Natural CO₂ sites in Italy show importance of overburden geopressure, fractures and faults for CO₂ storage performance and risk management

The study of natural analogues can inform the long-term performance security of engineered CO₂ storage. There are natural CO₂ reservoirs and CO₂ seeps in Italy. Here, we study nine reservoirs and establish which are sealed or are leaking CO₂ to surface. Their characteristics are compared to elucidate which conditions control CO₂ leakage. All of the case studies would fail current CO₂ storage site selection criteria, though only two leak CO₂ to surface. The factors found to systematically affect seal performance are overburden geopressure and proximity to modern extensional faults. Amongst our case studies, the sealing reservoirs show elevated overburden geopressure whereas the leaking reservoirs don’t. Since the leaking reservoirs are located within <10 km of modern extensional faults, pressure equilibration within the overburden may be facilitated by enhanced crustal permeability related to faulting. Modelling of the properties that could enable the observed CO₂ leakage rates finds high permeability pathways (such as transmissive faults or fractures) become increasingly necessary to sustain leak rates as CO₂ density decreases during ascent to surface, regardless of the leakage mechanism into the overburden. This work illustrates the value of characterising the overburden geology during CO₂ storage site selection to inform screening criterion, risk assessment and monitoring strategy.