Numerical simulation of acoustic fields in formation generated by linear phased array acoustic transmitters during logging while drilling

Abstract

The monopole acoustic transmitters of existing logging while drilling (LWD) tools rarely have directional radiation capacity. Only a small amount of energy is radiated to geological structures outside the borehole, which presents a significant challenge for acoustic reflection LWD. In this study, a method is proposed to enhance the amplitudes of P-P waves reflected from unknown reflection boundaries during LWD, based on the directional radiation technology of linear phased array (LPA) acoustic transmitters. Waveforms in the formation generated by a monopole acoustic transmitter and LPA acoustic transmitters are numerically simulated using the finite-difference time-domain and real axis integration methods. The influences of LPA parameters on formation P waves and the application effects of LPA acoustic transmitters in acoustic reflection LWD are also analyzed. The simulation results show that the characteristics of formation P waves when using the LPA acoustic transmitter differ from those when using the monopole transmitter. The main radiated acoustic beam of formation P waves can be controlled by adjusting the LPA parameters. The main beam angular width decreases with the increase of the element number; the main beam deflection angle is affected by the delay time between the excitation signals applied to adjacent elements; and the amplitude weighting of the excitation signals applied to all elements has a slight effect on the main beam angular width. With an appropriate choice of the LPA parameters, the amplitudes of reflected P-P waves are significantly increased in acoustic reflection LWD. Therefore, compared with the monopole acoustic transmitter, the LPA acoustic transmitter can be utilized to effectively improve the capacity of acoustic LWD tools for detecting the reflectors away from the borehole. The findings of this study provide a novel approach to designing next-generation acoustic reflection LWD tools.