Thermal overview of an area NW of the Larderello geothermal field, Italy.

A wide area enclosed between the ancient Etruscan to town of Volterra and the northern rim of the Larderello high enthalpy geothermal field (Tuscany, Italy) shows thermal features which suggest further investigations aimed at medium-high enthalpy geothermal energy exploitation. Thermal gradients are in the range 75-100 °C×km-1, while surface heat flow spans between 100 and 200 mW×m-2. Constrained by field data, unsteady forward simulations were performed to predict the spatial distribution of temperature and fluid circulation paths in a 6-7 km deep domain, under the assumption of impervious and isothermal top and bottom boundaries, lateral adiabatic faces and variable internal physical properties. Gridding specifications are based on industrial seismic profiles and deep exploratory geothermal wells along them, which allow temperature control down to about 3 km. The model area has been kept variable in a 20-30 by 6-7 km range, depending on simulation time requests, in an effort to test possible border effects. The results indicate that the present temperature and pressure distribution of hot fluids with depth in the northern border area of the Larderello field allows to hypothesize a fruitful exploitation of the medium-high enthalpy geothermal resources. Further to this, total heat loss values for the area were calculated as a tool to evaluate the potential energy production, as well as Nusselt number values for the area, to ascertain the possibility of a natural fluid convection in the reservoir formations.