Triaxial Induction Logging For Everyone: Credos And Caveats

Triaxial induction logging is an emerging technology in hydrocarbon and geothermal<br>reservoir studies, which has great potential in wide-ranging geotechnical applications. In the<br>technique, three component transmitters induce electrical current, which flows transverse and<br>longitudinal to the borehole. Receiver coils then measure three component secondary fields due<br>to these currents. Since triaxial fields are omnidirectional, they promise increased resolution of<br>the geoelectric section compared to traditional uniaxial logging. In particular, they offer the<br>ability to resolve anisotropic formation resistivity tensors or to detect vertical fracture systems, as<br>we demonstrate with numerical modeling. Likewise, the sophistication of the triaxial technique is<br>coupled with some caveats. Perhaps the greatest price of increased resolution of geoelectric<br>structure is an increased sensitivity to borehole effects. We have developed a computational<br>environment to evaluate 3D-electromagnetic sensitivities for high borehole-to-formation contrast<br>resistivities typically found in geothermal reservoirs. In particular, numerical modeling<br>demonstrates that a conductive borehole and its geometric features, such as breakouts, washouts,<br>or key seats, can mask target responses. Adaptive array weighting is a means of using known<br>borehole features to “design” an incident magnetic field, which optimizes the response of the<br>unknown structure. Adaptive array weighting should also compensate for tool eccentering<br>effects.