METHODOLOGICAL ASPECTS OF ELECTRICAL LOGGING DATA INTEGRATION FOR INVERSION BASED ON TWO-DIMENSIONAL AXISYMMETRIC FORMATION MODELS

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This work is dedicated to the development of a numerical inversion methodology for electrical logging data in sub-vertical and slightly inclined oil and gas wells. It examines the specific features of joint inversion of galvanic and induction logs combined in various configurations, depending on the equipment used. The axisymmetric two-dimensional geoelectric model of medium consists of homogeneous blocks separated with horizontal and coaxial cylindrical boundaries. The blocks are characterized by horizontal and vertical electrical resistivity, as well as dielectric permittivity. Numerical inversion is carried out until a minimal discrepancy is achieved between the measured signals and those simulated within the interpretive formation model. Unlike the traditional layer-based approach, this inversion method involves not only adjusting the sounding curves but also accounting for signal variations along the borehole. In this case, the resulting model aligns as closely as possible with the actual measurements. Inversion based solely on galvanic measurements can potentially yield unstable results near boundaries with high resistivity contrasts, underestimating the resistivity of layers adjacent to highly resistive ones. The ambiguity can be reduced through joint inversion of galvanic and induction logs, although this often requires the base formation model to be more complex. In particular, during drilling with fresh clay-based mud in reservoirs with mixed oil and mineralized formation water saturation, a low resistivity annulus forms. This zone is more electrically conductive than both the invaded zone and virgin formation due to the higher content of saline formation water. It cannot be detected through galvanic well logs analysis, but its influence on induction logs is significant. Therefore, the presence of an annulus may be a crucial factor to consider when constructing a geoelectric model of the formation that aligns with resistivity logs obtained with different logging methods. These specifics are illustrated with the results of inversion of real data measured in vertical wells in the intervals of Lower Cretaceous and Jurassic deposits of the Shirotnoe Priobie oil fields.