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CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 11/551,507 filed Oct. 20, 2006, still pending. 
     
    
     BACKGROUND 
       [0002]    The invention generally relates to a communicating through a barrier in a well. 
         [0003]    For purposes of enhancing production from a well, the well may be perforated. In a typical perforating operation, a perforating gun is run downhole on a conveyance mechanism, such as a string or wireline, to the location where a production zone is to be established. A firing head of the perforating gun typically is then remotely activated from the surface of the well for purposes of initiating a detonation wave on the gun&#39;s detonating cord that extends from the firing head. The detonation wave causes the gun&#39;s perforating charges (shaped charges, for example) that are in ballistic communication with the detonating cord to fire. When fired, the perforating charges produce corresponding perforation jets, which perforate the well casing (if the well is cased) and form perforation tunnels into the surrounding formation. 
         [0004]    Several different techniques have been used in the past for purposes of remotely activating the firing head from the surface of the well. For example, a firing head that is constructed to initiate a detonation wave in response to being impacted by a pin may be deployed downhole. The pin to activate the firing head may be dropped through the central passageway of a string (which conveys the perforating gun downhole) from the surface of the well. As another example, the firing head may be constructed to respond to tubing conveyed pressure (TCP), so that fluid pressure inside the string may be increased to a predetermined level to trigger the firing of the perforating gun&#39;s charges. As yet another example, the perforating gun may be run downhole on a wireline, and the firing head may be designed to fire the charges of the perforating gun in response to the appropriate electrical signal that is communicated downhole to the gun&#39;s firing head via the wireline. 
         [0005]    The above-recited techniques to activate the firing head assume that pathways (electrical, fluid, etc.) are available are available. However, communicating with a perforating gun or, in general, any downhole tool, may be challenging if fluid or electrical pathways are not available. In this regard, the presence of communication across barriers, such as fluids, cement, screens, packers, plugs, etc. may present challenges in establishing communication between the surface and a downhole tool and between downhole tools. 
         [0006]    Thus, there exists a continuing need for better ways to communicate with a tool that is located downhole in a well. 
       SUMMARY 
       [0007]    In an embodiment of the invention, a technique that is usable with a well includes generating first command-encoded impulse stimuli in the well to cause second command-encoded stimuli to emerge from a communication barrier in the well. The technique includes responding to the second command-encoded stimuli to initiate an activator of a downhole tool. 
         [0008]    In another embodiment of the invention, a system that is usable with a well includes a transmitter and a receiver. The transmitter generates first command-encoded impulse stimuli in the well to cause second command-encoded stimuli to emerge from a communication barrier in the well. The receiver responds to the second command-encoded stimuli to initiate an activator of a downhole tool. 
         [0009]    In yet another embodiment of the invention, a technique that is usable with a well includes communicating acoustic energy having most of its spectral energy at frequencies greater than approximately 300 Hertz into the well and activating a downhole tool in response to the communication of the acoustic energy. 
         [0010]    In yet another embodiment of the invention, a method that is usable with a well includes communicating a ballistic wave from a first point in the well to a second point in the well without propagating the ballistic wave along a detonating cord. The first and second points are separated by more than one foot. The technique includes responding to the ballistic wave near the second point to actuate a downhole tool. 
         [0011]    Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0012]      FIGS. 1 ,  3  and  7  are flow diagrams depicting techniques to communicate through downhole barriers according to embodiments of the invention. 
           [0013]      FIG. 2  is a schematic diagram depicting a system to communicate through one or more downhole barriers according to an embodiment of the invention. 
           [0014]      FIGS. 4 ,  5  and  8  depict wells according to embodiments of the invention. 
           [0015]      FIG. 6  is a schematic diagram of an activator according to an embodiment of the invention. 
           [0016]      FIGS. 9A and 9B  depict a technique to fire casing conveyed perforating guns according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIG. 1 , in accordance with an embodiment of the invention, a technique  10  for communicating with a downhole tool includes generating at least one high intensity impulse wave, pursuant to block  12 . The high intensity impulse wave(s) possess the capability of being communicated (block  14 ) through one or more “communication barriers,” such as (as a non-exhaustive exemplary list) one or more of the following in alone or in combination: a fluid layer, cement, a screen, a packer, a plug and the metal wall of a string. Although the communication barriers may block the communication of conventional stimuli, the high intensity impulse wave(s) propagate through the communication barrier(s) to allow initiation (block  16 ) of an activator of the tool, which is located downhole of the barrier(s). 
         [0018]    In accordance with some embodiments of the invention, the technique  10  may be implemented in a subsea or subterranean well system, an exemplary embodiment  50  of which is depicted in  FIG. 2 . As depicted in  FIG. 2 , the system  50  includes an impulse generator  58 , which, in accordance with some embodiments of the invention, may be a device that generates high intensity impulse waves, which are communicated downhole from the surface of the well. For example, in accordance with some embodiments of the invention, the impulse generator  58  may be in communication with a fluid layer in the well so that the generator  58 , when activated, forms a high intensity fluid impulse wave, or “fluid hammer,” in the fluid layer. The fluid hammer propagates from the impulse generator  58  through the fluid layer and the resulting energy from the initial fluid hammer may likewise propagate through one or more barriers  60  (in addition to the fluid layer) that intervene between the generator  50  and to an activator  64  of a downhole tool. In accordance with some embodiments of the invention, each of the high intensity impulse waves, such as the fluid hammer, which is generated by the impulse generator  58  may be an impulse of at least 2.7.N·m. As another example, the fluid hammer may be one or more pressure waves, each of which has a magnitude exceeding 200 pounds per square inch (psi) for a duration less than approximately 500 milliseconds (ms), in accordance with some embodiments of the invention. 
         [0019]    The impulse generator  58  may take on numerous forms, depending on the particular embodiment of the invention. For example, in accordance with some embodiments of the invention, the impulse generator  58  may be formed from a surface fluid pump and a valve. The valve may be controlled such that when the fluid pump builds up sufficient pressure in a surface flow path, the valve opens to communicate a high intensity impulse to the well fluid. 
         [0020]    In other embodiments of the invention, the impulse generator  58  may be formed from a mandrel that includes a piston head for the purposes of generating a fluid hammer in a fluid layer in communication with the piston head. The impulse generator  58  may be ballistically or pressure activated (as examples) in accordance with some embodiments of the invention. Thus, many variations are possible and are within the scope of the appended claims. 
         [0021]    Depending on the particular embodiment of the invention, the activator  64  may respond directly to a received impulse wave, without performing stimuli discrimination, other than not responding to impulse waves below a certain threshold. However, in other embodiments of the invention, the impulse waves may be encoded with a command, and for there embodiments of the invention, the activator  64  may decode received impulse waves for purposes of extracting and acting upon the encoded commands and not responding to other impulse waves and/or commands. 
         [0022]    As a more specific example, in accordance with some embodiments of the invention, the activator  64  may be a ballistic-based activator, such as the firing head of a perforating gun. Encoding a command into the impulse waves ensures that the firing head does not inadvertently fire in response to impulse waves that are not produced by the impulse generator  54  or impulse waves that are generated by the impulse generator  54  and are intended for a different activator. Furthermore, the encoding may involve encoding an address with the command for purposes of selecting one tool from another. This may be particularly advantageous, for example, when several perforating guns are part of a single string. 
         [0023]    For embodiments of the invention in which the activator  64  is a firing head, the activator  64  is capable of extracting a fire command from a series of impulse waves that encoded with a fire command. Upon verifying receipt of the fire command, the activator  64  initiates a detonation wave on a detonating cord  66  that extends from the activator  64  to a ballistic device  68 , which may be, for example, the first of many perforating charges to be fired in response the detonation wave. 
         [0024]    It is noted that the activator  64  may be an activator for a tool other than a perforating gun. For example, in accordance with other embodiments of the invention, the activator may be used for purposes of controlling operating of a packer, valve, plug, etc., as just a few examples. Furthermore, the activator may be ballistically-based or non-ballistically based, depending on the particular embodiment of the invention. For example, in some embodiments of the invention, the activator may respond to a command-encoded or non-command-encoded impulse wave by using fluid pressure to operate a downhole element (such as a sleeve of a valve, for example). 
         [0025]    For purposes of encoding a command into the impulse waves, the valve and/or the pump of the impulse generator  58  may be controlled by a computer  52 , which may include a processor  54  that executes program instructions  55  (stored in a memory  56  of the computer  52 ) for purposes of controlling the surface requirement to generate the impulses. In this regard, in accordance with some embodiments of the invention, the generation of the impulse waves may be precisely-regulated by the computer  52  for purposes of encoding the impulse waves with a command. 
         [0026]    In view of the foregoing, a technique  80  that is depicted in  FIG. 3  may be used to communicate through one or more barriers according to some embodiments of the invention. Pursuant to the technique  80 , an impulse generator is controlled (block  82 ) to generate command-encoded high intensity impulse waves. The impulse waves are then allowed to propagate through one or more barriers of the well, pursuant to block  86 . A command may then be decoded (block  90 ) from the impulse waves and an activator may be initiated (block  94 ) in response to the decoded command. Thus, for example, the activator  64  ( FIG. 2 ) may compare a time-signature of a series of receive pulses for purposes of identifying a particular command (a fire command, for example). Upon recognizing the command, the activator  64  may then be initiate a operation of a downhole tool. Other variations are possible and are within the scope of the appended claims. 
         [0027]    Referring to  FIG. 4 , in accordance with some embodiments of the invention, the above-described high intensity impulse wave communication technique may be used with a well, such as an exemplary well  100 . A string  120  may extend downhole inside a wellbore  110  of the well  100  and may include multiple perforating guns, such as an upper perforating gun  140  and a lower perforating gun  150 . The wellbore  110  may or may not be cased (via a casing string  112 ), depending on the particular embodiment of the invention. Each perforating gun  140 ,  150  has an associated initiator  64 . For example, the upper perforating gun has an associated initiator  64   1  for purposes of firing the perforating charges of the gun  140 , and the perforating gun  150  has an associated initiator  64   2  for purposes of firing its perforating charges. 
         [0028]    It is noted that the well  100  may be a subterranean or subsea well, depending on the particular embodiment of the invention. Additionally, the string  120  may be coiled tubing or formed from jointed sections, depending on the particular embodiment of the invention. 
         [0029]    As depicted in  FIG. 4 , in accordance with some embodiments of the invention, the impulse generator  58  is located at the surface of the well  100 . The impulse generator  58  may generate high intensity fluid-based impulse waves in the central passageway of the string  120  (as depicted in  FIG. 4 ). However, in accordance with other embodiments of the invention, the impulse generator  58  may generate high intensity fluid-based impulse waves in fluid that is located in an annulus of the well  100 . Thus, many variations are possible and are within the scope of the appended claims. 
         [0030]    Due to the energy of the impulses that are generated by the impulse generator  58 , the impulses overcome barriers such as screens, packers, string housing, etc. that may otherwise impede communication between the surface and either initiator  64   1  and  64   2 . 
         [0031]    Referring to  FIG. 5 , in another embodiment of the invention, a well  200  may be used for purposes of communicating through a downhole barrier. The well  200  has a similar design to the well  100  (with similar reference numerals being used), except that the well  200  does not include a surface-based impulse generator  58 . Instead, in this particular embodiment of the invention, the string  210  (replacing the string  120 ) includes a string-based impulse generator  204 . Thus, the generator  204  may be located downhole in the well away from the surface and may be in communication via wires, fluid, etc. with the surface of the well. A particular advantage of this arrangement is that the impulse generator  204  may be located in proximity to a particular fluid of the well to which the impulses are to be applied. Additionally, the downhole location of the generator  204  may bypass one or more downhole barriers. The impulse generator may be electrically, hydraulically, mechanically or ballistic-based, depending on the particular embodiment of the invention. 
         [0032]      FIG. 6  depicts an exemplary embodiment of the activator  64  when used as a firing head. The activator  64  may include a sensor  310  for purposes of detecting transmitted impulses. For example, in accordance with some embodiments of the invention, the sensor  310  may be an acoustic-based sensor for purposes of detecting energy that propagates through a wall of a string in which the sensor  310  is housed. A processor  314  is electrically coupled to the sensor  310  for purposes of analyzing the energy that is sensed by the sensor  310 . Thus, the processor  314  may, via the sensor  310 , analyze received pressure impulses and compare these impulses to a predetermined time-signature for purposes of decoding commands. In response to detecting a fire command, the processor  310  may activate a voltage booster  320 , which boosts a voltage that is provided by a downhole voltage source  322  for purposes of activating an exploding foil initiator (EFI)  326 . The activation of the EFI  326 , in turn, causes corresponding activation of the ballistic device  68 . As examples, the ballistic device  68  may be a pyrotechnic, a secondary explosive, etc. 
         [0033]    Other embodiments are possible and are within the scope of the appended claims. For example, referring to  FIG. 7 , in accordance with other embodiments of the invention, a technique  400  to communicate a command to a downhole activator uses high frequency acoustic energy, which is capable of propagating through one or more downhole communication barriers. As an example, in accordance with some embodiments of the invention, the high frequency acoustic energy may be energy that has most of its spectral energy located above approximately 300 Hertz (Hz). 
         [0034]    Pursuant to the technique  400 , command-encoded high frequency acoustic energy is generated (block  402 ) and allowed (block  410 ) to propagate through one or more downhole communication barriers. A command is then decoded (block  414 ) from the high frequency acoustic energy that is received on the other side of the barrier(s). An activator is then initiated (block  418 ) in response to the decoded command. 
         [0035]    The high frequency acoustic energy may be generated by a surface or a string-based generator, depending on the particular embodiment of the invention. The generator may include, for example, a mandrel that is attached to a pin that impacts a surface upon sufficient travel of the mandrel. The mandrel may therefore be moved back and forth to continually impact the pin and the surface to generate the acoustic energy. The mandrel may be hydraulically, mechanically or electrically, or ballistically driven, depending on the particular embodiment of the invention. 
         [0036]    Referring to  FIG. 8 , in accordance with some embodiments of the invention, the above-described techniques may be used in connection with a well  500 . The well  500  includes a casing string  502  that extends downhole to line a wellbore  510 . As depicted in  FIG. 8 , the casing string  502  may be fixed in place by a cement  504 . The casing string  502  includes casing conveyed perforating guns  520  (an upper casing conveyed perforating gun  520   a  and a lower casing conveyed perforating gun  520   b  being depicted as examples) which as their names imply, are perforating guns that are integrated into the casing string  502 . 
         [0037]    More specifically, each casing conveyed perforating gun  520  includes, as an example, radially extending fins  524  that are in general parallel to the longitudinal axis of the perforating gun  520  and contain perforating charges  530  (shaped charges, for example). A detonating cord  532  extends around the circumference of the casing conveyed perforating gun  520  for purposes of ballistically coupling the perforating charges  530  together. Thus, in response to a detonation wave propagating along the detonating cord  532 , the perforating charges  530  fire to form corresponding perforation jets that extend through the cement  504  and into the surrounding formation. 
         [0038]    A potential challenge in using casing conveyed perforating guns is that communication with the guns may be hampered by the manner in which the guns are assembled. More specifically, the casing string  502  includes joints, such as exemplary joints  516 . Thus, the casing string  502  is assembled at the surface by connecting corresponding casing string sections together at the joints. These joints, in turn, may restrict the communication with the casing conveyed perforating guns  520 . More specifically, it may be quite challenging to route a detonating cord through the casing joints  516 . Thus, this impairs the ability to fire the casing conveyed perforating guns  520  in a traditional top-down sequence in which the upper casing conveyed perforating gun  520   a  fires, then the lower perforating gun  520   b  fires, etc. To overcome the communication barriers that are imposed by the casing joints  516 , in accordance with some embodiments of the invention, ballistic waves are communicated between the casing conveyed perforating guns  520  without the use of detonating cords. 
         [0039]    More particularly, in accordance with some embodiments of the invention, each casing conveyed perforating gun  520  includes an acceptor charge  540  that is connected to the detonating cord  532  for the gun  520 . The acceptor charge  540  is constructed to respond to a ballistic wave that propagates from a ballistic source that is not connected to the acceptor charge  540  by a detonating cord. Thus, the acceptor charge  540  may be located several feet (more than one foot, for example), if not hundreds of feet from the source that generates the ballistic wave. In response to receiving the ballistic wave, the acceptor charge  540  initiates a corresponding detonation wave on the detonating cord  532 , which, in turn, fires the perforating charges  530  of the casing conveyed perforating gun  520 . 
         [0040]    In accordance with some embodiments of the invention, the firing of the perforating charges  530  of the upper casing conveyed perforating gun  520   a  is initiated via a tool  560 , such as a wireline tool that is lowered downhole via a wireline  559 . Other tools and other arrangements may be used to initiate the firing of the perforating charges  530  of the upper perforating gun  520   a,  depending on the particular embodiment of the invention. For example, in accordance with other embodiments of the invention, the firing of the perforating charges  530  of the upper casing conveyed perforating gun  520   a  may be accomplished via a wired connection, via a detonating cord, via pulses in a fluid, etc. Thus, many variations are possible and are within the scope of the appended claims. For the embodiment that is depicted in  FIG. 8 , the tool  560  contains a firing head  562  that initiates the firing of a donor charge  564  for purposes of producing a ballistic wave that propagates to the acceptor charge  540  of the upper casing conveyed perforating gun  520   a.    
         [0041]    After the detonation wave propagates along the detonating cord  532  to fire the perforating charges  530  of the upper casing conveyed perforating gun  520   a,  the detonation wave initiates the firing of a donor charge  550  of the gun  520   a.  This initiation, in turn, produces a ballistic wave, which propagates without aid of a detonating cord to the lower casing conveyed perforating tool  520   b.  The lower casing conveyed perforating tool  520   b,  in turn, includes an acceptor charge  540  that responds to the ballistic wave to generate a detonation wave on its detonating cord  532  to fire the perforating charges  530  of the lower casing conveyed perforating gun  520   b.  This detonation wave may also initiate the firing of a donor charge  550  of the lower casing conveyed perforating tool  520   b  to cause the firing of the perforating charges of a casing conveyed perforating gun that is located farther down in the well. 
         [0042]    Referring to  FIG. 9A , to summarize, in accordance with some embodiments of the invention, a technique  600  to fire casing conveyed perforating guns includes firing (block  604 ) an acceptor charge  540  in an upper casing conveyed perforating gun  520   a  to initiate a detonation wave on a detonating cord  532  of the upper perforating gun  520   a.  The detonation wave is used (block  608 ) to fire perforating charges of the upper casing conveyed perforating gun  520   a.    
         [0043]    The detonation wave is also used, pursuant to block  612 , to fire a donor charge  550  of the upper casing conveyed perforating gun  520   a  to create a ballistic wave, which propagates without aid of a detonating cord to the lower casing conveyed perforating gun  520   b.    
         [0044]    Referring to  FIG. 9B , the acceptor charge  540  and the lower casing conveyed perforating gun  520   b  is fired (block  614 ) in response to the ballistic wave to initiate a detonation wave on the detonating cord  532  of the lower casing conveyed perforating gun  520   b.  The detonation wave is used (block  618 ) to fire perforating charges  530  of the lower casing conveyed perforating gun  520   b.  The detonation wave may also be used (block  622 ) to fire a donor charge  550  of the lower casing conveyed perforating gun  520   b  to fire a gun below the gun  520   b,  in accordance with some embodiments of the invention. Thus, the above-described chain may be repeated for purposes of firing additional perforating guns. 
         [0045]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Summary:
A technique that is usable with a well includes communicating a ballistic wave from a first point in a well to a second point in the well without propagating the ballistic wave along a detonating cord. The first and second points are separated by at least one foot. The technique includes responding to the ballistic wave near the second point to activate a downhole tool.