Underground Energy Storage in Italy

Abstract: The decarbonization of energy systems requires storage solutions capable of compensating the seasonal mismatch between renewable electricity generation and demand. This issue is particularly relevant in mid-latitude industrial regions where renewables production is strongly seasonal and short-duration storage technologies are insufficient to ensure long-term energy balance. In this context, underground hydrogen storage (UHS) represents a promising option for large-scale and long-duration energy storage, provided that renewable energy requirements and geological storage suitability are assessed within a unified framework. This thesis investigates the feasibility of coupling photovoltaic electricity generation with UHS in northeastern Italy, using Friuli-Venezia Giulia (FVG) as a regional case study. The work integrates regional-scale energy modeling, geological screening, reservoir-caprock assessment, structural analysis, petrophysical characterization, fracture-network modeling, three-dimensional geological reconstruction, and laboratory experiments aimed at assessing the sensitivity of reservoir properties to reactive gas-rock interaction. A first-order energy balance model was developed to estimate the photovoltaic capacity required to replace fossil-fuel-based electricity production and imports, and to quantify the corresponding seasonal hydrogen storage demand. In parallel, potential subsurface storage sites in the Veneto-Friuli area were evaluated through a dedicated Underground Hydrogen Storage Suitability Index (UHSSI), used to rank candidate structures based on reservoir quality, caprock potential and site potential. The regional energy model indicates that replacing the fossil fuel used in the region would require the installation of a 6.8 GWp photovoltaic field and the underground storage of 9.13⋅10^8 Nm3 of hydrogen. Geological screening identifies the Arcade-Nervesa structure as the most promising candidate among the sites investigated. This structure includes a Cavanella Group reservoir overlain by the Tarzo Marls, which represents the main sealing interval. Three-dimensional structural modeling indicates a first-order equivalent hydrogen storage capacity of approximately 5.3⋅10^10 Nm3 at standard conditions, assuming a 30% cushion gas requirement. This value is about 58 times larger than the calculated regional hydrogen storage demand, indicating that the structure is volumetrically adequate at the screening scale. However, volumetric capacity alone is not sufficient to qualify a site for operational UHS. The Cavanella reservoir is heterogeneous, and fracture networks may enhance injectivity while also increasing uncertainty related to compartmentalization, preferential migration pathways, and containment security. Laboratory reactive-gas experiments indicate that exposure to CO2 under representative reservoir conditions can modify porosity and permeability, providing a conservative indication that gas-rock-brine interactions should be evaluated on a site-specific basis before operational UHS implementation. Overall, this thesis provides an integrated and quantitatively constrained framework for evaluating UHS as a seasonal energy storage strategy. The Arcade-Nervesa structure emerges as a geologically promising target, but further work is required before operational implementation, including dynamic reservoir simulations, caprock integrity testing, fault and fracture stability analysis, hydrogen-rock-brine experiments, microbiological assessment, and techno-economic evaluation.