Characterisation, quantification and modelling of CO2 transport and interactions in a carbonate vadose zone: application to a CO2 diffusive leakage in a geological sequestration context

Global warming is related to atmospheric greenhouse gas concentration increase and especially anthropogenic CO2 emissions. Geologic sequestration has the potential capacity and the longevity to significantly diminish anthropogenic CO2 emissions. This sequestration in deep geological formation induces leakage risks from the geological reservoir. Several leakage scenarios have been imagined. Since it could continue for a long period, inducing environmental issues and risks for human, the scenario of a diffusive leakage is the most worrying. Thus, monitoring tools and protocols are needed to set up a near-surface monitoring plan. The present thesis deals with this problematic. The aims are the characterisation, the quantification and the modelling of transport and interactions of CO2 in a carbonate unsaturated zone. This was achieved following an experimental approach on a natural pilot site in Saint-Emilion (Gironde, France), where diffusive gas leakage experiments were set up in a carbonate unsaturated zone. Different aspects were investigated during the study: natural pilot site description and instrumentation; the physical and chemical characterisation of carbonate reservoir heterogeneity; the natural functioning of the carbonate unsaturated zone and especially the set-up of a CO2 concentrations baseline; the characterisation of gas plume extension following induced diffusive leakage in the carbonate unsaturated zone and the study of gas-water-rock interactions during a CO2 diffusive leakage in a carbonate unsaturated zone through numerical simulations. The results show the importance of the carbonate reservoir heterogeneity characterisation as well as the sampling and analysing methods for the different phases. The baseline set-up is of main interest since it allows discrimination between the induced and the natural CO2 concentrations variations. The transfer of CO2 in a carbonate unsaturated zone is varying in function of physical and chemical properties. This transfer is done by diffusion and/or advection. Because the detection of the noble gases allows the prediction of CO2 plume arrival, the use of tracers in the sequestration site is of main importance. The chemical interactions have to be taken under account in transport models in order to predict delay factors and the impact of a CO2 leakage in a carbonate unsaturated zone. (author)