Borehole Effects on Coaxial and Coplanar Logs of Triaxial Tools in Laminated Formations with Anisotropic Shale Host

Abstract

This paper studies the borehole effect in the triaxial induction logs within sand-shale laminated models with isotropic and anisotropic shale laminae with transverse isotropy. This study compares results from a 3D Vector Finite Element program (with borehole) and a 1D-Analytic code (no borehole). In comparison with the coaxial configuration, the vertical coplanar logs show a stronger horning effect in front of the laminated pack boundaries; a more intense skin effect to the conductivity media; and a more prominent oscillation within the laminated formation. In addition, feature changes (angular or smooth shapes) occur on the coaxial and coplanar responses as the dipping angle varies. The sensitivities of the logs to the anisotropy and borehole are opposite, i.e., for small angles where the coaxial is least sensitive, the coplanar is most sensitive, and for large angles where the coaxial is most sensitive, the coplanar is least sensitive. The main physical cause of these opposite behaviors to the anisotropy and borehole effect is the same: the weight of the horizontal magnetic component of the horizontal dipole contribution on the coaxial and coplanar dipping logs since it is the only one of the four magnetic field components that has anisotropy sensitivity and strongest skin effect.