The CO2-Vadose project: Time-lapse geoelectrical monitoring during CO2 diffusion in the carbonate vadose zone

Experimental observations of electrical resistivity variations were carried out in a field site (CO2-Vadose project) in order to investigate the evolution of this geophysical property in response to possible leakages of geologically sequestered CO2. The dynamic evolution of electrical resistivity was measured using time-lapse electrical resistivity tomography during a sub-surface leakage of CO2 released under controlled conditions in a cavity of about 9 m3 located in an abandoned limestone quarry. The experimental CO2 induced leakage was performed to study the gas migration in a carbonate vadose zone. Initial electrical resistivity tomographies were carried out before the gas release in order to have reference values and to characterise the heterogeneity of the limestone massif around the injection room. Significant relative variations of resistivity were found in correlation with the CO2 concentrations, which allowed not only to detect a leakage of CO2 on free surfaces of the massif but also to follow the propagation of the CO2 plume inside the limestone massif. The limestone resistivity decrease observed in this study is probably caused by CO2 dissolution in the pore-water. The feasibility of this explanation was confirmed by laboratory measurements. Relative variations of resistivity and CO2 concentrations were exponentially correlated.