This paper examines the potential of Italian depleted gas reservoirs, particularly in the offshore Adriatic Sea region, for CO2 geological sequestration. A pairing publication will explore the option of converting these reservoirs into hydrogen storage sites. The relevant field data were taken from publicly available sources and categorized into main scenarios. Sensitivity analyses were performed on a set of simplified yet representative cases, varying the reservoir depth and volume, as well as the size of the confining aquifer. A 3D geological (static) model was set up for each scenario: the structural trap was assumed to be an anticline, and the reservoir made of turbiditic deposits. The geological models served as the input for 3D coupled geochemical-fluid flow (dynamic) numerical models, used for the simulation of the conversion of depleted gas reservoirs into CO2 storage facilities. The mineralogic composition of the reservoir rock and the chemical composition and pH of the formation water were taken from the literature. After reproducing the natural gas production phase, CO2 injection was simulated. Storage capacity and injectivity of the reservoirs, as well as the CO2 geochemical interactions over time, were assessed to identify the most promising reservoir characteristics. The storage safety issues, including the geomechanical response induced by underground CO2 injection, were not addressed as they were beyond the scope of the work. 

Keywords: CO2 geological sequestration, underground fluid storage, coupled reservoir simulation, trapping mechanisms, geochemical reactions.