Electrical anisotropy of terrigenous deposits: a brief overview of approaches to its determination from electrical logging data in vertical wells

Abstract

The article briefly reviews a number of publications on the problem of determining rocks electrical resistivity anisotropy (primarily of sedimentary genesis) using downhole resistivity logs. We provide the information on the main causes of sandy-clay sediments micro- and macroanisotropy according to Russian papers and the history of the model-based approaches to the resistivity anisotropy description according to Russian and foreign articles. We analyze the main approaches to the vertical resistivity estimation. Both new hardware solutions aimed at direct measurement of electromagnetic field components sensitive to anisotropy and methodological techniques for extracting this information from the signals of "conventional" resistivity logging tools are considered according to published studies. The hardware solutions are multicomponent measurements with inclined and perpendicular (to the tool axis) coils, as well as with coaxial toroidal coils. Methodological techniques include combining data measured by tools with different types of medium excitation, for example, induction and galvanic. A number of papers suggest a two-step scheme: the first step is to determine the horizontal resistivity using induction arrays data, and the second step is to determine the vertical resistivity using focused lateral logs. Such schemes are implemented using numerical inversion algorithms and in some cases by inventing transformations of measured signals. Adding anisotropy to the interpretational model is based on a priori data (core studies, microresistivity logs). In the absence of a priori data, anisotropy is added when it is impossible to reconcile the parameters of isotropic models built independently based on logs with different type of excitation. A special focus of the review is on the development of the theory of unfocused lateral logging (BKZ) method by Soviet and Russian scientists and on the history of research on the electrical anisotropy influence on the signals of gradient probes. Due to the widespread use of the method in the USSR, this topic is presented in a number of papers revealing the results of anisotropic geoelectric models theoretical studies, numerical and physical modeling of BKZ logs and methodological developments for their interpretation.