CO2 geological storage is considered ane af the aptions technolagically viable in arder to decrease the industrial emissians of this gas species that strongly contribute to the greenhause effect in the atmosphere (IPCC, 2005). As naturally, oíl and gas am stored in poraus rocks that have the same key geological features required far CO2 starage, so CCS aim to recreate a naturnl · process to trap carban diaxide far millians of years into deep saline aquifer ar depleted oil c..nd gas fields. Episodic release of CO2 fram undergraund can occur from either natural processes (i.e., mantle degassing, thermal decarbonation, volcanic areas) o·r industrial (geological storagé of CO2, CCS). Because of leakage can occur at every step of a ces project, monitoring assumes an importan! role not only during and after injection but also befare in arder to know the state of the entire site from deep underground (reservoir level) to surface. In this respect, the use of atmospheric dispersion numerical models helps predicting the dispersion of the CO2-enriched gas plume once emitted from underground and allows an accurate map of risk leve! through time under particular meteorological conditions. The aim of this PhD. project is to give a better understanding of eo2 behaviour in the unsaturated zone and its atmospheric dispersion. The results are obtained from experimental and numerical modelling of CO2 leakage from an artificial leakage site and from natural analogues applied to risk assessment for CCS sites. To reach the above-mentioned targets and to prove results, this research plan combines experimental activity on site and numerical modelling. The first part ofthe research plan consisted of a contribution in the PISCO2 project at the ES.CO2 facilities placed in Ponferrada (Spain) . The objective of this project was to predict how the injected CO2 will move laterally and vertically in the unsaturated zone and to determine the critica! parameters that will affect the ecosystems. This part has been published in a peer review journal and presentad in an international congress.
The second part of the PhD. project has focused on the study of two natural analogues in volcanic areas. Natural analogues provide evidences of the impact of CO2 leakage, for both sites two approaches have been followed: a) direct measurement of air concentration and b) numerical atmospheric modelling with the 1WODEE2 code. The study of eo2 emissions in natural systems provides a valuable information on the assessment and quantification of potential risks related to underground carbon storage leakage. Emissions of eo2 are studied in a iarge variety of geological environments, i.e., sedimentary basins, active and non-active volcanic areas, seismically-active regions, and geothermal fields. Because of the physics of carbon dioxide gas, e.g., colourless, odourless, higher density compared to air, its accumulation may re:oult hazardous and even lethal for life The objective of atmospheric dispersion modelling is to predict how will move the plume, generated by a leakage in natural degassing systems and ces sites Results obtained from this doctorate provide: •Caupling experimental and numerical data in arder to test new methods and!or nume,ical codes; •New elements to the knowledge of CO2 behaviours in the unsaturated zone as seen at PISCO2; •Nev,¡ clues on nan-invasive monitoring techniques far ces sites and na ural ana!ogues; •The usefulness of studying natural analogues; •The usefuiness of the SAP system, as a good method to evaluate high gas emissions from underground; •The efficiency of atmospheric modelling as a valuable methodology in the risk assessment; •The importance of risk assessment maps in active degassing areas; • The need of production of forecasting maps to evaluate dangerous scenarios . Numerical models (multiphase transport and atmospheric gas dispersion) are proved to be usefull tools to predict gas behaviour in the vadose zone and in the near-ground atm
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