Abstract:
An acoustic attenuator assembly for an acoustic tool for performing acoustic investigation of a subterranean formation is disclosed. The acoustic attenuator assembly includes a first end portion ( 205 ) and a second end portion ( 210 ) having a bore therethough to enable passage of an electrical line. The acoustic attenuator assembly further includes a fiber portion ( 230 ) disposed between the first and second end portion so that the fiber portion attenuates at least a portion of acoustic energy when the acoustic energy is received by one or both of the first ( 205 ) and second ( 210 ) end portions. The fiber portion ( 230 ) includes a continuous flexible portion that allows relative deflection of the first ( 205 ) and second ( 210 ) end portions.

Description:
RELATED APPLICATION 
     This application is a U.S. National Stage Application of International Application No. PCT/US2011/024593 filed Feb. 11, 2011, and is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates generally to well logging tools used in subterranean formations using acoustic waves and, more particularly, the present disclosure relates to methods and apparatus used to acoustically isolate the transmitter from the receiver. 
     Acoustic logging tools that are commonly used to characterize subterranean formations surrounding well bores. 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. 
     The transmitters generate a compressional waveform that travels through the wellbore 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 energy mode characterized by particle motion perpendicular to the direction of wave travel is also called flexure mode. 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. There is a need in the art to attenuate the flexure mode in acoustic logging tools. 
     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. 
     Acoustic isolators for downhole applications must be sufficiently flexible to attenuate acoustic waves traveling at or near the surface of the tool. Acoustic isolators must also be strong enough to survive running and retrieval operations, which may be by wireline or tubing conveyed means, for example. During these operations it is often required to push or pull heavy loads via the tool. There is a need in the art for improved isolators having advantageous frequency response characteristics, while having high tension and compression limits. 
    
    
     
       FIGURES 
       Some specific exemplary embodiments of the disclosure may be understood by referring, in part, to the following description and the accompanying drawings. 
         FIG. 1  is a general schematic representation of an acoustic well logging tool for performing acoustic investigation of a subterranean formation, in accordance with certain exemplary embodiments of the present disclosure. 
         FIGS. 2A ,  2 B,  2 C and  2 D are views of a schematic representation of an acoustic attenuation flex joint assembly, in accordance with certain exemplary embodiments of the present disclosure. 
     
    
    
     While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure. 
     DETAILED DESCRIPTION 
     The present disclosure relates generally to well logging tools used in subterranean formations using acoustic waves and, more particularly, the present disclosure relates to methods and apparatus used to acoustically isolate the transmitter from the receiver. 
     Illustrative embodiments of the present invention are described in detail herein. In the interest of clarity, not all features of an actual implementation may be described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the specific implementation goals, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of the present disclosure. 
     To facilitate a better understanding of the present invention, the following examples of certain embodiments are given. In no way should the following examples be read to limit, or define, the scope of the invention. Embodiments of the present disclosure may be applicable to horizontal, vertical, deviated, or otherwise nonlinear wellbores in any type of subterranean formation. Embodiments may be applicable to injection wells as well as production wells, including hydrocarbon wells. 
     Exemplary embodiments according to the present disclosure may provide for attenuators having fiber/epoxy terminations and having advantageous in removing or attenuating flexural tool arrivals at lower frequencies while having high tension and compression limits required for logging. Certain embodiments may provide for an attenuator flex joint for attenuating broad band frequency signals from acoustic sources and having high strength fiber epoxy terminations for added flexibility with the ability to attenuate flexural tool arrival of energy for added attenuation at lower frequencies. Certain embodiments may provide for increased joint flexibility while maintaining the tensile strength of the system, with increased joint flexibility providing improved low frequency signal attenuation. In certain embodiments, multiple flex joints may be connected together as required to achieve the desired signal attenuation. Devices and methods in accordance with certain embodiments may be used in one or more of wireline, measurement-while-drilling (MWD) and logging-while-drilling (LWD) operations. “Measurement-while-drilling” is the term for measuring conditions downhole concerning the movement and location of the drilling assembly while the drilling continues. “Logging-while-drilling” is the term for similar techniques that concentrate more on formation parameter measurement. 
       FIG. 1  is a general schematic representation of an acoustic well logging tool  100  for performing acoustic investigation of a subterranean formation, in accordance with certain exemplary embodiments of the present disclosure. An acoustic well logging tool  100  may be lowered into a wellbore  105 , which may contain a fluid  110 . The logging tool  100  may be conveyed into the wellbore  105  by a wireline  115 , a string of pipe, coiled tubing (wired or unwired), or an equivalent means. The acoustic well logging tool  100  may generally include a transmitter assembly  120 , an acoustic attenuator assembly  125 , and a receiver assembly  130 . The term “acoustic,” as used herein, 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. 
     The transmitter assembly  120  may include one or more of a monopole transmitter, dipole transmitters, and other multi-pole transmitters. The transmitter assembly  120  may include control electronics to control the triggering and timing of the acoustic energy output. The control electronics may fire the acoustic sources periodically, thereby producing acoustic pressure waves that propagate through the wellbore fluid. Part of the energy from the monopole transmitter is refracted at the wellbore interface and travels through the surrounding formation as a compressional wave (P-wave) and as a shear wave (S-wave). 
     In general operation, the transmitter assembly  120  may generate an acoustic signal  135  that propagates through the wellbore fluid  110  and into the surrounding formation  140 . The acoustic signal  135  may propagate through the formation  140  and be received by the receiver assembly  130 . The received acoustic signal can be used to determine the acoustic response of the formation  140 , which may indicate the properties of the formation, such as porosity, composition, and the presence of liquids or solids in the formation. 
     In some embodiments, the collected data and/or calculated data are stored in the tool for later analysis. In certain embodiments, the acoustic well logging tool  100  may be coupled to a telemetry transmitter (not shown) that communicates with the surface, providing telemetry signals and receiving command signals. A surface transceiver may be configured to receive transmitted telemetry signals and to transmit command signals downhole. Alternatively, the surface transceiver may be coupled to another portion of a rigging or drillstring of certain embodiments not shown. One or more repeater modules may be provided along the drill string to receive and retransmit the telemetry and command signals. The surface transceiver may be coupled to a logging facility that may gather, store, process, and analyze the telemetry information. 
     In certain embodiments, communication with the surface may also be provided through conductors of the wireline  115 , string of pipe, coiled tubing, or equivalent means. In some example systems, power may be transmitted to the acoustic well logging tool  100  and associated instrumentation through conductors of the wireline  115 , string of pipe, coiled tubing, or equivalent means from a power source at the surface. In other example systems, the acoustic well logging tool  100  may be powered from a downhole central power module, which may utilize a battery or other power source. 
     In order to effectively analyze the acoustic signal, the receiver of the receiver assembly  130  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 attenuator or isolator assembly  125  may be placed between the transmitter assembly  120  and the receiver assembly  130  to decrease the acoustic energy traveling directly between the transmitter and receiver without first passing through the formation  140 . 
       FIGS. 2A ,  2 B,  2 C and  2 D are various views of a schematic representation of an acoustic attenuator flex joint assembly  200 , in accordance with certain exemplary embodiments of the present disclosure. In certain embodiments, one or more acoustic attenuator flex joint assemblies  200  may be implemented in the acoustic attenuator assembly  125  illustrated in  FIG. 1 . The acoustic attenuator assembly  125 , for example, may be substantially an elongated, cylindrical assembly having a bore through the center and having one or more attenuator modules that are linearly affixed to each other. For example, multiple attenuator modules may be connected in series between the transmitter assembly  120  and the receiver assembly  130 . In certain embodiments, one or more acoustic attenuator flex joint assemblies  200  may be the only attenuator modules between the transmitter assembly  120  and the receiver assembly  130 . In certain embodiments, one or more acoustic attenuator flex joint assemblies  200  may be combined with one or more additional attenuator modules between the transmitter assembly  120  and the receiver assembly  130 . Various arrangements may 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 attenuator modules 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  130 , one or more attenuator modules may be installed below the receiver  130 . 
     Referring again to  FIGS. 2A-2D , the acoustic attenuator flex joint assembly  200  may be generally elongated and cylindrical when not in a flexed position. The assembly  200  may include a bore  205  that may be hydraulically isolated from the outside of the assembly  200 . The bore  205  may allow for the passage of electrical lines therethrough. In certain embodiments, the assembly  200  will be oil-filled during operation. The fluid may be a non-corrosive viscous oil, such as petroleum oil or a synthetic hydrocarbon fluid that may have negligible effects on the attenuation of acoustic signals through assembly  200 , while inhibiting corrosion and eliminating any affect from hydrostatic pressure in the wellbore, which could over-stress the internal parts of the assembly  200 . 
     The assembly  200  may include end caps  210  and  215  disposed at opposing ends. The end caps  210  and  215  may have a variety of designs for mating and/or coupling to other elements of the acoustic tool, depending on the implementation. For example, a threaded coupling may be included at end caps  210  and  215  to allow for the addition of more attenuator joints (not shown). 
     The end caps  210  and  215  may partially surround termination housings  220  and  225 , respectively. The end caps  210  and  215  may be threadedly connected, or otherwise coupled, to the termination housings  220  and  225 . The couplings may be sealed with seals  211  and  216 , which may be o-ring seals, for example. The couplings may further include additional seal  212  and back-up ring  217 . The back-up ring  217 , in some embodiments, will provide for higher pressure applications. 
     The termination housings  220  and  225  may surround or partially surround fiber  230 . The fiber  230  may be high-performance, high-strength fiber and may serve as the main structural back bone of the flex joint assembly  200 . By way of example without limitation, the fiber  230  may include one or more of high-performance liquid crystal polymer (Vectran) (LCP), Kevlar, and other high-performance, high-strength fibers with minimal creep measurement characteristics. In certain embodiments, the fiber  230  may include LCP (Vectran) as an acoustic/vibration dampening material. At each end of the flex joint assembly  200 , the fiber  230  may be molded with epoxy resin in the cone-shaped or generally cone-shaped ends. 
     The epoxy, by way of example without limitation, may be high-temperature, semi-flexible epoxy, with low shrinkage and with thixotropic characteristics. The ends of the fiber  230  may be terminated and coupled to the termination housings  220  and  225  with epoxy resin. At or near the center of the flex joint assembly  200 , the fibers  230  may be free from any epoxy resin in order to maximize the flexibility of the section. 
     Spacers  231  and  232  may be positioned between the cone-shaped ends of the fiber  230 . The spacers  231  and  232  may each comprise two cone-shaped halves in certain embodiments. The spacers  231  and  232  may comprise metal and may be designed to support the epoxy termination under load. 
     The assembly  200  may include a tubular  206  radially disposed within the fiber  230 . In certain embodiments, the tubular  206  may be the innermost tubular. O-rings  213  and  218  may be disposed between the tubular  206  and the termination housings  220  and  225 . The tubular  206  may be flexible at least at or near the center of the flex joint assembly  200  in order to allow for flexure of the assembly  200  as a whole. By way of example without limitation, the tubular  206  may include one or more of Teflon, nylon, and other types of engineering plastic, such as polyether ether ketone. 
     A resilient spacer  235  may be placed around or partially around the fiber  230  to further absorb the sound energy traveling through the attenuator section. For example without limitation, the resilient spacer  235  may be an elastomeric jacket, in the form of a split bobbin, or an equivalent means constructed of rubber, elastomer, or other means capable of further absorbing the sound energy traveling through the attenuator section while allowing for flexure. As nonlimiting examples, the resilient spacer  235  may include one or more of nitrile, hydrogenated nitrile, such as highly saturated nitrile (HSN) or Hydrogenated Nitrile Butadiene Rubber (HNDR), nitroxile, fluorocarbon (FKM, FPM), and any other material have similar properties. 
     Shells  240  and  245  may be connected together around or partially around the resilient spacer  235 . For example without limitation, the shells  240  and  245  may be metal half shells made from heavy weight tungsten material, stainless steel, brass or any suitable material, and may be bolted together about the resilient spacer  235  with bolts  250 . The shells  240  and  245  may act as inertial mass to further dissipate the acoustic energy propagating through the fiber  230 . The shells  240  and  245  may be circumferentially keyed into the cone shaped ends to transfer torque through the flex joint. The end caps, termination housings, and shells may also be designed to limit the amount of flexure that the assembly can withstand in a deviated borehole. The amount and type, including the hardness, of resilient spacer  235  may be designed to limit the amount of potential flexure of the assembly in a deviated borehole. 
     In certain embodiments, one or more of the end caps, termination housings, and shells may be coated on its outside surface with an attenuating material, such as fiberglass. In certain embodiments, one or more of the end caps, termination housings, and shells may be constructed from a corrosion resistant material, such as tungsten, stainless steel or a nickel based alloy. The lengths of the metal half shells and fiber portion can vary to make longer or shorter flex joints according to implementation needs. Of course, it will be understood that certain of the remaining elements of the assembly may also need adjustment to accommodate the varying design lengths. 
     The acoustic attenuator flex joint assembly  200  also may serve to attenuate acoustic signals ranging from below 100 Hz to over 10 kHz. As the signal  135  propagates out into the formation  140 , a recoil may be felt or absorbed in the body of the tool  100 . Acoustic wave isolation may occur when the signal is forced to travel down the body of tool  100  or wellbore fluid  110 . The inherent flexibility of the assembly  200  aids in the attenuation of acoustic signals between the transmitter and receiver. This flexibility could be a liability in other tools if the 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, such high axial loads have often caused substantial damage or failure in the acoustic attenuator components. If failure occurs, additional fishing operations are required to remove the now broken pieces of the tool from the wellbore. 
     However, the novel arrangement of components in the assembly  200  allows the tool as a whole to withstand high axial loading. For example, the acoustic attenuator flex joint assembly  200  may provide improved tensile strength such that it is capable of withstanding pull tests of over 80 klbs. When tension is applied, the tension may be carried through the fiber  230  and the epoxy termination. When a compressive load is applied, the compression may be carried through the termination housings  220  and  225  to the shells  240  and  245  after the gap between the members is closed. The gap may be closed when the resilient spacer  235  is compressed at high compression loads. 
     Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. The indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces.