Downhole Timing Recovery and Signal Detection

Abstract

The present invention relates to telemetry apparatus and methods, and more particularly to acoustic telemetry apparatus and methods used in the oil and gas industry. More specifically, the invention relates to a method for enhancing a received signal transmitted by acoustic telemetry through a drill string by modifying the received signal by a multiplication of the received signal with a second waveform.

Description

RELATED APPLICATIONS[0001]This application is related to and claims priority from U.S. Provisional Patent Application No. 61 / 118,501, filed Nov. 28, 2008, the entire contents of which are incorporated herein by reference for all purposes.FIELD OF THE INVENTION[0002]The present invention relates to telemetry apparatus and methods, and more particularly to acoustic telemetry apparatus and methods used in the oil and gas industry. More specifically, the invention relates to a method for enhancing a received signal transmitted by acoustic telemetry through a drill string by modifying the received signal by a multiplication of the received signal with a second waveform.BACKGROUND OF THE INVENTION[0003]As is known, acoustic telemetry is a method of communication in the well drilling and production industry. In a typical drilling environment, acoustic carrier waves from a downhole acoustic telemetry device are modulated in order to carry information via the drillpipe to the surface. Upon a...

AI Technical Summary

Benefits of technology

The invention provides a method for improving the detection and synchronization of an acoustic signal by modifying the time-bandwidth product of the transmitted linear acoustic chirp at the receiver. This method involves enhancing the received signal by a multiplication of the received signal with a second waveform, which can be a linear frequency upchirp or downchirp. The received acoustic signal can be a linear frequency chirp or linear frequency upchirp. The method compensates for limited chirp bandwidth and removes dispersion effects by calculating and optimizing an autocorrelation function. The received chirp signal is then selectively shifted in the frequency domain while maintaining baud rate to create a processed signal with an increased time-bandwidth. The processed signal can also be subjected to multiple frequency sweeps to further increase the time-bandwidth.

Problems solved by technology

Further still, other factors including significant surface and downhole noise from drilling, dispersion, phase non-linearity and frequency dependent attenuation as well as passband effects all contribute to the challenge of effectively enabling acoustic telemetry through the drillpipe.

Thus, as a result of the passband / stopband structure, the acoustic signal transmitted up the drillpipe to the surface must be limited in frequency bandwidth such that its frequencies fall within one or simultaneously two or more of the frequency passbands within the passband / stopband structure.

However, the main challenge in effective transmission up the drillpipe is the frequency dependant destructive interference of the signals within these passbands.

This interference arises from both the effect of the regular structure of the drill string (Drumheller), and signal reflections local to the acoustic transmitter.

The combination of the fine structure of the passband with the destructive interference of the transmitted signal by local reflections can result in narrow notches within the passband with attenuations of 10 dB or greater.

The bandpass filter properties of the downhole acoustic channel limit the frequency span available for the chirp placing a limit on the time bandwidth product.

This combination of constraints forms a boundary on the available performance of the acoustic telemetry system by limiting the shape of the auto-correlation waveform, making it more difficult to demodulate and detect the telemetry signal at higher data rates.

Images