oa Electromagnetic Monitoring of CO2 Storage in Deep Saline Aquifers– Numerical Simulations and Laboratory Experiments

To guarantee the safety of civilization and environment the storage of CO2 has to be monitored efficiently and reliable. The knowledge of petrophysical parameters and their contrasts is crucial to a resilient monitoring. Stipulated by the pressure and temperature regime in deep sequestration formations the stored CO2 occurs in supercritical state (scCO2). The influence of scCO2 on the electrical conductivity of the formation is not sufficiently known. To predict the contrast in electrical conductivity, estimates based on empirical equations and numerical simulations were implemented, and laboratory experiments were carried out. Several submodels were linked resulting in a concept for application, allowing calibration by measured data. Two-phase flow governing the physical storage of CO2 was simulated using the software packages COMSOL Multiphysics and Mod2PhaseThermo. The resulting non-stationary spatial distributions of saturation were transformed into distributions of electrical conductivity using Archie's law (1942) and the law of Waxman & Smits (1968). An increase of electrical conductivity by a factor of 2 to 10 has been predicted. An experimental set-up was developed and constructed which allows the experimental simulation of the sequestration process on a laboratory scale. Central element of the set-up is a measuring cell inserted into an autoclave allowing to monitor the average electrical conductivity of a sand sample. As a first step it could be proved that pure CO2 as well as the CO2 rich binary mixture of CO2 and water do not show any relevant electrical conductivity when a pressure of up to 130 bar has been employed. Experiments with CO2 flowing through a water saturated sample replacing the pore water were carried out with pressures up to 130 bar and temperatures up to 40°C. An increase of electrical conductivity by a factor of 27 to 33 occured when a pressure range up to 50 bar has been considered, which is dominated by a residual water content of 14 to 18%. The increase in electrical conductivity induced by the sequestration has also been demonstrated under supercritical conditions. All experimental data could be interpreted using Archie's law with sufficient reliability.