Methods and apparatus for downhole acoustic telemetry

Abstract

An improved method and apparatus for transmitting and receiving acoustic signals is presented that enables high speed transmission without repeaters. At the transmitter, a piezoelectric transducer is used to create acoustic waves through the drill string assembly walls. Transmission occurs at more than one passband and each carrier frequency undergoes different modulation and channel coding schemes as the channel permits. At the receiver with multiple accelerometers with known distance, the acoustic signal is processed using several signal diversity techniques to increase the signal-to-noise ratio. A lock-in amplifier extracts the attenuated signals, and a sensor fusion estimation algorithm is performed to decrease the effect of noise. In addition, the wave speed determined from the multiple accelerometer configuration is used for cancellation of echoes and reflections.

1 . Acoustic telemetry system for use with a drill string having MWD sensors and instruments, while conducting MWD operations comprising:
 an acoustic transmitter tool, comprising:
 a mandrel for securing the tool to the drill string, the mandrel having an annulus therethrough; 
 an annular acoustic transducer secured to the mandrel for generating acoustic waves for travelling longitudinally along the drill string; 
 a pressure barrel concentrically positioned within the annulus for housing electronics; and 
 electro-mechanical connections for communicating between the piezoelectric actuator and the electronics; and 
   a surface receiver.   
     
     
         2 . The system of  claim 1 , wherein the acoustic transducer can further comprise a piezoelectric actuator comprises a stack of a plurality of ceramic rings composed of lead zirconate titante (PZT). 
     
     
         3 . The system of  1 , wherein the acoustic transducer can further comprises a magneto-strictive transducer. 
     
     
         4 . The system of  claim 1 , wherein the electronics receives sensor data from the MWD sensors, and further comprises at least a voltage transformer, switching circuitry, and a microcontroller for converting binary signals to a modulated drive signal. 
     
     
         5 . The system of  claim 1 , wherein the surface receiver further comprises at least two accelerometers or acoustic emission sensors positioned and spaced longitudinally along the drill string for receiving and detecting acoustic signals transmitted from the tool. 
     
     
         6 . The system of  5 , wherein the surface receiver further comprises a lock-in amplifier. 
     
     
         7 . The system of  6 , wherein the lock-in amplifier further comprises:
 orthogonal oscillators;   phase-lock loop; and   a plurality of low-pass filters.   
     
     
         8 . The system of  claim 1 , further comprising an isolator at a downhole end of the tool, for minimizing acoustic interference. 
     
     
         9 . The system of  claim 1 , wherein the mandrel further comprises a bottom sub, a top sub, and an inner mandrel. 
     
     
         10 . The system of  claim 1 , wherein the pressure barrel is a beryllium copper pressure barrel. 
     
     
         11 . A method for downhole telemetry and acoustic transmission of MWD data for drilling operations using a drill string assembly, the method comprising the steps of:
 (a) providing an acoustic transmitter tool having a piezoelectric transmitter and positioning the acoustic telemetry tool down a wellbore;   (b) providing a surface receiver;   (c) prior to commencing the drilling operations, determining an appropriate acoustic transmission frequency;   (d) after determining an appropriate acoustic transmission frequency, commencing the drilling operations while conducting acoustic transmission using the appropriate acoustic transmission frequency;   (e) repeating the step of determining an appropriate acoustic transmission frequency as the drill string is elongated during the drilling operations to determine a subsequent acoustic transmission frequency;   (f) continue drilling operations and acoustic transmission using the subsequent acoustic transmission frequency; and   (g) repeating steps (e) and (f) as necessary until the drilling operations are complete.   
     
     
         12 . The method of  claim 11 , wherein providing a surface receiver further comprises configuring acoustic emission sensors on the drill string assembly to receive and detect acoustic signals transmitted from the acoustic telemetry tool. 
     
     
         13 . The method of  claim 12  wherein configuring acoustic emission sensors further comprises positioning at least two accelerometers longitudinally along the drill string assembly at known distances. 
     
     
         14 . The method of  claim 11 , wherein determining an appropriate acoustic signal transmission frequency prior to commencing drilling operations, further comprises:
 prompting the acoustic telemetry tool to transmit a broadband acoustic signal over all frequencies of interest;   receiving the broadband signal with the surface receiver; and   converting the broadband signal into electrical signals and processing the electrical signals to determine the appropriate acoustic transmission frequency.   
     
     
         15 . The method of  claim 11 , further comprising processing electrical signals at both the surface receiver and the piezoelectric transmitter to increase accuracy of MWD data received at the surface receiver. 
     
     
         16 . The method of  claim 15 , wherein processing electrical signals at the transmitter further comprises converting MWD data into a modulated drive signal for the transmitter involving channel coding, modulation, pulse-shaping, and up-conversion. 
     
     
         17 . The method of  claim 16 , wherein modulation further comprises convolutional encoding. 
     
     
         18 . The method of  claim 16 , wherein processing electrical signals at the surface receiver further comprises demodulating the modulated drive signal at the surface receiver.