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In petroleum exploration, reservoir navigation is used for reaching a productive reservoir and placing the borehole optimally inside the reservoir to maximize production. For proper well placement, it is necessary to calculate in real-time parameters of the formation we are drilling in, and the parameters of formations we are approaching. Based on these results, a decision to change the direction of drilling could be made. Modern logging while drilling (LWD) extra-deep and azimuthal resistivity tools acquire multi-component, multi-spacing, and multi-frequency data that provide sufficient information for resolving the surrounding formation parameters. These tools are generally used for reservoir navigation and real-time formation evaluation. However, real-time interpretation software very often is based on simplified resistivity models that can be inadequate and lead to incorrect geosteering decisions.The core of the newly developed software is an inversion algorithm based on a model of transversely-isotropic layered earth with an arbitrary number of layers. The following model parameters are determined in real time: horizontal and vertical resistivities and thickness of each layer, formation dip, and azimuth. The inversion algorithm is based on the method of the most-probable parameter combination. The algorithm has good performance and excellent convergence due to its enhanced capability of avoiding local minima. This capability enables interpretation of real-time resistivity data, including azimuthal and extra-deep measurements.A graphical user interface was developed to provide an interactive environment for each stage of the resistivity data interpretation process: preview of input resistivity logs, initial preprocessing and filtering of raw data, creation of initial guess, running inversion and viewing inversion results, and quality control indicators. Applications of the developed software will be shown on a series of synthetic examples and field data from the North Sea. This newly developed software is currently in use for real-time reservoir navigation and post-well analysis.

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