Wireline based colocated triaxial electromagnetic (EM) measurements (Omeragic, et al., 2015) with spacings up to 72 inches have verified their ability to accurately determine near-wellbore horizontal resistivity (Rh), vertical resistivity (Rv), and dip post-drilling. In conventional deep azimuthal (DAR) and ultra-deep azimuthal (UDAR) tools, non-colocated antennas tilted at ~45 degrees are typically used to synthesize pseudotriaxial responses. However, these arrangements mix the formation’s EM signal with systematic errors and tilt-angle variations, making calibration challenging.

In contrast, colocated triaxial measurements (as outlined in the wireline tools above) are inherently more robust against tool-related errors, independent of formation resistivity, and can be optimized for higher signal-to-noise ratios. Additionally, colocated triaxial antennas are more compact, allowing multiple antenna spacings to be integrated into the drill collar closer to the bit.

A novel, modular (multi-sub), logging-while-drilling (LWD) tool system has been developed to leverage multi-depth, colocated, and calibrated triaxial electromagnetic (EM) measurements. This Multi-Depth Azimuthal Resistivity (MDAR) system delivers more accurate inverted formation resistivity boundaries and interfaces, enabling improved geosteering, enhanced petrophysical interpretation, and better subsurface model building.

This paper describes the architecture and performance advantages of the new MDAR LWD triaxial EM system, which has a significant improvement in signal-to-noise ratio and measurement accuracy when compared to the current LWD DAR and UDAR architecture deployed in the market.

The MDAR architectural design, which has the smallest measurement offset to bit available in the industry, enables a considerable reduction in the “Inversion Time to Bit”, without compromising the measurement quality. “Inversion Time to Bit” is proposed as a new benchmarking indicator which is a function of Rate of Penetration (ROP), Telemetry, Computing Power and Inversion reference point. A low Inversion Time to Bit is achieved by utilizing a near-bit transmitter referenced inversion enabled by the innovative tri-axial transmitter and receiver arrays, in parallel with the industry standard mid-point referenced inversion, where the highest measurement sensitivity is apparent. The approach of combining two inversion reference points for a holistic interpretation is an industry first.

This increase in measurement sensitivity, when compared to a tilted antenna architecture, will be highlighted using both synthetic and real-world examples, delivering significant improvements in distance-to-boundary detection and layer vertical resolution, also marking a step-change in reducing boundary uncertainty in LWD applications.

The paper will also illustrate how the MDAR architecture approach, together with the triaxial colocated antenna, unlocks additional answer products and geosteering techniques, including petrophysical adjacent deliverables such as bed-boundary free resistivity outputs and look-ahead sensitivity.