Study of the Low-Frequency Dispersion of Permittivity and Resistivity in Tight Rocks

Abstract

Obviously, in the low-frequency domain, the dielectric permittivity dispersion bears abundant information about the matrix, shale, fluid and pores, and even the geometry of the pores, their distribution, and the connectivity of the pore throat. For the case of tight reservoirs, a low frequency should be more useful to disclose the mysteries hidden in the rocks. This paper presented an experiment on tight limestone from Daanzhai Group, Jurassic formation, Sichuan Basin, and discussed the results in detail. In this paper, we mainly focused on the characteristics of the permittivity dispersion and its relationships with AbstractThis study investigated the frequency dispersion of the resistivity and permittivity in tight rocks of the Daanzhai Group, Jurassic formation, Sichuan Basin. Tight limestone cores were used in the experiment, and the frequency varied from 0.1Hz to 1kHzunder laboratory conditions. The permittivity and resistivity decrease as the frequency increases, but the changing rates of these two parameters are different, as the permittivity exhibits a more obvious dispersion than the resistivity at both low and high frequency. Over the whole frequency range, the change rate of the resistivity is very small and can even be neglected. The change rate of the permittivity, however, is very significant, and its difference reached several orders of magnitude. Besides this phenomenon, the frequency dispersion degree increases as the water saturation and concentration increase. Therefore, we establish a group of equations to describe this relationship between the frequency dispersion and water saturation, and water concentration. Unlike Archie's equation, which need multiple experimental parameters, the new formula can directly calculate the saturation from R p and C p or from D R and D C , only requiring two variables. Furthermore, frequency scanning test still implicates abundant information, which perhaps characterizes the pore structure, porosity, permeability and other properties of rocks or formations.Keywords: litho-electrical experiment, resistivity, permittivity, frequency dispersion