Seismic and geoelectric modeling studies of parameters controlling CO2 geostorage in saline formations

Abstract

A possible CO2 geostorage in deep saline formation demands the development of a monitoring strategy using techniques of surface seismic reflection and electrical resistivity tomography (ERT) in boreholes. For this purpose, the resolution and sensitivity of both techniques are studied numerically regarding changes induced by CO2 sequestration.We calculated the effect of partially replacing brine with supercritical CO2 on seismic velocities and densities by using the Gassmann equation. Resulting seismic responses and reflection amplitude changes are quite strong in some cases. For thin CO2 layer (thickness<wavelength) the CO2 anomaly can be delimited laterally quite easier than vertically due to possible reflection overlap from the upper and lower CO2 boundaries.To maximize the ERT resolution, we applied the optimized and tripotential electrode configurations with current flows and potential measurements in different spatial orientations. We performed several comparative 2.5D modelling studies assuming different CO2 saturations, plume scenarios, burial depths, electrode configurations and aspect ratios. The results generally reveal the capability of the techniques to map the storage targets (CO2 plumes, saline host formation and impermeable caprock) with different degrees of resolution, smearing and artefacts. The results show the resolution superiority of the optimized arrays over the other arrays.The highest sensitivity to CO2 saturation changes is found for low saturation case in the seismic and for intermediate and high saturation in the geoelectric case. Therefore, a combination of both methods may be necessary for sn accurate determination of CO2 saturation in situ.A supercritical CO2 plume can be resolved as long as its dimensions are larger than the applied electrode spacing and seismic wavelength, respectively. Thinner layers are problematic, particularly in the ERT technique due to its smearing nature. In the North German Basin strongly the salinity increases strongly with depth, and thus causes a dramatical decrease in the resistivity contrasts and anomalies. The depth effect on the seismic resolution is two-fold: on one hand, the contrast in velocity and density between brine and CO2 increases with depth and enhances the reflection strength accordingly; on the other hand attenuation leads to an increase of wavelength causing a resolution decline.The consideration of seismic results in the inversion of ERT data strongly improves the resolution of the resulting tomograms and the CO2 estimation. It also enables reasonable monitoring (even at aspect ratio<1) of CO2 migrations in large areas and thus reduces the costs of the expensive monitoring wells.