FAST AND AUTOMATIC INVERSION OF LWD RESISTIVITY MEASUREMENTS FOR PETROPHYSICAL INTERPRETATION

摘要

This paper describes an extension of a recentlydeveloped fast inversion method (Pardo and Torres-Verdín (2015)) for estimating a layer-by-layerelectric resistivity distribution from logging-whiledrilling(LWD) electromagnetic inductionmeasurements. The well trajectory is arbitrary andthe developed method is suitable for any commerciallogging device with known antennae configurations,including tri-axial instruments.There are two key novel contributions in this work:First, the three-dimensional (3D) transverselyisotropic (TI) formation is now approximated by asequence of various “stitched” one-dimensional (1D)sections rather than by a single 1D reduced model.This provides added flexibility in order toapproximate complex 3D formations. Second, weintroduce the concept of “negative apparentresistivities” in the inversion method. By using thevalues of attenuation and phase differences thatcorrespond to a “negative” resistivity in ahomogeneous formation, the amount of data lostwhen converting magnetic fields into apparentresistivities is minimized, thus leading to a morerobust inversion method that also convergencesfaster.The developed inversion method can be used tointerpret LWD resistivity measurements and to adjustthe well trajectory in real (logging) time. Numericalinversion results of challenging synthetic and actualfield measurements confirm the high stability andsuperior approximation properties of the developedinversion algorithm. Because of the efficiency,flexibility, and stability of the inversion algorithm,formation-evaluation specialists can readily employit for routine petrophysical interpretation andappraisal of complex LWD and wireline resistivitymeasurements acquired under general geometricaland geological constraints.