A new logging-while-drilling azimuthal electromagnetic measurement for highly resistive coal mines

Abstract Logging-while-drilling (LWD) azimuthal electromagnetic measurement (AEM), capable of detecting and imaging formation structures around the wellbore, has become a vital tool for navigating complex reservoirs. However, this technology faces challenges such as weakened signals, significant noise interference, and reduced detection scope in highly resistive formations. To address these challenges, this paper presents a new LWD AEM method for highly resistive coal mines. The key distinctions from current AEM tools are the utilization of higher frequencies and extended coil spacing to ensure proper electromagnetic (EM) wave decay in resistive media. First, by stretching the distance between receivers to 10 and 20 inches (25.4 and 50.8 cm), a notable expansion of the upper limit of resistivity range is obtained. Additionally, the operating frequency has been increased to 5 MHz to boost the strength of phase-shift (PS) signal and reduce the noise effect. A boundary detection method based on high-frequency PS in highly resistive coal mines is then established, significantly improving signal strength and depth of detection. Various operating frequencies are also optimized to increase the effective curves in highly resistive formations. Finally, a coal mine LWD AEM tool design is designed, providing five effective resistivity curves and 12 effective geological signal curves for highly resistive formations. With a resistivity detection limit of 5000 Ω·m and a depth of detection limit of 4 m, this instrument serves as a solid foundation for advancing research and development in coal mine geosteering and instrumentation.