Time lapse geoelectrical and geochemical monitoring of CO2 migration in a carbonate vadose zone - lessons learned from the CO2-Vadose and DEMO-CO2 projects (Saint Emilion, France).

The surface and subsurface monitoring of underground CO2 leakage is a complex task because of the natural heterogeneities in the distribution of soil and substrate properties such as porosity, permeability, mineralogy, water saturation and organic content; together with diurnal and seasonal weather condition variations. The different potential migration pathways and mechanisms, and the local sources and sinks of free CO2 render the establishment of mass balance at the natural experiment scale very challenging; precluding robust impact assessment of CO2 leakage on the environment to be made. The CO2-Vadose and DEMO-CO2 projects aimed at building knowledge and methodologies for apprehending CO2 leakage in the context of a carbonate vadose zone. More than 6 years of experiments and technical developments have led to a refined understanding of the CO2 migration throughout a high porosity-permeability limestone formation, through the use of time-lapse geoelectric monitoring and geochemical tracers. Based on numerical simulations of fluid flow in porous media, a monitoring set up was designed with optimized probe placement and sampling calendars. CO2+tracer gas was injected in different locations of the experimental site, and gas compositions were monitored and the data interpreted in a common 4D reference frame. The comparison of the results from numerical simulations and leakage experiments allow to track advection and diffusion driven migrations, and preferential migration pathways in response to underground petrophysical heterogeneities. Building on the expertise gained from the development of the experimental site, a prototype of automated gas geochemistry monitoring was designed, built and put into operation. This presentation will guide you through the scientific and technical achievements of several years of experimental work in the underground quarries of the St Emilion village.