ERT monitoring of gas injection into water saturated sands: Modelling and inversion of cross-hole laboratory data

A set of laboratory experiments were performed in order to investigate the capacity of the electrical resistivity tomography method to detect and monitor the circulation of gaseous CO2 into geological media. The experiments consisted of injecting reactive (CO2) or inert (N2) gases into reactive (carbonate) or inert (silica) sands saturated with water. The laboratory setup was a metric-scale cylindrical tank instrumented with electrodes placed on vertical rods, hence simulating a borehole to borehole monitoring network. The results demonstrated that the ERT method can efficiently detect, locate and image gas circulation plumes. Two main behaviors were identified: either an increase of the medium resistivity due to the invasion of the porous space by an electrically insulating gas phase, or a resistivity decrease induced by dissolution processes that enhance ionic concentrations in the saturating water. The observed anomalies showed varying geometric shapes depending on which type of sand was used. A 3D cellular automaton model was built and managed to reproduce different gas plume circulation patterns and the electrical resistivity response associated. This modeling work confirmed the link existing between the shapes of the resistivity anomalies, the gas plume geometry and the particle size distribution of the sand. Through a numerical sensitivity analysis, several measuring protocols were tested and developed in order to improve the imaging capacity of the electrical resistivity method. Results demonstrated that the most efficient protocol was different depending on the shape of the gas plume, thus stressing the importance of having a priori knowledge on the geological, lithological and hydraulic characteristics of the subsurface, in order to correctly assess the most likely gas plume geometry and hence design the most adapted monitoring protocol.