The Geothermal System of Vulcano Island: New insights from integrated geophysical methods and Laboratory data

The Aeolian Islands, located in the southern Tyrrhenian Sea on the western coast of Italy, like several other volcanic islands, confront challenges when it comes to electricity and water supply, dependant on the surrounding mainlands at high expenses. Yet, several geoscientific surveys and drillings have confirmed the presence of geothermal systems, that could turn this situation around. Previous research on the geothermal resource of Vulcano, one of the seven islands making up the Aeolian archipelago, did not produce conclusive results as to the feasibility of its power production plant project, although the reported temperatures from existing exploratory wells, were sensibly high (198° C ~236 m in Vu2bis well). In order to detect the most advantageous site to realize a geothermal power plant, it is fundamental to know the permeability distribution across the island of Vulcano. Recent studies evidenced heterogeneities between in-depth and lateral permeabilities, implying a discontinuous and complex hydrogeological network; making it difficult to localise the pathways through which hydrothermal fluids ascend to the surface. The aim of this work is to target the most suitable potential drilling spots by detecting hydrothermal fluid upwelling paths, linked to higher temperatures at shallower depths, essential for implementing the future geothermal power plants cogenerating electricity and energy for seawater desalination on the island. To achieve this objective, a multidisciplinary approach is adopted, including geophysical methods, notably Electrical Resistivity Tomography (ERT) coupled with Magnetotellurics (MT). Moreover, are measured and analysed the thermal conductivities, electrical resistivities, and elastic moduli of eleven (11) outcropping rock samples, representative of the encountered lithologies in the VP1 and IV1 wells, drilled in the 1980s. Geophysical data allowed to localize hot fluid ascension paths, coinciding with the main areas of fumarolic activity between Baia di Levante and Faraglione, crossed by the NNE-SSW tectonic lineament. The laboratory data, on the one hand, provided us with information on rocks’ physical properties useful for drilling operations, and on the other hand, complementary elements for the interpretation of geophysical data. The integrated analysis of geological, geophysical and laboratory data delivered a better understanding of the structural framework of our investigated area, presenting lava flows as the potential drilling target with sufficient spatial coverage and modest physical properties. These results do not only represent another step forward in our knowledge of the superficial part of Vulcano's geothermal system, but can also serve as a reference for the exploration of neighbouring islands, and even other volcanic islands elsewhere in the world. However, they pave the way to questions concerning the influence of mineralogical composition and geological context of rock samples and their representativeness of in-situ conditions. Therefore, the mineralogical analysis of samples and well logging in the study area are recommended for a better constrain of these findings. Additionally, a tailored ecological operational scheme for geothermal energy extraction is proposed, in particular considering closed-loop borehole heat exchangers operating under Organic Rankine Cycle (ORC) systems.