Channel Characterization for Acoustic Downhole Communication Systems
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AbstractThe ability to communicate between downhole and surface instruments became a critical need as well operators monitor flow rate, temperature, and pressure data to facilitate well performance optimization and maintenance. The use of wire lines for communication between downhole and surface is common, but these installations present cost, maintenance, and reliability issues. Wireless communications technology using acoustic waves is an interesting alternative to these wired systems. While the acoustic technology offers great benefits, a clear understanding of its propagation aspects inside the wells is lacking.A testbed was built to investigate the propagation of acoustic signals over production pipes. The testbed comprises an acoustic tool that transmits data to the well surface without cables, an internally-developed receiver unit, and five segments of 7 inch production tubing that form a pipe string. Acoustic waves propagate in this setup by vibrating the pipes body, without interfering with the surrounding medium. Moreover, to study the effect of cemented pipes on wave propagation, the third pipe segment was encased in concrete. An impulse signal was fed to the acoustic tool, and channel impulse response readings were taken along the pipe string. The measurements were analyzed to understand the propagation aspects of acoustic waves.This work shows that acoustic waves experience dispersion and frequency-dependent attenuation over the pipe string; the pipe string appears as a frequency selective channel. The concrete segment filters out a considerable amount of energy in the higher frequency band and introduces further attenuation and dispersion. Signal processing algorithms are proposed to reduce the distortion and dispersion introduced by the pipe string channel on the acoustic waves.Technical contributions include: finding the impulse response of the channel along the pipe string, investigating the power delay profile, power spectral density, and signal-to-noise ratio measures, and studying the channel dispersion parameters.
