Abstract:
A perforating system has a perforating module comprising a unitary body of explosive. The explosive is contained within a non-explosive casing, or liner, having formed indentations and a cover thereover. The indentations, which will transform into explosively formed penetrators (EFP&#39;s) upon detonation, have a perimeter shape that allows for improved packing density, e.g., a hexagonal perimeter, which results in relatively little “dead space” wherein no perforating penetrators are generated. In operation, the module provides a relatively dense shot pattern and substantially reduced amount of post-detonation debris that could clog the perforations and/or require remedial clean-up or repeat perforation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to the design of perforating tools for use in creating perforations in wellbores to improve the flow of fluids from the wellbore.  
         [0003]     2. Description of the Related Art  
         [0004]     Perforation guns are used within wellbore holes to increase the permeability of the formation surrounding the wellbore. In general, perforation guns producing greater numbers of perforations are considered to be more effective than those producing fewer perforations. It is therefore often desired to maximize the number of penetrating jets within a segment of the wellbore. This may be difficult, however, because there are limitations relating to placement of the charges used for perforation. Standard shaped charges have an outer housing formed of metal or another material that encloses the high explosive charge. The shaped charge holder has openings that have typically circular perimeters. When packing the charges in an adjoining manner in the charge tube, interstitial spaces are unavoidably left between the neighboring charges as a result their shape. This packing results in “dead spaces,” that is, areas from which no perforating product, i.e., no jets, is/are provided, between the charges, and limits the density with which the charges can be packed.  
         [0005]     There are a number of known styles and designs for perforation guns. There are, for example, strip guns that include a strip carrier upon which are mounted a number of capsule charges. The capsule charges are individually sealed against corrosive wellbore fluids. Also known are hollow carrier guns that have a sealed outer housing that contains unencapsulated shaped charges. In each case, the shaped charges are arranged such that they will detonate in a radially outward direction to form a specific pattern of perforations.  
         [0006]     An alternative perforation gun design is described in U.S. Pat. No. 5,619,008 to Chawla et al. In this design, a two-layer liner serves to sheath discontinuous loadings of explosive material. The liner is configured with indentations that are each aligned with an individual loading of the explosive material. Upon detonation of the loadings of explosive material, these indentations act in the manner of a shaped charge, creating a directed jet of liner material. The indentations have a circular perimeter and are spaced apart from one another, leaving significant “dead space” between them. Following detonation and any resulting perforation, the housing that surrounds the charges is not completely destroyed and forms debris. This debris is undesirable, both because it must be removed by wireline or by other means in a secondary operation, and because it may clog the perforations that are formed by the perforation operation, thereby making the perforations less effective and sometimes necessitating repeat perforation operations. The Chawla et al. invention thus suffers from problems relating to both “dead space” and debris creation.  
         [0007]     The present invention addresses the problems of the prior art.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a perforating device that produces multiple perforating penetrators from a single high explosive charge. In one embodiment, the perforating module has a central rod with a surrounding cylinder of high explosive. The cylinder of high explosive is contained within a liner having formed indentations. The liner may be of any suitable material, such as a non-explosive material including, for example, an elemental metal or alloy, a composite, a ceramic, a thermoplastic or thermo set polymer, or the like. Finally, a cylindrical outer cover is disposed about the liner. In one embodiment, the indentations are linearly contiguous to one another. In another embodiment, the indentations each have a perimeter that is triangular, square, hexagonal, or octagonal and are disposed in an adjoining fashion to one another.  
         [0009]     In operation and as a result of detonation of the explosive material, the module forms penetrators of liner material that propagate into the formation in a direction that is, in one embodiment, substantially perpendicular to the longitudinal axis of the wellbore. The module thus is capable of providing a relatively dense shot pattern with little or no “dead space” between the locations from which the penetrators are formed. This results in an effective perforation of a wellbore segment.  
         [0010]     During the detonation, the constituent components of the module, including in some embodiments the high explosive, the liner, and the outer cover, are largely destroyed. As a result, the amount of debris resulting from the detonation is reduced or eliminated, as compared with the amount of debris produced by many conventional perforation devices.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     For greater understanding of the invention, reference is made to the following detailed description of the embodiments of the present invention, taken in conjunction with the accompanying drawings in which reference characters designate like or similar elements throughout the several figures of the drawings.  
         [0012]      FIG. 1  is a side, cross-sectional view of a wellbore containing an exemplary perforation system constructed in accordance with the present invention.  
         [0013]      FIGS. 1   a  and  1   b  illustrate a pair of alternative constructions for perforation systems constructed in accordance with the present invention.  
         [0014]      FIG. 2  is a side, cross-section depiction of a single perforation module of the perforation system shown in  FIG. 1 .  
         [0015]      FIG. 3  is an exterior view of the module shown in  FIG. 2 .  
         [0016]      FIG. 4  is a detail view of a portion of the liner of an exemplary perforation module showing further details concerning the indentations.  
         [0017]      FIG. 5  is a detail view of a portion of the liner of an exemplary perforation module showing an alternative shape for the indentations.  
         [0018]      FIG. 6  is a side cross-section of the portion of liner shown in  FIG. 5 , taken along lines  6 - 6 .  
         [0019]      FIG. 7  depicts an exemplary shot pattern that is created by the perforation module shown in  FIGS. 2 and 3 .  
         [0020]      FIG. 8  illustrates an alternative embodiment for a perforation module in accordance with the present invention having triangular indentations.  
         [0021]      FIG. 9  illustrates a further alternative embodiment for a perforation module in accordance with the present invention having square indentations.  
         [0022]      FIG. 10  depicts a portion of the surface of the liner of a perforation module that utilizes octagonal indentations.  
         [0023]      FIGS. 11-14  illustrate an exemplary initiation sequence for a single penetrator of a perforation module in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     The present invention relates to devices and methods for perforating wellbores. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein.  
         [0025]      FIG. 1  illustrates an exemplary perforation system  10  that is configured in accordance with one embodiment of the present invention. The perforation system  10  is disposed within a wellbore  12  that has been drilled through the earth  14  and a hydrocarbon-bearing formation  16 . Portions of the wellbore  12  are cased by a steel casing  18  that is secured within the open wellbore hole by cement  20 .  
         [0026]     The hydrocarbon-bearing formation  16  contains two oil-bearing strata  22 ,  24 , which are separated by a layer of water  26 . A layer of water  28  also separates the lower oil stratum  24  from a stratum of gas  30 . It is noted that this arrangement of strata in formation  16  is presented only by way of example and that those skilled in the art will recognize that the actual composition and configuration of formations varies.  
         [0027]     The perforation system  10  is disposed into the wellbore  12  on a conveyance string  32 . The conveyance string  32  may be of any known construction for conveying a tool into a wellbore, including a drill pipe, wireline, production tubing, coiled tubing, and the like. The perforation system  10  includes one or more perforating modules that are used to perforate portions of the surrounding formation  16 . In the described embodiment, there are three perforating modules  34 ,  36 ,  38  that are secured to one another in series. There may, of course, be more or fewer than three modules, depending upon the desired length of wellbore to be perforated. Additionally, it is pointed out that there may be intermediate sections of tubing, or subs  37  (see  FIG. 1   a ) interposed between the individual modules  34 ,  36 , and  38 , to provide a desired spacing therebetween. In practice, the subs  37  are desirably non-explosive. If desired, the modules  34 ,  36 , and  38  may alternatively be secured to one another so as to form an unbroken, contiguous series of modules.  FIG. 1   b  illustrates a further alternative perforation system arrangement wherein the perforation modules  34 ,  36 , and  38 , of the system are interconnected directly to one another in series.  
         [0028]     An exemplary individual module  40  is depicted in  FIGS. 2 and 3 . The module  40  is representative of each of the three modules  34 ,  36 , and  38  shown in  FIG. 1 . As will be described in further detail below, the module  40  creates a plurality of perforating penetrators from a single explosive charge. The penetrators travel in a direction substantially normal or orthogonal to the longitudinal axis of the wellbore. Advantageously, this arrangement may significantly increase shot density and simultaneously reduce the amount of debris left in the wellbore, relative to many conventional perforation systems. In one embodiment, the module  40  includes a support member such as a central rod  42  having upper and lower axial ends  44 ,  46 . The upper and lower axial ends  44 ,  46  are provided with threaded connections, as is known in the art, so that they may be secured to the conveying string  32  (see  FIG. 1   b ) or to an adjoining module. The central rod  42  is composed of a central load bearing portion  41  and an outer detonation layer  43 . The load-bearing portion  41  of the central rod  42  may be a section of pipe, rod or other load bearing structure. In one embodiment, the load-bearing portion  41  of the central rod  42  is formed of steel. In another embodiment, if the perforation device  10  is not to be withdrawn from the wellbore  12  after detonation, the load-bearing portion  41  of the central rod  42  is formed of a frangible or combustible material that will be readily destroyed during the detonation of the perforating device  10 . Ceramic is just one example of a suitable frangible material.  
         [0029]     The detonation layer  43  comprises, in this embodiment, a primasheet of a type known in the art for initiation of detonations. The load-bearing portion  41  of the central rod  42  may also contain an axial passage  48  along its length to contain electrical wiring (not shown) that is necessary for initiation of the detonation layer  43  which, in turn, results in detonation of the body  50  of high explosive material. The detonation layer  43  may be initiated with a control signal either manually or utilizing some preprogrammed device. For example, suitable initiating systems can include using electrical signals transmitted from the surface via wiring (not shown) in the axial passage  48  to initiate detonation cord (not shown) disposed within the axial passage  48 , by increasing hydraulic pressure in the wellbore, or by the dropping of a drop bar (not shown) into the axial passage  48 , as is used conventionally with tubing conveyed perforation guns. Other initiating systems can utilize timers or well bore parameter sensitive devices (e.g., pressure, temperature, depth, etc. Initiation systems for detonating perforating guns are known in the art and will not be discussed in further detail.  
         [0030]     Surrounding the central rod  42  is a substantially unitary body  50  of high explosive material that explosively forms the perforating penetrators using the liner  52 . Suitable high explosive materials may include, for example, conventionally-employed high explosives such as RDX, HMX and HNS. While the size of the module is not a critical aspect thereof, it may be convenient to configure the module  40  such that it is a cylinder about 12 inches in length and about 4.5 inches in diameter. However, the length and diameter may be varied according to the dimensions of the wellbore  12  or other factors. A tube  51  of cardboard or a similar material is disposed between the central rod  42  and the high explosive body  50 .  
         [0031]     The liner  52  surrounds the body  50  of high explosive and is configured to form a plurality of perforating penetrators. The penetrators formed by the liner  52  may travel in a direction generally perpendicular to the longitudinal axis of the wellbore, although modifications in direction may also be achieved in other embodiments of this invention. In one embodiment, the liner  52  may be, in this embodiment, a cylindrical and non-explosive liner formed of a metal, such as, for example, tantalum. Alternatively, the liner  52  may be made from extruded copper, tungsten, steel, depleted uranium, aluminum, or another elemental metal or alloy. In other embodiments blends of elemental metals or alloys with materials such as lead, graphite, and zinc stearate may also be employed. In still other embodiments blends or alloys of aluminum with either titanium or hafnium may be used. Additionally, a frangible material may be used to form the liner  52  in order to further reduce the likelihood that the formed penetrator will plug the perforation created in the surrounding formation. Such may include, for example, the use of pressed, sintered metallic powders, such as those described in U.S. Pat. No. 6,012,392, which is incorporated herein by reference in its entirety, and metal/matrix composites.  
         [0032]     The size, shape, velocity and other characteristics of the perforating penetrators formed by the liner  52  may be controlled, in part, by adjusting the surface contours of the liner  52 . In one embodiment, a plurality of linearly contiguous indentations  54  is formed into the liner  52 . As used herein, the phrase “linearly contiguous” means that the perimeters of every indentation shares at least one common side with an adjacent indentation. In some embodiments a majority of each indentation is linearly contiguous with adjacent indentations, and in other embodiments essentially all of each indentation is linearly contiguous with adjacent indentations. In one embodiment, each indentation  54  has an axis that is substantially perpendicular to the exterior surface of the liner  52 , where such exterior surface is substantially parallel to the longitudinal axis of the wellbore. In other embodiments such indentation axis may be significantly greater or less than ninety degrees to the exterior surface of the liner  52  and/or to the longitudinal axis of the wellbore, in order to direct the penetrators in a specific direction, according to the purposes and goals of the perforation operation.  
         [0033]      FIG. 4  depicts further details concerning one embodiment of the indentations  54 . In this embodiment, each indentation  54  has a hexagonal outer perimeter  56  and therefore adjoins a neighboring indentation  54  on each of its six sides, i.e., all of its six sides are linearly contiguous with neighboring indentations  54 . Because of this fact, there are no “dead spaces” between the indentations  54  from which it is inferable that there is no area from which a penetrator is not, or could not be, transformed. A small linear ridge  58  is formed at each of the adjoining contact areas of the neighboring indentations  54 . A hexagonal shape for the perimeter  56  of the indentations  54  is one possible arrangement, which may offer the additional benefit that, by approximating the shape of a circle, a penetrator that is relatively radially uniform is, upon detonation of the body  50  of high explosive, developed therefrom. Additionally, the hexagonal shape of the perimeter  56  permits relatively closer packing of the indentations  54  to form an adjoining, interlocking honeycomb effect. As a result, the “dead space,” that is unavoidable when indentations having circular perimeters are employed, is thereby greatly reduced or eliminated. A further advantage of the honeycomb arrangement of the indentations  54  is that the perforations created may, as a result, be spaced equally in all directions, that is, in circumferential, axial, vertical, and horizontal directions, such as to significantly reduce the possibility of failure of the surrounding casing  18  upon perforation. A high density of perforations may therefore be achieved from the use of such linearly contiguous and interlocking indentations that cover essentially the entire outer surface area of the module  40 . For example, a pattern of hexagonal indentations that are two inches in diameter, i.e., hexagons that can be inscribed within a two-inch diameter circle, may in some embodiments generate a shot pattern of 51 perforations per linear foot of the wellbore from the surface of a 4.5-inch diameter module  40 . In contrast, a similarly sized, conventional carrier-type perforating gun, using conventional shaped charges, will typically provide only about 18 perforations per linear foot. Thus, this embodiment illustrates a capability to increase the perforated area by a factor of three. The size and number of hexagonal indentations  54  may be varied, depending upon factors such as the diameter of the module  40  relative to the size of the annular space between the perforation system  10  and the casing wall  18 ; the properties of the formation in which the perforation gun is being used; the presence or absence of fluid in the annular space; the selection of liner material and explosive; and the like. Those skilled in the art will be able to determine optimal configurations based upon such skill and with, at most, routine experimentation to ensure success.  
         [0034]      FIGS. 4, 5  and  6  show additional possible configurations for the liner to enable formation of effective penetrators therefrom. As illustrated therein, the indentations  54  each define a cavity  60 . While the perimeter of the indentations may influence the shape of the cavity  60 , it is not necessarily determinative thereof. Thus, in certain embodiments the shape of the cavity  60  may be of a generally conical or pyramidal configuration, as shown in  FIG. 4 , or of a generally spherical or parabolic configuration, as depicted in  FIGS. 5 and 6 . The cavity  60  provides a formation distance for a penetrator to form. The cavity  60  provides an apex  62 , i.e., point of greatest indentation, opposite the opening defined by perimeter  56 . In this embodiment, the cavity  60  has six equal planar triangular sides  70 . The sides  70  adjoin one another along junction lines  72 , forming a cavity  60  that is symmetrical along certain axes. The indentations  54  may be formed into the essentially planar liner  52  by stamping, forging or by other known means. Thereafter, the sheet may be formed into a cylinder by bringing opposing ends together and then welding or otherwise connecting the ends. The high explosive body  50  may then be cast into the space between the liner  52  and the inner cardboard tube  51 .  
         [0035]     An alternative method for forming the high explosive body  50  is by pressing a billet to a desired length and diameter, and then machining the billet to match the hexagonal indentations  54  at the outer surface of the liner  52 . A long axial hole is then drilled into the center of the billet and sized to accommodate the tube  51 . As those skilled in the art are aware, a billet of high explosive is a mass of high explosive material that has been pressed or cast into cylindrical shape. Pressed billets can be machined to a desired shape, while cast billets are formed to the desired shape, such as, in this case, a cylinder with an axial passage therethrough.  
         [0036]      FIG. 5  illustrates an alternative design for the indentations  54 , here designated  54 ′. The indentations  54 ′ still have a hexagonal perimeter  56 . However, the side surfaces defining the cavity  60  are smooth and rounded. In side cross-section, the cavity  60  forms a dome-like cap or parabola, as  FIGS. 5 and 6 , respectively, depict. The radius and apex of each dome-like cavity  60  depend upon the liner thickness and desired formation distance, with the goal that a penetrator may be transformed therefrom that is optimal for creating a large perforation in the wellbore casing  18 . In alternative embodiments, other cavity shapes, such as a conical shape, may be employed.  
         [0037]     Circumferentially surrounding the liner  52  is a cover  64  that protects the liner  52  and other parts of the module  40  from the harsh wellbore environment. In one embodiment, the cover  64  is a generally cylindrical construction having planar inner and outer surfaces. The cover  64  may be formed of, for example, a thermoplastic or thermoset polymer that is resistant to high wellbore temperatures. The cover  64  may be relatively thin, having a thickness of, for example, just 0.05 inch, and light in weight, such that it will not unduly interfere with the creation of the penetrators from the indentations  54  or  54 ′. In some embodiments, an elemental metal or alloy, composite material, thermoplastic or thermoset polymer, or glass, for example, may be used to form the cover  64 . The cover  64  overlies the adjoining ridges  58  between neighboring indentations  54  or  54 ′ (see  FIG. 6 ). There is a space disposed between the cover  64  and the ridges  58  to permit the indentations  54 ,  54 ′ to fully develop into penetrators upon detonation. Such space may be relatively small, for example, about 5 mm. Air, at atmospheric pressure, may be trapped within the cavities  60  of the indentations  54 ,  54 ′ between the cover  64  and the outer surface of the liner  52 . The distance between the apex  62  of each indentation  54  or  54 ′ and the outer cover  64  provides a stand-off for each indentation  54  or  54 ′ such that a penetrator can more fully develop prior to contact with the well casing  18  (see  FIG. 1 ).  
         [0038]     Upper and lower end caps  66 ,  68  (see  FIG. 3 ) are secured to the cover  64  and liner  52  of the module  40  and serve to help encapsulate and protect the contents of the module  40 , particularly the explosive body  50 , from fluids within the wellbore  12  prior to detonation.  
         [0039]     In operation, the perforation system  10  is lowered into the wellbore  12  until the modules  34 ,  36 ,  38  of the perforation system  10  are aligned with the desired strata  22 ,  24 , and  30 , respectively, of the formation  14 . The modules  34 ,  36 ,  38  of the perforation system  10  are then detonated to create penetrators that perforate the casing  18 , cement  20  and formation  14 . Following perforation of the formation  14 , the remains of the perforation system  10  may be removed from the wellbore  12  by pulling upwardly on the conveyance string  32 . It is anticipated that, in many embodiments, the perforation modules  34 ,  36 ,  38  will be substantially or totally consumed in the detonation.  
         [0040]     During detonation of the perforation modules  34 ,  36 ,  38 , directional penetrators are formed by the indentations  54 ,  54 ′. Because the mechanism of the creation of this type of directional explosively formed penetrator (EFP) is well known in the art, it will not be described here in any detail. It is noted, however, that the detonation sequence of each module  34 ,  36 ,  38 , begins at the top end proximate to the central rod  42  and proceeds simultaneously in axially downward and radially outward directions. Each liner indentation  54 ,  54 ′, when acted upon by the advancing detonation wave, forms a robust EFP, which is particularly well suited for making large and shallow perforation holes in sandy or soft formations. While conventional shaped charges form a relatively fast-moving, low mass jet that accomplishes the perforation, followed by a relatively slow-moving slug that thereafter carries the mass of the remaining charge liner but does not take part in the actual perforation, the EFP penetrator of the present invention carries essentially all of the mass of the liner  52  forming the indentation  54  or  54 ′. This means that the liner mass effectively forms part of the penetrator and takes an active part in the perforation, increasing the relative effectiveness thereof. In one embodiment it has been found that the perforations that result from indentations  54  or  54 ′ having hexagonal perimeters very closely approximate those created from indentations having circular perimeters.  
         [0041]      FIG. 7  illustrates an exemplary shot pattern that may be formed upon detonation of the perforation module  40  within a section  80  of the wellbore  12 .  FIG. 7  depicts the sidewall of the wellbore section  80  in cylindrical projection with the upper end of the section  80  depicted as line  82  and the lower end of the section  80  shown as line  84 . The illustrated wellbore section  80  has a length (L) of approximately one foot. There are fifty-one (51) perforations  86  disposed within the wellbore section  80 , which have been created by penetrators formed from the indentations  54  or  54 ′ of the perforation module  40 . In practice, those skilled in the art frequently desire perforations having diameters, as measured at the inner surface of the well casing, ranging from about 10 to about 22 mm, but larger or smaller perforations may alternatively be obtained by simply varying the size of the indentations. It is noted that the fifty-one (51) perforations  86  are arranged in six horizontal rows  88   a ,  88   b ,  88   c ,  88   d ,  88   e , and  88   f  of eight perforations  86  each. Adjacent rows  88  of perforations  86  are shown herein as horizontally staggered from one another, such that perforations  86  in one row are located diagonal to, i.e., offset diagonally in relation to, perforations  86  in adjacent rows. For example, referring to  FIG. 7 , perforation  86   b  in row  88   b  is located diagonal to penetrations  86   a  and  86   c  in row  88   a . This staggered pattern is frequently advantageous. Because the penetrations  86  are more densely concentrated than perforations from conventional shaped charge perforation devices, the staggered arrangement may help to avoid overlapping of adjacent perforations. This is desirable because, if there were numerous such overlaps, the resultant effect of a linear cut in the casing  18  could theoretically produce a casing failure, such as a casing collapse. The staggered arrangement may therefore desirably avoid such an undesirable event. In another embodiment, some of the indentations may be configured of a material that does not suitably form penetrators, in order to reduce the number of penetrators and, therefore, the number or density of perforations obtained thereby. Such an embodiment may be acceptable in certain applications, wherein relatively increased amounts of post-detonation debris are not problematic.  
         [0042]     Alternative to indentations having hexagonal perimeters, other perimeter shapes may be selected, desirably such that the perimeters may be adjoined in a linearly contiguous fashion. For example, the indentations may be configured to have triangular, square, or octagonal perimeters.  FIGS. 8 and 9  illustrate alternative embodiments wherein such triangular and square perimeter indentations, respectively, are used.  FIG. 8  depicts an exemplary perforation module  90  having triangular perimeter indentations  92 . As may be seen, the triangular perimeter indentations are located in an adjacent manner such that each of the three sides of a given perimeter borders a side of a neighboring perimeter. Thus, “dead space” between the indentations  92  has thereby been eliminated.  
         [0043]      FIG. 9  depicts an exemplary perforation module  94  having square perimeter indentations  96 . These indentations  96  are arranged in several horizontally-disposed rows, e.g.,  98   a ,  98   b ,  98   c . Adjacent rows of indentations  96  are staggered relative to one another, i.e., offset by half a square, such that indentations  96  in each row are located with their apices diagonal to the apices of indentations  96  in the adjacent row.  
         [0044]     It will be understood by those in the art that each perimeter shape will impart some effect on the configuration of the cavity formed by an indentation, and therefore of the penetrator that will be formed from collapse of the cavity as a result of detonation. Factors such as the fabrication method, and capabilities and limitations thereof, of the liner wherein the indentations are formed, and the material of which the liner is composed, will desirably be taken into account when selecting the perimeter shape and associated packing parameters. For example, triangular and square perimeter indentations may, because of their shape, not collapse as readily during detonation as do hexagonal perimeter indentations in a perforation module wherein all materials and detonation factors are the same. However, modification of such factors may, in some embodiments, offset such disadvantages or even turn such a tendency into an advantage.  
         [0045]      FIG. 10  depicts a portion of an exemplary liner surface for a perforation module wherein octagonal perimeter indentations are used. As may be seen in  FIG. 10 , octagonal perimeter indentations cannot completely cover a given area without leaving some “dead space” between the indentations. In this aspect, their use may be less advantageous, in some embodiments, than the use of hexagonal, square or triangular-shaped indentations. However, octagonal perimeter indentations may more readily approximate the collapse sequence and penetrator transformation of indentations having a circular perimeter, and thus may obtain an advantage overtriangular and circular perimeter indentations in certain embodiments.  FIG. 10  depicts a liner surface section  100  having a plurality of octagonal perimeter indentations  102  that adjoin, i.e., are linearly contiguous to, one another at four of their eight sides  104 . The remaining four sides  106  of the octagonal perimeter indentations  102  define square areas  108  as interstitial spaces. If desired, the interstitial square areas  108  may themselves be indented, in the manner of square indentations  96  (see  FIG. 9 ), to provide for additional formed penetrators.  
         [0046]     Turning now to  FIGS. 11 through 14 , an exemplary initiation sequence is illustrated for a single formed penetrator from a perforation module  40 .  FIG. 11  is a cross-sectional view of the indentation  54  prior to detonation of the perforation module  40 . The indentation  56  is formed in liner  52  that surrounds the high explosive body  50 . In this embodiment a thermoplastic cover  64  surrounds liner  52 . The module  40  is disposed within a section of wellbore casing  18  surrounded by cement  20 . Fluid  57  resides in the annular space that is between the casing  18  and the radially exterior portion of the cover  64 .  FIG. 12  depicts the beginning portion of the detonation wherein the material forming metallic liner  52  has begun to collapse or coalesce within the space formerly occupied by the cavity  60  of indentation  54 . The cover  64  atop the indentation  54  has begun to bow outward and thin out. In  FIG. 13 , the detonation process has progressed to the point where a generally spherical penetrator  110  has been formed from the material making up the liner  52 . The casing  18  and fluid  57  are essentially sheared through by the penetrator  110 .  FIG. 14  depicts an advanced stage of the detonation with the penetrator  110  now in a primarily plastic phase and perforating the cement  20  on its way to the formation (not shown).  
         [0047]     In summary of the foregoing description, those skilled in the art will appreciate that the design of the perforation system  10  thus provides a number of advantages over conventional perforation systems. Included among these, first, is the fact that the linearly contiguous packing of the indentations combined with the unitary body of high explosive produces a greater number of perforating penetrators over a given axial length of a module  40  and reduced amount of “dead space,” as compared with conventional perforation systems using shaped charges and indentations that are physically separated and/or have circular perimeters. The greater number of penetrators results in a desirably greater density in the post-detonation perforation shot pattern. Second, the invention provides for a substantial reduction in debris formed during the perforation operation. And third, the perforation module  40  may be created or manufactured and customized relatively easily, without the need for time-consuming placement and orientation of individual shaped charges, as with conventional systems.  
         [0048]     Those skilled in the art will recognize that numerous modifications and changes can be made to the illustrative designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.