“Theory and numerical simulation of induction and MWD resistivity tools in anisotropic dipping beds,” The Log Analyst, Vol. 39, No. 1, pp. 24-37, 1998.
Shale formations as well as thinly laminated sequences of shale and sand are anisotropic formations often encountered in electromagnetic logging. Also, many wells are deviated from the direction perpendicular to the formation beds. The combined effect of dip and anisotropy on the readings of induction and measurement-while-drilling (MWD) resistivity tools is studied. Solutions of Maxwell''s equations in anisotropic media are obtained, and computer codes are developed to model responses of induction and MWD resistivity tools. Numerical results show that 1) the induction and MWD tool responses in thin sand-shale sequence and in anisotropic beds show different characteristics; 2) in the case of an MWD tool, the anisotropy can be identified by the difference between the phase-based and the amplitude-based apparent resistivities and the difference increases with the dip angle; 3) although a simple equation can be used to account for anisotropy in thick beds, it becomes inaccurate in thin anisotropic beds. A typical correction chart is presented to demonstrate that the apparent resistivity at the center of a thin anisotropic bed is very different from the apparent resistivity predicted by that equation.