The shift from conventional vertical drilling to complex, multi-target trajectories has changed the requirements for downhole navigation. Today’s oil and gas operators often navigate dispersed reservoirs from a single surface location, demanding centimeter-level precision thousands of meters below the surface.
At the core of these operations is measurement while drilling (MWD) technology. The directional modules within MWD tools rely on precision inertial sensors to track borehole inclination, azimuth and position in real time. However, as well profiles become more aggressive, the environmental stresses on these sensors—specifically temperature and vibration—have reached the limits of legacy technology.
Constraints of legacy and conventional MEMS
For decades, quartz accelerometers were the industry standard for inclination measurement in MWD tools due to their high precision. However, their high cost, complex integration requirements and bulky form factors are increasingly incompatible with the miniaturized, digital architectures of modern bottom-hole assemblies. Moreover, quartz accelerometers are mechanical devices that are known to be fragile, and any failure during operation can negatively affect the equipment uptime.
While MEMS (micro-electro-mechanical systems) accelerometers offer a more reliable, compact, cost-effective and digitally ready alternative, the energy sector has been slow to adopt them for two primary reasons:
- Thermal stability: Most industrial MEMS are qualified only to +85°C. Downhole environments routinely exceed +150°C, reaching +175°. At these extremes, standard MEMS devices suffer from degraded accuracy and systematic bias failure.
- Vibration rectification error (VRE): During active drilling, sensors are subjected to random vibrations. In conventional open-loop MEMS accelerometers, these disturbances induce a DC bias shift (VRE). For a directional module, even a minor angular deviation caused by VRE can translate into significant borehole positioning errors over the length of the well.
Closed-loop architecture for better VRE
To address these challenges, Tronics Microsystems (a TDK Group company) has introduced the AXO315T series. Released for full production since June 2025, this family of closed-loop, digital and high-temperature MEMS accelerometers is engineered specifically for the MWD environment.
The AXO315T series is powered by a closed-loop architecture. This design provides a 10x improvement in vibration rectification error compared to traditional open-loop MEMS accelerometers. This architectural advantage allows for continuous, accurate inclination measurements even during high-vibration drilling phases, eliminating the need to stop the drillbit motor during measurement.
Performance validation and reliability
The series consists of two primary variants designed to meet specific mission profiles:
- AXO315T0: Operates from -30°C to +150°C with a typical residual bias error of 0.8 mg.
- AXO315T1: Extends to +175°C, with a residual bias error of 1.7 mg across the full temperature range.
To guarantee performance in the field, Tronics executed an extensive qualification protocol on a population of more than 250 devices. Reliability was validated through High-Temperature Operating Life testing exceeding 1,000 hours. Crucially, testing was performed at +165°C and +185°C, respectively, providing a critical "guard band" (safety margin) above the maximum rated operating temperatures.
Furthermore, the sensors were subjected to random vibration levels of 20 g rms (50-2000 Hz) and sine sweep vibrations reaching 50 g peak across all three axes at maximum operating temperatures.
Digital integration and system benefits
Beyond environmental robustness, the AXO315T series is designed to simplify the electronics stack within the directional module. The sensors feature a 24-bit SPI digital output and a hermetic ceramic J-Lead package.
By eliminating the need for external analog-to-digital converters and specialized signal conditioning, drilling tool manufacturers can reduce the overall footprint of their electronics, allowing for more compact MWD tool designs.
As the industry transitions away from legacy quartz sensors, the criteria for MWD navigation components have become more stringent. The AXO315T series represents a convergence of quartz-level precision with the scalability and digital readiness of MEMS technology. For next-generation drilling operations, success depends on sensors that can maintain accuracy and reliability in the most demanding downhole conditions.
