3D Forward Modeling of Borehole-to-Surface Electromagnetic Method with Steel Casing Based on Cylindrical Grid and Analysis of Effective Detection Depth

The borehole-to-surface electromagnetic (BSEM) method is widely employed in oil and gas exploration and downhole monitoring. However, the strength of the ground observation signals of the BSEM method is affected by the metal steel casing in the well. To investigate the response characteristics of the BSEM method under metal casing conditions, this study performed three-dimensional BSEM forward modeling based on a cylindrical grid. The finite volume method was adopted to discretize and solve the governing equations of the electromagnetic field, and the cylindrical grid was partitioned in accordance with the axisymmetric geometric features of the wellbore-casing system, thereby achieving high-precision adaptation to the well structure. To explore the impact of metal casing in an alternating electromagnetic field, four typical models were established: a linear source, a long metal wire, a metal casing, and a casing with a cement sheath. The characteristics of ground signals under low-frequency alternating emission conditions were systematically studied. By comparing the simulation results with the 1D analytical solution, this method was verified to have high numerical accuracy, which can accurately reflect the responses of a metal casing and multiple media interfaces to the alternating electromagnetic field. Based on comparative analysis, the differences in underground electromagnetic field distributions among different source models and their applicable ranges were clarified, and the applicable scenarios and effective detection depths of different models in actual monitoring were explored. This research provides numerical simulation cases to investigate the role of metal casings in BSEM observations, and also lays a theoretical foundation for the interpretation of downhole electromagnetic data, which is of positive significance for improving the effect of applying BSEM technology in oil and gas exploration.