneto Region (NE Italy). Approximately 400 wells up to 1 km deep have been drilled since 1950s, and 170 wells are currently active exploiting 14*106 m3/y of thermal water. The water has a temperature from 65°C to 86°C, and it is used for balneotherapy feeding 240 pools. Therefore, the EGF represents an important thermal field of Italy, and its recreational tourism is a significant economic resource for the Veneto region. The EGF is the outflow area of a regional hydrothermal system (Pola et al., 2015). The water is of meteoric origin infiltrating 100 km to the north of the EGF in the Veneto Prealps. The water flows towards the South in a Mesozoic carbonate aquifer reaching a depth of 3 km and warming to 100°C due to a normal geothermal gradient. The flow is enhanced by the highly permeable damage zone of the regional Schio-Vicenza fault systems (SVFS). In the EGF area, the water intercepts a local network of fractures associated to an interaction zone between the SVFS. The water rises quickly through the open fractures, and subsequently it spreads horizontally within fractured layers located at depth from 300 m to 1 km. The aim of this research is to understand the role of SVFS on the regional fluid flow, as well as the influence of the local fractures mesh on the upwelling of the hot water. The FEFLOW code is employed to reproduce the conceptual model of the thermal system in a 3D numerical model using the Equivalent Porous Medium Approach. The regional geological setting is simplified by 3 planar hydrostratigraphic units with a thickness from 1.4 km to 2 km. The units are simulated using 26 layers of constant thickness, and the values of their hydro-thermal properties are estimated using literature values. In addition, the damage zones of the faults are modelled increasing the local permeability by 1 order of magnitude compared with the protolith, while the permeability in the central part of the domain is increased by 2 order of magnitude representing the fracture mesh. Preliminary results show the development of convective cells along the damage zones and the increase of the temperature in the EGF subsurface. However, the modelled temperature is lower than that estimated into the reservoir. Therefore, a sensitivity analysis will be performed to evaluate the impact of the permeability (i.e., permeability of the units, ratios between the permeability of the protolith and the fractured rock and between horizontal and vertical permeability) on the modelled temperature in the EGF. This impact will be evaluated comparing the results with the subsurface temperature measured with thermal logs in the wells.