Résumé
Geological storage of CO2 is still a recent technology and many questions remain open, particularly for saline formations. Geological storage in accessible saline formations is, in fact, expected to become over time more important than that in depleted hydrocarbon reservoirs. The Maguelone shallow experimental site, located near Montpellier (Languedoc, France) has been used over the past few years to perform CO2 injection experiments. The geology, petrophysics and hydrology of this site are well known from previous studies. The presence of small saline coastal reservoirs bounded above and below by clay-rich layers provides an opportunity to study a saline formation for geological storage at field laboratory scale with a set of hydrogeophysical (seismic, electrical, sonic, pressure) and geochemical (pH, minor and major ion concentrations) methods, either downhole or at surface. Series of experiments can be run at moderate costs from the shallow depth of one of these reservoirs (13-16 m), offering flexibility for testing different monitoring configurations, performing repeated injection releases with variable injection parameters and type of gas (e.g., N2, CO2), and cross-calibrating the monitoring methods. Moreover, additional methods/boreholes can be easily implemented at this experimental site. Three N2 injections were thus undertaken at Maguelone in 2012 to measure the site response to neutral gas injection. An experiment involving the release of CO2 was successively conducted in January 2013. A volume of 111 m3 of CO2 was injected during 3.5 hours. Both the N2 and CO2 gas plumes were detected by all monitoring techniques, and the response to gas propagation was instantaneous. Integrating the lesson learned from past injection experiments, the next stage of the project will allow to establish the best guidelines for CO2 injection and post-injection monitoring and, in perspective, not only to detect the CO2 plume but to quantify CO2 migration in the subsurface.