Geochemical monitoring of CO2 and CH4 injection in a carbonate shallow aquifer

In order to ensure the safety of future CO2 or natural gas geological storage sites, the implementation of monitoring system is essential. The aquifers located near the storage areas are key places of study as they represent locations where gas may accumulate before leaking into the atmosphere. In this context, the Aquifer-CO2Leak project is dedicated to the development of monitoring tools and methodologies for CO2 and CH4 detection within the saturated zone, as well as understanding the behaviour of these gases in a carbonate aquifer. Based on numerical simulations results, an induced leakage experiment was designed and conducted on the experimental pilot site of Saint-Emilion in France. Water was saturated with a gas mixture of CO2, CH4, He and Kr in a specific designed tank, and was then injected in the aquifer through an injection well. Monitoring was conducted by collecting periodic samples through 3 observation wells located at a maximal distance of 10m. Analyses of concentration and 13C of dissolved inorganic carbon (DIC) and methane (CH4) were realized in laboratory. The experiment shows that 13C of DIC is a more sensitive tool than DIC concentrations to monitor CO2 plume distribution. These two parameters allow the determination of the physicochemical processes taking place in the aquifer which involve CO2. The CH4 content and isotopic measurements were more difficult to realise. At such low contents (< 5mol.L-1) several measurement methods have been tested and will be presented.