1. School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China; 2. Key Laboratory of Gas Geology and Gas Control in Henan Province-National Key Laboratory Cultivation Base Co-constructed by Provincial Departments, Jiaozuo, Henan 454000, China; 3. China Petroleum Logging Co., Ltd, Xi'an, Shaanxi 710077, China; 4. GME & Geochemical Surveys, BGP Inc., CNPC, Zhuozhou, Hebei 072751, China
Abstract：In recent years, transient electromagnetic through-casing technology has become one of the hotspots in electromagnetic prospecting. Since the conductivity and magnetic permeability of inhomogeneous media in wellbores and formation change drastically on small scales (casing, mud, etc.), efficient forward modeling for three-dimensional (3D) transient electromagnetic through casing is greatly important to simulate and eliminate the influence of metal casing on the electromagnetic field. In the calculations of forward modeling, the solution region is discretized in the cylindrical coordinate system to effectively reduce the number of grids and thus reduce the solution scale of the equation system, and the 3D cased-hole transient electromagnetic forward modeling is realized on the basis of the finite volume method. By simulating and analyzing the influence of parameters such as casing conductivity, relative magnetic permeability, thickness, and inner diameter on the electromagnetic field, a foundation is laid for correcting the influence of metal casing. The comparative analysis of the response results of trapezoidal waves, half-sine waves, and triangle waves de-monstrates that the transmitting waveform using trapezoidal waves performs the best. Moreover, the analysis on the influence of the duration of each stage of trapezoidal waves on electromagnetic response results reveals that the shorter waveform stabilization time has a significant impact on the early response results; the impact of the turn-on time on the response results can be ignored, and shorter turn-off time leads to stronger electromagnetic response. The research conclusions can provide technical support for research on transient electromagnetic through-casing data processing technology.
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