Geological, hydrogeological and thermal characterization of deep subsoil (Marche Region, Italy) linked to use of low enthalpy geothermal heat pumps

It’s well known that geothermal energy is a clean and sustainable source of energy, directly coming from the Earth. Geothermal energy resources range from hot water and hot rock found a few miles beneath the Earth’s surface, and down even deeper to the extremely high temperatures. Between the common uses of geothermal energy, direct use systems provide direct heat for residential, industrial and commercial uses with many advantages both for environment and for heating bill, because it is much less expensive than using traditional fuels. In this regard the aim of the PhD project, in collaboration with Ergon Bluenergy Group, is to increase the knowledge of the subsoil of Marche Region1 through its geological, hydrogeological and thermal3 characterization. This study, is of primary importance to understand the correct depth of well drilling to extract the necessary amount of heat for buildings energy need and to perform low enthalpy geothermal plants, linked to use of geothermal probes and heat pumps at a specific depth of investigation (until 400 m a.s.l.). Initial studies are focused on three important parameters and their connections, useful to find T values essentials to create a thermal modelling of the subsoil: lithology, depth and thermal conductivity. Data from well profiles drilled to find hydrocarbon in Marche Region2, show that in Jesi, Macerata and S.Costanzo areas with varying depths (from 100 and 308,50 m a.s.l.) and lithologies, corresponds variable T values from 20°C and 28°C (for clay lithologies) and 34°C (for sandy clay lithologies). This means that, with more porous and permeable lithologies and in particular with the presence of water circulation in sediments at same depths, T values strongly increase. Another important factor to consider is thermal conductivity, that notably increase from impermeable (e.g. clay) to permeable lithologies as seen in thermal conductivity (W/mK) vs lithologies tables. An important example should be the Fossombrone area4, where there are calcareous sediments at shallow depths to indicate, probably, higher T values than those seen in Jesi, Macerata and S.Costanzo areas (with clay and sandy clay lithologies) at the same depths, presumed on the base of the higher thermal conducivity values of the limestones in respect to clays.