Patent Publication Number: US-9891336-B2

Title: Acoustic isolator for downhole tools

Description:
CLAIM OF PRIORITY 
     This application is a U.S. National Stage Filing under 35 U.S.C. 371 from International Application No. PCT/US2014/032003, filed on 27 Mar. 2014, and published as WO 2014/160858 A1, on 2 Oct. 2014, which claims the benefit of U.S. Provisional Application Ser. No. 61/806,092, filed on Mar. 28, 2013, which applications and publication are incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present application relates generally to methods and apparatus for providing acoustic isolation between components in downhole tools. 
     BACKGROUND 
     A wide variety of logging tools are often used to evaluate parameters of that a wellbore being drilled, the formation surrounding that wellbore, and/or the fluids within the wellbore. Where such logging tools rely upon acoustical measurements, there is often a need to isolate the sensors of acoustical signals from other components within the logging system. One clear example of such tools are acoustic logging tools which generate acoustic signals through a transmitter at one location on the tool (or in the tool string) and which travel through the formation to a receiver at a spaced location on the tool. Depending on the tool, the receiver may be spaced a few feet from the transmitter, or may be spaced 20 feet or more from the transmitter. 
     When such a system is operated, different types of waves propagate within the well and/or formation, including pressure waves (P-waves), shear waves (S waves), Rayleigh waves, mud waves and Stoneley waves. Of these wave types, P-waves and S-waves in particular, if unimpeded, can propagate along the body of the acoustic logging tool in a manner that would mask or otherwise adversely affect measurements by the acoustic receiver. Accordingly, there is a need to attenuate and/or slow down such propagation along the logging tool body so as to not adversely affect the measurements being made at the receiver. Additionally, the external contours of an acoustic isolator can couple acoustic energy between the logging tool and the formation surrounding the borehole, reducing the fidelity of the acoustic measurements. Accordingly, for some applications, an acoustic isolator having an external profile which approximates continuous and symmetrical surfaces would be advantageous. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments are illustrated by way of example and not limitation in the following figures, in which: 
         FIG. 1  depicts a schematic representation of an acoustic logging tool on an example configuration that can benefit from the methods and apparatus described herein. 
         FIG. 2A-2B  depict an example acoustic isolation structure, depicted in  FIG. 2A  from an external view; and depicted in  FIG. 2B  in a cross-sectional view. 
         FIGS. 3A-B  depict an alternative configuration of an acoustic isolation structure depicted in  FIG. 3A  from an external view; and depicted in  FIG. 3B  in a cross-sectional view. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description describes example embodiments of the disclosure with reference to the accompanying drawings, which depict various details of examples that show how the disclosure may be practiced. The discussion addresses various examples of novel methods, systems and apparatuses in reference to these drawings, and describes the depicted embodiments in sufficient detail to enable those skilled in the art to practice the disclosed subject matter. Many embodiments other than the illustrative examples discussed herein may be used to practice these techniques. Structural and operational changes in addition to the alternatives specifically discussed herein may be made without departing from the scope of this disclosure. 
     In this description, references to “one embodiment” or “an embodiment,” or to “one example” or “an example” in this description are not intended necessarily to refer to the same embodiment or example; however, neither are such embodiments mutually exclusive, unless so stated or as will be readily apparent to those of ordinary skill in the art having the benefit of this disclosure. Thus, a variety of combinations and/or integrations of the embodiments and examples described herein may be included, as well as further embodiments and examples as defined within the scope of all claims based on this disclosure, as well as all legal equivalents of such claims. 
     The present disclosure addresses multiple embodiments of an acoustic isolator, and an acoustic logging tool which incorporates the acoustic isolator. The acoustic isolator is configured to minimize acoustic transmissions which could otherwise adversely affect acoustical measurements being made by acoustic receiver. The described acoustic isolators include a plurality of longitudinally arranged mass members coupled to a central supporting structure. In the depicted examples, both the central supporting structure and the mass members are configured to allow the acoustic isolator to provide some degree movement or deflection within the isolator, such as relative longitudinal movement between adjacent mass blocks and/or some degree of axial deflection over a range of motion. In the depicted examples this movement or deflection is facilitated in part by cooperative configuration of the structures used to couple each mass member to another mass member. 
       FIG. 1  depicts a schematic representation of an acoustic logging tool  100 . Logging tool  100  is suspended from a wireline  102  through use of a cable head assembly  108 , in one example operating configuration well-known in the art. Acoustic logging tool  100  is suspended within a borehole  112  penetrating a formation  114 . In other examples, acoustic logging tool  100  might be incorporated into a tubular string, which may be for example, in a logging while drilling (LWD) drillstring disposed within a wellbore to perform drilling or reaming operations. In either configuration or form of operation, the various mechanisms and methods for providing power and/or signals to the logging tool, and for processing of signals received by the logging tool are well known to those skilled in the art. 
     Acoustic logging tool  100  includes a transmitter section, indicated generally at  104 , housing acoustic transmitters  116  and  118 . While in the depicted tool two transmitters are shown, either only a single transmitter or more than two transmitters may be utilized. Such transmitters may be constructed similarly to one another, or different configurations of transmitters known to those in the art may be utilized. In some example systems, one or more of the provided transmitters may be configured to emit acoustic signals essentially around the circumference of the transmitter section  104 . 
     Acoustic logging tool  100  also includes a receiver section, indicated generally at  106 ; which in this example includes only a single receiver, indicated generally at  120 . As with transmitters, either a greater or lesser number of receivers may be provided, and such receivers can either be a single configuration or of multiple configurations. In some example systems, multiple receivers will be angularly disposed around the lateral periphery of the receiver section. For example, a group of eight receivers might be disposed in essentially a single plane that extends generally perpendicular to the longitudinal axis through acoustic logging tool  100 , with the receivers oriented at essentially 45° increments around the tool periphery. 
     As can be seen from the schematic representation of  FIG. 1 , transmitter section  104  is retained in spaced relation relative to receiver section  106  through an acoustic isolation section, indicated generally at  110 . Acoustic isolation section  110  can be constructed, for example, in accordance with the example embodiment as will be discussed with respect to  FIGS. 2A-B . Acoustic isolation section  110  does not need to be entirely of a structure providing acoustic isolation along its entire length; as once an acoustical path is defined which is sufficiently disrupted, or which sufficiently retards or attenuates the problematic acoustic signals, then additional structures may be provided as needed for other purposes, for example to establish the desired spacing between the transmitter section  104  and receiver section  106 . As will be apparent to those skilled in the art, there can be two sources of energy propagating through an acoustic logging tool: energy resulting directly from the transmitter(s) or other components and propagating directly through the tool string; and energy external to the tool recoupling to the logging tool through the borehole fluid. 
     Referring now to  FIGS. 2A-B , the figures depict a portion of an acoustic isolator  200  such as could advantageously be used in logging tool  100  of  FIG. 1 . Acoustic isolator  200  has an exterior surface formed of a plurality of mass blocks  202 A-E, which are coupled together by “dog-bone”-shaped connectors, as indicated at  204 . Each mass block  202  is a structural element which may be formed of a suitable, relatively higher mass, material. For example metal or metallic compounds, such as stainless steel, Iconel alloys, or tungsten, can be used, as well as many other comparable materials providing appropriate strength and weight which will be apparent to those skilled in the art having the benefit of this disclosure. 
     Each mass block  202  contains a respective central bore  206  which cooperatively form a central passageway, indicated generally at  208 , when the mass blocks  202  are assembled as shown. Each mass block  202  also contains a plurality of appropriately configured recesses, as indicated typically at  210 , proximate an external surface, each recess  210  configured to engage a respective portion of a dog bone connector  204 . In most examples, even after coupling of the mass blocks together through a dog bone connector  210 , the relative configuration of the dog bone connectors  204  and the recesses  210  provides some degree of longitudinal movement, and preferably also some degree of axial deflection, between adjacent mass blocks  202 , The depicted “dog bone” shaped connector is only one example of a connector that may be utilized to enable the identified longitudinal movement and/or axial deflection over a range of motion. The function of this movement and/or deflection will be addressed later herein. In other examples, also as will be addressed later herein, the dog bone connectors may be coupled, such as through bolts, to both of two adjacent mass blocks. Other configurations of connectors can be envisioned. In many such alternative configurations, both space efficiency and secure limiting of the maximum motion will achieved through use of connector components that have regions of a relatively greater dimension that engage each mass block relative to the dimension of a central region that extends between the two mass blocks. 
     As can also be seen in  FIGS. 2A-B  in some examples, each dog bone connector  204  will be rigidly coupled to only one mass block  210 . In most embodiments, each dog bone connector  204  is configured with a convex external profile such that when the connector is an operating configuration, as depicted in  FIGS. 2A-B , a generally uniform cylindrical surface is exposed. In many examples, the recess and dog bone connector will be cooperatively formed to facilitate the described longitudinal movement and axial deflection, while at the same time limiting torsional movement. For example, the dog bone connector and the recess may define both a longitudinally extending gap  232  and an axially extending gap  230  to accomplish such. The dimensions of these gaps (and the dimension of the space between joined mass blocks) may be configured to achieve a desired design balance between a maximum logging load limit (increased by relatively increased gap dimensions) and a maximum radius of curvature of the tool structure (restricted by relatively reduced gap dimensions). 
     Each mass block  202  is spaced from an adjacent mass block  202  by an elastomeric member  214  providing a resilient seal between the adjacent mass blocks  202 . As can be seen in  FIG. 2B , in some configurations the elastomeric member  214  may have provisions for additional seals, such as o-ring seals, as indicated generally at  216 . This resulting spacing between mass blocks avoids a vibration path between blocks. In some examples, the elastomeric members  214  might be constructed to enhance the acoustic isolation between the mass blocks. 
     Referring now particularly to  FIG. 2B , each mass block  202  is assembled in a respective fixed position relative to a slotted central tube  218 , which extends through passageway  208  formed by individual bores  206  in each mass block  202 . Central tube  218  is again formed of a structural material, such as an Iconel alloy, and includes a plurality of slots, as indicated typically at  220 . As can be seen in the Figure, in this example, the slots are arranged in both longitudinally and radially spaced relation around all sides of the central tube, and each aperture radially overlaps with at least one longitudinally adjacent aperture. Thus, slots  220  are sized and arranged to define a nonlinear path for vibrations traversing central tube  218 . For example, in the depicted example, slots are presented in pairs on opposing sides of central tube  218 , and the next adjacent slots are also presented in pairs on opposing sides of central tube  218 , but are positioned at a 90° offset from the preceding slots. Additionally, the slots are of dimensions such that they overlap one another so as to preclude a linear path for vibrations. Many other configurations and/or dimensions of slots, or other structural configurations to provide only a nonlinear vibrational path through central tube  218 , might be utilized in place of the depicted structure. One advantage of the described slot configuration is that it also facilitates (and allows control of) the flexing of central tube  218 , and thereby the relative deflection of the mass blocks secured to the tube. 
     Each mass block  202  is structurally secured to central tube  218  through a locking wedge  222  (which in many examples will have a discontinuity to facilitate compression of the wedge) which is compressed against an inclined shoulder  224  defining a portion of each mass block central bore  206 . This compression is achieved through an annular locking nut  226  which threadably engages, at  228 , a respective mass block  202 . As will be apparent to those skilled in the art, increased threaded engagement of annular locking nut  226 , causes wedge block  222  to compress against central tube  218 , serving to both secure mass block  202  to central tube  218 , and to also acoustically couple the mass block to the central tube. 
     As a result of the above-described structure, the only direct acoustic path through the acoustic isolator  200  is along the slotted central tube  218 . In this configuration of acoustic isolator  200 , the flexural slowness is a function of the transverse motion of the mass and of the spring structure provided by the described structure. Some example configurations in accordance with the example structure described herein should be able to achieve a flexural wave slowness of at least 2500 microseconds per foot. Additionally, the mass and spring structure achieved by acoustically isolated mass blocks coupled to a flexible central tube defining a nonlinear acoustic path can be configured to mechanically filter out high-frequency flexural tool wave components, and to allow essentially only low flexural tool wave frequencies, for example below 200 Hz, to propagate along the spacer. 
     In other examples, dampening of the center tube and/or of the central fluid path there through may be provided. For example, a dampening member may be placed to engage the center tube, such as a coating or sleeve of tungsten rubber may be provided on either the interior or exterior surface of the central tube ( 218 ), to further attenuate any waves traveling down the tube. In some examples, it may also be desirable to attenuate any Stonely wave energy in the fluid channel within the central tube, or in other passageways in the system. Sintered metal may be provided in the central tube (or in any other passageway in the tool) to allow fluid and pressure communication while attenuating such energy. The permeability of such sintered metal may be selected in a manner known to those skilled in the art. 
     As noted previously, the configuration of the dog bone connectors with the respective recesses  210  in each mass block  204  allow acoustic isolator  204  to deflect over a range of motion to a selected point. Once the flexing between two adjacent mass blocks reaches that selected point, each dog bone connector will engage surfaces defining the recess of the respective mass blocks, and the system will then become more rigid. In one example configuration, the central tube  218  can be configured to accept over 2300 pound loads, and the flexing that comes therewith, before the dog bone connectors and mass blocks fully engage one another to significantly increase the stiffness, tensile strength, and torsional strength of the acoustic isolator. 
     In the depicted example, a plurality of mass blocks are provided at each of a plurality of longitudinal positions along the acoustic isolator, in this example, at a first longitudinal location, three dog bone connectors (and the associated structures) are provided at 120 degree circumferential spacing. And also in this example, at the next longitudinal location, there are again three dog bone connectors (and associated structures) at 120 degree circumferential spacing, but the orientation is offset 60 degrees from the connectors each of the longitudinally adjacent longitudinal locations (i.e., those next “above” and “below”). Of course other distributions of connectors may be used in different examples. 
     Referring now to  FIGS. 3A-B , therein is depicted an alternative construction for an acoustic isolator  300 . Acoustic isolator  300  functions in accordance with the basic principles described above relative to acoustic isolator  200 . Accordingly, this description will address the two primary differences between acoustic isolator  300  and previously described acoustic isolator  200 , i.e., the solid mounting of each dog bone connector, indicated generally at  304 , to both respective mass blocks, indicated typically at  302 ; and a bolted compression assembly mechanically and acoustically coupling each mass block  302  to central tube  306 . 
     In this example, wherein at least a first bolt  308 A will secure a dog bone connector  304  to a first mass block  302 , and a second bolt  308 B will connect that dog bone connector  304  to another mass block  302 , the dog bone connector will preferably be configured with appropriate spaces surrounding each bolt such that flexural loads will result in flexing of one mass block relative to another in a manner similar to that described relative to acoustic isolator  200 . In some cases, the spacing provided to facilitate this relative movement between the bolts  308  and a respective dog bone connector  304  may be filled with an elastomer or other substance to preclude entry of contaminants and to thereby preserve the described movable relationships. 
     Acoustic isolator  300  also includes bolts which extend from the outside directly to engage apertures  312  in central tube  306  to both physically attach and acoustically couple each mass block  302  to central tube  306 . In one example configuration, as depicted, such coupling bolts  308  may be provided around the periphery of the respective mass block, such as at 90° intervals around the periphery and arranged in a common plane perpendicular to the longitudinal axis of the longitudinal isolator. 
     In the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that embodiments of the invention necessarily require all or even multiple of such features.