Dielectric dispersion science
As multifrequency electromagnetic waves travel from the transmitter to receiver antennas on Dielectric Scanner service’s articulated pad, changes occur in amplitude and phase that are directly related to the dielectric permittivity and conductivity of the formation. Because there is a large contrast between the permittivity of water and that of oil and of rock, the tool makes a direct measurement of the water volume that is independent of the water salinity.
Depending on the texture of the rock, the permittivity and conductivity differ for different frequencies of the electromagnetic waves. This dielectric dispersion cannot be measured by conventional single-frequency electromagnetic tools. The log on the left, emulating a previous-generation single-frequency electromagnetic propagation tool, cannot account for textural variation and overestimates the invaded zone resistivity Rxo (Track 3). The right-hand log from Dielectric Scanner service analyzes the multiple frequencies to correctly match the water-filled porosity to the total porosity in the water-filled sand (Track 4), as confirmed by the matching Rxo values in Track 3.
The permittivity and conductivity measurements made at each frequency are interpreted using a petrophysical model. The output parameters of the model are water-filled porosity (hence water saturation if the total porosity is known), water salinity, and textural effects. In carbonate reservoirs, dielectric dispersion is related to the cementation and saturation factors that are represented by the exponents m and n in the Archie saturation equation. In shaly sands dielectric dispersion is related to the shale volume, represented by the cation exchange capacity.