Frequency-Dependent and Anisotropic Dielectric Properties of Fractured Rocks During CO 2 -Replacing Brine Based on Numerical Simulation

Dielectric properties with high sensitivity to the types and saturations of fluids are important means to assess the quality and leakage risk of carbon dioxide (CO2) sequestration in reservoirs. However, the understanding of frequency-dependent and anisotropic dielectric responses in fractured rocks during CO2-replacing brine, which is the key to accurately inverse the dielectric survey data from fractured saline aquifers during CO2 sequestration, is still missing. In this study, we investigate, via dedicated numerical simulations, for the first time, the frequency-dependent dielectric properties (electrical conductivity, relative permittivity, and dielectric loss factor) of a fractured rock during CO2-replacing brine in two directions: vertical and parallel to CO2 migration. The results show that, with increasing frequency , conductivities and relative permittivities increase and decrease, respectively, while the variations in loss factors present a bell-shaped trend. Enhancing the CO2 saturation is shown to reduce these dielectric properties and their dispersion amplitudes. More importantly, the reducing trend of the three properties with enhancing CO2 saturation occurs the maximum gradient at various frequencies. Analyses and discussion of the simulation results suggest that the frequency-dependent dielectric properties in the direction orthogonal to CO2 migration are more stable to characterize CO2 sequestration, and the three properties should be respectively chosen as the most reliable detecting parameters according to the operating frequency of dielectric tools with single frequency or narrow band. The results not only provide new insights into the frequency-dependent and anisotropic dielectric behaviors in fractured rocks during CO2 migration but also are helpful in assessing CO2 sequestration in fractured saline layers.