A New Generation of LWD Geosteering Electromagnetic Resistivity Tool Providing Multi-Layered Bed Boundary Detection, Anisotropy Determination, and Azimuthal Resistivity Measurements for Accurate Well Placement and Formation Evaluation

Abstract

Over the past few years, ultra-deep electromagnetic (EM) resistivity tools have been primarily used to enable proactive geosteering and reservoir mapping. The ability to detect formation properties with a depth of investigation from 25 to 225 ft away from a wellbore enables optimization of well placement and helps to maximize reservoir exposure. As a result of the low frequencies and long spacings used by ultra-deep-reading tools, the corresponding inversion cannot resolve thinly bedded layers near the wellbore. Conventional azimuthal resistivity tools are then used to evaluate such missing details from the ultra-deep tools. Still, the conventional tools do not completely close the technology gap of providing high-fidelity, multilayer bed-boundary detection near the wellbore because of hardware limitations. A new generation of logging-while-drilling (LWD) geosteering resistivity tool is introduced, adopting enhanced antenna designs from the ultra-deep EM resistivity tool and comprising a shallow and a deep antenna collar. The shallow collar contains an antenna array of multiple coaxial transmitters and tilted receivers to produce azimuthal resistivities and geosignals. In addition, a pair of co-located, tilted transmitters enable formation anisotropy determination at any wellbore deviation. The deep collar is equipped with two tilted transmitters. By connecting the two collars, the system can detect formation bed boundaries within the range of 1 to 30 ft away from the wellbore, depending on the selected operating frequency and transmitter-to-receiver spacing. This paper discusses the design principle of the new generation resistivity tool. The design has three major advantages over existing resistivity technologies: tool measurements have a detection range of up to 30 ft to a formation bed boundary with a favorable resistivity contrast; the corresponding distance-to-bed-boundary (DTBB) inversion achieves very high resolution of thin layers near the wellbore; and the tool measures formation anisotropy and dip at any wellbore deviation. In addition, it provides greater signal-to-noise ratio (SNR) in its resistivity and geosignal measurements compared to previous designs. Several field trials have validated and exhibited the tool performance in calculating real-time resistivity anisotropy comparable to a 3D wireline induction reference. The trials also demonstrated azimuthal resistivity and geosignal measurements that matched other LWD resistivity tools, and a detection range of 1 to 30 ft for the multi-layered inversion, which matched offset well logs and real-time resistivity logs. Inversion results from deep to very shallow provide the flexibility to address a range of geosteering objectives and improve the economics of field development.