Patent Publication Number: US-2009230969-A1

Title: Downhole Acoustic Receiver with Canceling Element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/341,771 filed on Dec. 22, 2008, which is a continuation-in-part of U.S. patent application Ser. No. 11/776,447 filed on Jul. 11, 2007 which claims priority to Provisional U.S. Patent Application No. 60/914,619 filed on Apr. 27, 2007 and entitled “Resistivity Tool.” This application is also a continuation-in-part of U.S. patent application Ser. Nos. 11/676,494; 11/687,891; 61/073,190. All of the above mentioned references are herein incorporated by reference for all that they contain. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of downhole oil, gas and/or geothermal exploration and more particularly to the fields of acoustic tools for tool strings employed in such exploration. 
     Engineers in the oil, gas, and geothermal fields have worked to develop machinery and methods to effectively obtain information about downhole formations, especially during the process of drilling. Logging-while-drilling (LWD) refers to a set of processes commonly used in the art to obtain information about a formation during the drilling process. Such information may be used by downhole tool string components or be transmitted to the earth&#39;s surface. 
     Information regarding acoustic characteristics of a formation may be valuable to a drilling operation. Acoustic characteristics of a formation may include the speed of sound as it travels through varying subsurface formations or the location of objects or interfaces from which sound waves may rebound. 
     In acoustic systems, waves may be generated by a transmitter. Acoustic tools may be classified as sonic, seismic, or other designations depending on the frequency of the waves. This transmitter may comprise a hammer, an explosive element, a siren, a jar, a piezoelectric source, a magnetostrictive element, an eccentric rotor, a drill bit, or other means known in the art. This transmitter may be on the surface or may be located downhole. This transmitter generates an elastic wave which may propagate through an earthen formation. This elastic wave may interact with layers or other objects within the formation. 
     A receiver is then used to measure the wave at a different location and also to measure any incident waves that may have formed. A receiver is generally in the form of a geophone, a hydrophone, or an accelerometer. A geophone may comprise a magnetic mass moving within a wire coil to generate an electrical signal. A hydrophone may comprise a piezoelectric transducer that generates electricity when subjected to a pressure change. An accelerometer may comprise a cantilever beam with a proof mass that deflects in the presence of non-gravitational accelerations. 
     One difficulty with this method commonly arises when the transmitted wave makes contact with the borehole. This may create a tube wave that may travel along the borehole with less attenuation than the wave traveling through the earthen formation. Another difficulty may arise when the tool string makes contact with the borehole and the transmitted wave begins to travel in the form of an acoustic wave along the length of the tool string. An acoustic wave traveling through the metal of the tool string will generally travel at a much higher rate of speed than a wave traveling through the formation. 
     Some have attempted to reduce this problem by creating a tortuous path along the tool string for the waves to travel between the transmitter and receiver. This may cause the waves traveling along the tool string to significantly attenuate or to arrive so late that they may be readily recognized. However, the waves traveling along the tool string do still arrive and may still interfere depending on the length of time required for a reflected wave of interest to arrive at the sensor. Others have attempted to insert a dampening layer such as a fluid or gel at certain locations along the tool string to dampen the wave. 
     The prior art contains references to drill bits comprising acoustic transmitters and receivers. U.S. Pat. No. 7,334,661 to Pabon, et al., which is herein incorporated by reference for all that it contains, discloses an acoustic logging tool sleeve with a preferably discontinuous, alternating structure that is acoustically opaque in some zones, and acoustically transparent in others. The sleeve may be modular, with several stages connected together. The multiple stages provide a sleeve that may be useful with a variety of borehole logging tools to reduce or eliminate the transmission of noise to the receiving elements. 
     U.S. Pat. No. 7,032,707 to Egerev, et al., which is herein incorporated by reference for all that it contains, discloses a plurality of heavy mass irregularities attached to an inner wall of a drill collar to 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 pipe may be of a soft material such as rubber to reduce transfer of acoustic noise along the drill string. 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 cylindrical 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. 
     U.S. Pat. No. 6,082,484 to Molz, et al., which is herein incorporated by reference for all that it contains, discloses an acoustic attenuator that suppresses acoustic signals traveling along the body of a measurement-while-drilling (MWD) tool, making it possible to obtain acoustic measurements relating to underground formations. Shaped cavities (spherical or cylindrical) filled with a fluid have a resonance frequency that is tuned to be within the band of interest thereby attenuating acoustic signals traveling through the body at these resonance frequencies. The staggered arrangement of the cavities increases the path length for the acoustic signals and provides further attenuation. Attenuation may also be accomplished by use of a composite consisting of cylindrical layers of two different materials with thicknesses that attenuate selected frequencies. Additional attenuation is provided by lengthening the path length of a seismic signal passing through the more competent of the two materials of the composite. 
     BRIEF SUMMARY OF THE INVENTION 
     A downhole tool string assembly may comprise an acoustic transmitter. In various embodiments, the transmitter may be a sonic transmitter, a seismic transmitter, an ultrasonic transmitter or other acoustic transmitter known in the art. The transmitter may comprise a hammer, an explosive element, a siren, a jar, a piezoelectric source, a magnetostrictive element, an eccentric rotor, or a drill bit. The transmitter may form part of a downhole tool string component, or may be a surface element. The transmitter may be adapted to generate an acoustic wave capable of traveling through an earthen formation and reflecting off or interacting with layers and other objects that may be located within the formation. The transmitter may be powered by a downhole battery, turbine, a power transmission system or combinations thereof. The transmitter may produce a characteristic wave such that the characteristic wave may be differentiated from noise generated by the primary downhole tool string. 
     At least one acoustic receiver adapted to measure acoustic waves within the formation may be disposed on the tool string and spaced apart from the transmitter. The receiver may be disposed on the same tool string as the transmitter or may be disposed on a separate tool string in a separate borehole. In various embodiments, the receiver may comprise a geophone, a hydrophone, or an accelerometer. A plurality of receivers may be oriented orthogonal each other and spaced circumferentially around the tool string. The plurality of receivers may also be spaced evenly and axially along the tool string. 
     At least one acoustic transducer adapted to create a canceling acoustic wave may be disposed on the tool string proximate the receiver and attached to a canceling signal generator. The acoustic transducer may comprise a piezoelectric crystal or magnetic coil device capable of creating an acoustic wave. The acoustic wave created by the transducer may be capable of canceling an acoustic wave traveling along the borehole or along the tool string as experienced at the receiver. This canceling may allow the receiver to measure less of a wave traveling along the borehole or tool string and more of a wave traveling through the formation. A downhole telemetry network may actuate or control the acoustic transducer via the canceling signal generator. The downhole telemetry network may comprise inductive couplers located in joints comprised within the tool string. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side-view diagram of an embodiment of a downhole tool string assembly. 
         FIG. 2  is a side-view diagram of an embodiment of two downhole tool string assemblies. 
         FIG. 3  is a side-view diagram of another embodiment of a downhole tool string assembly. 
         FIG. 4  is a side-view diagram of another embodiment of a downhole tool string assembly. 
         FIG. 5  is a perspective diagram of an embodiment of a plurality of receivers integrated into a tool string component. 
         FIG. 6  is a cross-sectional diagram of an embodiment of a receiver integrated into a stabilizer blade. 
         FIG. 7  is an axial cross-sectional diagram of an embodiment of a receiver integrated into a stabilizer blade. 
         FIG. 8   a  is a cross-sectional diagram of a close-up view of embodiments of a receiver and transducer. 
         FIG. 8   b  is a perspective diagram of embodiments of a receiver and transducer. 
         FIG. 8   c  is another perspective diagram of embodiments of a receiver and transducer. 
         FIG. 8   d  is another perspective diagram of embodiments of a receiver and transducer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a downhole tool string  101  is shown suspended by a derrick  102  in a borehole  150 . A tool string component  100  is shown as part of the tool string  101  and may be in communication with surface equipment  103  through a downhole telemetry network  122 . The downhole telemetry network  122  may comprise inductive couplers located in joints  170  of the tool string. The tool string component  100  comprises a receiver  105 . The receiver  105  may comprise a geophone, a hydrophone, or an accelerometer. The receiver  105  may be adapted to measure waves within the surrounding earthen formation  130 . Based upon the frequency of those waves, they may be referred to as sonic waves, ultrasonic waves, seismic waves or other designations known in the art. 
     A transmitter  110  is shown that may be adapted to generate waves in the earthen formation  130 . In the embodiment shown, the transmitter is located on the surface of the formation  130 . The transmitter  110  may comprise a hammer, an explosive element, seismic source, a siren, a jar, a piezoelectric source, a magnetostrictive element, an eccentric rotor, a drill bit, or any other device known in the art capable of propagating a wave through an earthen formation  130 . The transmitter  110  may generate a wave  115  which travels through the earthen formation  130  and may be measured by the receiver  105  at the tool string component  100 . The transmitter  110  and receiver  105  may be actuated or controlled by a downhole telemetry network  122 . 
     If the wave  115  hits the borehole  150  a tube wave or acoustic wave  125  may be formed. A tube wave is a wave that may travel along the length of the borehole  150  and be measured by the receiver  105 . The tube wave may propagate along the borehole  150  with a wavelength much longer than the diameter of the borehole  150 . The tube wave may have less attenuation than the wave  115  traveling through the earthen formation  130 . If the tool string  101  makes contact with the borehole  150 , an acoustic wave may begin to travel along the length of the tool string  101 . The acoustic wave traveling through the metal of the tool string  101  will generally travel at a much higher rate of speed than the wave  115  traveling through the earthen formation  130 . Thus, the tube wave or acoustic wave  125  may interfere with the receiver&#39;s  105  ability to measure the wave  115 . 
       FIG. 2  depicts another embodiment where the transmitter  110  is located on a second tool string  201  suspended in a second borehole  250 . In this embodiment, the transmitter  110  is shown generating a wave  115  which travels from the second borehole  250  through the earthen formation to the first borehole  150  where it is measured by the receiver  105 . The wave  115  may create a tube wave or acoustic wave  125  when it comes in contact with the borehole  150  that may travel along the borehole  150  if a tube wave or along the tool string  101  if an acoustic wave and cause interference at the receiver  105 . 
       FIG. 3  depicts another embodiment where the transmitter  110  is located on the same tool string  101  as the receiver  105 . In this embodiment, the transmitter  110  is shown generating waves  115  that may travel along the sides of the borehole  150  and be measured by the receiver  105 . A tube wave or acoustic wave  125  may also be generated that may travel along the borehole  150  if a tube wave or along the tool string  101  if an acoustic wave and may cause interference at the receiver  105 . 
       FIG. 4  depicts another embodiment where the transmitter  110  is located on the same tool string  101  as the receiver  105 . In this embodiment, the transmitter  110  is shown generating waves  115  which may reflect off of various boundary layers and other objects within the earthen formation and then be measured by a plurality of receivers  105 . The various receivers  105  may be orientated in different directions to measure different aspects of the wave  115 . A tube wave or acoustic wave  125  may also be created which may travel along the borehole  150  if a tube wave or along the tool string  101  if an acoustic wave and cause interference at the receivers  105 . The tube wave or acoustic wave  125  may be read by a sensor  310 . It is believed that the tube wave or acoustic wave  125  as read by the sensor  310  may be cancelled by an opposite nulling wave generated proximate the receiver  105 . The transmitter  110  and/or receivers  105  may be powered by a downhole turbine  303 , battery  304  or power transmission system  305 . 
       FIG. 5  is a perspective diagram of an embodiment of a tool string component  100 . In some embodiments the tool string component  100  may comprise receivers  105  mounted to the tool string component  100 . Such receivers  105  may be adapted to detect and measure vibrations or other waves propagating to the tool string component  100 . Receivers  105  may comprise geophones, hydrophones, or accelerometers. 
     In some embodiments, the tool string component  100  may comprise a stabilizer  450 . The stabilizer  450  may comprise a plurality of stabilizer blades  455  that may further comprise pockets  460  adapted to hold a receiver  105  or plurality of receivers  105 . When the tool string component  100  is disposed in a borehole, at least one stabilizer blade  455  may extend to contact the formation which may allow better coupling of receivers  105  to the borehole. 
       FIG. 6  is a cross-sectional diagram of an embodiment of a receiver  105  integrated into a stabilizer blade  455 . The receiver  105  may be a three-component geophone  501 . The stabilizer blade  455  may have a pocket  460  adapted to receive at least three downhole geophones wherein each geophone  520 ,  521 ,  522  receives signals on different orthogonal axes. The first geophone  520  may be adapted to receive and measure signals in the y-direction  530  with respect to a three-dimensional coordinate system, the second geophone  521  may be adapted to receive and measure signals in the z-direction  531  with respect to a three-dimensional coordinate system, and the third geophone  522  may be adapted to receive and measure signals in the x-direction  532  with respect to a three-dimensional coordinate system. 
     The stabilizer blade  455  may also comprise at least three piezoelectric transducers wherein each piezoelectric transducer  550 ,  551 ,  552  correlates with a respective geophone  520 ,  521 ,  522 . Each piezoelectric transducer  550 ,  551 ,  552  may produce a nulling wave on a different orthogonal axis. The first piezoelectric transducer  550  may be adapted to produce a nulling wave in the y-direction  530  with respect to a three-dimensional coordinate system, the second piezoelectric transducer  551  may be adapted to produce a nulling wave in the z-direction  531  with respect to a three-dimensional coordinate system, and the third piezoelectric transducer  552  may be adapted to produce a nulling wave in the x-direction  532  with respect to a three-dimensional coordinate system. 
     In some embodiments, each piezoelectric transducer  550 ,  551 ,  552  may be individually driven by a canceling signal generator  570  to cancel the effects of a tube wave traveling along the length of the borehole or an acoustic wave traveling along the length of the tool string. The canceling signal generator  570  may be in communication with surface equipment  103  (see  FIG. 1 ) comprising a primary signal generator. The primary signal generator may send a signal to be transmitted in the form of an acoustic wave by the transmitter  110  (see  FIG. 1 ). The surface equipment  103  may also comprise a comparator. The comparator may read the signal generated by the primary signal generator and lock into that signal. The comparator may then communicate to the canceling signal generator  570  which signals need to be transmitted through the piezoelectric transducers  550 ,  551 ,  552  to attempt to cancel unwanted signals. When actively driven by a canceling signal generator  570  the piezoelectric transducers  550 ,  551 ,  552  may have a nulling effect on the geophones  520 ,  521 ,  522 . The primary signal generator and comparator may also be kept downhole. 
     In some embodiments, the canceling signal generator  570  may be connected to at least one piezoelectric sensor  560 . The piezoelectric sensor  560  may read tube waves traveling along the borehole or acoustic waves traveling along the length of the tool string. The piezoelectric sensor  560  may then communicate to the canceling signal generator  570  what signals need to be transmitted through the piezoelectric transducers  550 ,  551 ,  552  to attempt to cancel those signals. 
       FIG. 7  is an axial cross-sectional diagram of an embodiment of a receiver  105  integrated into a tool sting component  100 . In this embodiment, the receiver  105  comprises three geophones  620 ,  621 ,  622  each on different orthogonal axes with respect to a three-dimensional coordinate system. In this embodiment, the receiver  105  also comprises three piezoelectric transducers  650 ,  651 ,  652  which each correlate with a respective geophone  620 ,  621 ,  622 . Each piezoelectric transducer  650 ,  651 ,  652  may produce a nulling wave on a different orthogonal axis. 
       FIG. 8   a  shows a close-up view of embodiments of a receiver  820  and transducer  850 . The receiver  820  may be adapted to receive and measure acoustic waves. The transducer  850  may be adapted to produce a nulling acoustic wave to substantially cancel unwanted acoustic waves at the receiver  820 . 
       FIG. 8   b  depicts other embodiments of the receiver  820  and transducer  850 . The embodiment of the receiver  820  shown comprises a piezoelectric crystal  825  intermediate two conductive plates  826  and  827 . The two conductive plates  826  and  827  are wired to a measuring device (not shown) by two wires  828  and  829  respectively. As pressure is applied to the two conductive plates  826  and  827  in the form of an acoustic wave an electrical current is generated in the piezoelectric crystal  825 . That electrical current may be sent into the two wires  828  and  829  and may further be measured by the measuring device. The embodiment of the transducer  850  shown also comprises a piezoelectric crystal  855  intermediate two conductive plates  856  and  857 . The two conductive plates  856  and  857  are wired to a canceling signal generator (not shown) by two wires  858  and  859  respectively. The canceling signal generator may send an electrical current into the two wires  858  and  859  and thus cause the piezoelectric crystal  855  to expand and contract. This expansion and contraction may produce a nulling acoustic wave to substantially cancel unwanted acoustic waves at the receiver  820 . 
       FIG. 8   c  depicts a cross-sectional view of other embodiments of the receiver  820  and transducer  850 . In the embodiments shown, the receiver  820  may comprise a frame  832  housing a coil of wire  835  that may be connected to a measuring device (not shown). A magnet  833  may be housed within the coil of wire  835 . As a wave hits the frame  832 , the magnet  833  may oscillate within the coil of wire  835  thus causing an electrical current to form within the coil of wire  835 . That electrical current may be measured by the measuring device. The embodiment of the transducer  850  shown may also comprise a frame  862 . The frame  862  may house a coil of wire  865  that may be connected to a canceling signal generator (not shown). A magnet  863  may be housed within the coil of wire  865 . The canceling signal generator may send an electrical current into the coil of wire  865  and thus cause the magnet  863  to oscillate within the coil of wire  865 . This oscillation of the magnet  863  may produce a nulling wave to substantially cancel unwanted waves at the receiver  820 . 
       FIG. 8   d  depicts a cross-sectional view of other embodiments of the receiver  820  and transducer  850 . In the embodiments shown, the receiver  820  may comprise a frame  832  housing a coil of wire  835  that may be connected to a measuring device (not shown). A magnet  833  may be housed within the coil of wire  835 . As a wave hits the frame  832 , the magnet  833  may oscillate within the coil of wire  835  thus causing an electrical current to form within the coil of wire  835 . That electrical current may be measured by the measuring device. The embodiment of the transducer  850  shown may comprise a piezoelectric crystal  855  intermediate two conductive plates  856  and  857 . The two conductive plates  856  and  857  are wired to a canceling signal generator (not shown) by two wires  858  and  859  respectively. The canceling signal generator may send an electrical current into the two wires  858  and  859  and thus cause the piezoelectric crystal  855  to expand and contract. This expansion and contraction may produce a nulling acoustic wave to substantially cancel unwanted acoustic waves at the receiver  820 . 
     Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.