Influence of CO2 on the electrical conductivity and streaming potential of carbonate rocks

Minimally intrusive geophysical methods are required to monitor CO 2 leakages from underground storage reservoirs. We investigate the impact of gaseous CO 2 on both electrical conductivity and electrokinetic properties of two limestones during their drainage. These data are contrasted with measurements performed on one clay-free sandstone. The initial NaCl brine concentrations before drainage (from 8.5 to 17.1 mMol/L) correspond to the limit between freshwater and slightly brackish water. These values are representative to saturated brine formations inside which CO 2 can be stored. Using these water salinities, the surface conductivity of the samples represents less than 5% of the overall electrical conductivity. A CO 2 release leads to an increase of the electrical conductivity of the rock during drainage in limestones and no change in sandstone. This increase in the electrical conductivity is due to the dissolution of calcite with the concomitant release of Ca 2+ and HCO 3 − in the pore water. It is not due to the CO 2 dissociation in the pore water in the pore pressure range 0-0.5 MPa and at a temperature of T = 20°C. The measurements of the streaming potential show a substantial decrease of the streaming potential coupling coefficient and zeta potential magnitudes after a CO 2 release in carbonates. This observation is explained by the increase of the ionic strength of the pore water in the course of the experiment. This change can be used, in turn, to determine calcite dissolution rates from the measurement of the electrokinetic properties.