A Novel Approach for Studying the CO₂-Rock Interactions and Estimating the In-Situ CO₂ Volume Using Multifrequency Dielectric Technique

Carbon dioxide (CO₂) is injected into underground reservoirs for several purposes, such as CO₂ sequestration or enhancing the oil recovery (EOR). Various techniques are implemented to investigate the interaction between the CO₂ and rock surfaces. In this work, we propose the use of a multifrequency dielectric approach to understand CO2-rock interactions and monitor CO₂ saturation. This study investigates CO2-rock interactions using carbonate and sandstone core samples. The cores were characterized by measuring porosity, permeability, and mineral composition. CO₂ flooding experiments were carried out under confining and back pressures of 2500 psi and 1500 psi, respectively. Dielectric measurements were performed on dry and CO2-saturated samples across a wide frequency range (1 MHz to 3 GHz). Low- frequency measurements (1-100 MHz) captured interfacial polarization and rock-fluid interactions, revealing the reactivity and surface complexity. High-frequency measurements highlighted dipolar polarization, enabling CO₂ saturation estimation. The output of dielectric measurements was used to determine the in-situ CO₂ saturation over time and distance from the injection point. The carbonate samples have an average porosity of 18% and the average sandstone porosity is 20%. The mineralogical composition of carbonate rocks shows around 97% calcite and a few percentages of Illite and Quartz. The sandstone samples composite of 62% Quartz, 23% Plagioclase, and traces of Illite, Ankerite, Chlorite, and Kaolinite minerals. The dielectric profiles carried out on the CO2/brine saturated cores exhibited a highly dispersive dielectric behavior, with relative permittivity decreasing from 45 at 1 MHz to 7.7 at 100 MHz. After 10 minutes, the dielectric constant in the carbonate sample decreased by a factor of 3.5 within the 1-10 MHz frequency range, indicating strong temporal changes likely due to fluid redistribution or chemical interactions. In contrast, the sandstone sample showed almost no change over the same frequency range, reflecting greater dielectric stability and lower reactivity. The change in dielectric constant at low frequency is consistent with the fact that carbonate is more reactive compared to sandstone rocks, hence, more dielectric changes are expected for the carbonate samples. The dielectric results were combined with the rock properties and core flooding outcomes to estimate the in situ volume of CO₂. This work presents for the first time the use of a multifrequency dielectric technique to monitor the CO₂- rock interaction during CO₂ sequestering in carbonate and sandstone formations. The permittivity and conductivity profiles were utilized to estimate the in situ volume of CO₂ trapped as a function of time and distance. Overall, the work findings contribute to a deeper understanding of CO₂ rock interactions and estimating the in- situ CO₂ volume, potentially leading to more effective strategies for CO₂ sequestration.