Response analyses on the drill-string channel for logging while drilling telemetry

Abstract

Downhole acoustic telemetry (DAT), using a long drill string with periodical structures as the channel, is a prospective technology for improving the transmission rate of logging while drilling (LWD) data. Previous studies only focused on the acoustic property of a free drill string and neglected the coupling between pipes and fluid-filled boreholes. In addition to the drill-string waves, a series of fluid waves are recorded in the DAT channel, which has not been investigated yet. Unpredictable channel characteristics result in lower transmission rates and stability than expected. Therefore, a more realistic channel model is needed considering the fluid-filled borehole. In this paper, we propose a hybrid modeling method to investigate the response characteristics of the DAT channel. By combining the axial wavenumbers and excitation functions of mode waves in radially layered LWD structures, the channel model is approximated to the 1-D propagation, which considers transmission, reflection, and interconversion of the drill-string and fluid waves. The proposed 1-D approximation has been well validated by comparing the 2-D finite-difference modeling. It is revealed that the transmitted and converted fluid waves interfere with the drill-string wave, which characterizes the DAT channel as a particular coherent multi-path channel. When a fluid-filled borehole surrounds the drill string, the channel responses exhibit considerable delay as well as strong frequency selectivity in amplitude and phase. These new findings suggest that the complexity of the channel response has been underestimated in the past, and therefore channel measurements on the ground are unreliable. To address these channel characteristics, we apply a non-coherent demodulation strategy. The transmission rate for synthetic data reaches 15 bps in a 94.5 m long channel, indicating that the acoustic telemetry is promising to break the low-speed limitation of mud-pulse telemetry.