Enhancing Extremely High Resistivity Inversion through Electromagnetic Propagation Resistivity Measurements

Abstract Recent advances on the logging-while-drilling (LWD) electromagnetic (EM) propagation resistivity tools, especially designed for deep resistivity boundary detection and azimuthal reading, have enabled proactive drilling applications such as geosteering in many different environments. However, in some rare geological scenarios, for example, an anhydrite cap rock overlaying a carbonate reservoir, even the latest LWD EM tools can struggle and produce inaccurate formation mapping results. Such formations have extremely high resistivity coupled with low resistivity contrast to adjacent layers, leading to significantly reduced EM measurement sensitivity. The reduced sensitivity is further compounded by the presence of minor noise from tool electronics and drilling operations, contaminating the total received signals. Therefore, conventional inversion methods fail to extract correct formation properties under these high-resistive formation conditions. To address this challenging task, a higher frequency channel at deep transmitter-receiver (TR) spacing is enabled to achieve greater sensitivity to highly resistive formations. Also, a novel inversion scheme is proposed by employing the high-frequency deep azimuthal measurements and pre-existing shallow measurements at relatively short TR spacing. Since shallow measurements often have a higher signal-to-noise ratio (SNR), incorporating them into our inversion process ensures accurate near-wellbore resistivity determination and stabilizes the inversion iterations. Finally, the new inversion includes the so-called geosignals from the deep TR spacing, significantly enhancing SNR and sensitivity to bed boundaries in extremely high-resistivity formations. The improved inversion scheme, utilizing the modified LWD EM tool, was evaluated using both synthetic and field examples. Results demonstrate great inversion performance, providing more accurate and stable resistivity mapping in formations with high resistivity and low resistivity contrast compared to the existing methods. The innovative approach has been further validated through ongoing field trials and has successfully assisted operations in optimizing the wellbore placements under such demanding formation conditions.