Abstract:
The acoustic isolator assembly of the present invention comprises a elongated cylindrical body suited for connection to an acoustic array and subsequent disposition within a wellbore. According to one embodiment of the present invention, the acoustic isolator comprises a plurality of cylindrical isolator modules that are coaxially arranged to form the body of the tool. Each isolator module comprises a spring disposed within an outer housing. The separate isolator modules are attached to one another by connecting rods around which are disposed a plurality of metal spacers. The isolator module further comprises mechanical stops that limit the deflection of the spring during high axial loading. These features enable the acoustic isolator assembly to withstand the high loading that may be applied during logging operations. Therefore, the isolator modules are capable of supporting high compressive and tensile loads without suffering permanent deformation of the springs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to well logging tools used in oil and gas wells to characterize a subterranean formation using acoustic waves transmitted and received by the tool. More specifically, the present invention relates to methods and apparatus used to acoustically isolate the transmitter from the receiver while providing sufficient structural strength to withstand common handling and use, including pushing and pulling of the tool.  
           [0003]    Acoustic logging tools that are used to characterize subterranean formations surrounding well bores are well known in the art. In general, acoustic logging tools operate by broadcasting an acoustic signal into a formation from one or more transmitters located at one position on the tool and receiving the signal with one or more receivers located at a second location on the tool. Properties of the received acoustic signal, such as travel time, frequency, amplitude, and attenuation, are then used to characterize the surrounding formation.  
           [0004]    The transmitters generate a compressional waveform that travels through the borehole fluids and into the surrounding formation. The acoustic wavefields propagate through the formation in a variety of modes, the most important being compressional waves, or “P-waves”, and transverse shear waves, or “S-waves”. P-waves are characterized by particle motion in the direction of wave travel while S-waves are characterized by particle motion perpendicular to the direction of wave travel. The various modes of propagation are distinguishable by their relative velocities. The velocities of both P-waves and S-waves depend on the elastic constants and the density of the medium through which the waves travel.  
           [0005]    Ideally, the only acoustic signals received by the tool&#39;s receivers would be those signals that are transmitted by the tool that have traveled through the formation. However, if not properly isolated, the receiver will also detect other signals, sometimes referred to as “tool noise” or “road noise”. This undesired noise can interfere with the ability of the tool to render an accurate representation of the acoustic response of the formation. This noise is typically energy, more specifically vibrations, traveling within or on the surface or body of the logging tool. The noise may be a high or low frequency noise, such as that created by the transmitters or by contact of the logging tool with the wellbore. Therefore, it is desirable to isolate the receivers of a well logging tool from extraneous sources of acoustic signals, namely the transmitters and the rigid body of the tool.  
           [0006]    U.S. Pat. No. 3,190,388, issued Jun. 22, 1965 to Moser et al., teaches a simple acoustic well logging tool with a signal attenuating structure. The logging tool comprises an outer housing made of steel and a centrally disposed bar that supports the acoustic apparatus and is connected to the outer housing at the top and bottom of the tool. To attenuate the acoustic signal, the outer housing incorporates a series of generally U-shaped, helical grooves cut into the cylindrical metal housing on both the inside and outside diameter. The grooves are of a depth greater than one-half of the wall-thickness of the housing so as to interrupt the direct travel of the signal through the housing. The grooves are arranged so that the spacing between the inner and outer grooves lengthwise of the housing is less than one quarter of the wave length of the principal frequency of the acoustic signal. The grooves may be filled with a high-density material, such as lead, to increase the weight of the housing to further inhibit acoustic transmission. An alternate method of constructing the housing using a plurality of circular openings through the wall of the outer housing is also disclosed. The acoustic transducers are mounted on the centrally mounted, hollow bar that is constructed of Teflon™, or some other material with a low velocity characteristic.  
           [0007]    U.S. Pat. No. 5,036,945, issued Aug. 6, 1991, to Hoyle et al. discloses a sonic well tool having a first and second attenuation and delay apparatus for attenuating and delaying the signal traversing the tool body. The first attenuation and delay apparatus includes interleaved rubber and metal like washers for attenuating compressional and flexural waves propagating along the body, and further includes a bellows section having a corrugated shape and a thin traverse dimension. The second attenuation and delay apparatus includes mass loading rings surrounding the housing of the well tool, and also includes a bellows section having a corrugated shape and a thin traverse dimension.  
           [0008]    U.S. Pat. No. 5,229,553, issued Jul. 20, 1993 to Lester et al. discloses an acoustic isolator for use with a well logging tool having transducers in a first and third tool segment, which are to be acoustically isolated from receivers in a second and fourth tool segment. The acoustic isolator consists of vertebrate links composed of spools, encased by resilient boots, which spools are arranged end-to-end in tandem configuration. A plurality of split shells interconnect the spools by externally gripping the boots covering the end portions of the respective adjacent spools.  
           [0009]    The design of acoustic isolators for downhole applications involves two requirements that are seemingly mutually exclusive. The first of these requirements is that the tool be sufficiently flexible to attenuate acoustic waves traveling at or near the surface of the tool. The second requirement is that the tool be strong enough to survive running and retrieval operations, which may be by wireline or tubing conveyed means. During these operations it is often required to push or pull heavy loads via the tool. It is desirable that this extreme loading not have any permanent deleterious effects on the performance of the isolator. Additionally, because of the nature of well logging operations, including the environment in which it occurs, it is desired to have a tool with a simple, robust design.  
           [0010]    Typical acoustic well logging tools that incorporate acoustic isolators are long, flexible cylinders. Occasionally, well logging tools become stuck in the well bore and have to be retrieved using force. The typical acoustic isolator can not withstand the forces normally observed during these operations and recovery of the tool often results in destruction of, or major damage to, the tool. Therefore, a need exists in the art for an improved acoustic isolator capable of high rates of signal attenuation in normal logging operations and capable of withstanding high axial forces common to fishing or recovery operations.  
         SUMMARY OF THE INVENTION  
         [0011]    The acoustic isolator assembly of the present invention comprises a elongated cylindrical body suited for connection to an acoustic array and subsequent disposition within a wellbore. According to one embodiment of the present invention, the acoustic isolator is a component of a well logging tool and is linearly disposed between an acoustic transmitter and an acoustic receiver. The acoustic isolator serves to restrict the propagation of acoustic signals along the length of the logging tool, therefore reducing the amount of tool noise or road noise received by the receiver.  
           [0012]    According to one embodiment of the present invention, the acoustic isolator comprises a plurality of cylindrical isolator modules that are coaxially arranged to form the body of the tool. Each isolator module comprises a spring disposed within an outer housing. The separate isolator modules are attached to one another by connecting rods around which are disposed a plurality of nodal masses.  
           [0013]    The isolator module further comprises mechanical stops that limit the deflection of the spring during high axial tension, or compression, loading. These features enable the acoustic isolator assembly to withstand the high loading that may be applied during logging operations. Therefore, the isolator modules are capable of supporting high compressive and tensile loads without suffering permanent deformation of the springs.  
           [0014]    The springs are preferably encapsulated with a material having a low velocity characteristic, such as a rubber or phenolic compound. Holes may also be drilled radially through the coils of the spring to further inhibit the transmission of acoustic signals. These holes may also be filled with rubber or a similar material, as previously discussed. The outside diameter of the outer housing may be coated with a dissimilar material, such as fiberglass or another low velocity characteristic material that can withstand the wellbore environment. This coating preferably interferes with any signal traveling along the outer surface of the tool. Further, the outside surface of the tool provides an irregular surface that also aids in the attenuation of signals along the outside surface.  
           [0015]    The acoustic isolator assembly provides a centrally located bore for electrical connection between the receiver and transmitter. The entire assembly may be filled with oil during operation. The oil prevents corrosion and eliminates any stresses from hydrostatic pressure in the wellbore.  
           [0016]    Therefore, the acoustic isolator of the present invention effectively attenuates the higher frequency compressional waves through a series of isolator modules containing springs and attenuates the lower frequency transverse waves through the overall acoustic response of the system that is greatly improved through flexurally-limber isolator modules and nodal masses. The arrangement of the components also allows the tool to withstand high axial loading. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The nature, objects, and advantages of the present invention will become more apparent to those skilled in the art after consideration of the following detailed description in connection with the accompanying figures wherein:  
         [0018]    [0018]FIG. 1 is a general schematic representation of an acoustic well logging tool;  
         [0019]    [0019]FIG. 2 is one embodiment of a well logging tool constructed in accordance with the present invention; and  
         [0020]    [0020]FIG. 3 is a cross-sectional view of one embodiment of an acoustic isolator module constructed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Referring to FIG. 1, an acoustic well logging tool  10  is lowered into a wellbore  12  filled with a fluid  16 . The logging tool can be conveyed into the wellbore  12  by a wireline  14 , or a string of pipe. The acoustic well logging tool  10  generally comprises a transmitter assembly  18 , an acoustic isolator assembly  20 , and a receiver assembly  22 . The term “acoustic” as used in describing the present invention is intended to generally describe the nature of the tool as one that employs the use of sound waves of any frequency and is not limited to any specific frequency range, unless specifically stated or claimed.  
         [0022]    In general operation, the transmitter assembly  18  generates an acoustic signal  26  that propagates through the wellbore fluid  16  and into the surrounding formation  24 . The acoustic signal  26  propagates through the formation  24  and is received by the receiver assembly  22 . The received acoustic signal can be used to determine the acoustic response of the formation  24 , which can indicate the properties of the formation, such as porosity, composition, and the presence of liquids or solids in the formation.  
         [0023]    In order to effectively analyze the acoustic signal, the receiver  22  must be very sensitive, therefore any extraneous signal that is received has the propensity to cause interference with the desired signal and degrade the performance of the tool. The acoustic isolator assembly  20  is placed between the transmitter assembly  18  and the receiver assembly  22  to decrease the acoustic energy traveling directly between the transmitter and receiver without first passing through the formation  24 .  
         [0024]    Referring to FIG. 2, the acoustic isolator assembly  22  is substantially an elongated, cylindrical assembly having a bore through the center and comprising one or more isolator modules  28  that are linearly affixed to each other. One preferred embodiment of the isolator assembly  22 , comprises five isolator modules  28  connected in series between a transmitter assembly  18  at the upper end  34  and a receiver assembly  22  at the lower end  36 . It is preferred that the transmitter assembly  18  and the receiver assembly  22  are affixed as close as possible to the ends  34 ,  36  without the inclusion of an adapter joint or other component between the transmitter and receiver components and the isolator assembly.  
         [0025]    Although one preferred embodiment of an isolator assembly  22  constructed in accordance with the present invention uses a series of five isolator modules  28 , other arrangements may also find utility depending on the amount of signal attenuation desired and the type of signal interference sought to be attenuated. It is contemplated that any number of isolator modules  28  may be used to make up a single tool. It is also possible that in the event of signal interference coming from below the acoustic receiver  22 , one or more isolator modules  28  may be installed below the receiver  22 .  
         [0026]    Now referring to FIG. 3, each isolator module  28  comprises a spring  38 , an outer housing  40 , a connector rod  52 , an resilient spacer  54 , and a metal spacer  56 . Spring  38  fits inside the outer housing  40  and is threadably, or otherwise connected at the upper end  37 . Resilient spacer  54  and metal spacer  56  are positioned on connector rod  52 , with the resilient spacer  54  between the rod  52  and the metal spacer  56 . Seal  60  is arranged so that the inner bore  62  of the module is hydraulically isolated from the outside of the tool.  
         [0027]    Spring  38  is a linear spring having a first end  48  and a second end  50 , each end being adapted for connection to a connector rod  52 . Spring  38  is preferably made from a hollow, cylindrical piece of corrosion resistant alloy, such as a stainless steel or a nickel based alloy. A helical cut is made through the wall of the hollow cylinder to form a spring. This helical cut may be a single or multiple lead helix. The pitch of the helical cut is determined by the desired stiffness of the spring. One preferred embodiment is a double lead helical spring having a resultant stiffness of 10,000 to 30,000 pounds per inch. The spring  38  is coated with a resilient material  39 , both on its inner and outer diameter and in between the spring coils  70 . Therefore, each spring coil  70  is separated from adjacent coils by a layer of resilient material  39 . The spring  38  may also have radial holes  72  that penetrate the spring coils  70 , which may also be filled with a resilient material  39 . The resilient material is preferably a moldable, durable material such as rubber, Viton™, or other elastomer. For effective attenuation the preferred resilient material should have a durometer (shore A scale) between 50 and 100.  
         [0028]    Outer housing  40  is an elongated, hollow cylinder having a first end  42  adapted to be attached to the outside of spring  38  and a second end  44  having a circumferential shoulder  46 . Outer housing  40  is preferably coated on its outside surface with an attenuating material, such as fiberglass. The second end  44  is adapted to receive a circumferential seal  60  on its inside diameter to seal against the connector rod  52 . The outer housing  40  is preferably constructed from a corrosion resistant material, such as stainless steel or a nickel based alloy. A gap  61  of between 0.010 in. and 0.100 in. is preferably maintained between the outside surface of the spring  38  and the inside diameter of the outer housing  40 .  
         [0029]    Connecting rod  52  is a hollow, cylindrical member also preferably constructed from a corrosion resistant material such as a stainless steel or a nickel based alloy. Each end of the connector rod  52  is threaded to engage the spring  38 . An resilient spacer  54  is circumferentially mounted on the outside of the connector rod  52 . The resilient spacer  54  is constructed of a resilient material, such as rubber or another elastomer such as Viton™. The resilient spacer  54  forms a ring of resilient material around the outside of the connector rod  52 . Metal spacer  56  is also a hollow cylinder and is installed around the resilient spacer  54 . The metal spacer  56  is preferably constructed from a high density material, such as tungsten carbide or lead and serves as a nodal mass to help attenuate the acoustic signal as will be discussed below. A preferable metal spacer  56  has a density of at least one to three times the density of steel.  
         [0030]    Each isolator module of the attenuating assembly has a centrally located bore that allows for the passage of electrical connections between the receiver and transmitter. The entire acoustic tool assembly is preferably filled with fluid during operation. The preferable fluid is a non-corrosive viscous oil, such as petroleum oil or a synthetic hydrocarbon fluid. The oil has negligible effects on the attenuation of acoustic signals through the isolator modules and it inhibits corrosion within the modules and eliminates any affect from hydrostatic pressure in the wellbore, which could over-stress the internal parts of the isolator module.  
         [0031]    The tool described above serves to attenuate both high frequency and low frequency acoustic signals ranging from below 500 Hz to over 10,000 Hz. These signals can be attenuated through primarily two methods. The first method of attenuation is acoustic wave isolation which occurs when the signal is forced to travel across an interface of two different materials having significantly different acoustic impedances. The second method of attenuation is acoustic wave absorption that occurs when the signal is forced to travel through materials that tend to absorb the vibrations. These methods can be considered at the component level for higher frequency applications but must be analyzed macroscopically for low frequency operations where the wavelength may be as long as the tool itself. In these low frequency applications the overall flexibility of the tool has a major impact on signal attenuation. It should also be noted that the irregular outside surface of the tool also helps attenuate any signal traveling along the tool surface.  
         [0032]    High frequency acoustic signals enter the isolator module  28  at the upper end  48  of the spring  38 . The signal will then travel through the spring  38  and the outer housing  40 . Sound waves traveling axially down the spring  38  will be attenuated by the multiple interfaces between the spring  38  and the resilient material  39 . Sound waves traveling solely through the spring  38  will have to follow the coils of the spring  38 , thereby greatly increasing the travel distance of the signal. Any signal traveling through the outer housing  40  will have to cross seal  60  to continue down the tool and therefore will have to cross two attenuating interfaces. The resultant signal then passes through the connector rod  52  and into another isolator module  28 .  
         [0033]    Low frequency acoustic signals will tend to vibrate the entire tool in waves having very long wavelengths. The combination of the flexibility of the spring  38  and the metal spacers  56  help to damp out and attenuate this signal. Because the metal spacers  56  are made of a high density material, they operate as nodal masses that absorb low frequency acoustic energy. A portion of the energy of the acoustic signal is absorbed by the motion of the spacers  56  and, as the signal travels the length of the tool, it further attenuates as it encounters additional metal spacers  56 .  
         [0034]    The inherent flexibility of the system aids in the attenuation of acoustic signals between the transmitter and receiver. This flexibility can also be a liability if the acoustic logging tool becomes stuck in a wellbore and has to be retrieved. Downhole tools that become stuck in the wellbore are often subjected to high axial loads (i.e. pushing and pulling) in an attempt to jar loose the stuck tools. In previous tools these high axial loads have often caused substantial damage or failure in the acoustic isolator components. If failure occurs, additional fishing operations are required to remove the now broken pieces of the tool from the wellbore.  
         [0035]    The novel arrangement of components in the isolator module  28  allows each module, and consequently the tool as a whole, to withstand high axial loading. Referring to FIG. 3, when a compressive load is applied in the direction of arrow  62 , spring  38  will compress. The compression of spring  38  will close compression gaps  64  until the housing  40  contacts the metal spacer  56 . Once these gaps  64  are closed, the load is transferred directly from the outer housing  40  to the metal spacer  56  and into the next outer housing  40 . The deflection of the spring  28  is limited so that the resilient material  39  remains molded to the spring  38  and the spring  38  does not plastically deform.  
         [0036]    If a tension load is applied opposite arrow  62 , the spring  38  will elongate. Because the outer housing  40  is threadably attached to the spring  38  at the upper end  37 , the spring  38  can only elongate until tension gap  66  is closed. Once the tension gap  66  is closed the load is carried through the outer housing  40  and spring  38  is not allowed to be damaged due to excessive stretch. In both the tension and compression loading scenarios, the attenuation of the tool is severely decreased but once the load is released the tool will function properly. If the components are constructed from the preferable high-strength corrosion resistant materials the tool can withstand tension and compression loads of up to 100,000 lbs.  
         [0037]    While the above represents the preferred embodiment of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made herein without departing from the scope of the invention as claimed.