DIELECTRIC EFFECTS IN PYRITE-RICH CLAYS ON MULTIFREQUENCY INDUCTION LOGS AND EQUIVALENT LABORATORY CORE MEASUREMENTS

摘要

In the last decade, shale gas has become a leading domestic energy source in the United States. New formation-evaluation measurements and interpretation methods have been developed for accurate characterization of such unconventional reservoirs. Here, we propose an interpretation method for induction logs in shale formations that accounts for the effects of clay minerals, clay-sized particles, and conductive minerals on the conductivity and permittivity estimates. Modern induction tools measure the in-phase and quadrature signals with equal accuracy. The quadrature signal is expected to be small and positive, providing information for skin-effect corrections in conductive media. However, several downhole induction measurements show large, negative quadrature signals in certain shale formations. Based on a new dielectric inversion of such logs, these shale formations exhibit high dielectric-permittivity values and frequency dispersion of electrical-conductivity and dielectric-permittivity estimates.A hypothesis attributed the large dielectric effects to the presence of dispersed pyrite nodules, which are generated together with kerogen production, and large volume fraction of dispersed clay-sized grains and clay particles in shales. Dispersed clay-sized grains and clay particles exhibit surface conductance that enhances conductivity and gives rise to interfacial polarization (IP) phenomena, commonly referred as membrane polarization, in the presence of an external electric field. On the other hand, dispersed conductive minerals possess highly mobile charge carriers (holes and electrons) and are generally surrounded by ionic charge carriers that together lead to metallic IP phenomena, also known as electrode polarization, in an external electric field. These dispersed inclusions reradiate as electric dipoles, generating a large polarization signature, which manifests itself as very large and dispersive effective dielectric permittivity. Further, these IP phenomena significantly influence charge-carrier migration processes that cause conductivity dispersion in such geomaterials.The IP effects of dispersed conductive minerals were tested in controlled laboratory experiments on glass-bead packs containing known volume fractions of dispersed pyrite and graphite inclusions. These laboratory experiments used a multifrequency, triaxial electromagnetic (EM) induction cell that was developed for measuring the complex-conductivity tensor of full-diameter cores. The experimental results supported the hypothesis of large dielectric permittivity and frequency dispersion of EM properties for mixtures containing dispersed conductive mineral inclusions.The frequency-dependent effective-dielectric permittivity and electric conductivity of geomaterials containing conductive minerals, clay-sized grains, and clay particles are computed using a newly developed electrochemical model. The model allows an inverse application for extracting water saturation, brine conductivity, and clay surface conductance free from the IP effects of clay-sized grains, clay particles, and conductive minerals. This model successfully quantifies the conductivity and permittivity estimated from regular array and triaxial induction logs acquired in shale formations. The dielectric inversion of induction measurements followed by the new model inversion offers a novel petrophysical insight into shale formations. This processing technique may be applied to all induction logs that record the in-phase (R) and quadrature (X) signals wherever they have been or will be acquired in hydrocarbon-rich shale formations around the world.