FAST AND AUTOMATIC INVERSION OF LWD RESISTIVITY MEASUREMENTS FOR PETROPHYSICAL INTERPRETATION
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This paper describes an extension of a recently developed fast inversion method (Pardo and Torres-VerdÍn (2015)) for estimating a layer-by-layer electric resistivity distribution from logging-whiledrilling (LWD) electromagnetic induction measurements. The well trajectory is arbitrary and the developed method is suitable for any commercial logging device with known antennae configurations, including tri-axial instruments. There are two key novel contributions in this work: First, the three-dimensional (3D) transversely isotropic (TI) formation is now approximated by a sequence of various "stitched" one-dimensional (1D) sections rather than by a single 1D reduced model. This provides added flexibility in order to approximate complex 3D formations. Second, we introduce the concept of "negative apparent resistivities" in the inversion method. By using the values of attenuation and phase differences that correspond to a "negative" resistivity in a homogeneous formation, the amount of data lost when converting magnetic fields into apparent resistivities is minimized, thus leading to a more robust inversion method that also convergences faster. The developed inversion method can be used to interpret LWD resistivity measurements and to adjust the well trajectory in real (logging) time. Numerical inversion results of challenging synthetic and actual field measurements confirm the high stability and superior approximation properties of the developed inversion algorithm. Because of the efficiency, flexibility, and stability of the inversion algorithm, formation-evaluation specialists can readily employ it for routine petrophysical interpretation and appraisal of complex LWD and wireline resistivity measurements acquired under general geometrical and geological constraints.