Numerical simulation of opposing-coils transient electromagnetic method for through-casing resistivity logging

Accurate evaluation of remaining oil saturation in high-water-cut oilfields requires through-casing resistivity logging technology. Conventional electrode-based and induction-based methods suffer from poor electrode contact and strong signal attenuation caused by the steel casing. The transient electromagnetic (TEM) method offers deep detection depth and continuous measurement, but its late-time response suffers from poor vertical resolution due to volumetric averaging, making it difficult to identify thin layers and interfaces. To address these issues, this paper applies the opposing-coils transient electromagnetic method (OCTEM) to through-casing logging. OCTEM adopts a dual-transmitter single-receiver coaxial configuration: two transmitting coils with equal but opposite currents, and the receiving coil placed on the zero-flux plane of the primary field to directly measure the pure secondary field from the formation. A two-dimensional axisymmetric finite-element time-domain (FETD) model is employed to simulate the transient response in a cased borehole consisting of mud, steel casing, and formation, and analyze the effects of casing parameters and instrumental asymmetry on responses. Numerical results show that the logging signal is related to both the distance from the interface and the conductivity contrast. Compared with conventional TEM, OCTEM can accurately locate conductivity interfaces, resolve thin layers, and improve vertical resolution. This method provides a basis for through-casing resistivity logging and explores a feasible pathway for evaluating remaining oil saturation in high-water-cut oilfields.