Three-dimensional sensitivity analysis of induction logging in anisotropic media

An investigation of the sensitivity of two-coil coaxial (2C-40) and focused (6FF40) induction logging sondes shows that for deviated wells, cylindrical symmetry about the borehole vanishes as transverse isotropy is introduced into the formation. For horizontal wells, the sensitivity functions for both tools become increasingly complicated with increasing coefficients of anisotropy. In addition, the sign of the sensitivity changes from predominantly negative to positive with increasing anisotropy. This indicates that for formations with moderate-to-high degrees of anisotropy, the magnetic field amplitude will decrease with a positive perturbation in conductivity, which is the opposite of what occurs in the isotropic case. Increasing anisotropy is also demonstrated to cause greater sensitivity in the axial response function near the source and receivers, thus limiting the depth into the formation that is sensed. The effect of anisotropy on the transverse response function is shown to be direction dependent. In deviated wells, the effect of anisotropy on the sensitivity increases with increasing deviation angle. The sensitivity, pattern is skewed and inclined along the deviation direction, but is increasingly out of alignment with the borehole as the relative deviation increases.