Induction tools are designed to measure formation conductivity. The measured in-phase voltage is linearly proportional to the formation conductivity at moderate to high formation resistivities, and gradually becomes non-linear due to skin effect as formation resistivity decreases. The quadrature (out-of phase) component of the voltage is also measured on many modern induction instruments. It is often used to provide a skin-effect correction to the R-signal. Induction tools operate at frequencies of a few tens of kilohertz. At these frequencies, dielectric effects usually can be neglected. However, strange induction logs have been encountered over the past two decades with large, negative quadrature signals with a character that could only be explained by a high dielectric permittivity. The observed large dielectric permittivities show considerable dispersion (variation as a function of frequency). The dielectric polarization processes and time-delayed dissipation may be mathematically described by complex- valued permittivities and/or conductivities. At a single operating frequency, such a generalization makes no physical sense. However, at two or more frequencies an alternative complex parameterization may be more realistic than the simple, conventional formulation, especially in shales where the movement of ions in an electric field is the dominant effect. This permittivity effect led us to revisit basic induction processing. A new inversion algorithm was developed that simultaneously converts the induction in-phase and quadrature signals into dielectric permittivity and electric conductivity. Skin-effect correction is intrinsically included in this new algorithm. The processing algorithm requires a stable and highly accurate quadrature signal from the induction tool, which limits the application to the longer arrays of modern array induction tools. The observed elevated permittivities have been encountered only in a small number of shale regions. These regions are usually surrounded by shales with negligible permittivities. The cause of the high permittivity has been attributed to the presence of conductive minerals (pyrite or graphite) that build up as a result of kerogen formation and exposure to elevated temperature and pressure. To our surprise, some shales with unusually large dielectric effects have been proven to be rich gasproducing zones. This observation led to exploring additional shales with high permittivities. However, so far this search has yielded mixed results: some gas-producing shales have not shown such high permittivities while others have. Hence the induction quadrature signal by itself will not conclusively identify gas-producing shales; it merely may act as a first indication flag to encourage further log analysis with complementary measurements. Core studies from several these different shales are currently underway. As results are gathered, the chemistry sheds new light on the petrophysics of organic matter in the shales and their widely varying response to lowfrequency electromagnetic signals. The half-century-old induction technology continues to provide scientific and technologic challenges.
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