A combined seismic and geoelectrical monitoring approach for CO2 storage using a synthetic field site

This paper presents an approach to assess the methods of 2D elastic seismic full waveform inversion (FWI) and electrical resistivity tomography (ERT) for monitoring storage of CO2 in deep saline formations. As at a real site the subsurface is not fully known, this approach uses a synthetic field site. Densities and saturations are obtained from a numerical simulation of the injection process and are introduced into geophysical forward models which simulate the geophysical data acquisition from the surface (FWI) and boreholes (ERT). These synthetic geophysical data are then evaluated with respect to changes in CO2 saturation and are compared to the fully known CO2 saturation of the numerical process model to verify the geophysical monitoring methods. Results show that both methods are capable of detecting a thin CO2 phase body in about 2,200 m depth for a synthetic site in the North German Basin. Inverted CO2 saturations are in good agreement, however, both methods cannot resolve the CO2 phase edges of less than 4 m saturated thickness. The maximum error of estimated CO2 saturation is 10 % for the FWI method and 15 % for the ERT method, if accurate baseline models are available. The FWI method sensitivity on the baseline model is tested by a sensitivity analysis demonstrating high sensitivity on bulk density, but low sensitivity on fluid densities and porosity. ERT results are considerably improved using structural information from the FWI as constraint.