The impact of CO2 on the electrical properties of water bearing porous media - laboratory experiments with respect to carbon capture and storage

We conducted a detailed experimental investigation of the effect of CO2 injection on the electrical conductivity of water bearing porous media, needed for an improved geophysical monitoring of CO2 storage reservoirs. Therefore, we developed an experimental set-up that allows to investigate electrical characteristics of the injection process as well as the impact of dissolved CO2 on pore water conductivity. We found that a gaseous, fluid and supercritical pure CO2 phase bears no relevant conductivity at pressures up to 13 MPa and temperatures up to 50 degrees C. When CO2 dissolves in pore water, pressure-dependent dissociation processes can double the pore water conductivity, that can be used in leakage detection. This is quantified by an adaptation of Archie's law. The empirical adaptation and the experimental data are confirmed by combined geochemical-geoelectrical modelling. Furthermore, water-saturated sand samples were investigated while CO2 displaced the pore water at pressures up to 13 MPa and temperatures up to 40 degrees C. A decrease in electrical conductivity by a factor of up to 33 was measured, corresponding to a residual water saturation of 14-19%. Qualitatively, a decrease was also demonstrated under supercritical conditions. As an integrative interpretation, a conceptual model of electrical rock properties during CO2 sequestration is presented.