Numerical Modeling for Excitation and Coupling Transmission of Near Field Around the Metal Drilling Pipe in Lossy Formation

The wireless transmission of the logging signal by using metal drill pipe as a dipole antenna with an extremely asymmetric feeding is one of the potential techniques for logging (or measurement) while drilling (LWD, or MWD) and has drawn a lot of attention in recent decades. In this paper, a precise numerical model based on numerical mode matching (NMM) theory has been built for solving the very low-frequency radiation of the asymmetrical feeding dipole antenna in the layered background medium. First of all, the feeding gap of the antenna has been equalized as a magnetic current loop, and NMM numerical solutions have been applied to expand the field excited by the equivalent source, and then, the characteristics of input impedance and the near field radiated by the dipole have been calculated based on the field solutions. Furthermore, the results have been compared with the corresponding results of the “method of moments” and the “equivalent transmission line model,” respectively. The inference of the relevant parameters, such as the asymmetry of the feeding, the working frequency, the conductivity of the formation, the size of the insulating gap, the size of the casing, etc., on the transmission performance of the signal of electromagnetic MWD (EM-MWD) system has been discussed. In addition, the method for receiving the LWD signal transmitted to the ground is also evaluated, and an adaptive impedance matching scheme in transmitting terminal is proposed based on the numerical analysis in this paper.