Numerical simulation of acoustic field in logging-while-drilling borehole generated by linear phased array acoustic transmitter

Abstract

In actual monopole acoustic logging-while-drilling (LWD), the acquisition of formation velocities is rather difficult because of a strong collar wave interference. In this study, a method is proposed to enhance the amplitudes of formation gliding waves in an LWD borehole, based on the directional radiation technology of linear phased array (LPA) acoustic transmitters. Waveforms in the LWD borehole generated by a monopole acoustic transmitter and LPA acoustic transmitters are numerically simulated using the finite-difference method. The influence of the LPA parameters on the various waves in the LWD borehole is also analysed. The simulation results show that the guided waves in the LWD borehole generated by the LPA acoustic transmitter have the same type but different relative amplitudes compared to those generated by the monopole transmitter. The angular width and deflection angle of the main radiated acoustic beam in the borehole can be controlled by adjusting the LPA parameters. As the delay time between the excitation signals applied to adjacent elements is increased, the deflection angle of the main radiated acoustic beam in the borehole satisfies the generation conditions for the gliding compressional (P) and shear (S) waves, successively. When the delay time is set such that the deflection angle is equal to the first (or second) critical angle of the formation, the amplitude of the gliding P (or S) wave is greatly enhanced. Moreover, the amplitude of the gliding P (or S) wave increases linearly as the element number increases under such conditions. Therefore, compared to the monopole acoustic transmitter, the LPA acoustic transmitter can be utilized to effectively improve the reliability of the acoustic LWD tool in measuring formation velocities. This study establishes a theoretical foundation for the next-generation acoustic LWD tool development.