Thermal modelling of the Larderello geothermal field (Tuscany, Italy).

We present a 3-D thermal model of the Larderello geothermal field (Tuscany) to evaluate (1) the extent and contribution of the heat transfer mechanisms (conduction vs. convection) at the intermediate-upper crust levels, (2) the variability of the heat and mass fluxes entering from below and (3) the crucial role of the formation permeability. The model, composed by three main layers, considers the upper 10 km of the crust to better constrain the simulations with experimental data from borehole, fluid inclusion studies and hypocentral distributions. Several sets of simulations were carried out with different bottom boundary temperatures and different formation permeabilities for the two deeper layers. The results indicate that the present temperature (T) and pressure distributions in the Larderello field require deep reservoir rocks with higher permeability than the overlying capping units and underlying intermediate crust. Permeability values of 1 mDarcy for the reservoir rocks are enough to allow fluid convection, if the temperature at 10 km depth is as high as 500 ± 50°C. The presence of localized zones with formation permeability 50-100 times higher than the surrounding rocks strongly favours the migration of overpressurized fluids, which episodically break through the overburden, feeding the presently exploited geothermal fields.