Intermediate-high frequency dielectric permittivity of oil-wet rock and its wettability characterization

Wettability has complex effects on the physical properties of reservoir rocks. The wettability of rocks should be characterized accurately to explore and develop oil and gas. Researchers have studied the rock wettability by dielectric spectra which contained abundant information. To study the rock wettability from dielectric dispersion, four rock samples with different wettabilities were used to design an experimental measurement flow. The relative dielectric permittivity in the frequency range of 100 Hz–10 MHz and nuclear magnetic resonance T2 spectra of the samples were obtained. Subsequently, the wettabilities of the rocks were verified by the T2 spectra. The dielectric dispersions of the samples under different conditions were analyzed. Furthermore, the simulated-annealing (SA) algorithm was used to invert the wettability and related parameters of the rocks by a dielectric dispersion model. The results indicated that the dielectric permittivity of lipophilic rocks is lower than that of hydrophilic rocks, and that the dielectric permittivity of hydrophilic rocks decreases faster as the frequency increases. The dielectric permittivity in the high-frequency band is associated with the water content. The rock wettability parameters obtained via inversion agreed well with the T2 spectra, and the saturation index of the rocks. The errors between the rock permittivity calculated by the inverted parameters and the experimentally measured values were minor, indicating that rock wettability could be accurately characterized using dielectric dispersion data.