Acoustic isolator for downhole applications

Abstract

A plurality of heavy mass irregularities attached to an inner wall of the drill collar attenuate waves traveling through the collar. The plurality of heavy mass irregularities are spaced and sized for the maximum attenuation of acoustic pulses in a predetermined frequency range. The mass irregularities may be rings secured to the inner surface of the collar by neck pieces, extending outwardly from the outer circumference of the ring. The mass irregularities may be made of steel or tungsten and are between six and ten in number. The spacing of the irregularities may lie between twelve and fourteen centimeters. A center pipe may be included to isolate the irregularities from the fluid flow associated with the drilling operation. The pipe may be of a soft material such as rubber to reduce transfer of acoustic noise along the drill string. The irregularities may be in an oil based fluid with the pipe fitting closely in the center of the rings. In another embodiment of the invention, each of the mass irregularities is attached to the drill collar over substantially the entire length of the mass irregularity, enabling the attenuation of high frequencies. In yet another embodiment of the invention, the attenuator comprises a substantially cylindrical body with a plurality of recesses on the inside and/or outside of the cylindrica body, with the length of the recesses selected to provide attenuation within a specified band. In another embodiment of the invention, the attenuator comprises a plurality of sections each having an inner diameter and an outer diameter, each section acting like a waveguide with an associated passband and reject-band.

1 - 15 . (canceled)  
   
   
       16 . An apparatus performing acoustic investigations of a subsurface geological formation penetrated by a borehole comprising: 
 (a) a drilling tubular conveyed in the borehole;    (b) an acoustic transmitter supported by a longitudinal body, said transmitter generating acoustic signals in the body, the borehole and the subsurface formations,    (d) an acoustic receiver spaced apart from the transmitter and supported by the body which receives said acoustic signals, said acoustic receiver located on said drilling tool on a side of the acoustic transmitter opposite to the drill bit; and    (d) an attenuator located on a substantially cylindrical portion of the body having an inner diameter and an outer diameter, between said acoustic transmitter and said acoustic receiver which attenuates said acoustic signals in the body within a predetermined frequency range;    wherein said attenuator comprises at least one of: (i) a plurality of spaced-apart masses having a predetermined spacing and size for attenuation of said acoustic signals in the body, said spaced apart masses attached to an inner wall of the cylindrical portion of the body, (ii) at least one recess having a length and a depth on an inner wall or an outer wall of the cylindrical portion of the body for attenuation of said acoustic signals in the body.    
   
   
       17 . The apparatus of claim  15  wherein the drilling tubular is selected from the group consisting of (i) a drillstring, and, (ii) coiled tubing.  
   
   
       18 . The apparatus of claim  15  wherein the attenuator comprises a plurality of spaced apart masses wherein said predetermined frequency range comprises 10 khz to 20 khz.  
   
   
       19 . The apparatus of  claim 16  wherein the attenuator comprises a plurality of spaced apart masses wherein material of said masses is selected from the group consisting of (i) steel rings, and, (ii) tungsten rings.  
   
   
       20 . The apparatus of  claim 16  wherein the attenuator comprises a plurality of spaced apart masses wherein said plurality of masses is between six and ten.  
   
   
       21 . The apparatus of  claim 16  wherein the attenuator comprises a plurality of spaced apart masses and further comprising a center pipe fitting closely against an inner periphery of said masses for preventing contact between a borehole fluid and said masses.  
   
   
       22 . The apparatus of  claim 21  wherein said center pipe is made of an elastomeric material.  
   
   
       23 . The apparatus of  claim 16  wherein the attenuator comprises a plurality of spaced apart masses and wherein said spacing of the masses is within the range of twelve to fourteen centimeters.  
   
   
       24 . The apparatus of  claim 16  wherein the attenuator comprises a plurality of spaced apart masses and wherein said spacing of the masses is within the range of twelve to fourteen centimeters.  
   
   
       25 . The apparatus of  claim 16  wherein the attenuator comprises a plurality of recesses on a wall of the cylindrical portion, said recesses having a length between 5 cm. and 11 cm.  
   
   
       26 . A method for attenuation of acoustic waves traveling through a substantially cylindrical body conveyed on a drilling tubular carrying a drilling tool for drilling of a borehole in earth formations, the method comprising: 
 (a) attaching a plurality of spaced-apart masses having a spacing selected for acoustic attenuation within a predetermined frequency to an inner surface of a said cylindrical body; and    (b) preventing fluid communication of drilling mud with said plurality of mass irregularities.    
   
   
       27 . The method according to  claim 26  wherein said preventing fluid communication further comprises securing a pipe inside a periphery of said plurality of mass irregularities.  
   
   
       28 . The method according to  claim 27  wherein said spacing between said mass irregularities is between twelve to fourteen centimeters.  
   
   
       29 . The method according to  claim 26  wherein said comprise a material selected from steel and tungsten.  
   
   
       30 . A method of performing acoustic investigations of a subsurface geological formation penetrated by a borehole while drilling said wellbore comprising: 
 (a) conveying a logging tool having a substantially cylindrical body on a drilling tubular;    (b) activating a transmitter on the body for generating acoustic signals in the formation, borehole and the body;    (c) attenuating signals passing through the body using an attenuator comprising a plurality of spaced-apart masses on an inside wall of the body, said masses being spaced apart a preselected distance to attenuate signals within a specified frequency range;    (d) using a receiver on a side of the attenuator opposite the transmitter for receiving signals trough the formation and the attenuated signals through the body.    
   
   
       31 . The method of  claim 30  wherein said specified frequency range comprises 10 khz to 20 khz.  
   
   
       32 . The method of  claim 30  wherein said plurality of masses comprises a material selected from (i) steel rings, and, (ii) tungsten rings.  
   
   
       33 . The method of  claim 30  further comprising using a center pipe fitting closely against an inner periphery of said masses for preventing contact between a borehole fluid and said plurality of masses.  
   
   
       34 . A method of performing acoustic investigations of a subsurface geological formation penetrated by a borehole while drilling said wellbore comprising: 
 (a) conveying a logging tool having a substantially cylindrical body on a drilling tubular;    (b) activating a transmitter on the body for generating acoustic signals in the formation, borehole and the body;    (c) attenuating signals passing through the body using an attenuator comprising a plurality of recesses on at least one of an inner wall, and an outer wall of the body, said recesses having a preselected length to attenuate signals within a specified frequency range; and    (d) using a receiver on a side of the attenuator opposite the transmitter for receiving signals through the formation and the attenuated signals through the body.    
   
   
       35 . The method of  claim 34  wherein said recesses have a length between 5 cm. and 11 cm.  
   
   
       36 . The method of  claim 34  wherein at least one of the plurality of recesses has a length different from a length of another of the plurality of recesses.  
   
   
       37 . A method of performing acoustic investigations of a subsurface geological formation penetrated by a borehole while drilling said wellbore comprising: 
 (a) conveying a logging tool having a substantially cylindrical body on a drilling tubular;    (b) activating a transmitter on the body for generating acoustic signals in the formation, borehole and the body;    (c) attenuating signals passing through the body using an attenuator comprising a plurality of interconnected sections each having an inner diameter and an outer diameter, said inner and outer diameter selected to attenuate signals passing through the body within a prespecified range of frequencies; and    (d) using a receiver on a side of the attenuator opposite the transmitter for receiving signals through the formation and the attenuated signals through the body.    
   
   
       38 . The method of  claim 37  wherein said inner diameter of each said section ranges from 2″ to 6″ and said outer diameter of each said section ranges from 4″ to 10″.