Present approaches for interpreting the resistivity responses assume circular and symmetric invasion of the formation around the tool axis. However, invasion is influenced by gravity and formation heterogeneity, which...
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Although the directional sensitivity and deep investigation depths of directional logging-while-drilling (LWD) resistivity tools have led to their wide use in detecting bed boundaries for well placement, their multico...
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Although the directional sensitivity and deep investigation depths of directional logging-while-drilling (LWD) resistivity tools have led to their wide use in detecting bed boundaries for well placement, their multicomponent electromagnetic (EM) measurements also have important applications in formation evaluation. An area of current interest is the evaluation of resistivity anisotropy and formation dip in low-angle wells to better quantify hydrocarbon in place. We present a multistep inversion-based workflow for the interpretation of resistivity anisotropy and formation dip from the directional EM measurements. A 1D parametric inversion is used to construct a layer-cake, transversely isotropic formation model that has outputs of horizontal and vertical resistivities (Rh and Rv), formation dip, and layer thicknesses. The procedure takes advantage of predominant sensitivities of different groups of measurements to various formation parameters. After identifying boundary positions, we first invert Rh only from the standard resistivity logs and then add Rv and dip to the inversion model successively, each time incorporating a different group of measurements in the inversion. This significantly improves the robustness of the inversion. We also develop a method of assigning confidence levels to the inversion results for log quality control (LQC), using data misfits and the rate of dip change. The workflow can be used both to process recorded-mode data and to perform real-time interpretation, while drilling, from a subset of input channels. In addition, the algorithm can be applied to conventional LWD resistivity logs to improve their vertical resolution in low-angle wells and to extract Rh and Rv in high-angle wells. The inversion algorithm has been validated, first with synthetic cases constructed from field logs and then with field data in vertical and deviated wells. The examples show that resistivity anisotropy and dip information can be consistently determined from
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