Seismic and Electromagnetic Tomography to Image the CO2 Plume in a Cross-well Synthetic Experiment

CO2 storage in geological structures represents an immediate solution to reduce the greenhouse effect caused by the emission of polluting gases. To this purpose, the best conditions can be found in depleted oil and natural gas fields, methane-bearing coal beds and saline aquifers. In any case it is necessary, during and after the injection, to monitor the effective CO2 distribution in the subsoil, to detect any possible migration outside the reservoir. In this work we simulated cross-well investigations in a realistic saline aquifer model to determine the sensitivity of the joint seismic-electromagnetic (EM) method for monitoring the CO2 plume in the reservoir. To this aim, we performed numerical modelling of seismic and EM wave propagation in the geological formations to simulate a realistic survey, and used traveltime and attenuation tomography as a monitoring tool of the study area. The combined use of these methods can give more reliable results, especially when the interpretation is based on suitable cross-property relations between seismic velocity and conductivity. In particular, the electrical conductivity of reservoir rocks is highly sensitive to changes in brine and CO2 saturation. The sensitivity is lower for P-wave velocities and quality factors, while the S-wave velocity remains nearly constant.