The electrical permittivity (ε) of rock has been measured and applied in petrophysical evaluation for decades. With the new-generation tools the popularity and application of ε has increased in the past years. One of the advances of the new-generation tools is the measurement of ε at multiple frequencies (f), also known as the dispersive permittivity (ε (f)). Drawbacks with these tools are that they respond to the invaded zone and the data must be acquired on wire-line and therefore is not always accessible.
Logging While Drilling (LWD) propagation resistivity records the phase shift and attenuation between two receivers and responds differently to the electrical rock properties (resistivity, permittivity and permeability). The characteristics of the amplitude decay and the phase shift can therefore, under given conditions, be used to extract both the electrical permittivity and the resistivity from the measurements.
The goal with this paper is to show that electrical permittivity and its dispersion can be extracted from LWD resistivities. The work is motivated by the fact that information about ε and ε(f) is hidden in the LWD resistivities, so why not extract it and use it? The permittivity can be used in the petrophysical evaluation, it is acquired in real time and can be used to identify bypass zone, as geological marker and for geo-steering. The LWD resistivity accuracy will also be improved by replacing the empirical based assumption about ε with the more correct value on ε in the LWD processing. This improvement in accuracy of the LWD resistivity will be significant in rocks with large permittivity (e.g. organic-rich source rock).
This paper applies classical electromagnetic theory and shows how to extract permittivity from LWD resistivity and contain examples that illustrate results on data from different LWD tools. The examples show that the LWD permittivity and its dispersion fit very well with data from commercial wireline tools. Limitations of present techniques and further application of the LWD permittivity and dispersion will be discussed.