Borehole Effects and Correction in OBM With Dip and Anisotropy for Triaxial Induction Tools

Abstract

Abstract The latest advance in resistivity logging is the multi-spacing triaxial induction tool. In addition to the conventional induction logs, a triaxial induction tool provides added capability to measure formation anisotropy (horizontal and vertical resistivities), relative dip, and azimuthal angles over a wide range of logging conditions. The anisotropy measurement is very important in low-resistivity pay analysis, while the dip and azimuth measurements have many important geophysical and geological applications. However, due to the physics of the triaxial induction measurements, the borehole effects are significant even for oil-base mud (OBM) conditions. These borehole effects must be corrected in order to obtain accurate measurements of the formation properties. Through extensive modeling studies, we learned that borehole effects depend on many parameters, including hole diameter, tool eccentric distance and azimuth, mud resistivity, formation horizontal and vertical resistivities, formation relative dip, and azimuth angles. Removing the borehole effects is a very difficult and complicated three dimensional (3D) electromagnetic inversion problem, which involves many free parameters. We have developed an inversion algorithm for OBM filled boreholes that robustly solves for horizontal and vertical resistivities, relative dip and azimuth, tool eccentric distance and azimuth. The model for the inversion is an eccentric tool in a borehole through an anisotropic formation with a dip angle. These inverted parameters can then be used to derive the borehole correction for the triaxial measurements. We demonstrate the algorithm through theoretical examples and field logs. The algorithm is very efficient and fast enough to be implemented as real-time well site answer product.