Electrochemical Impedance Spectroscopy for the Dielectric Characterization of Crude Oils and Their Maltene Fractions

The dielectric characterization of crude oils and their fractions, particularly asphaltenes and maltenes (saturated and aromatic hydrocarbons and resins) are essential for understanding critical phenomena in the petroleum industry, such as oil aggregation, colloidal stability, and electrical conductivity. This work advances the dielectric characterization of crude oils and their fractions using electrochemical impedance spectroscopy over a wide frequency range, combining equivalent-circuit analysis with rigorous model validation. Measurements were performed on two crude oil samples (A and M) from the same source rock but from different reservoir rocks and their corresponding maltene fractions at 25 ± 0.1 °C using an electrochemical cell with carbon-steel electrodes separated by a 5 mm gap. Impedance spectra were acquired over the frequency range of 10 kHz to 10 mHz with a logarithmic distribution of 10 points per decade using an AC perturbation amplitude of 200 mV. Nyquist diagrams exhibit single semicircular arcs for all samples, a characteristic feature of highly resistive dielectric systems. Quantitative analysis through equivalent circuits revealed a clear resistivity hierarchy: Rct (A Oil) < Rct (A Maltene) < Rct (M Oil) < Rct (M Maltene), indicating that the presence of resins in the M Maltene and asphaltenes + resins in the M Oil strongly influences dielectric behavior. Samples from M Oil exhibit significantly higher charge-transfer resistance, suggesting that M Oil is denser and more concentrated in polar organic compounds (resins and asphaltenes), making it structurally more complex. Asphaltene removal results in a substantial increase in the insulating character of the maltene matrix, demonstrating that asphaltenes act as charge-active centers, facilitating conduction through mechanisms such as charge hopping. The results reinforce the importance of asphaltenes in modulating charge-transport properties and dielectric stability in petroleum systems, with significant implications for industrial applications related to stability, aggregation, and transport of oils.