The Geothermal District Heating System on the Grado Island (North-eastern Adriatic Sea).

The feasibility of a geothermal district heating pilot-plant on Grado Island, northeastern Adriatic Sea (Italy), was the result of the Grado geothermal Project, Phase 1, completed in 2008 by the Regione Friuli Venezia Giulia, mostly supported by European Union funding. The reservoir characterization and the preliminary geothermal potential assessment rely on the geophysical prospect and on the exploration borehole drilled down to 1110 m. These investigations confirmed the existence of an untapped low-enthalpy geothermal reservoir within the Mesozoic carbonate platform buried beneath about 1 km of Paleogene and Neogene sediments, in correspondence of the structural highs along the coastal areas. The well production potential from a fossil, seawater confined aquifer, having a temperature of about 45-50 °C, was estimated to be about 140 tons/h. In 2012, as part of the Grado geothermal Project, Phase 2, an integrated gravity and seismic geophysical prospecting, including multi-offset VSPs, was conducted in downtown Grado and in its surrounding lagoon. The target was to extend the investigation of the geothermal reservoir and to provide adequate information on the faults/fracture systems interesting the buried external Dinaric thrust front. The results, here presented, allowed operators to locate the second well of the geothermal doublet, planned to feed the district heating system of public buildings on the island. The drilling of the second borehole is due by June 2014 and the deployment of the main distribution network by September 2014. Borehole geophysics, interference test measurements between the two wells located at distance of one km, and 3-D thermo-fluid dynamic numerical modeling will optimize the production and fluid re-injection and help to design the heat exchangers and to manage the sustainability of the geothermal plant.