Natural CO₂ fluids in Italy: implications for the leakage of geologically stored CO₂

A principle concern for engineered CO₂ storage is long-term security. Surface leakage (‘seepage’) of injected CO₂ to the surface is economically and environmentally undesirable. Italy is a region of intense natural CO₂ degassing; 308 CO₂ seeps are catalogued which exhibit different surface characteristics, and a number CO₂ rich reservoirs were discovered when drilling for hydrocarbons. These seeps and reservoirs provide excellent natural analogues for seeps that might arise from breached carbon stores. This thesis explores the geological controls on the crustal plumbing of CO₂ fluids to model the processes governing CO₂ seep locations and distribution, and characteristics; and their consequences on human health risk. Risk of human death from accidental CO₂ poisoning at all seep types is low (10-8 yr-1) and several factors influence risk of human mortality. Seeps distribute on two spatial scales; on a local scale (<5 km) seeps are clustered and aligned with subsidiary geologic structures, while on large scales seep clusters are discrete, and align with regional structures. Within clusters, seep locations are influenced by fault maturity, the presence of lithological boundaries and seep manifestation, which is determined by the flow properties of the outcropping lithology and local topography. Sealing and seeping CO₂ reservoirs are identified, and their geological characteristics compared. Italian reservoirs successfully retain large CO₂ columns at a range of reservoir conditions. Reservoirs which have hydrostatic pressure conditions in the overburden, determined from well logs, are located close to surface CO₂ seeps and recent extensional faults. Where there is significant overpressure above hydrostatic in the overburden, there are no seeps present above the reservoir structure. Overpressure of reservoir fluids may enhance fluid flow rates but is not a necessary condition for CO₂ leakage. Geothermal conditions influences the style of leakage at depths and towards the surface. Total CO₂ degassing from dry Italian CO₂ seeps is 3.5 ± 0.5 Mt(CO₂)yr-1. It would take thousands of years for the effectiveness of a commercial scale store to be significantly reduced if it leaks to form a single seep with the mean flux rates modeled in Italy. If a seep cluster develops, the storage effectiveness will reduce more rapidly, and could negate engineered CO₂ storage as a climate mitigation strategy. The research presented in this thesis contributes to a body of knowledge which directly informs site selection procedure for carbon storage and maximise the long term storage potential for CCS. Thorough scientific understanding of the geological processes governing fluid escape is crucial to assure the scientific, political and public communities that safe, long-term carbon storage can be realised as an effective climate mitigation technology.