GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.3 181 Settings of numerical simulations and optimization procedure. With the aim to simulate the temperature distribution in the four geothermal systems under study, site-specific geological models were created including the available stratigraphic information fromwells, active seismic imaging as well as velocity and resistivity data attained in tomographic and magnetotellurics studies. The heat and mass transfer processes acting in the subsurface have been described by the combination of the continuity and momentum equations coupled with the energy conservation equation. The lithothermal units were treated as a homogeneous and downward anisotropic porous material (Pasquale et al. 2011) in which mixing laws were applied to estimate the effective thermal and hydraulic properties accounting for the in-situ conditions. In the heat transfer equation, the thermal energy carried by the geothermal fluids and the heat released by radiogenic elements and by young magmatic intrusions have been accounted for. The solution of the differential equations has been approximated through the finite element method on a tetrahedral mesh grid. Sensitivity analysis revealed that the key parameters having major control on the thermal structure are the reservoir permeability and the magma emplacement temperature. As example, the optimization procedure of the numerical model of Ischia is shown in Figure 1. The available deep temperatures were used as control points on which the overall misfit was calculated by the Normalized Root Mean Square Error (NRMSE). Results and conclusions. The style and magnitude of fluid circulation in the upper crust and the relationships between hot magmatic intrusions and hydrothermal systems have been investigated for a variety of reservoir permeability, magma emplacement depth and temperature as well as deep geometries. Thermal convection dominates in the magmatic environments if rocks permeability is greater than about 10 -15 m 2 , resulting in a spatial redistribution of thermal energy which notably differs from a purely conductive thermal model. As soon as the magma is emplaced in the crust, it warms the hosting rocks and large-scale convective cells develop after few thousand years. The large vertical heat flux transports the thermal energy from the heat source to the top boundary of the reservoirs faster than it can be dissipated through the conductive caprock units. This condition determines vigorous horizontal fluxes which in turn control the lateral extension of the thermal plume. Although most geothermal systems are associated with up-flow paths, there are some geothermal fields, like the Long Valley caldera, where up-flow appears over- printed by a more vigorous cross-flow. Under these circumstances, temperature inversions arise due to fluid flow within highly permeable, horizontal aquifers. Topography may provide an additional kick to the fluid circulation as fluids flow from higher elevations towards lower ones due to a difference in the pressure heads. In order to maintain reservoir temperatures above 150°C – 200°C or greater for long times, the heat source requires consecutive magmatic pulses because the cooling time of upper-crustal, middle-sized granitoids is quite fast. It is particularly difficult to unravel the thermal evolution of the heat source from convection-dominated thermal profiles because the hydrogeological noise clears the deep thermal signals. Conversely, conductive thermal profiles may record more clearly the thermal effects of deep magmatic sources. For example, the temperature data coming from Acoculco and Larderello areas show a characteristic convex upward trend. These temperatures can only be fitted by a transient simulation accounting for an increase of specific heat flow with time. The transient simulations suggest for both the sites a relatively young age of the magmatic intrusion ranging from 0.1 Ma to 0.01 Ma or even younger. Acknowledgments. This research was performed within the framework of four main National (Italian) and European projects. The GEOTHERMAL ATLAS OF SOUTHERN ITALY Project is one of six Projects constituting the Program “CNR per il Mezzogiorno” of the Italian National Research Council. The IMAGE Project has received funding from the European Community’s Seventh Framework Programme under grant agreement No. 608553. The DESCRAMBLE and the GEMex Projects have received foundings from the European Union’s Horizon 2020 research and innovation programme under the grant agreement No. 640573 and No. 727550, respectively.