Physico-chemical processes and impacts of CO₂ leaks on near-surface hydrosystems during geological storage: Experimental in-situ and laboratory approaches

CO₂ is known to be one of the main greenhouse gases that actively contributes to global warming. In order to limit the impacts that its increases in the atmosphere cause, different methods are proposed such as carbon capture and geological storage. One of the main qualities of a storage site is that it has a minimum probability of leakage. Therefore, it is essential to develop monitoring tools and protocols in order to assess their importance and plan corrective actions. The main objective of this thesis is to study the physicochemical processes and the impacts of a CO₂ leak on near-surface hydrosystems (vadose zone (VZ) and aquifer) in the context of an Oligocene carbonate reservoir system, located in the Aquitaine basin. This study followed two main approaches: i) an experimental approach on the pilot site of Saint- Emilion (Gironde, France) where a CO₂ injection took place directly into the aquifer through a borehole; ii) a core-scale laboratory approach to study the interactions of the CO₂-H₂O-CaCO₃ system. This work also brings a new experimental approach by coupling geochemical and geophysical methods on the two scales of investigation. The baseline, carried out through the VZ the year before the CO₂ injection experiment, provided an understanding of the natural evolution of this gas during a hydrogeological cycle. Water transfers by diffusivity as well as aquifer recharge were also studied using geoelectric measurements in the field and laboratory. The transfer time turns out to be dependent on the physical properties of the medium (porosity, effective permeability and water saturation). In addition, over a year, the natural concentrations of major elements of VZ remain constant and the hydrosystem is in equilibrium with respect to calcite. Following the leakage caused by CO₂ in the aquifer and according to our results, certain physiochemical parameters (such as electrical conductivity, pH and concentrations of Ca²⁺, HCO₃^- and CO₂ (aq)) turned out to be good sufficiently sensitive and reactive indicators that can be used on a geological CO₂ storage site in order to monitor any physicochemical disturbance. Numerical simulations made it possible to better understand the reaction and transport processes taking place at the scale of the pilot site. Hydrodynamic dispersion turns out to be the main parameter controlling the transport of major elements by mechanical dispersion and molecular diffusion through the saturated zone. Core-scale experiments have observed and quantified the influence of CO₂ effects on the electrical behavior of carbonate rock. The results obtained allow a better understanding of flow processes and in-situ parameters (capillary pressure, porosity, etc.) controlling the propagation of a gas plume through a carbonate aquifer. The factors influencing the geochemical response of groundwater to a CO₂ leak are also studied. Thus, a better definition of the relationship between CO₂ concentrations, electrical conductivity and pH is provided, making it possible to pass from the core scale to the pilot scale and vice versa. All the results presented in this thesis work highlight the importance of characterizing the petrophysical heterogeneity of the carbonate reservoir. It is also essential to establish a baseline of the saturated zone and across the VZ, allowing natural variations in CO₂ to be distinguished from those induced by a leak. Combining field observations and laboratory demonstrations increases the chances of detecting a potential CO₂ leak at a geological storage site. (author)