Induction tools are designed to measure formation conductivity.The measured in-phase voltage is linearlyproportional to the formation conductivity at moderateto high formation resistivities, and gradually becomesnon-linear due to skin effect as formation resistivitydecreases. The quadrature (out-of phase) component ofthe voltage is also measured on many modern inductioninstruments. It is often used to provide a skin-effect correctionto the R-signal.Induction tools operate at frequencies of a few tens ofkilohertz. At these frequencies, dielectric effects usuallycan be neglected. However, strange induction logs havebeen encountered over the past two decades with large,negative quadrature signals with a character that couldonly be explained by a high dielectric permittivity.The observed large dielectric permittivities show considerabledispersion (variation as a function of frequency).The dielectric polarization processes and time-delayeddissipation may be mathematically described by complex-valued permittivities and/or conductivities. At asingle operating frequency, such a generalization makesno physical sense. However, at two or more frequenciesan alternative complex parameterization may be morerealistic than the simple, conventional formulation, especiallyin shales where the movement of ions in an electricfield is the dominant effect.This permittivity effect led us to revisit basic inductionprocessing. A new inversion algorithm was developedthat simultaneously converts the induction in-phase andquadrature signals into dielectric permittivity and electricconductivity. Skin-effect correction is intrinsicallyincluded in this new algorithm. The processing algorithmrequires a stable and highly accurate quadrature signalfrom the induction tool, which limits the application tothe longer arrays of modern array induction tools.The observed elevated permittivities have been encounteredonly in a small number of shale regions. These regionsare usually surrounded by shales with negligiblepermittivities. The cause of the high permittivity hasbeen attributed to the presence of conductive minerals(pyrite or graphite) that build up as a result of kerogenformation and exposure to elevated temperature andpressure. To our surprise, some shales with unusuallylarge dielectric effects have been proven to be rich gasproducingzones.This observation led to exploring additional shales withhigh permittivities. However, so far this search has yieldedmixed results: some gas-producing shales have notshown such high permittivities while others have. Hencethe induction quadrature signal by itself will not conclusivelyidentify gas-producing shales; it merely may actas a first indication flag to encourage further log analysiswith complementary measurements.Core studies from several these different shales are currentlyunderway. As results are gathered, the chemistrysheds new light on the petrophysics of organic matterin the shales and their widely varying response to lowfrequencyelectromagnetic signals. The half-century-oldinduction technology continues to provide scientific andtechnologic challenges.
展开▼
