Numerical simulations of drill-string responses for downhole acoustic telemetry

Abstract

In petroleum engineering, one major challenge for logging while drilling is that the transmission rate of downhole telemetry is too low for real-time evaluations. Downhole acoustic telemetry (DAT) uses acoustic guided waves in a drill string with periodic structures to transmit measurement data, expected to break through low-speed telemetry limitations. However, the lack of effective modeling methods makes the channel response difficult to predict, severely restricting equipment development and field applications. We propose a novel modeling approach to study the responses of the DAT channel. Excitation and propagation mechanisms of the mode waves in the DAT channel are investigated. The 2-D modeling issue is approximated to the 1-D plane-wave propagation along the borehole, considering transmission, reflection, and interconversion of the drill-string and fluid waves. The coefficients of each wave component and the full-wave channel function are derived from the transfer matrix. The efficiency and accuracy of the proposed means are validated by comparison with the finite difference. Numerical results show that the fluid-solid coupling leads to a generalized multipath effect in the DAT channel, making the acoustic responses exhibit discrete bandgaps and nonlinear phase distortion. This study provides a basis for frequency band selection and design for DAT systems.