Effects of formation structure on directional electromagnetic logging while drilling measurements

Directional electromagnetic (EM) logging while drilling (LWD) tools have been widely used in well navigation for its azimuthal sensitivity and deep depth of detection (DOD). In this paper, an efficient 2.5D finite difference method (FDM) associated with non-uniform meshes and parallel computing is applied to evaluate the formation structure effects on the EM tools. The algorithm is validated by comparing the numerical results with analytical/3D FEM solutions in 1D and 2D heterogeneous formations. The effects of complex formation structures, e.g., faults, curved boundaries and unconformities are further analyzed by using the FDM code. Two new curves GPF and GAF are defined by subtracting the geosignals in a 1D layered model from that of a 2D fault model to quantitatively evaluate the fault detection capability. Numerical results show that the EM tools are sensitive to faults, especially the low dip angle ones. The depth of detection to fault reduces from 10.6 m to 4.0 m as the dip angle increases from 15° to 45°. Numerical cases also indicate that the EM tool responses can be affected by the sharp slope curved boundaries, while the effects of the gentle slope (<0.2) boundaries can be neglected. Furthermore, examples in unconformity and oil/water contact formation demonstrate that a 2D structure can affect the signal amplitude significantly.