Monopole collar wave characteristics for acoustic logging while drilling in fast formations in the frequency and spatial domains

Abstract

Abstract Collar waves generated by the presence of the massive steel collar can interfere with or even cover formation signals, which has been a challenge for designing acoustic logging while drilling (ALWD) tools. In order to seek the means for the suppression of collar waves, it is necessary to study their propagation characteristics. In this study, considering azimuthally symmetric modes only, we compare the radial distribution of the excitation intensity for the first-order collar mode (hereinafter referred to as the collar wave) with varying frequencies obtained by solving elastic wave equations. And the characteristics of the particle vibration trajectories contributed by the collar wave are investigated. It is found that the radial position corresponding to the excitation peak of the collar wave depends on frequencies. As the frequency increases, the peak gradually moves from the inner wall to the outer wall in the collar. According to this feature, it is concluded that interior grooves cut on the drill collar are more suitable for weakening the collar wave than exterior grooves with lower frequencies. On the contrary, in the high-frequency range, exterior grooves attenuate the collar wave more effectively. To confirm these conclusions, we design numerical models and validate the collar wave attenuation by finite difference time-domain simulations. Furthermore, by analyzing vibration properties of particles contributed by the collar wave with varying frequencies, we reveal that the polarization of collar wave motions at the low frequency is similar to that of the longitudinal wave, while at the high frequency it becomes the transverse-wave-like vibration. Inspired by the observation, it is proposed that the collar wave can be weakened by using a steel collar containing an interlayer whose material differ from that of the steel collar. Two layered models are designed and the synthetic waveforms are numerically simulated to examine the proposal. The comparison of wave amplitudes indicates that the collar with an axial interlayer attenuates more collar waves than the collar containing a radial interlayer in the low-frequency range, while in the higher frequency range, the radial layered structures are better for collar wave suppression. Finally, the feasibility analysis of ALWD in the high-frequency range is carried out. The analysis illustrates that when the source frequency is within 20–25 kHz, the excitation amplitude of the second-order collar mode is relatively weak and the first-order collar mode is mainly concentrated near the outer surface of the collar. Appropriate exterior grooves cut on the collar may lead to a clear arrival of formation signals.