Abstract:
A downhole tool for acoustically imaging a subterranean formation having a transmitter and receiver and which includes an acoustic isolator between the transmitter and receiver. The acoustic isolator includes attenuation elements with rounded ends and that are connected in series by compression fittings. The compression fittings include sleeves that threadingly engage lock nuts and compressively engage the rounded ends. The contact interfaces between the compression fittings extend along a curved path thereby limiting acoustic transmission along the series of attenuation elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present disclosure relates in general to a system for use in imaging a subterranean borehole. More specifically, the present disclosure relates to a downhole imaging system having an acoustic isolator for attenuating acoustic signals in the system. 
     2. Description of Prior Art 
     Geological data concerning subterranean formations is often gathered with an imaging technique. The data obtained usually relates to formation resistivity, formation porosity and/or permeability, identification of formation strata and the like. Zones of entrained hydrocarbons and reservoir production capabilities can be determined using this data. Often, the imaging is obtained with a downhole logging tool, which is deployed into a well that extends into the subterranean formation. Example downhole tools for imaging include resistivity tools, nuclear magnetic resonance (NMR) devices, and acoustic sensors. Resistivity tools usually include electrodes on one portion of the tool that are energized to emit a current into the formation, which is measured with sensors on another part of the tool. NMR devices release radiation that scatters from the formation, which is analyzed for assessing formation details. Similar to radiation devices, acoustic devices analyze acoustic data that reflects from the formation. Acoustic imaging tools though are susceptible to erroneous readings when the acoustic signals travel from the transmitter along the tool housing and directly to receivers in the tool. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are examples of acoustic tools used for imaging in a subterranean wellbore. In one example the tool includes a transmitter, a receiver, and an acoustic isolator disposed between the transmitter and receiver. In this embodiment, the acoustic isolator includes an attenuation element having a rounded end, and a compression element that couples with the rounded end and along a contact interface that defines a curved path. The compression element can include a lock ring having an profiled surface that is oblique to an axis of the acoustic isolator, and wherein the contact interface is on the profiled surface. In this example, the compression element further includes an annular lock nut having a lip that projects radially inward and retains the lock ring in a position to maintain contact between the lock ring and the attenuation element. The compression element may optionally further have an annular sleeve having an end coupled with the lock nut and an opposing end that circumscribes an adjacent attenuation element. In an alternative, the rounded end of the attenuation element is made up of a first end, the attenuation element having a second end distal from the first end and wherein the second end is rounded. In this example, the acoustic isolator may further have a plurality of attenuation elements coupled together in series by a plurality of sleeves and lock rings that have an axial side that is profiled oblique to an axis of the tool to define a profiled side. The contact interface can be between the lock rings and rounded ends of the attenuation elements. In an embodiment, opposing ends of adjacent attenuation elements insert into a one of the sleeves, and wherein lock rings are wedged between opposite ends of each of the sleeves and the opposing ends of adjacent attenuation elements. Lock nuts may be included that are threaded onto the ends of the sleeves for wedging the lock rings against the ends of the attenuation elements. The tool may further include an anti-rotation pin that inserts through a side wall of a sleeve and attaches into the rounded end, so that the attenuation element is rotationally coupled with the sleeve. 
     Also disclosed herein is an acoustic tool for imaging in a subterranean wellbore that includes an acoustic transmitter, an acoustic receiver spaced axially away from the transmitter, and an elongate attenuation element that is between the transmitter and receiver. The attenuation element includes an end that is spherically shaped, and that is inserted into an open end of a sleeve. A communication path is defined by a transmission of acoustic signals between the transmitter and receiver, and a lock ring contacts the end of the attenuation element. Further, a contact interface between the lock ring and the attenuation element, which follows a curved route, is in the communication path and defines a reduction in cross sectional area of the communication path. In an example, the end of the attenuation element has a first end and a second end that is rounded and inserted into an open end of a sleeve. This example of the tool further includes a multiplicity of attenuation elements, a multiplicity of sleeves and lock rings for coupling the attenuation element in series, and thereby forming a multiplicity of contact interfaces that each attenuate acoustic signals that propagate along the communication path. The tool may further include an oil filled tube inserted into a bore that extends axially through each of the attenuation elements, and lines in the tube that selectively couple with the transmitter and receiver. Optionally, the lock ring spaces the sleeve radially away from the attenuation element. 
     In another example, an acoustic tool for imaging in a subterranean wellbore is disclosed that includes a tool body, an acoustic transmitter coupled with the body, an acoustic receiver coupled with the body, an acoustic path along the body and between the transmitter and receiver, and a series of acoustic attenuation members that are in the acoustic path and that make up a coupling between adjacent members having a curved interface and that define a substantial decrease in a cross sectional area of the acoustic path. The attenuation members include sleeves, attenuation elements each having opposing ends that are rounded and wherein opposing ends of adjacent attenuation elements insert into open ends of sleeves, and lock rings that retain the ends of the attenuation elements in the sleeves. Lock nuts may further be included that wedge the lock rings against the rounded ends of the attenuation elements. In an alternate embodiment, the curved interface defines a loop that circumscribes an axis of the tool. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is side partial sectional view of an example of a logging tool in a wellbore and having an acoustic isolator and in accordance with the present invention. 
         FIG. 2  is a side sectional view of an example of the acoustic isolator of  FIG. 1  and in accordance with the present invention. 
         FIG. 3  is a perspective view of an example of an attenuation element for use with the acoustic isolator of  FIG. 2  and in accordance with the present invention. 
         FIG. 4  is a perspective view of an example of a sleeve for use with the acoustic isolator of  FIG. 2  and in accordance with the present invention. 
         FIG. 5  is a perspective view of an example of a lock ring for use with the acoustic isolator of  FIG. 2  and in accordance with the present invention. 
         FIG. 6  is a perspective view of an example of the acoustic isolator of  FIG. 2  and in accordance with the present invention. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
       FIG. 1  shows in side partial sectional view an embodiment of a downhole tool  10  deployed in borehole  12 . In the example of  FIG. 1 , borehole  12  intersects a subterranean formation  14 , where the downhole tool  10  is used for acoustically interrogating the formation  14  from within borehole  12 . Included on the body of the downhole tool  10  is a transmitter  16  shown generating acoustic waves  18  that propagate from the tool  10 , through borehole  12 , and into the formation  14 . Some of the acoustic waves  18  reflect from within the formation  14 , and propagate back to acoustic receivers  20 , shown provided on a part of the tool  10  different where the transmitter  16  is located. The acoustic sensors  20  sense and record the waves  18 . As is known, analyzing the waves  18  can yield information about the formation  14 ; such as the presence and/or amount of hydrocarbons in the formation  14 . 
     An example of an acoustic attenuator  22  is shown included with the tool  10  and disposed between the transmitter  16  and receivers  20 . Further in the example of  FIG. 1 , the downhole tool  10  is deployed on a wireline  24  in the borehole, wherein optionally the wireline  24  can be coiled tubing or any other conveyance means for lowering and raising a tool within the borehole  12 . Further shown is a wellhead assembly  26  at the opening to the wellbore  12  and on surface  28 . As shown, wireline  24  is threaded through the wellhead assembly  26  from surface and into the wellbore  12 . 
       FIG. 2  provides a side sectional view of an example of the acoustic attenuator  22 , which is shown as having a bore  30  that extends therethrough and substantially aligned with an axis A x  of the attenuator  22 . Also provided in  FIG. 2  are attenuation elements  32 , which as shown in perspective view in  FIG. 3 , are generally elongate elements. Attenuation elements  32  have opposing ends  34 , a portion of each of which approximates a spherical shape. The mid portion  36  of the attenuation element  32  of  FIGS. 2 and 3  has an outer radius that is about the same as the radius of each of the ends  34  from the axis A x . The radius of the attenuation element  32  decreases between the mid portion  36  and each of the spherical ends  34  to define profiles  37  between these portions of the attenuation element  32 . In the example of  FIG. 2 , each profile  37  has a generally curved shape from its low point and up to the maximum radius portions of each end  34 . Whereas the profile  37  increases generally linearly from its low point and to the maximum radius region of the mid portion  36 . Keyholes  38  are shown formed radially inward into the attenuation element  32  and from an outer surface of the ends  34 . 
     Referring back to  FIG. 2 , an anti-rotation pin  40  is shown having a radially inward projecting end that projects into the keyhole  38 . An end the anti-rotation pin  40  distal from keyhole  38  is illustrated coupled with a sleeve  42 ; where sleeve  42  circumscribes respective ends  34  of two adjacent attenuation elements  32 . In the example of  FIG. 2 , inserting the anti-rotation pins  40  through the sleeves  42  and into the attenuation elements  32  limits relative rotation of attenuation elements  32 . Openings  44  are provided in the sleeve  42  to allow insertion of the anti-rotation pins  40 . Referring now to  FIG. 4 , a perspective view of the sleeve  42  is shown having the openings  44  formed through its sidewall, and a main bore  46  extending axially through the sleeve  42 . A ridge  48  is shown within bore  46  and projects radially inward from the outer surface of the bore  46  and proximate an axial mid portion of the sleeve  42 . Further in the embodiment of  FIG. 4  are threads  49  formed in the bore  46  and at the outer axial portions of sleeve  42 . 
     Lock rings  50  are shown inserted in spaces between sleeves  42  and outer peripheries of ends  34 . As explained in more detail below, the lock rings  50  in combination with the sleeves  42 , are used for coupling together adjacent attenuation elements  32 . Referring back to  FIG. 2 , lateral sides of lock rings  50  abut shoulders  51  in sleeves  42  that are defined where the radius of the bore  46  reduces and projects radially inward. As shown in perspective view of  FIG. 5 , each lock ring  50  is made up of C-shaped halves  52 ,  54  that when joined as shown form an axial bore  56  through the lock ring  50 . Bore  56  tapers radially inward from a lateral side  57  and defines an oblique surface. As shown in  FIG. 2 , a portion of the oblique surface is in contact with the outer radial surface of the end  34  of attenuation element  32 . Strategically profiling the outer surface of end  34  and bore  56  defines a contact interface  58  between lock ring  50  and attenuation element  32 . In one example the contact interface extends along a curved path, and may optionally circumscribe axis A x , thereby forming a continuous loop. Referring back to  FIG. 1 , an advantage of limiting contact between the element  32  and lock ring  50  to a thin contact interface  58  is that the communication path that extends directly between the transmitter  16  and receiver  20  has a significantly reduced cross-sectional area. Reducing the cross-sectional area of the acoustic communication path between the transmitter  16  and receivers  20  significantly attenuates the acoustic signal between the transmitter  16  and receivers  20 . 
     Referring back to  FIG. 2 , an annular lock nut  60  is shown connected via a threaded connection  62  to the openings of each sleeve  42 . An axial end of each lock nut  60  is in contact with a side of lock ring  50  opposite its lateral side  57 , thereby axially retaining the lock ring  50  in its place as shown, and in contact with the ends  34  and the attenuation elements  32 . Further in the example of  FIG. 2 , an axial gap  64  is shown between adjacent attenuation elements, wherein in one example wellbore fluid fills the gap  64  thereby enhancing the attenuation capabilities of the acoustic attenuator  22 . Optionally, a tube  60  may be inserted into gap  30 , and filled with an oil, which in one example may be dielectric. As such, wires  68  can be inserted into the tube  66  and reduce the risk of electrical shorting due to the presence of the dielectric fluid. 
       FIG. 6  illustrates in perspective view an assembled example of the acoustic attenuator  22  wherein ends  34  of an attenuation element project axially outward from lock nuts  60  shown threadingly engaged with oppositely located sleeves  42  on the acoustic attenuator  22 . In one non-limiting example of operation, the presence of the series of acoustic elements  32  within the acoustic attenuator  22  severely attenuates any waves  18  that may attempt to propagate along the body of the tool  10  so that acoustic events sensed by the receivers  20  can be isolated to those waves  18  that reflect from the formation  14  rather than propagating lengthwise across the body of the tool  10 . 
     The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.