Spectral induced polarization of the three-phase system CO2 - brine - sand under reservoir conditions

The spectral complex conductivity of a water-bearing sand during interaction with carbon dioxide (CO2) is influenced by multiple, simultaneous processes. These processes include partial saturation due to the replacement of conductive pore water with CO2 and chemical interaction of the reactive CO2 with the bulk fluid and the grain-water interface. We present a laboratory study on the spectral induced polarization of water-bearing sands during exposure to and flow-through by CO2. Conductivity spectra were measured successfully at pressures up to 30 MPa and 80 degrees C during active flow and at steady-state conditions concentrating on the frequency range between 0.0014 and 100 Hz. The frequency range between 0.1 and 100 Hz turned out to be most indicative for potential monitoring applications. The presented data show that the impact of CO2 on the electrolytic conductivity may be covered by a model for pore-water conductivity, which depends on salinity, pressure and temperature and has been derived from earlier investigations of the pore-water phase. The new data covering the three-phase system CO2-brine-sand further show that chemical interaction causes a reduction of surface conductivity by almost 20 per cent, which could be related to the low pH-value in the acidic environment due to CO2 dissolution and the dissociation of carbonic acid. The quantification of the total CO2 effect may be used as a correction during monitoring of a sequestration in terms of saturation. We show that this leads to a correct reconstruction of fluid saturation from electrical measurements. In addition, an indicator for changes of the inner surface area, which is related to mineral dissolution or precipitation processes, can be computed from the imaginary part of conductivity. The low frequency range between 0.0014 and 0.1 Hz shows additional characteristics, which deviate from the behaviour at higher frequencies. A Debye decomposition approach is applied to isolate the feature dominating the data at low frequencies. We conclude from our study that electrical conductivity is not only a highly sensitive indicator for CO2 saturation in pore space. When it is measured in its full spectral and complex form it contains additional information on the chemical state of the system, which holds the potential of getting access to both saturation and interface properties with one monitoring method.