Impedance Spectroscopy of Petroleum Fluids at Low Frequency Lamia Goual* Department of Chemical and Petroleum Engineering, UniVersity of Wyoming, 1000 East UniVersity AVenue, Laramie, Wyoming 82071 ReceiVed October 7, 2008. ReVised Manuscript ReceiVed January 19, 2009 Impedance spectroscopy enables fast and nondestructive dielectric characterization of petroleum fluids during different stages of exploration and processing. The majority of existing studies focused on frequencies above 10 3 Hz. The advantage of considering smaller frequencies is that more information on the electrical conductivity of the sample can be extracted. In this work, dielectric relaxation measurements on Boscan asphaltenes and maltenes are performed by impedance spectroscopy in the low-frequency range of 40-10 6 Hz. The data are analyzed within the formalism of complex impedance. As frequency increases, the system shifts from highly conducting below ∼10 Hz to highly insulating above 10 5 Hz. A comparison of impedance with electric modulus and permittivity suggests that only one primary relaxation mechanism exists between 10 and 10 5 Hz because of conductance effects. The alternating-current (AC) conductivity is almost independent of frequency below 10 5 Hz and increases with increasing frequency above this range. Extrapolation of AC conductivity to zero frequency provides an estimate of direct-current (DC) conductivity, from which the molecular size of asphaltenes can be determined. In addition, the onset of asphaltene aggregation can be detected from the variations of DC conductivity with asphaltene concentration in toluene. Thus, low-frequency dielectric relaxation is a simple and powerful tool to predict asphaltene-associated problems in the field. 1. Introduction Dielectric relaxation techniques have been widely employed to investigate relaxation processes in petroleum fluids and the dependence of their characteristics on asphaltene aggregation. Impedance spectroscopy is particularly attractive because it enables fast and nondestructive dielectric characterization of petroleum fluids during different stages of exploration and processing. 1,2 Dependent upon the frequency range, the complex electrical behavior of asphaltenes may result from their dipolar response, their conductive response, or both. The dipolar component is usually associated with the orientation of perma- nent dipoles 1,3,4 and can be described by the Cole-Cole formalism, 5 whereas the conductive component reflects the translation of charges among the π orbitals of polynuclear aromatics. 6,7 Previous dielectric relaxation studies on various crude oils showed a dipolar relaxation in the megahertz region and a low- frequency conductivity. 1,2 The dipolar relaxation of asphaltenes in toluene depends upon their aggregation state. In addition to a dipolar relaxation observed in the megahertz region, Sheu and co-workers 8 recorded a second peak in the dielectric loss spectra of asphaltene aggregates at 10 5 Hz, possibly a result of caging effects in large clusters, which restricted their rotational dynam- ics. The same authors claimed that, in such systems, the measured dipoles are largely induced dipoles and that the electrical conductivity may originate from rapid charge exchange between asphaltene aggregates. This mechanism is also known as Maxwell-Wagner interfacial polarization. 9 Syunyaev et al. 3 reported the existence of three relaxation peaks in the dielectric loss spectra of pyrolysis tar in benzene using time-domain dielectric spectroscopy. The peaks were observed in the frequency range of 0.2-20 × 10 6 Hz (first band), 20-250 × 10 6 Hz (second band), and 250-2500 × 10 6 Hz (third band). The relaxation processes were attributed to dipole-orientation polarization and described by the Cole-Cole formalism. Using the same technique on a vacuum residue in toluene, Evdokimov and Eliseev 4 recorded two peaks in the dielectric loss spectra at 80 × 10 6 and 220 × 10 6 Hz, respectively. The first peak was related to the relaxation of large particles and complexes, while the second peak was due to the relaxation of macromolecular asphaltene and resin components. Almost all previous dielectric relaxation studies were per- formed at frequencies above 10 3 Hz to avoid electrode polariza- tion effects. However, these effects can be minimized with the advent of new technology and proper calibration procedures. To our best knowledge, no dielectric relaxation spectra of petroleum fluids are currently available below 10 3 Hz. The * To whom correspondence should be addressed. Telephone: 307-766- 3278. E-mail: lgoual@uwyo.edu. (1) Tjomsland, T.; Hilland, J.; Christy, A. A.; Sjoblom, J.; Riis, M.; Friiso, T.; Folgero, K. Comparison of infrared and impedance spectra of petroleum fractions. Fuel 1996, 75 (3), 322–332. (2) Folgero, K. Broad-band dielectric spectroscopy of low-permittivity fluids using one measurement cell. IEEE Trans. Instrum. Meas. 1998, 47 (4), 881–855. (3) Syunyaev, R. Z.; Kaprov, S. A.; Kaprova, V. V. Study of the disperse structure of petroleum by time-domain dielectric spectroscopy. Chem. Technol. Fuels Oils 2001, 37 (2), 131–133. (4) Evdokimov, I. N.; Eliseev, N. Y. Electrophysical properties of liquid hydrocarbon media. Chem. Technol. Fuels Oils 2001, 37 (1), 39–43. (5) Cole, K. S.; Cole, R. H. Dispersion and absorption in dielectrics. J. Appl. Phys. 1941, 9, 341–351. (6) Maruska, H. P.; Forster, E. O.; Enard, J. H. Electrical transport processes in heavy hydrocarbon fluids. IEEE Trans. Dielectr. Electr. Insul. 1985, 20 (6), 947–955. (7) Siffert, B.; Kuczinski, J.; Papirer, E. Relationship between electrical charge and flocculation of heavy oil distillation residues in organic medium. J. Colloid Interface Sci. 1990, 135 (1), 107–117. (8) Sheu, E. Y.; Storm, D. A.; Shields, M. B. Dielectric response of asphaltenes in solvent. Energy Fuels 1994, 8, 552–556. (9) Maxwell, J. C. A Treatise on Electricity and Magnetism; Courier Dover Publications: New York, 1954; Vol. 1. Energy & Fuels 2009, 23, 2090–2094 2090 10.1021/ef800860x CCC: $40.75  2009 American Chemical Society Published on Web 02/19/2009