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 GP_F and GA_F 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.

定向电磁（EM）随钻测井（LWD）工具因其方位角灵敏度和探测深度（DOD）而被广泛用于井导航。在本文中，应用与非均匀网格和并行计算相关的有效 2.5D 有限差分法 (FDM) 来评估地层结构对 EM 工具的影响。该算法通过将数值结果与 1D 和 2D 异质地层中的解析/3D FEM 解决方案进行比较来验证。复杂地层结构的影响，例如断层、弯曲边界和不整合面，通过使用 FDM 程序进一步分析。两条新曲线GP _F和GA _F通过从二维故障模型中减去一维分层模型中的地理信号来定量评估故障检测能力来定义。数值结果表明，电磁工具对断层敏感，尤其是低倾角断层。随着倾角从 15° 增加到 45°，到断层的探测深度从 10.6 m 减少到 4.0 m。数值案例还表明，EM 工具响应会受到陡坡曲线边界的影响，而缓坡（<0.2）边界的影响可以忽略不计。此外，不整合和油/水接触地层的例子表明，二维结构可以显着影响信号幅度。