Patent Publication Number: US-2022213767-A1

Title: Modular perforation tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application Ser. No. 62/848,856 filed May 16, 2019 and U.S. Provisional Patent Application Ser. No. 63/011,082 filed Apr. 16, 2020, which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments of the present invention generally relate to well perforation in oil and gas recovery. Specifically, embodiments of a modular perforation tool are described herein. 
     BACKGROUND 
     Oil wells are holes drilled into the earth, sometimes under water. The holes may descend essentially straight into the earth, or may advance at an angle, or even sideways. The well provides a conduit for oil and gas, or indeed any flowable resource, to flow out of a geologic formation into the hole and up to the surface. The hole is often lined with a support material to keep the flow path open as fluids flow into the conduit. The support material can be a pipe, a concrete lining, or other support. Oil and gas flow through tiny fissures, cracks, and capillaries in the geologic formation under the influence of a pressure drop between the formation and the hole. As the fluids move through the formation, solids can be entrained in the fluid movement that can be lodged into the cracks and fissures, slowing recovery of fluids from the formation. As fluids are produced from the formation, the pressure of the formation can decline, reducing the pressure drop causing the fluids to flow. 
     In some cases, flow from the geologic formation can be enhanced by forming conduits from the well wall into the formation. These are openings that extend from the well wall into the formation providing an open pathway substantially immune to plugging with solids to promote free flow of oil and gas into the well from the formation. These conduits are typically formed by inserting a perforation tool into the well to fire explosive charges shaped to propel blast force in a desired pattern toward the well wall, often through a well casing and/or lining. The focused blast force pulverizes and removes a portion of the geology, rock or other geology, at the well wall and a radial distance from the well wall into the formation, sometimes up to 60 inches, to form a conduit. In most cases, more than one such conduit is formed in the well wall at different azimuths and depths depending on the production needs of the well. 
     A perforation tool, sometimes called a perforating gun, is a complex tool that delivers a firing impulse to one or more explosive charges positioned by the perforating gun to direct the blast from the shaped charge in a desired direction. The explosive charges are contained and held in position by a cage, and a firing circuit is electrically connected to the shaped charges. Transportation of such assemblies is heavily regulated worldwide, due to the propensity to set off an explosion. In most cases, the firing impulse sub-assembly must be transported separately from the explosive sub-assembly, and the tool assembled in the field. Such restrictions complicate delivering perforation tools to drill sites. Additionally, securing the shaped charges in a desired position in the perforating gun is challenging. Often, the shaped charges must be installed at specific positions in the perforating gun, restricting flexibility in applying the blast. A perforating gun is needed in the industry that is transportable and has more flexibility of operation. 
     SUMMARY 
     Embodiments described herein provide an apparatus, comprising a housing; a plurality of frames that fit inside the housing, each frame having a cylindrical shape with a central axis and a plurality of liners, each liner having an axis perpendicular to the central axis, wherein the axes of the liners of each frame are disposed in a plane perpendicular to the central axis, and the frames are axially stackable; an electrical conductor disposed along a central passage of each frame; a plurality of shaped charges secured in the liners of the frames; a bulkhead member disposed in the housing and forming a seal with the housing; and an initiator module disposed in the housing with the bulkhead member between the initiator module and the plurality of frames 
     Other embodiments described herein provide a perforation tool, comprising a cylindrical housing having a plurality of perforation grooves formed therein; a plurality of charge frames that fit inside the housing, each charge frame having a cylindrical shape with a central axis and a plurality of liners positioned adjacent to one of the perforation grooves, each liner having an axis perpendicular to the central axis, wherein the axes of the liners of each charge frame are disposed in a plane perpendicular to the central axis, each liner has a retention member that extends around a circumference of the liner, and the charge frames are axially stackable; a plurality of shaped charges secured in the liners of the charge frames; a bulkhead member disposed in the housing and forming a seal with the housing; and an initiator module disposed in the housing with the bulkhead member between the initiator module and the plurality of charge frames. 
     Other embodiments described herein provide a perforation tool, comprising a cylindrical housing; a first perforation assembly disposed inside the housing, that first perforation assembly comprising a first frame for shaped charges, an initiator module, and a bulkhead member between the frame and the initiator module; and a second perforation assembly disposed inside the housing, the second perforation assembly comprising a second frame for shaped charges electrically coupled to the initiator module of the first perforation assembly by an electrical conductor disposed through a central passage of the second frame. 
     Other embodiments described herein provide a frame for housing shaped charges, comprising a body having a central axis, with a passage through the body along the central axis and a plurality of openings oriented transverse to the central axis, each opening comprising a wall having circular cross-section and diameter that declines from a first end of the opening to a second end of the opening, the first end of the opening having a circular rim and the second end of the opening having a port that fluidly couples the opening with the passage, each opening having a notch in the wall, the notch having a first side wall and a second side wall opposite the first side wall, with a restraint extending from the first side wall across the notch toward the second side wall. 
     Other embodiments described herein provide a frame for housing shaped charges, comprising a body having a central axis, with a passage through the body along the central axis and a plurality of openings oriented transverse to the central axis, each opening comprising a wall having circular cross-section and diameter that declines from a first end of the opening to a second end of the opening, the first end of the opening having a circular rim and the second end of the opening having a port that fluidly couples the opening with the passage, each opening having two rectangular notches in the wall on opposite sides, the two notches aligned along the central axis, with a restraint extending across each notch along the circumference of the opening. 
     Other embodiments described herein provide a frame for housing shaped charges, comprising a body having a central axis, with a passage through the body along the central axis and a plurality of openings oriented transverse to the central axis, each opening comprising a wall having circular cross-section and diameter that declines from a first end of the opening to a second end of the opening, the first end of the opening having a circular rim and the second end of the opening having a port that fluidly couples the opening with the passage, each opening having two rectangular notches in the wall on opposite sides, the two notches aligned along the central axis, with a restraint extending across each notch along the circumference of the opening, the body having a first end with a plurality of first alignment features and a second end opposite the first end with a plurality of second alignment features that match the first alignment features. 
     Other embodiments described herein provide an initiator module, comprising a first member having a contact ring with a tapered surface and a central opening; and a second member comprising a circuit plate coupled to the second member in a transverse orientation by a plurality of clips; and a central plug holding a detonator that protrudes through the central opening of the first member. 
     Other embodiments described herein provide an initiator module, comprising a first member having a contact ring at a first end of the initiator module, the contact ring having a tapered surface and a central opening; and a second member comprising a circuit plate coupled to the second member by a plurality of clips; and a central plug holding a detonator that protrudes through the central opening of the first member, wherein the initiator module has a cylindrical shape with a central axis, the circuit plate is oriented along a plane perpendicular to the central axis, the initiator module has a second end opposite from the first end, and the initiator module further comprises an electrical connector that protrudes from the second end. 
     Other embodiments described herein provide an initiator module, comprising a first member having a contact ring at a first end of the initiator module, the contact ring having a tapered surface and a central socket defining a central opening; and a second member comprising a circuit plate coupled to the second member by a plurality of clips; and a central plug holding a detonator that fits within the central socket of the first member, wherein the initiator module has a cylindrical shape with a central axis, the circuit plate is oriented along a plane perpendicular to the central axis, the initiator module has a second end opposite from the first end, and the initiator module further comprises a cylindrical electrical connector that protrudes from the second end. 
     Other embodiments described herein provide a bulkhead member for a perforation tool, the bulkhead member comprising a cylindrical body made from a structurally strong material the cylindrical body having a first end, a second end opposite the first end, and a central passage formed along an axis thereof from the first end to the second end; and an electrical conductor disposed in the passage along the axis and having a first end with a rounded prong that extends into a socket formed in the first end of the cylindrical body, and a second end that has an electrical coupling disposed at the second end of the cylindrical body. 
     Other embodiments described herein provide a bulkhead member for a perforation tool, the bulkhead member comprising a cylindrical body made from a structurally strong material the cylindrical body having a first end, a second end opposite the first end, and a central passage formed along an axis thereof from the first end to the second end; and an electrical conductor disposed in the passage along the axis and having a first end with a rounded prong that extends into a socket formed in the first end of the cylindrical body, and a second end that has an electrical coupling disposed at the second end of the cylindrical body, wherein the first end of the electrical conductor is electrically connectable to an initiator module for the perforation tool and the second end of the electrical conductor is electrically connectable to an initiator module for the perforation tool and to a charge module for the perforation tool. 
     Other embodiments described herein provide a bulkhead member for a perforation tool, the bulkhead member comprising a cylindrical body made from a structurally strong material the cylindrical body having a first end, a second end opposite the first end, and a central passage formed along an axis thereof from the first end to the second end; an electrical conductor disposed in the passage along the axis and having a first end with a rounded prong that extends into a socket formed in the first end of the cylindrical body, and a second end that has an electrical coupling protruding from the second end of the cylindrical body, wherein the first end of the electrical conductor is electrically connectable to an initiator module for the perforation tool and the second end of the electrical conductor is electrically connectable to an initiator module for the perforation tool and to a charge module for the perforation tool; and an insulating member disposed in the central passage between the cylindrical body and the electrical conductor. 
     Other embodiments described herein provide a method of downhole processing, comprising positioning a perforating charge in a perforating tool; positioning a booster charge adjacent to the perforating charge; deploying the perforating tool in a well; and activating the perforating charge by directly transferring ballistic energy from the booster charge to the perforating charge. 
     Other embodiments described herein provide a method of downhole processing, comprising positioning a plurality of perforating charges in a perforating tool; positioning a plurality of booster charges adjacent to the perforating charges, each booster charge adjacent to a corresponding perforating charge; deploying the perforating tool in a well; activating a first booster charge of the plurality of booster charges; activating each booster charge by direct transfer of ballistic energy from a neighboring booster charge; and activating each perforating charge by direct transfer of ballistic energy from the corresponding booster charge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, may admit to other equally effective embodiments. 
         FIG. 1A  is a side view of a perforation tool according to one embodiment. 
         FIG. 1B  is a detailed view of a portion of the perforation tool of  FIG. 1A . 
         FIG. 1C  is a cross-sectional view of a portion of the perforation tool of  FIG. 1A . 
         FIG. 1D  is a more detailed cross-sectional view of a portion of the perforation tool of  FIG. 1A . 
         FIG. 2A  is a cross-sectional view of a perforation tool in a transportation configuration according to another embodiment. 
         FIG. 2B  is a detail view of a portion of a perforation tool, according to one embodiment. 
         FIG. 2C  is a cross-sectional view of an initiator module according to another embodiment. 
         FIG. 3A  is an isometric view of a charge frame according to another embodiment. 
         FIG. 3B  is a cross-sectional view of a portion of the charge frame of  FIG. 3A . 
         FIG. 3C  is a different isometric view of the frame of  FIG. 3A . 
         FIG. 3D  is an isometric view of a frame according to another embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     The modular perforation tools described herein are configured to be transportable in a single parcel, and to be scalable to any desired degree in the field. The tools described herein are also feature configurable discharge directionality and spacing.  FIG. 1A  is a side view of a perforation tool  100  according to one embodiment. The perforation tool  100  is configured to integrate into a tool for use in a well bore. The perforation tool  100  has a first end  102  and a second end  104  opposite from the first end  102 . Each end is configured to attach to a drill string, by threaded connection, bolting, or other attachment mode. The apparatus shown in  FIG. 1A  displays an outer housing  106 , which is in three identical segments  106 A,  106 B, and  106 C, which are fit together at joints  107 A and  107 B. Each of the segments  106 A,  106 B, and  106 C has an initiator end  108  and a bulkhead end  110 . The initiator end  108  can house an initiator and the bulkhead end  110  can house a bulkhead, each of which is further described below. Each segment  106 A,  106 B, and  106 C also has a charge region  112 , generally between the initiator end  108  and the bulkhead  110 , which houses shaped charges that perform the perforation of the well bore when activated. The outer housing  106  of the perforation tool has a generally cylindrical shape with circular cross-section at most locations along its length, and a central cylindrical axis  103 . Thus, the initiator end  108  of each segment  106 A,  106 B, and  106 C encloses a cylindrical interior in which a cylindrical object can be disposed. Likewise, the bulkhead end  110  of each segment  106 A,  106 B, and  106 C encloses a cylindrical interior, and the charge region  112  also encloses a cylindrical interior. 
       FIG. 1B  is a detail view of a portion of the perforation tool  100  of  FIG. 1A . This view shows one of the segments  106 A,  106 B,  106 C, of the outer housing  106 . The outer housing  106  has a wall thickness that varies according to the function of the particular location in the outer housing  106 , some parts thicker and some parts thinner. In particular, each charge region  112  features two perforation grooves  114 , a first perforation groove  114 A and a second perforation groove  114 B, which are thin portions of the outer housing  106  configured to admit a discharge impulse of gas from one or more shaped charges located in the interior of the charge region  112  adjacent to each perforation groove  114 . Each perforation groove  114  is a band formed around the circumference of the outer housing  106 , generally with wall thickness that is less than a wall thickness at another location in the housing. The band proceeds around the circumference of the outer housing  106  in an azimuthal direction, generally along a plane transverse to the central axis. Each perforation groove  114  has a flat central portion  116  and a sloped wall  118  on either side of the central portion  116  that connects the central portion  116  to an outer wall  120  of the outer housing  106 . The first perforation groove  114 A has a first central portion  116 A, a first sloped wall  118 A and a second sloped wall  118 B, opposite from the first sloped wall  118 A. The second perforation groove  114 B has a second central portion  116 B, a third sloped wall  118 C and a fourth sloped wall  118 D, opposite from the third sloped wall  118 C. The sloped walls  118  may be flat or curved, and may form an angle with the central portion of 20-80 degrees. In this case, the sloped walls  118  all form an angle of 45 degrees with the respective central portions  116 . 
     The central portions  116  of the perforation grooves  114  have the same width along the axial direction of the perforation tool  100 . The width of the central portions  116  is selected to provide a window for discharge gas penetration of the central portion tailored to result in a discharge impulse, upon charge activation, that travels from the central portion into the well bore wall a desired distance with a desired dispersion profile. If the internal diameter of the outer housing  106  at the central portion  116  is D and the width of the central portion  116  is W, a ratio D/W is general from about 1 to about 100, such as from about 2 to about 10, for example about 3. 
     Here, there are two perforation grooves  114 , but the outer housing  106  can have any desired number of perforation grooves  114 , as will be further explained below due to the modularity of the perforation tool  100 . A minimum spacing between the perforation grooves  114  is generally set by interior structures of the perforation tool  100 . Here, the spacing between the perforation grooves  114  is about the same as the width of the central portions  116 . Each segment  106 A,  106 B, and  106 C of the outer housing  106 , in this case, has a bulkhead region  122  extending from the bulkhead end  110  of the segment to the first sloped wall  118 A of the first perforation groove  114 A, a first central portion  116 A extending from the first sloped wall  118 A to a second sloped wall  118 B of the first perforation groove  114 A, opposite the first central portion from the first sloped wall  118 A, a spacing region  126  extending from the second sloped wall  118 B of the first perforation groove  114 A to the third sloped wall  118 C of the second perforation groove  114 B, a second central portion  116 B extending from the third sloped wall  118 C to the fourth sloped wall  118 D, opposite the second central portion  116 B from the third sloped wall  118 C, and an initiator region  128  extending from the fourth sloped wall  118 D to the initiator end  108  of the segment. 
       FIG. 1C  is a cross-sectional view of the perforation tool  100  of  FIG. 1A . The outer housing  106  of the perforation tool  100 , in this case, has three segments  106 A,  106 B, and  106 C, as mentioned above. The first and third segments  106 A and  106 C are shown here largely empty in order to present the internal wall structure of the outer housing  106  clearly. The second segment  106 B mainly features the internal structures of the perforation tool  100 . 
     The perforation tool  100  includes an initiator module  130 , a plurality of charge modules  132 , in this case two charge modules  132  matched with the number of perforation grooves  114  in the outer housing  106 , and a bulkhead member  134 . Two bulkhead members  134  are shown in the apparatus of  FIG. 1C . The charge modules  132  are arranged between a bulkhead member  134  and the initiator module  130 . The initiator module  130  is a cylindrical module with an outer radius substantially the same as an inner radius of the outer housing  106  at the initiator region  128 . 
     The wall of the outer housing  106  at the initiator region  128  has a first section  136 , a second section  138 , and a third section  140 . The first section  136  starts at the initiator end  108  of the outer housing  106  and extends to the second section  138 . The second section  138  is between the first section  136  and the third section  140 . The inner diameter of the outer housing  106  tapers in the first section  136  from a first diameter at an end of the first section  136  to a second diameter at the second section  138 . The first diameter is less than the second diameter. The tapered portion of the inner wall of the outer housing  106  provides a tapered surface  139  for engaging with the initiator module  130 . 
     The initiator module  130  comprises a circuit plate  142  disposed in a container  144 . The container  144  comprises a first member  146  and a second member  148 . The first and second members  146  and  148  can be separated to extract the circuit plate  142  from the container  144 . The initiator module  130  has a generally cylindrical shape with a first end  152  and a second end  153 . The first member  146  has a contact ring  150  at the first end  152 . The contact ring  150  has a tapered surface  154  that matches the tapered wall of the outer housing  106  in the first section  136  of the initiator region  128  thereof. The tapered surface  154  of the contact ring  150  has an outer radius that matches the inner radius of the wall of the outer housing  106  in the first section  136 , the outer radius linearly declining toward the first end  152  such that the tapered surface  154  contacts the tapered wall of the outer housing  106  in the first section  136 . The outer radius can generally have any declining profile that matches with the inner radius of the wall of the outer housing  106 , such as curved or angled. It should be noted that, instead of providing tapered surfaces for engagement of the contact ring  150  with the wall of the outer housing  106 , the wall of the outer housing  106  could have a shelf extending from a right cylindrical wall, and the first member could have a contact ring, or contact portion, with a straight surface to engage with the wall and an end to contact the shelf. 
     The first member  146  of the container  144  has a central opening  156  that accommodates a detonator  158  that is disposed in the initiator module  130 , and when so disposed, protrudes through the central opening  156 . The second member  148  includes a central plug  160  that is aligned with the central opening  156  and holds the detonator  158  in position. The second member  148  also includes a plurality of clips  162  that hold the circuit plate  142  in a position transverse to the central axis  103 . The central plug  160  can be formed integrally with the second member  148 , or may be a separate member that engages with the rest of the second member  148  and with the detonator  158  and the first member  146 . 
     The first member  146  has a central socket  155  that defines the central opening  156  and extends along an axis of the initiator module  130 . The central plug  160  fits within the central socket  155  to assemble the first member  146  and the second member  148 . The central plug  160  and central socket  155  may be electrically conductive to provide electrical coupling from the initiator module  130  to the charge module  132 , as further described below. Alternately, one or both of the central plug  160  and the central socket  155  may be non-conductive materials, for example polymer, with conductive members such as wires or rods disposed therein, for example in grooves in the surface of either or both of the central plug  160  and the central socket  155 , or disposed within the body of either or both of the central plug  160  and the central socket  155 . 
     The central socket  155  has a nub  145  that extends slightly beyond the end of the detonator  158  and encloses the detonator  158 . The central socket  155  has three sections, a first section that engages with the central plug  160 , a second section that holds the detonator  158 , and a third section defined by the nub  145 . Each section is cylindrical in profile. The first section has an inner diameter larger than an outer diameter of the detonator  158  to accommodate the central plug  160 . The second section has an inner diameter substantially the same as the outer diameter of the detonator  158  in order to hold the detonator  158 . Here, the first and second sections have the same outer diameter, but the outer diameter of the two sections could be different. The third section, defined by the nub  145 , has an inner diameter substantially the same as the outer diameter of the detonator  158 , and an outer diameter less than the outer diameter of the first and second sections. The nub  145  has a hollow end to provide fluid coupling through the end of the nub  145  for ballistic coupling to a charge module  132 . The nub  145  fits within an electrical coupling of a charge module  132 , as further described below, to provide electrical coupling to the charge module  132 . 
     Here, no wires are shown for connecting the detonator  158  to electricity. The detonator  158  may receive electrical energy by contact with the central plug  160 , for example if the detonator  158  has electrical contacts built in. Alternately, the detonator  158  may have wires. In this case, the wires are disposed through a central opening  161  of the second member  148  to make electrical connection with contacts of the circuit plate  142 . 
       FIG. 1D  is a cross-sectional view of the charge modules  132  of the perforation tool  100 . Each charge module  132  includes a frame  164  housing a plurality of shaped charges  166 , each shaped charge disposed in a liner  168  that engages with the frame  164 . Each frame  164  has a cylindrical shape with a central axis  165  aligned with the central axis  103  of the perforation tool  100  when the frames  164  are installed in the perforation tool  100 . The liners  168  are disposed in openings  170  of the frame  164 , the openings  170  arranged along a plane  172  perpendicular to the central axis  103 . The plane  172  is here represented as a dotted line, since the view of  FIG. 1D  shows the plane edge-on. Each frame  164 , in this case, has three such openings  170  spaced 120° apart around the azimuth of the frame  164 . The liners  168  are cup-shaped to fit the shape of the charges. The frames  164  are positioned in the interior of the outer housing  106  such that the openings  170 , liners  168 , and charges are positioned adjacent to a perforation groove  114  of the outer housing  106 . Depending on the size of the charges to be used, the frames  164  can be configured to hold any convenient number of charges in openings spaced uniformly around the azimuth of the frame. Thus, two, three, four, five, or more charges can be positioned in a frame similar to these frames  164  by configuring the openings of the frames. 
     The frames  164  are made of a polymer material such as polypropylene, polyurethane, or another strong polymer material. The frames  164  can be made by molding, sculpting (for example laser scribing), or 3D printing. As shown in  FIGS. 1B-1D , the frames  164  are stackable such that any number of frames  164  can be disposed in a perforation tool. Usually the number of frames  164  disposed in the perforation tool will be consistent with the number of perforation grooves  114  in the outer housing  106 , but a greater or lesser number of frames  164  can, in principle, be disposed in the outer housing  106 . The frames  164  are also rotatable within the outer housing  106  to orient the discharge direction in any desired way. The perforation grooves  114 , which proceed completely around the housing  106 , generally allow orienting a shaped charge in any azimuthal direction. The perforation grooves  114  are equally susceptible to penetration at every azimuth by the shaped charges located within the frames  164  adjacent to the perforation grooves  114 . Rotatability of the frames  164  supports pointing the shaped charges held by the frames  164  in any direction within the perforation tool, without limitation. In this way, the housing  106  of the perforation tool  100  does not need to be positioned or oriented according to a rotation angle to allow jets from the shaped charges to penetrate in a desired direction. 
     Each of the frames  164  has a central passage  174  axially oriented in the center of the frame  164 . An electrical conductor  176  is disposed through the central passage  174  of each frame  164 . In this case, each frame  164  has an electrical conductor  176  disposed in the central passage  174  thereof. Each electrical conductor  176  contacts an inner wall  178  of the passage  174 , but a space can be provided between the electrical conductor  176  and the inner wall  178  in some cases. Such space can be a continuous annular space, or may be discontinuous, if desired. 
     Each electrical conductor  176  has a first end  180  and a second end  182  opposite the first end  180 . The first end  180  has a coupling  184 . The coupling  184  has a first diameter and the rest of the electrical conductor  176  has a second diameter. In this case, the first diameter is greater than the second diameter. The coupling can receive a connection portion of another module, such as the initiator module  130  or another frame  164 . The second end  182  of each electrical conductor  176  protrudes beyond the frame  164 , providing a connection that can connect the frame  164  to another frame  164 , an initiator module  130 , or a bulkhead member  134 . When the charge modules  132  and initiator module  130  are installed in the perforation tool  100 , the coupling  184  of a frame  164  receives the portion of the detonator  158  that protrudes through the first member  146  of the initiator module  130 . Here, the central passage  174  has a first section  186  with a first diameter and a second section  188  with a second diameter. The first diameter, in this case, is greater than the second diameter. Here, the second diameter of the second section  188  of the central passage  174  is substantially the same as the outer diameter of the electrical conductor  176 , while the first diameter of the central passage  174  is substantially the same as an outer diameter of the coupling  184 . Thus, an exterior surface of the coupling  184  contacts the first section  186  of the central passage  174  and the rest of the electrical conductor  176  contacts the second section  188  of the central passage  174  such that there is essentially no space between the electrical conductor  176  and the central passage  174 . The coupling  184  in this case has a tapered outer surface where the coupling  184  joins the rest of the electrical conductor  176 . The tapered outer surface of the coupling contacts a similarly tapered portion of the inner wall  178  of the central passage  174  that joins the first section  186  and second section  188  of the central passage  174 . As noted above, this construction allows for space to be provided between the electrical conductor  176  and the second section  188  of the central passage  174 , if desired. 
     A capsule  190  is positioned within the electrical conductor  176  for each frame  164  adjacent to the openings  170  of the frame, and adjacent to the shaped charges  166 . The capsule  190  is made of a combustible material that provides an ignition source for the shaped charges. Unlike conventional boosters used in downhole tools, the capsule  190  has no direct physical connection to an energy source such as a detonator cord. The capsule  190  is merely a shaped volume of combustible material, perhaps contained in a wrapping, that fits within the electrical conductor  176 . The capsule  190  is, thus, wireless. The capsule  190  is slipped into the electrical conductor  176  before the frame  164  is disposed in the container  144 . Here, the electrical conductor  176  has a ridge  192  at one end of the electrical conductor  176  to aid in dependable placement of the capsule  190  with respect to the shaped charges. In this case, the ridge  192  is located at the second end  182  of the electrical conductor  176 , but the ridge  192  could be positioned near the first end  180  of the electrical conductor  176 . 
     To hold the capsule  190  in place inside the electrical conductor  176 , a restraint  194  is positioned inside the electrical conductor  176  at a desired location. The restraint  194  can be a removable member, such as a snap ring, or a retractable member, which can extend around the entire circumference of the electrical conductor  176  or partway around the circumference. In any event, the restraint  194  can be removed or repositioned to allow placement of the capsule  190  between the restraint  194  and the ridge  192 . In  FIG. 1D , the capsule  190  is smaller than the space between the ridge  192  and the restraint  194 . 
     In other embodiments, the capsule  190  may be sized to extend substantially from the first end  180  to the second end  182  of the electrical conductor  176 . In such cases, the capsule  190  can be held in place between the ridge  192  of a first electrical conductor  176  and the ridge  192  of a second electrical conductor  196 . Thus, referring to  FIG. 1D , a capsule  190  might extend from a ridge  192  at the second end  182  of a first electrical conductor  176  to the first end  180  of the first electrical conductor  176 , and may be held in place between the ridge  192  of the first electrical conductor  176  and the second end  182  of a second electrical conductor  176  disposed against the first end  182  of the first electrical conductor  176 . Use of a separate restraint  194 , as shown in  FIG. 1D , allows for use of smaller capsules  190 . 
     An opening in the electrical conductor  176 , adjacent to a similar opening in the inner wall  178  of the central passage  174 , provides fluid coupling from the interior of the electrical conductor  176  through the frame  164  to the shaped charges  166 . Each electrical conductor  176  provides fluid coupling along the length thereof from the detonator  158  to the capsules  190 . The electrical conductor  176  thus provides electrical and ballistic coupling from the initiator module  130  to the shaped charges  166 . When the circuit plate  142  provides an electrical impulse, the electrical impulse is transmitted to the detonator  158  and to a first electrical conductor  176  of a first frame  164  adjacent to the initiator module  130 . The electrical impulse is transmitted by the first electrical conductor  176  through the first frame  164  to the capsule  190  of the first electrical conductor  176 . The detonator  158  combusts upon application of the electrical discharge from the circuit plate, and the ballistic discharge from the detonator  158  travels down the first electrical conductor  176  to the first capsule  190 , which combusts. Discharge from the first capsule  190  discharges the shaped charges  166  adjacent to the first capsule  190 . Discharge from the first capsule  190  also travels down the first electrical conductor  176  to the second end thereof, along with the electrical impulse from the firing of the circuit plate  142 . The second end of the first electrical conductor is mated with the coupling  184  of a second electrical conductor  176  of a second frame  164 , providing fluid and electrical communication from the first electrical conductor  176  to the second electrical conductor  176 . The ballistic discharge, and electrical impulse, travel from the first electrical conductor  176  of the second electrical conductor  176 , to the second capsule  190 , which discharges and ignites the shaped charges adjacent to the second capsule  190 . The discharge from the shaped charges  166  is configured to pierce the perforation grooves  114  of the outer housing  106  with enough energy to continue to the well bore and through the bore wall into the formation. In this way, any number of shaped charges  166  can be discharged using the structure of the perforation tool  100  shown in  FIGS. 1A-1D . 
     The structures described above allow for activation of perforating charges in a perforating tool without use of wires to transfer energy. The detonator is activated in the initiator module by electrical stimulation, but then a direct transfer of ballistic energy from the detonator activates the first capsule, which in this case is a booster charge. Ballistic energy transfer from activation of the first booster charge activates a first perforation charge adjacent to the first booster charge. Ballistic energy transfer from activation of the first booster charge also activates a second booster charge adjacent to the first booster charge. Ballistic energy transfer from the second booster charge, in turn, activates a second perforation charge adjacent to the second booster charge. In this way, an entire perforation gun made up of an number of perforating charges can be activated essentially wirelessly. The only wired connection in such a structure is from the firing circuit of the circuit plate to the detonator in the initiator module. In the event a wireless initiator module is used with the structures above, the entire perforating tool is wireless. 
     The circuit plate  142  is also electrically connected to the electrical conductor  176 , which provides electrical power through the frames  164 . The initiator module  130  includes a connector  141  with a central passage to receive the detonator  158  therein. The connector  141  protrudes from the initiator module  130  like the detonator  158  and electrically couples to the coupling  184  of the electrical conductor  176 . The plug  160  inserts into the connector  141 , and the detonator  158  held in the plug  160  passes through the connector  141  and protrudes into the electrical conductor  176  when the initiator module  130  is coupled to the charge module  132 . 
     The bulkhead member  134  is a dense structurally strong object made, for example, of steel. The bulkhead member  134  has a cylindrical body  147  in this case, but in general is shaped to match the cross-sectional profile of the outer housing  106 . The bulkhead member  134  is disposed in the outer housing  106  in the bulkhead region  122  thereof. As shown in  FIG. 1C , the bulkhead member  134  is inserted into the bulkhead region  122  of the second segment  106 B of the outer housing  106  such that a first end  167  of the bulkhead member  134  couples with the protrusion of the second end  182  of the electrical conductor  176  by a bulkhead conductor  177  inserted through a central passage  175  of the bulkhead member  134  that extends from the first end  167  to a second end  163  of the cylindrical body  147  of the bulkhead member  134 . As noted above, the cylindrical body  147  of the bulkhead member  134  is made of a structurally strong material, which may be electrically conductive or non-conductive. The bulkhead conductor  177  has a first end  173  and a second end  171  opposite from the first end  173 . The first end  173  has a coupling  169  that protrudes at the first end  167  of the bulkhead member  134  and receives an electrical conductor from another module, such as the second end  182  of the electrical conductor  176  of the frame  164  or the connector  141  of the initiator module  130  that protrudes from the initiator module  130 . The second end  171  of the conductor  177  is a prong-like structure with a rounded tip extending into a socket  179  at the second end  163  of the bulkhead member  134 . The socket  179  is configured to couple only to the second member  148  of the initiator module  130  such that the circuit plate  142  installed in the second member  148  is substantially protected from detonation of the shaped charges in the charge module  132 . In  FIG. 1C , two bulkhead members  134  are installed at either end of the assembly to isolate the discharge of the charge modules  132 . 
     The bulkhead member  134  is inserted into the housing  106  and abuts a ledge  159  of the outer housing  106  that marks a boundary between the bulkhead region  122  of the outer housing  106  and the charge region  112 . The bulkhead member  134  is a cylindrical object, in this case, but is generally shaped according to the profile of the housing  106 . The bulkhead member  134  has a retention shelf  157  that engages with an edge  143  of the first segment  106 A of the outer housing  106 , which modularly engages with the second segment  106 B, as described above. The edge  143  of the outer housing  106  securely captures the bulkhead member  134  between the edge  143  of one housing segment and the ledge  159  of another housing segment, holding the bulkhead member  134  securely in the bulkhead region  122  of the housing segment. 
     A first seal is disposed in a first seal groove  153  of the bulkhead member  134  near the first end  167  thereof and between the bulkhead member  134  and the second housing segment  106 B. A second seal is disposed in a second seal groove  151  disposed between the first housing segment  106 A, near the edge  143 , and the bulkhead member  134 . The first seal groove  153  is disposed in a side wall of the bulkhead member  134  that extends from the retention shelf  157  to the first end  167  of the bulkhead member  134  and has a first diameter. The second seal groove  151  is disposed in a side wall of the bulkhead member  134  that extends from the retention shelf  157  toward the second end  163  of the bulkhead member  134  and has a second diameter. The retention shelf  157  is thus located between the first seal groove  153  and the second seal groove  151 , and the first and second seal grooves  153  and  151  provide sealing between the bulkhead member  134  and respective first and second housing segments  106 A and  106 B. The first diameter of the side wall of the bulkhead member  134 , between the first end  167  and the retention shelf  157 , is larger than the second diameter of the side wall, between the retention shelf  157  and the second end  163 , such that the retention shelf  157  can provide secure positioning of the bulkhead member  134  in the outer housing  106 . 
     The bulkhead member  134  has a recess  149  formed in the first end  167  thereof. The central passage  175  of the bulkhead member  134  is formed at a central area of the recess  149 . The recess  149  facilitates inserting and removing the conductor  177  in the central passage. The second end  163  of the bulkhead member  134  is designed, overall, to abut the initiator module  130 , while the first end  167  of the bulkhead member  134  is designed to interface with a charge module  132 . Here, the first end  167  of the bulkhead member  134  does not abut the charge module  132 , since the conductors of the two components protrude and engage without the modules abutting. By reducing the distance the conductors of the two modules protrude, the charge module  132  could be disposed close enough to the bulkhead member  134  to abut the first end  167  of the bulkhead member  134 . It should be noted that the bulkhead member  134  is radially symmetric, so rotational orientation of the bulkhead member  134  in the outer housing  106  does not affect its operation. 
       FIG. 2A  is a cross-sectional view of a perforation tool  200  in a transportation configuration according to another embodiment. The perforation tool  200  packages an initiator module  202  with a charge module  204  in one package for shipment. The initiator module  202  is like the initiator module  130  of  FIGS. 1C-1D , with a two-piece container  206  housing a circuit plate  208  and a detonator  210 . The detonator  210  is located at a first end  212  of the initiator module  202 , and the initiator module  202  is coupled to a bulkhead member  214  at a second end  216  of the initiator module  202 . The bulkhead member  214  is between the initiator module  202  and the charge module  204  in the perforation tool  200 , with the detonator  210  spaced apart from the charge module  204 , such that the perforation tool  200  can be shipped with detonator and charges in the same package, without the possibility of unintended ignition. An electrical connector  217  protrudes from the second end  216  of the initiator module  202  and engages with a socket  219  of the bulkhead member  214  with a bulkhead conductor  221  positioned in the socket  219  and extending through the bulkhead member  214 . Here, the electrical connector  217  is cylindrical, but in other embodiments the electrical connector  217  may be prongs or brackets that engage with one or more similarly shaped sockets  219 . The electrical connector  217  provides electrical continuity between the bulkhead member  214  and the initiator module  202 . The bulkhead member  214  may be electrically conductive. In such cases, an insulating member  223  is disposed through the bulkhead member  214  between the cylindrical body  147  and the bulkhead conductor  221 , thus preventing short circuiting through the bulkhead member  214 . The insulating member  223  is disposed in the central passage  175  of the cylindrical body  147  ( FIG. 1C ), and in this case continuously lines the central passage  175 . Thus, in this case, the insulating member  223  is a liner. In other cases, the insulating member  223  may be a spacer instead of, or in addition to, being a liner. In such cases, the insulating member  223  may be discontinuous or may have holes or openings that provide space between the cylindrical body  147  and the bulkhead conductor  221 . In the event the bulkhead member  214  is not electrically conductive, the insulating member  223  can be omitted. 
     The electrical connector  217  could be a separate member from the initiator module  202 . In one case, the electrical connector  217  could be a electrical contact band that fits within the shaped socket  219 .  FIG. 2B  is a detail view of a portion of a perforation tool, according to one embodiment.  FIG. 2B  shows an embodiment in which a electrical contact band  217 A is disposed in the shaped socket  219  to provide electrical connection from the circuit plate  208  to the bulkhead conductor  221 . The electrical contact band  217 A is a generally cylindrical electrically conductive member that fits within the shaped socket  219  and provides electrical connection under a 360 degree rotation of members in electrical communication therewith. The electrical contact band  217 A has an open middle so that the bulkhead conductor  221  can be inserted into the middle of the electrical contact band  217 A. The electrical contact band  217 A, in this case, specifically provides electrical connection with the bulkhead conductor  221  under any rotation of the bulkhead conductor  221 . 
     In  FIG. 2A , the circuit plate  208  is shown electrically connected to the connector  217  by a wire  240 , optionally terminating in a contact pad  242  that connects with the connector  217 . In  FIG. 2B , an optional pogo pin  209  can be provided extending from the circuit plate  208  to provide a robust electrical connection from the circuit plate  208  to the connector  217  of  FIG. 2A  or the electrical contact band  217 A of  FIG. 2B . In  FIG. 2B , electrical connection from the circuit plate  208  to the electrical contact band  217 A is made at the ring end of the electrical contact band  217 A facing the circuit plate  208  by providing a cylindrical pogo pin  209  with a tapered tip and a diameter larger than an inner diameter of the ring end of the electrical contact band  217 A. Upon deploying the initiator module  202  with the charge module  204 , the tapered tip of the pogo pin  209  nests in the ring end of the electrical contact band  217 A, and the spring-loaded pogo pin  209  depresses to provide an engaging force between the pogo pin  209  and the ring end of the electrical contact band  217 A, thus creating a robust electrical connection between the circuit plate  208  and the bulkhead conductor  221 . While the pogo pin  209  of  FIG. 2B  is shown engaging with the electrical contact band  217 A in the center of the ring end of the electrical contact band  217 A, the electrical contact could be made using a pogo pin such as the pogo pin  209  at any convenient location between the circuit plate  208  and the bulkhead conductor  221  using wires and contact pads or recesses in any convenient configuration. An RCA connection can also be used in place of the pogo pin  209 . 
     Here, the electrical contact band  217 A is shown as a cylindrical member. The electrical contact band  217 A may have solid cylindrical walls, or may have vertically slotted walls in some embodiments. In other embodiments, the electrical contact band  217 A may have a generally cylindrical shape with a first diameter at the ends of the electrical contact band and a second diameter near a middle of the electrical contact band, where the second diameter is less than the first diameter, for example a cylindrical shape with a slight narrowing in the middle. Use of such a shape along with vertical slotting can provide a flexible and secure connection with the electrical contact band  217 A. 
     There are two charge modules  204  in the perforation tool  200 , but as described elsewhere herein, more or fewer charge modules  204  can be used. The charge modules  204 , bulkhead member  214 , and initiator module  202  are configured with an outer housing  213 , which has a bulkhead region  218 , a charge region  220 , and an initiator region  222 , similar to the outer housing  106 , and segments thereof  106 A,  106 B, and  106 C, described in connection with  FIGS. 1A-1D . To prepare the perforation tool  200  for operation, an initiator module is inserted into the interior of the outer housing  213  in the initiator region  222 , coupling the detonator  210  of the initiator module with the charge module  204 . The initiator module  202  can be disconnected from the bulkhead member  214  and inserted into the initiator region  222 . Alternately, an initiator module of another unit like the perforation tool  200  can be coupled at the initiator region  222 , coupling two charge regions together in an extended perforation tool. The modularity and safe shipping configuration of the perforation tool  200  allows a plurality of such tools to be shipped to a field location and assembled with ease into an operational configuration. 
       FIG. 2C  is a cross-sectional view of an initiator module  250  according to another embodiment. The initiator module  250  is adapted for use with a circuit plate of a different configuration. The initiator module  250  has a first member  252  that accommodates a central plug  260  that holds a detonator (not shown), similar to the central plug  160  of  FIG. 1C . In this case, the central plug  260  is a separate member that is attached to the first member  252  by a fastener  262 , in this case, a screw. 
     The initiator module  250  also has a second member  254 , which contains a circuit plate  256 , held to the second member  254  by circuit plate clips  258 . Outer clips  270  of the second member  254  engage with recesses  272  of the first member  252  to releasably attach the first and second members  252  and  254  together. 
     In this case, the initiator module  250  is configured to accommodate wires to connect the detonator to contacts located at a periphery of the circuit plate  256 . Here, the first member  252  has a central opening  274  to allow wires from the detonator to pass through the central opening  274  into a passage  276  of the first member  252 . The central opening  274  has a flared entrance  275  to simplify feeding wires of the detonator through the central opening  274 . The passage  276  includes a curved guide  278  that extends from the central opening  274  toward the second member  254  and radially outward away from the central opening  274 , along a curved path, toward the periphery of the first member  252 . The passage  276 , with the guide  278 , steers detonator wires to the periphery of the first member  252  to simplify connection of the wires to contacts at the periphery of the circuit plate  256 . 
       FIG. 3A  is an isometric view of a frame  300  according to another embodiment. The frame  300  can be used as one of the frames  164  in the charge module  132  of the perforation tool  100  of  FIGS. 1A-1D . The frame  300  has a generally cylindrical shape with a central axis  302 . In this case, the frame  300  is a single unit with a first end  304  and a second end  306  opposite from the first end  304 . The central axis  302  extends from the first end  304  to the second end  306 . Each of the first end  304  and the second end  306  have a generally circular shape as the ends of the cylindrical shape of the frame  300 . 
     A side  308  of the frame  300  connects the first end  304  to the second end  306 . A plurality of recesses  310  are formed in the side  308 . The recesses  310  are generally circular and extend into the side  308  toward the central axis  302  and have diameter that generally declines toward the central axis  302 . Each recess  310  generally defines an outer wall  312  of the recess  310 , which is a surface extending from the side  308  toward the central axis  302 . The outer wall  312  maintains a generally circular cross-section while tapering in dimension toward the central axis  302 . This outer wall  312  will generally take a shape that facilitates placement of charges in the recesses  310  that have shapes designed to project a jet of ballistic discharge radially outward from the central axis  302 . The recesses  310  are generally arranged in a coplanar arrangement, where a plane defined by the centroids of the three recesses  310  is perpendicular to the central axis  302  of the frame  300 . 
     Where the outer wall  312  of the recess  310  approaches the first end  304  and second end  306  of the frame  300 , a notch  314  is provided in the side  308  of the frame  300  from the outer wall  312  of the recess  310  to the respective first end  304  or second end  306 . The notch  314  provides a foundation for a restraint  316  that operates to secure a shaped charge in the recess  310 . The restraint  316  extends from a side wall  318  of the notch  314  in a direction generally across the notch  314  toward the opposite side wall of the notch  314 , which faces the side wall  318 . The notch  314 , in this case, has a rectangular profile that extends radially inward from the side  308  of the frame  300  toward the central axis  302 . Each notch  314  thus has two side walls  318 , which are substantially parallel, and a floor  320  connecting the two side walls  318  and perpendicular to both side walls  318 . The floor  320  of each notch  314  is planar here, but could be curved in other embodiments. As noted above, each end of the frame  300 , the first end  304  and the second end  306 , has a notch  314 , and the notches  314  at the two ends  304  and  306  are aligned in a direction parallel to the central axis  302 . The two side walls  318  of each notch  314  are substantially parallel, and substantially parallel to a plane midway between them that runs through the central axis  302 . Thus, the two side walls  318  do not run in a radial direction in this embodiment. Alternately, the two side walls  318  could run in radial directions, in which case the two side walls  318  would not be parallel. The side walls  318  are generally flat, but may be any convenient shape, curved, angled, beveled, and the like. 
     There are two restraints  316  for each recess  310 , one disposed in each opposing notch  314  of a recess  310 . The restraints  316  of a recess  310  extend from opposite side walls  318 , which means that one of the restraints  316  of a recess  310  extends from a side wall  318  on a first side of a plane bisecting the notch  314 , and the other restraint  316  extends from a side wall  318  on a second side of the plane bisecting the notch  314 . The restraints extend from their respective side walls  318  along the edge of the recess  310 . Each restraint  316  extends substantially parallel to the floor  320  of the notch  314 . The restraints  316  are curved in this case, following the circular perimeter of the recess  310  in opposite directions. The restraints  316  are designed to engage with a groove in the rim of a charge liner. The restraints  316  flex outward as the charge liner is pressed into the recess  310 , and snap into the groove of the charge liner to securely hold the charge in place. It should be noted that the restraints  316  of a recess  310  are located at the same elevation above the floor  320  of their respective notches  314 , but may be at different elevations, for example if two grooves are provided in the charge liner to engage the two restraints  316 . 
     The frame  300  has a nub  322  that extends from the first end  304  of the frame  300 . The nub  322  is cylindrical here, but may be any convenient shape. For example, the nub  322  can have the shape of any regular polygon, for example square, triangular, pentagonal, and the like. A circular opening  325  at the end of the nub  322  leads to a passage through the frame  300  along the central axis  302 . The passage may be the same as the passage  174  described above in connection with the perforation tool  200 . The passage is not visible in  FIG. 3A , but extends through the frame  300  to the second side  306  thereof to provide a conduit for the electrical conductor  176  described above. 
     Adjacent to the center of the floor  320  of each notch  314  of the frame  300 , extending from the first side  304  and substantially perpendicular thereto, is a first alignment feature  324 . Since there are three notches  314  in the frame  300 , there are three first alignment features  324 . In general, one or a plurality of first alignment features  324  may be provided. The alignment features  324  can have any arrangement on the first end  304 . For example, the alignment features  324  could be located between the notices  314  close to the edge of the first end  304 , or near the nub  322 . Alternately, two alignment features  324  could be used arranged on opposite sides of the nub  322 . Any convenient arrangement of alignment features  324  could be used. The alignment features  324  here are posts that extend perpendicular to the first end  304  and are uniformly distributed around the nub  322 . These posts have a plus shape in cross-section. Any type of alignment feature  324  could be used. For example, bumps, ridges, and other kinds of protrusions can be used, and can be mixed with recesses and/or notches, and/or posts with different cross-sectional profiles, for example square, hexagonal, or with an irregular shape. The alignment features  324  mate with alignment features on the second end  306  of the frame  300  to maintain alignment of two frames  300 , one to the other. The alignment features  324  can also be used, if desired, to constrain rotation of one or more frames  300  within a perforation tool such as the perforation tool  100 . 
     Where cylindrical connectivity features are used to connect a first frame with a second frame, as described herein, the first and second frame are operable in any rotational alignment. The axial features of the frames maintain electrical and fluid communication between the first frame and the second frame, so that the frames can be continuously rotated, one with respect to the other, and operability of the frames maintained. Where a discharge of a shaped charge creates a torque, the frames may relatively rotate following a discharge, potentially changing the direction of subsequent discharges. If discharge is intended in certain directions, rotation of a frame following a discharge can send subsequent discharges in unintended azimuthal directions. The alignment features described herein can be used to constrain unintended rotation of the frames, one with respect to another 
       FIG. 3B  is a cross-sectional view of a restraint  316  of  FIG. 3A . The restraint  316  has a generally irregular shape in cross-section, with a first end  340  that has a rectangular shape and a second end  342  opposite from the first end  340  that has an irregular shape. The restraint  316  has a first surface  344  and a second surface  346  perpendicular to the first surface  344 . The first surface  344  has a first width, and the second surface  346  has a second width less than the first width. The restraint  316  has a third surface  348  also perpendicular to the second surface  346 , and therefore parallel to the first surface  344 , with the second surface  346  connecting the first surface  344  and the third surface  348 . The third surface  348  has a third width less than the first width. The restraint  316  has a fourth surface  350  perpendicular to the first surface  344 , and therefore parallel to the second surface  346 . The fourth surface has a fourth width that is less than the second width. The restraint  316  has a fifth surface  352  that connects the third surface  348  to the fourth surface  350 . The fifth surface  352  has a straight portion  354  connected to the third surface  348  and a curved portion  356  connected to the fourth surface  350 . The straight portion  354  forms an angle with the third surface  348  that depends on the length of the third surface  348 , and may be between about 120° and about 170°, for example about 150°. The curved portion  356  may be circular, or otherwise rounded in a specific, or non-specific way. Here, the curved portion  356  has a radius of curvature that is about the same as the fourth length. The radius of curvature of the curved portion  356  depends on the dimensions of the other surfaces. The cross-sectional profile of the restraint  316  is selected to mate with a groove in the charge cup that holds the shaped charge, and to have a shape that successfully secures the restraint  316  in the groove. 
     Referring again to  FIG. 3A , each restraint  316  extends from a base  317  attached to the side wall  318  of the notch  314 . The base  317  has curved surfaces that connect the various surfaces of the restraint  316  to the side wall  318 . Each curved surface of the base  317  connects from the side wall to one of the surfaces of the restraint  316  described above. The base  317  thus has a dimension that matches a corresponding dimension of the restraint  316  where the base  317  meets the restraint  316 , and increases in areas of the base  317  approaching the side wall  314 . The base  317  is thus thicker than the restraint  316  to provide a strong foundation for attaching the restraint to the side wall  314 . 
     Each restraint  316  has a latch feature  329  at a distal end of the restraint  316 . In this case, the latch feature  329  has the shape of a triangular prism with two parallel right triangular surfaces connected by rectangular surfaces. The right angle of the latch feature  329  abuts the first surface  344  of the restraint  316  described above in connection with  FIG. 3B . The latch feature  329  has a thickness, distance of separation of the triangular surfaces, that is less than the first width of the first surface  344 . In this case, the thickness of the latch feature  329  is about half the first width, but can be more or less than half the first width in other embodiments. The right angle of the latch feature  329  is positioned near and end  331  of the restraint  316 , and the latch feature  329  rises from the right angle to an edge  333 . A surface of the latch feature  329  slopes downward from the edge  333  along the restraint  316 , such that the tallest part of the latch feature  329 , having the edge  333 , is near the end  331  of the restraint  316 , and the latch feature  329  narrows in a direction along the restraint  316  away from the end  331 . 
       FIG. 3C  is an isometric view of the frame  300  looking toward the second end  306 . The second end  306  has an opening  326  that fluidly communicates with the nub  322  of  FIG. 3A  on the first end  304  by the passage extending through the frame  300 . The opening  326  has a diameter larger than the diameter of the passage, as described in connection with the perforation tool  100 . The diameter of the opening  326  is substantially the same as the outer diameter of the nub  322 , so the nub  322  can fit into the opening  326  to stack multiple frames  300  together. The nub  322  can also fit with other components of a perforation tool, as described above. The opening  326  also accommodates coupling of other components of a perforation tool. 
     A plurality of second alignment features  328  are formed in the second end  306  of the frame  300 . These alignment features  328  can engage with the first alignment features  324  of the first end  304 . There are six second alignment features  328  on the second end  306 , where there are only three alignment features  324  on the first end  304 . As above, in general, one or a plurality of second alignment features may be provided. The six alignment features  328  here are recesses shaped to match the posts at the first end  304  of the frame  300  to allow two frames  300  to be engaged in two different relative orientations. The recesses  310  of two adjacent frames  300  coupled together can be aligned, pointing in the same direction, or the recesses  310  can be azimuthally displaced by 30°. The relative orientation of adjacent frames can be selected in this way by providing alignment features having any desired relationship to allow different relative orientations. It should be noted that the spacing of alignment features can be any useful spacing to allow positioning adjacent frames with a variety of different angular displacements. 
     Each recess  310  of each frame  300  also features a finger notch  330 . The finger notch  330  is formed in the side  308  of the frame  300 . The finger notch  330  is formed in the side  308  between the first and second ends  304  and  306  of the frame, on only one side of the recess  310 . The finger notch  330  extends through the outer wall  312  of the recess  310 , providing access to the recess  310  from the side. The finger notch  330  simplifies the process of extracting charges, for example spent charges, from the frame  300  by providing a lateral extraction method that minimizes stress on the restraints  316  as the charge is removed from the frame  300 . The finger notch  330  can have any convenient shape. Here, the notch  330  has a flat floor  332  and substantially straight side walls  334 . The side walls  334  are not parallel here, but there is no reason they could not be parallel. 
       FIG. 3D  is an isometric view of a frame  380  according to another embodiment. This isometric view is from the same perspective as the view of  FIG. 3C . The frame in  FIG. 3D  is a variant that has only two of the recesses  310 . The two recesses  310  of the frame  380  are also arranged in the same coplanar fashion as the recesses  310  of the frame  300  in  FIG. 3C . Here, two recesses  310  are arranged at an angle α that is from 120° to 180°. Such frames  350  can be made with different angles α such that charge geometry for a single perforation tool can be distributed among a plurality of such angles and then selected using any suitable firing method. A single perforation tool can be fitted with an assortment of frames  300  and frames  350  to provide flexibility regarding the directions chosen for perforation 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.