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CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to provisional application Ser. No. 61/015,730 filed Dec. 21, 2007. 
     
    
     BACKGROUND 
       [0002]    The present application generally relates to a downhole initiator, and more particularly, to an initiator for an oil or gas well environment, which contains a safety barrier to prevent inadvertent firing of the initiator. 
         [0003]    Explosives typically are used in an oil or gas well for such purposes as perforating a well casing and forming perforation tunnels in a surrounding formation to enhance the productivity of the well. More specifically, a well tool called a perforating gun typically is run downhole in the well on a conveyance mechanism, such as a wireline, slickline, coiled tubing string, jointed tubing string, etc. When the perforating gun is in an appropriate position adjacent to the formation to be perforated, perforating charges (shaped charges, for example) of the perforating gun are fired to create perforating jets, which penetrate the casing and form the perforation tunnels in the formation. 
         [0004]    A typical wireline-based perforating gun may include an initiator that is constructed to fire perforating charges of the gun after the initiator detects the appropriate command that is communicated downhole to the perforating gun from the surface of the well. The initiator may include an igniter, such as a semiconductor bridge (SCB), hot wire, exploding bridgewire (EBW) or TiB igniter, which is energized by the initiator after the initiator detects the command. When energized, the igniter sets off an explosive to begin a chain of explosive events that ultimately results in the initiation of a detonation wave on a detonating cord. The detonation wave causes the perforating charges (which are connected to the detonating cord) to fire. 
         [0005]    Care typically is exercised for purposes of preventing inadvertent firing of the perforating charges. However, challenges remain in preventing an unintended triggering event, such as an electrostatic discharge (ESD) or a radio frequency (RF) signal, from causing inadvertent firing of the perforating charges. 
       SUMMARY 
       [0006]    In an embodiment of the invention, a perforating gun that is usable with a well includes at least one perforating charge and an initiator. The initiator includes a ballistic train to fire the perforating charge(s). The initiator is adapted to misalign components of the ballistic train to disarm the initiator and realign the components to arm the initiator. 
         [0007]    In another embodiment of the invention, a technique that is usable with a well includes providing an initiator to fire at least one perforating charge and preventing inadvertent firing of the perforating charge(s), including misaligning components of a ballistic train of the initiator. 
         [0008]    In yet another embodiment of the invention, an initiator assembly includes a ballistic train to fire an end device in a well and an actuator to misalign components of the ballistic train to prevent inadvertent firing of the end device. 
         [0009]    Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]      FIG. 1  is a schematic diagram of a well illustrating a perforating system according to an embodiment of the invention. 
           [0011]      FIGS. 2 and 3  are flow diagrams depicting techniques to prevent inadvertent firing of the perforating system of  FIG. 1  according to embodiments of the invention. 
           [0012]      FIG. 4  depicts an initiator assembly in an unarmed state according to an embodiment of the invention. 
           [0013]      FIG. 5  depicts the initiator assembly in an armed state according to an embodiment of the invention. 
           [0014]      FIG. 6  is a schematic diagram of a MEMS-based actuator of the initiator assembly according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
         [0016]    As used here, the terms “above” and “below”; “up” and “down”; “upper” and “lower”; “upwardly” and “downwardly”; and other Like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship as appropriate. 
         [0017]    Referring to  FIG. 1 , a well  10  (a subsea or subterranean well, as examples) in accordance with embodiments of the invention includes a wellbore  12  that extends downhole through one or more formations. The wellbore  12  may or may not be lined with a casing string  14 , depending on the particular embodiment of the invention. Furthermore, the wellbore  12  may be the main wellbore (as shown) or a lateral wellbore, depending on the particular embodiment. 
         [0018]    For purposes of enhancing the productivity of the well  10 , a perforating system may be run into the well  10  to perforate the casing string  14  (assuming the wellbore  12  is cased) and the surrounding formation. More specifically, a perforating gun  20  may be run downhole on a conveyance mechanism, which is generally denoted in  FIG. 1  by reference numeral “ 16 .” Depending on the particular embodiment of the invention, the conveyance mechanism  16  may be a wireline, slickline, coiled tubing, jointed tubing, etc. Thus, many variations are contemplated and are within the scope of the appended claims. 
         [0019]    The perforating gun  20  contains perforating charges  24  (shaped charges, for example), which are outwardly directed (radially or tangentially directed, as examples) to perforate the casing string  14  (if the wellbore  12  is cased) and form corresponding perforation tunnels into the surrounding formation. More specifically, the perforating charges  24  may be arranged in a particular phasing pattern (a helical or spiral phasing pattern, missing arc helical phasing pattern, a planar phasing pattern, etc.), depending on the particular perforating application. Furthermore, the perforating gun  20  may be, as examples, a hollow carrier gun in which the perforating charges  24  are protected by a sealed tube or an encapsulated perforating gun in which the perforating charges  24  are individually encapsulated or sealed. 
         [0020]    The perforating charges  24  are ballistically coupled to an initiator  22  of the perforating gun  20 . As a more specific example, the perforating charges  24  may be connected to one or more detonating cords (not shown) that are operatively coupled to the initiator  22 . 
         [0021]    In general, the initiator  22  is responsible for firing the perforating charges  24  in response to the detection of a command (herein called the “fire command”) that may be generated at the surface of the well  10  by a surface controller  30  (for example) for purposes of arming the initiator  22  and causing the initiator  22  to fire the charges  24 . The surface controller  30  may communicate the fire command downhole to the initiator  22  via signals that are communicated over one or more wires of a wireline (as a non-limiting example). Alternatively, the surface controller  30  may transmit the fire command downhole, along with an address of the perforating gun  20 . In this regard, the perforating gun  20  may be one of several downhole perforating guns that are specifically addressed in communications from the surface. Wired or wireless stimuli that are generated at the surface of the well  10  may be used to communicate the fire command and possibly an address of the perforating gun  20  (if multiple perforating guns are present). It is assumed hereinafter that for these embodiments of the invention the fire command is intended for the perforating gun  20  and thus, for example, the fire command is associated with an address that targets the perforating gun  20 . 
         [0022]    The stimuli that are used to communicate the fire command to the perforating gun  20  may take on a number of different forms and may be electrical, mechanical or mechanical stimuli, as just a few non-limiting examples. As more specific examples, a fire command may be communicated downhole to the initiator  22  via up and down movement of the perforating gun  20  by movement of the conveyance mechanism  16 ; via an electrical signal that is communicated downhole on a wireline; via hydraulic pressure (tubing conveyed pressure or pressure pulses, as examples); via an electromagnetic signal that is communicated downhole on a tubing string; etc. Regardless of the particular form of the stimuli, in response to detecting the fire command, the initiator  22  initiates a detonation wave on a detonating cord, and the detonation wave propagates on one or more detonating cord(s) to the perforating charges  24  to cause the charges  24  to fire. 
         [0023]    The initiator  22  contains certain safety features to ensure that the perforating charges  24  do not inadvertently fire. More specifically, the initiator  22  may contain one or more electrical switches for purposes of isolating a power source (a downhole battery, power communicated downhole via a wireline, a downhole pressure, etc.) from the final initiation component, such as an igniter of the initiator  22 , until the initiator  22  detects the fire command. In general, to fire the perforating charges  24  once the fire command is detected, the initiator  22  activates the igniter to initiate a sequence of explosions in a ballistic train of the initiator  22 , which ultimately results in the initiation of the detonation wave on the detonating cord. 
         [0024]    As described herein, as an added safety barrier, in its unarmed state, the initiator  22  physically interrupts the ballistic train so that the firing of an explosive (such as a primary explosive, for example) on one end of the ballistic train does not result in the firing of an explosive on the opposite end of the ballistic train, which would initiate the detonation wave on the detonating cord. More specifically, the initiator  22  includes an actuator assembly  21  that is constructed to misalign components (explosives, for example) of the ballistic train to establish the unarmed state of the initiator  22 . Therefore, even if an unintended triggering event, such as imparted radio frequency (RF) and/or electrostatic discharge (ESD) energy, initiates the firing of the first explosive (a primary explosive, for example) of the ballistic train, the discontinuity in the ballistic train terminates the chain of explosive events, thereby preventing unintended firing of the perforating gun  20 . 
         [0025]    To summarize,  FIG. 2  depicts a technique  30 , in accordance with embodiments of the invention, for arming and disarming a perforating gun. The technique  30  includes providing a perforating gun that includes perforating charges and an initiator that has a ballistic train, pursuant to block  32 . Explosives in the ballistic train are misaligned (block  34 ), and the perforating gun is run downhole, pursuant to block  36 . The explosives are then aligned (block  38 ) in response to a determination (diamond  37 ) that the initiator  22  is to be armed. For example, the initiator  22  may determine that the initiator  22  is to be armed in response to detecting the above-described fire command. After the initiator  22  is armed, the technique  30  includes initiating the firing of the ballistic train, pursuant to block  39 , for purposes of firing the perforating charges  24 . 
         [0026]    As a more specific example,  FIG. 3  depicts an exemplary technique  40  that may be performed by the initiator  22  (see  FIG. 1 ) in accordance with some embodiments of the invention. Referring to  FIG. 3  in conjunction with  FIG. 1 , upon detecting the fire command (pursuant to diamond  42 ), the initiator  22  moves (block  44 ) a primary explosive of the ballistic train into alignment with the remaining part of the ballistic train. The initiator  22  then electrically connects an energy source to an igniter of the initiator  22 , pursuant to block  46 , for purposes of initiating the firing of the ballistic train, which results in the initiation of the detonation wave on the detonating cord and the firing of the perforating charges  24 . 
         [0027]      FIG. 4  depicts an exemplary initiator assembly  50  in an unarmed state in accordance with some embodiments of the invention. Referring to  FIG. 4  in conjunction with  FIG. 1 , for this example the initiator assembly  50  includes the initiator  22 , a downhole energy source  96  and a detonation cord  90  that is operatively coupled to the perforating charges  24 . As examples, the downhole energy source  96  may be a battery, a power cable that extends from the surface of the well, an AC and/or DC converter that converts energy supplied through a downhole power cable, etc. Regardless of the particular form of the downhole energy source  96 , the downhole energy source  96  for this example provides electrical power that may be used to initiate the firing of a ballistic train  60  of the initiator  22 . It is noted that in other embodiments of the invention, another source, such as wellbore pressure, may be used to provide a force that activates an igniter or other mechanism to initiate the firing of the ballistic train. Thus, many variations are contemplated and are within the scope of the appended claims. 
         [0028]    The ballistic train  60  includes a primary explosive  74  and a secondary explosive  87 , which for this example are physically misaligned (as shown in  FIG. 4 ) in the unarmed state of the initiator assembly  50 . In this context, misalignment of the explosives  74  and  87  means that the explosives  74  and  87  are positioned so that firing of the primary explosive  74  (which is the first explosive in the ballistic train for this example) does not initiate firing of the secondary explosive  87 . The misalignment of the explosives  74  and  87  is to be contrasted to the alignment of the explosives  74  and  87  (as depicted in an armed state of the initiator assembly  60  in  FIG. 5 ), which means that the explosives  74  and  87  are positioned so that firing of the primary explosive  74  initiates the firing of the secondary explosive  87 . Alternate/additional to misalignment, components of the ballistic train can be separated or have barriers places there between. 
         [0029]    The initiator  22  includes one or more sensors  64  for purposes of detecting the fire command, which may be communicated downhole through pressure pulses in the fluid of the well  10 , electromagnetic signaling, seismic signaling or acoustic signaling, as a few non-limiting examples. The signals that are detected by the sensor(s)  64  may be processed by one or more controllers  62  of the initiator  22  for purposes of determining whether the fire command has been detected. In some embodiments of the invention, two controllers  62  may independently verify detection of the fire command before further action is taken to arm the initiator assembly  50  and fire the perforating charges  24 . 
         [0030]    In other embodiments of the invention, the fire command may be communicated downhole via signal, on a wireline. Therefore, for these embodiments of the invention, the sensors  64  may be replaced by a wireline telemetry interface. 
         [0031]    The initiator  22  controls electrical communication between the energy source  96  and an igniter  71 . As an example, this electrical communication may be controlled by a switch  68 , which remains open (as depicted in  FIG. 4 ) until the controller(s)  62  intend to fire the perforating charges  24 . When the igniter  71  is energized (due to the closing of the switch  68 ), the igniter  71  forms a projectile that impacts the primary explosive  74  to initiate firing of the explosive  74 . 
         [0032]    Depending on the particular embodiment of the invention, the igniter  71  may be a semiconductor bridge (SCB), hot wire, exploding bridgewire (EBW) or TiB igniter. In some embodiments of the invention, the igniter  71  may be an exploding foil initiator (EFI). In yet other embodiments of the invention, the igniter may be a non-electrical-based igniter, such as a pressure activated igniter, as a non-limiting example. 
         [0033]    In accordance with embodiments of the invention, the igniter  71  and the primary explosive  74  form a unit  70  that is translated along an axis  86  of motion by the actuator assembly  21  (see  FIG. 1 ) of the initiator  22 . In this regard, in response to the controller(s)  62  detecting the fire command, the controller(s)  62  communicate an electrical signal to the actuator assembly  21  to cause the assembly  21  to translate the unit  70  along the axis  86  until the primary explosive  74  is aligned with the secondary explosive  87 , as depicted in an armed state of the detonating assembly  50  in  FIG. 5 . In accordance with some embodiments of the invention, upon detection of the fire command, the controller(s)  62  first activate the actuator assembly  21  to align the primary  74  and secondary  87  explosives and subsequently close the switch  68  to establish electrical communication between the downhole energy source  96  and the igniter  7 . 
         [0034]    The actuator assembly  21  may include a microelectromechanical system (MEMS)-based actuator  80 , which moves an actuating member  84  that is attached to the unit  70  for purposes of translating the unit  70  along the axis  86 . In accordance with some embodiments of the invention, the MEMS-based actuator  80  along with the actuating member  84  and the circuitry of the initiator  22  (such as the controller(s)  62 , the sensor(s)  64 , the switch  68 , etc.) may be fabricated on a monolithic semiconductor substrate, although other packaging and/or fabrication techniques may be used in accordance with other embodiments of the invention. As non-limiting examples, the MEMS-based actuator  80  may be an electromagnetic, electrostatic, piezoelectric or thermal MEMS device, depending on the particular embodiment of the invention. 
         [0035]    As a more specific example, in accordance with some embodiments of the invention, the MEMS-based actuator  80  may be a comb-drive electrostatic actuator, which is depicted for purposes of example in  FIG. 6 . It is noted that the activator  80  of  FIG. 6  is only an example, as other types of MEMS-based activators are contemplated and are within the scope of the appended claims. Referring to  FIG. 6  in conjunction with  FIG. 4 , for these embodiments of the invention, the MEMS-based actuator  80  includes a stator  81  and the actuating element  84  that is constructed to translate in a controlled manner relative to the stator  81 . The actuating element  84  is attached to a tray  130  that holds the unit  70 . 
         [0036]    The actuating element  84  includes longitudinally extending fingers  124  that are received into corresponding longitudinal slots  108  of the stator  81 . The stator  81  and actuating element  84  are conductors, and a voltage is produced between the stator  81  and the actuating element  84  to produce a force that repels or attracts the actuating element  84  with respect to the stator  81 , depending on the polarity of the voltage. Thus, to physically misalign the actuating element  84  with respect to the stator  81 , an appropriate voltage is applied to attract the actuating element  84  to the stator  81 , and likewise, to physically align the explosives, the opposite voltage is applied to attract the actuating element  84  to the stator  81 . 
         [0037]    As depicted in  FIG. 6 , at the end farthest from the stator  81 , the actuating element  84  is attached to the tray  130 , which is mounted to the unit  70 . As shown in  FIG. 6 , the unit  70  is misaligned with the secondary explosive  87  (which may be below the tray  130 , as shown) in the initiator assembly&#39;s unarmed state. When the appropriate voltage is applied to repel the actuating element  84  with respect to the stator  81 , the unit  70  becomes aligned with the secondary explosive  87  to transition the initiator assembly  50  into the armed state. In accordance with some embodiments of the invention, the fingers  124  contain underlying metallic layers, which may be electrically isolated by a dielectric layer from the upper portion of the fingers  124  for purposes of maintaining electrical contact with an underlying metal layer that is connected to the switch  68 . Thus, when the switch  68  closes, power is communicated through the metal layer and through the conductive layers of the fingers  84  to the igniter  71  of the unit  70 . 
         [0038]    Other embodiments are within the scope of the appended claims. For example, the initiator assembly may be used in connection with a tool other than a perforating gun in accordance with other embodiments of the invention. More specifically, the initiator assembly may be used in connection with any downhole tool that operates in response to the firing of an explosive, a “one shot” tool (a one shot packer or a one shot valve, as non-limiting examples). 
         [0039]    The advantages of the initiating systems and techniques that are disclosed herein may include one or more of the following. The initiating system is protected from inadvertent firing due to radio frequency (RF) signals or electrostatic discharge (ESD). A two barrier safety system is provided. A safety barrier is disclosed, which facilitiates the use of a primary explosive to set off a secondary explosive. The components of the initiator  22  may be integrated to facilitate complete assembly of the perforating gun in the shop. A primary explosive may be used in the ballistic train for simpler and more reliable initiation, due to the isolation of the primary explosive from the remainder of the ballistic train in the unarmed state of the detonating system. 
         [0040]    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 perforating gun that is usable with a well includes at least one perforating charge and a initiator. The initiator includes an explosive ballistic train to the perforating charge(s). The initiator is adapted to physically misalign components of the ballistic train to prevent inadvertent firing of the perforating charge(s) and physically realign the components to arm the ballistic train.