Abstract:
The borehole of many wells, including oil and gas production wells, is frequently cased with a steel or similar metal casing. In order to extract oil or other material existing within the surrounding geologic formation, it is necessary to puncture the casing. Currently, this is accomplished with tubes (guns) containing explosive charges being lowered into the well bore and detonated, causing the tube and well casing to be punctured and the geologic formation shattered. The guns are made from high strength, thick-walled and machined metal. This invention discloses a multi-layered or composite tube that enhances the directional orientation of the explosive charges utilizing less costly and more easily fabricated material. The invention also discloses a gun having properties to allow the desired directionally oriented perforation by the explosive charge without being deformed and jammed within the well casing. Other advantageous are also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICTION  
       [0001]    This application claims the benefit of and priority to U.S. Provisional Application 60/431,446, filed Dec. 5, 2002 and entitled Well Perforating Gun. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of Use  
           [0003]    Well completion techniques normally require perforation of the ground formation surrounding the borehole to facilitate the flow of interstitial fluid (including gasses) into the hole so that the fluid can be gathered. In boreholes constructed with a casing such as steel, the casing must also be perforated. Perforating the casing and underground structures can be accomplished using high explosive charges. The explosion must be conducted in a controlled manner to produce the desired perforation without destruction or collapse of the well bore.  
           [0004]    Hydrocarbon production wells are usually lined with steel casing. The cased well, often many thousands of feet in length, penetrates varying strata of underground geologic formations. Only few of the strata may contain hydrocarbon fluids. Well completion techniques require the placement of explosive charges within a specified portion of the strata. The charge must perforate the casing wall and shatter the underground formation sufficiently to facilitate the flow of hydrocarbon fluid into the well as shown in FIG. 1. However, the explosive charge must not collapse the well or cause the well casing wall extending into a non-hydrocarbon containing strata to be breached. It will be appreciated by those skilled in the industry that undesired salt water is frequently contained in geologic strata adjacent to a hydrocarbon production zone, therefore requiring accuracy and precision in the penetration of the casing.  
           [0005]    The explosive charges are conveyed to the intended region of the well, such as an underground strata containing hydrocarbon, by a multi-component perforation gun system (“gun systems,” or “gun string”). The gun string is typically conveyed through the cased well bore by means of coiled tubing, wire line, or other devices, depending on the application and service company recommendations. Although the following description of the invention will be described in terms of existing oil and gas well production technology, it will be appreciated that the invention is not limited to those applications.  
           [0006]    2. Existing Technology  
           [0007]    Typically, the major component of the gun string is the “gun carrier” tube component (hereinafter called “gun”) that houses multiple shaped explosive charges contained in lightweight precut “loading tubes” within the gun. The loading tubes provide axial and circumferential orientation of the charges within the gun (and hence within the well bore). These tubes allow the service company to preload charges in the correct geometric configuration, connect the detonation primer cord to the charges, and assemble other necessary hardware. This assembly is then inserted into the gun as shown in FIG. 2. Once the assembly is complete, other sealing connection parts are attached to the gun and the completed gun string is lowered into the well bore by the conveying method chosen. The gun is lowered to the correct down-hole position within the producing zone, and the charges are ignited producing an explosive high-energy jet of very short duration (see FIG. 3). This explosive jet perforates the gun and well casing while fracturing and penetrating the producing strata outside the casing. After detonation, the expended gun string hardware is extracted from the well or released remotely to fall to the bottom of the well. Oil or gas (hydrocarbon fluids) then enters the casing through the perforations. It will be appreciated that the size and configuration of the explosive charge, and thus the gun string hardware, may vary with the size and composition of the strata, as well as the thickness and interior diameter of the well casing.  
           [0008]    Currently, cold-drawn or hot-rolled tubing is used for the gun carrier component and the explosive charges are contained in an inner, lightweight, precut loading tube. The gun is normally constructed from a high-strength alloy metal. The gun is produced by machining connection profiles on the interior circumference of each of the guns ends and “scallops,” or recesses, cut along the gun&#39;s outer surface to allow protruding extensions (“burrs”) created by the explosive discharge through the gun to remain near or below the overall outside diameter of the gun. This method reduces the chance of burrs inhibiting extraction or dropping the detonated gun. High strength materials are used to construct guns because they must withstand the high energy expended upon detonation. A gun must allow explosions to penetrate the gun body, but not allow the tubing to split or otherwise lose its original shape (FIG. 4.) Extreme distortion of the gun may cause it to jam within the casing. Use of high strength alloys and relatively heavy tube wall thickness has been used to minimize this problem.  
           [0009]    Guns are typically used only once. The gun, loading tube, and other associated hardware items are destroyed by the explosive discharge. Although effective, guns are relatively expensive. Most of the expense involved in manufacturing guns is the cost of material. These expenses may account for as much as 60% or more of the total cost of the gun. The oil well service industry has continually sought a method or material to reduce this cost while also seeking to minimize the possibility of misdirected explosive discharges or jamming of the expended gun within the well.  
           [0010]    Although the need to ensure gun integrity is paramount, efforts have been made to use lower cost steel alloys through heat-treating, mechanical working, or increasing wall thickness in lower-strength but less expensive materials. Unfortunately, these efforts have seen only limited success. Currently, all manufacturers of guns are using some variation of high-strength, heavy-wall metal tubes.  
         SUMMARY OF INVENTION  
         [0011]    The existing technology, requiring use of heavy-wall, high-alloy metal tubing to minimize gun wall failure, does not completely address the dynamic nature of the short duration, high-temperature, and high-pressure energy pulse used in the perforation process. Current technology suggests that ultimate material strength or strain to failure ratio determines the ability to withstand the high energy pulse. Selecting a material upon its ultimate tensile strength and then fracture, will include the measure of material properties similar to a balloon being inflated until the rubber can no longer hold the pressure and then ruptures catastrophically. The existing technology has been to minimize this problem by increasing the strength and wall thickness of the gun until the internal pressure is successfully contained during perforation. Gun wall thickness is also required to prevent wall collapse due to the high static pressures encountered in deep wells. This static pressure, however, is less than the outward and internally generated pressure from explosive detonation.  
           [0012]    This invention, therefore, includes a novel gun design and method of manufacture utilizing the shock absorptive (impact strength) properties of materials in contrast to the selection of material based upon ultimate tensile strength. For the purpose of illustration, steel can be compared to taffy. If stretched slowly, taffy continues to grow thin and elongate; but, if pulled very rapidly, it will break before any significant elongation occurs. Most common high-carbon steels easily fracture when struck at low temperatures, but these same steels will exhibit predictable ultimate tensile strengths if placed in tension and loaded slowly. Add alloying elements to these steels, and they no longer easily fracture, but will exhibit similar ultimate tensile strengths when loaded to failure as high-carbon, unalloyed steel.  
           [0013]    The outer surface of the gun tube is the most highly stressed area and is placed in pure tension during the brief but highly intense pulse of explosive energy upon detonation (FIG. 5). Prior to the invention subject of this disclosure, gun material has been homogeneous and monolithic, resulting in immediate and unimpeded (unbuffered) transfer of the high-energy pulse from the interior circumference to the outer surface of the gun. Imperfections near or at the outer surface of the steel tube will become stress risers, and impact fractures can occur. Of particular note here are the scallop recesses that are machined into the surface of the guns at the very points of maximum pressure (FIG. 6). These planned surface irregularities may very well exacerbate the fracture problem. In addition, the use of a high-strength monolithic material frequently results in burrs adjacent to the points where the explosive charge exits the gun. These burrs protrude outward from the outer surface of the gun, and can cause the gun to jam in the casing or retard the effectiveness of the explosive charge intended to penetrate through the casing and fracture the formation.  
           [0014]    Existing technology uses guns constructed of solid, homogeneous material having no engineered energy arrestors or cracking arrestors. In addition, the current industry practice of cutting scallops into the outer gun surface sharply interrupts the surface continuity of the gun. This scalloped outer material will significantly decrease the gun&#39;s ability to withstand tensile shock.  
           [0015]    Existing technology typically requires an alloyed and, preferably, a heat-treated steel (quenched and tempered) to ensure adequate shock absorption or resistive strength in the gun wall. These materials are expensive and have a limited number of producers. Mill runs are required, and logistical problems are inherent in ordering and shipping. Economical alternatives to the heavy wall tubing are limited. Alloy additions or mechanical/thermal treatments are relatively expensive. The restricted space within a down-hole well casing also limits the ability to increase wall thickness. The relatively limited number of sources and the special material requirements limit opportunities for cost saving.  
           [0016]    Efforts to achieve cost savings by increasing the batch size of casing wall mill runs restricts the flexibility to modify individual gun designs based on material type, wall thickness, recess design, and gun strings to accommodate the characteristics of strata and well casings encountered in the field. This limitation can hamper the effectiveness of the gun string and cause expensive delays in well production. Therefore, the objects of this invention are as follows:  
           [0017]    To support the design and construction of a gun capable of withstanding the short but high-energy pulse of an explosion without requiring use of expensive materials with high ultimate tensile strengths.  
           [0018]    To support the design and construction of a gun having shock absorptive and energy transfer characteristics, thereby reducing the occurrence of a catastrophic failure due to imperfections or a latent structural flaw in the gun material.  
           [0019]    To create internal shock or crack arrestors in the gun to reduce gun failure and misdirected explosive discharges.  
           [0020]    To reduce the amount of material machining, particularly the precision machining of outer scallops on the gun.  
           [0021]    To reduce stress risers created at the scallops during the detonation of an explosive discharge.  
           [0022]    To reduce the formation of burrs on the gun.  
           [0023]    To reduce the cost of fabrication or simplify the fabrication process to allow increased sources of supply.  
           [0024]    To allow reduction of space between the outer surface of the gun and the inside surface of the casing, thereby increasing the effective focus or channel of the explosive pulse.  
           [0025]    To facilitate the modification of gun size and configuration for individual applications.  
           [0026]    Other benefits included in the scope of the invention will also become apparent to those skilled in the art. 
       
    
    
     SUMMARY OF DRAWINGS  
       [0027]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention. These drawings, together with the general description of the invention above and the detailed description of the preferred embodiments below, serve to explain the principles of the invention.  
         [0028]    [0028]FIG. 1 illustrates the affect of the explosive discharge from a well perforating gun penetrating through the well casing and into the surrounding geologic formation.  
         [0029]    [0029]FIG. 2 illustrates typical alignment of scallops and explosive charges within the gun utilizing existing technology.  
         [0030]    [0030]FIG. 2A illustrates a cross-section view of the gun and the typical placement of the explosive charges held within the loading tube.  
         [0031]    [0031]FIG. 3 illustrates the detonation of the shaped charge from the loading tube penetrating through the gun wall (using existing technology) and into the geologic structure.  
         [0032]    [0032]FIG. 4 illustrates a typical cracking of a gun caused by use of existing technology for gun wall fabrication.  
         [0033]    [0033]FIGS. 5A and 5B are cross-sectional depictions of the existing technology of machined scallops and the formation of burrs.  
         [0034]    [0034]FIG. 6 is a cross-sectional depiction of a gun wall that shows how existing technology can contribute to gun wall cracks.  
         [0035]    [0035]FIG. 7 illustrates an embodiment of the invention comprised of an engineered sequence of layered materials.  
         [0036]    [0036]FIG. 8 illustrates an embodiment of the invention showing use of perforated tubing, thereby eliminating machining of scallops.  
         [0037]    [0037]FIG. 8A illustrates a cross section view of the layered wall construction.  
         [0038]    [0038]FIG. 9 illustrates a detailed embodiment of the invention employing laminates for extra strength.  
         [0039]    [0039]FIG. 9A illustrates a detailed embodiment of the invention employing energy absorption zones.  
         [0040]    [0040]FIG. 9B illustrates an embodiment of the invention utilizing precut holes and wrapped layers.  
         [0041]    [0041]FIG. 10 and FIGS. 10A through 10F illustrate detailed embodiments of the invention employing various designs for precut recesses in gun wall layers.  
         [0042]    [0042]FIG. 11 illustrates alternate designs for precut recesses of the invention.  
         [0043]    [0043]FIG. 12 illustrates a further embodiment of the invention.  
         [0044]    [0044]FIG. 12A depicts a side sectional view of the invention depicted in FIG. 12 an improved scallop configuration using a multi-layered gun tube.  
         [0045]    [0045]FIG. 12B depicts a side sectional view of the prior art machined scallop.  
         [0046]    [0046]FIG. 12C further illustrates recesses with the walls of perforating guns subject of the invention.  
         [0047]    [0047]FIG. 13 illustrates attachment of end fittings to perforating guns subject of the invention. 
     
    
       [0048]    The above general description and the following detailed description are merely illustrative of the subject invention, additional modes, and advantages. The particulars of this invention will be readily suggested to those skilled in the art without departing from the spirit and scope of the invention.  
       DETAILED DESCRIPTION OF INVENTION  
       [0049]    The invention disclosed herein incorporates novel engineering criteria into the design and fabrication of well perforating guns. This criterion addresses multiple requirements. First, the gun material&#39;s (steel or other metal) ability to withstand high shocks delivered over very short periods of time (“impact strength”) created by the simultaneous detonation of multiple explosive charges (“explosive energy pulse” or “pulse”) is more important than the material&#39;s ultimate strength. This impact strength is measurable and is normally associated with steels with 200 low carbon content and/or higher levels of other alloying elements such as chromium and nickel. Second the shock of the explosion transfers its energy immediately to the outside surface of the tubing. Any imperfections, including scallops, will act as stress risers and can initiate cracking and failure.  
         [0050]    [0050]FIG. 1 illustrates the basic casing perforation operation in which the tool and fabrication method disclosed in this specification are utilized. The gun  200  is suspended within the well bore  110  by a coil tube or wire line device  250 . The charges (not shown) contained within the gun are oriented in 90 degrees around the circumference of the gun. The explosive gas jet  450  produced by detonation of the charge penetrates  236  through the wall  210  of the gun  200  and well casing  100  creating fractures  930  in the adjacent strata  950 . Penetration of the gun wall is intended to occur at machined recesses  220  in the wall  210 . The recesses are fabricated in a selected pattern around the circumference of the gun.  
         [0051]    It is desirable to use various arrangements or orientations of the charges (“shots”) and with varying numbers of charges within a given area (“shot density”). This allows variation in the effect and directionality of the explosive charges. Shots are typically arranged in helical orientation (not sh 16 own) around the wall of the gun  200  as well as in straight lines parallel to the axial direction of the gun tube. The arrangements are defined by the application and the design engineers&#39; requirements, but are virtually limitless in variation. Guns are typically produced in increments of 5 feet, with the most common gun being about 20 feet. These guns may hold and fire as many as 21 charges for every foot of gun length. Perforation jobs may require multiple combinations of 20-foot sections, which are joined together end to end by threaded screw-on connectors.  
         [0052]    [0052]FIG. 2 illustrates the basic components of the gun  200  and the relationships between the gun wall  210 , loading tube  310 , charges  420 , and detonation cord  421 . The longitudinal axis  115  of the gun is parallel to the axis of the borehole (not shown). The line shown as  2 A- 2 A illustrates the location of the sectional view depicted in FIG. 2A.  
         [0053]    [0053]FIG. 2A is a sectional top view of the gun  200 . The relationship of the gun wall  210  to the loading tube  310 , containing the charge  420 , and the longitudinal axis  115  is illustrated. The loading tube and charge(s) are located within the annulus  215  of the gun wall  210 . Also shown is a recess or scallop  220  machined into the outer surface of the gun wall at locations specified to be immediately adjacent to each explosive charge. The recess  220  includes a flat bottom  229  and walls orthogonal  228  to the bottom. The charge  420  includes the explosive charge  410 , shape charge body  324 , primer vent  325  and retainer cone  326 .  
         [0054]    It will be appreciated that differing well conductions, casings, strata, and so on create the need for varying configurations and properties of the loading tubes, charges, and mounting hardware.  
         [0055]    The high-energy explosive gas jet that is produced when a charge detonates is illustrated in FIG. 3. The duration of this explosive event is only of an extremely small fraction of a second and can be considered to be an explosive pulse occurring at detonation. During the violent and explosive energy pulse, the charge casing, loading tubes, and other gun components are subjected to an immediate, non-uniform change in pressure and temperature. The detonation cord  421  ignites the explosive  410  at the primer vent  325  within the non-combusting shaped charge body  324 . The entire explosive within the charge ignites nearly instantaneously. Ignition within the shaped charge focuses an explosive jet  450  of expanding hot gas radially outward  452  toward the gun wall  210 . The gun wall proximate to the short duration explosive jet or energy pulse contains a machined recess or scallop  220 . The explosive jet  450  perforates  236  through the machined scalloped gun wall (having a decreased thickness) and continues through the narrow space  180  between the gun wall  210  and the well casing  100 . The explosive jet energy  450  also perforates  136  the well casing  100 . The energy of the jet pulse  451  creates one or more shock waves  455  that fracture  930  the geologic formation  950 . It will be appreciated that the amount of energy required to penetrate the gun body is reduced by the reduced thickness provided by the scallops. The machined scallops also diminish the protrusion of burrs  233  beyond the gun wall. These burrs are created from remnants of the gun wall  210  pushed out from the outer surface as the energy pulse  450  pushes through from the interior and the shaped charge  420 .  
         [0056]    [0056]FIG. 4 illustrates a typical cracking of a failed gun experienced in the existing technology. The machined scallops  220  are fabricated weak points to facilitate the perforation of the gun wall  210  at specified locations and to retard or contain the formation of burrs (not shown) from the outer wall surface of the gun. The operation of the gun utilizes nearly simultaneous detonation of explosive charges, subjecting the separate locations of the gun wall to short, violent explosive pulses  450 . The proximity of multiple scallops, designed for increased charge or shot density, can result in an unintended portion of the gun wall to fail, thereby degrading the directionality and quantity of energy reaching the well casing  110  and the geologic strata. Further, such catastrophic gun wall failure may cause the deformation of the gun preventing it from being removed from the well bore. FIG. 4 illustrates a straight-line failure in the form of the splitting or cracking  295  of the gun wall  210  and the expansion  452  of the gun wall into contact or near contact with the well casing  110 . The failure can often occur due to the proximity of the scallops  220  and the resulting energy pulse exit points  236 . This failure occurs simultaneously with the detonation of the explosives creating the multiple energy pulses  450  through the well casing and into the geologic formation. Although the failure may occur in orientations other than a straight line, most events occur between the machined scallops  220  and jet exit points  236  separated by the shortest distance.  
         [0057]    [0057]FIG. 4 also illustrates the direction  452  of the gun wall or gun diameter expansion is radially outward from the longitudinal axis  115  of the gun and the well bore. This is the direction in which minimal spacing  180  between the casing and gun wall is desired. Therefore there little tolerance  181  for expansion of the gun wall or the formation of outward protruding burrs.  
         [0058]    [0058]FIG. 5A illustrates that the direction  850  of static force upon the gun wall  210  caused by the increased down hole pressures. This force also applies to the bottom  229  of the machined scallop  220  where the gun wall  210  has reduced thickness. FIG. 5B illustrates the direction of energy  452  and stress sustained by the gun wall  210  during detonation. The stress is greatest near the exit point  236  of the jet  450 . This increased force is represented by the respective length of the vector arrows ( 455   454   452 ). This will typically be the location and the site of any resulting material failure (such as cracking or bending), and wall failure will radiate from this point in a direction  296  through the wall  210 . The failure will often start proximate to the outer wall surface and propagate radially into the wall. One limitation of the existing gun wall technology, therefore, is the number of explosive charges per foot that can be placed within a gun (shot density).  
         [0059]    The catastrophic failure illustrated in FIG. 4 occurs in the very short nanosecond duration of the explosive pulse. Failure is not a function of the ultimate material strength of the gun wall, but rather the limited mechanical ability to transfer or absorb the shock of the high energy burst. FIG. 5A illustrates the external static load (illustrated by vector arrows  850  of uniform magnitude and placenent) existing across the surface of the wall  210 . FIG. 5B illustrates the non uniform and outward directed explosive force (represented by vector arrows  455   454   452  of non-uniform magnitude) occurring during the short duration of the detonation of the explosive charges. The failure occurs in this very short time period as the static load illustrated in FIG. 5A is overcome by the dynamic outward explosive force illustrated in FIG. 5B. During this short time period, there will be a dramatic dynamic shift of load forces and immediate return or near return to the original load force. The impact strength of the gun wall, that is its ability to withstand the immediate dynamic shift in load, may be a factor independent of the ultimate load (tensile) strength of the gun wall. FIGS. 5A and 5B also illustrate the orientation of the static force  850  and explosive jet  450  and force  455   454   452  to the loading tube wall  310 , the charge  324  and the longitudinal axis  115 . FIG. 5A illustrates the bottom  229  and side wall  228  of the machined recess. FIG. 5B also illustrates the jet  450  exit  236  through the gun wall and vectors  296  showing dispersion of energy though the wall.  
         [0060]    [0060]FIG. 6 is a cross sectional view of the gun wall  210  illustrated in FIG. 5B. For clarity, the loading tube is not shown. The machined scallop  220  on the outer surface of the wall will be the location of the exit point of the explosive jet energy pulse (not shown). The orientation of the scallop and the shaped charge  420  is shown. Cracks or defects  294  propagating in the gun wall  210  proximate to the scallop due to the impact of the explosive energy pulse are shown. As in FIG. 5B, the non uniform and outward directed explosive force occurring during the short duration of the detonation of the explosive charges is represented by vector arrows  455   454   452  of non-uniform magnitude. The cracks (wall failure)  294  normally occur in conventional guns at the machined scalloped recess edges or wall  228  or the scallop bottom surface  229  proximate to the jet exit point. The cracks typically initiate from the outside diameter of the gun wall and propagate in a traverse direction  296  through the wall and radially into the wall. Since the internal loading is maximized at the machined scalloped location  220 , being the intended jet exit point, and dissipates as it travels away from the scallop, there is a tendency or frequency of the multiple crack failures propagating from separate scallop locations to linkup to produce a zipper effect along the line of charge locations. FIG. 4 illustrates this type of catastrophic failure  295 .  
         [0061]    The design criteria specified by the invention can be used to create an alternative gun tube construction that eliminates many of the problems and costs of the heavy walled tubing currently used. Although multiple embodiments of new gun material selection and construction are within the scope of this invention, attention should be first directed to the design and fabrication of gun tubing utilizing multiple layers of material. This method includes fabrication by layering or lamination of materials around a radius encompassing the longitudinal axis of the gun tube.  
         [0062]    [0062]FIG. 7 illustrates the construction of a gun wall  210  comprised of four material layers ( 210 A  210 B  210 C  210 D). The orientation of each layer is parallel or at a constant radius to the longitudinal axis  115  of the gun ( 200 ) and the well bore (not shown). The thickness of each layer or tube  231 D  231 C  231 B  231 A may be varied. The diameter of the annulus  215  formed within the inner tube may also be varied. The outer surface of each respective tube layer may be varied in construction to facilitate binding and retard delamination. Such designs may facilitate the strength characteristics of the gun wall in alternate directions, such as traverse or longitudinal directions. It is known that multi-layered constructions can have numerous advantageous over conventional, monolithic material constructions. It will be appreciated that this invention does not limit the number of layers, the composition of individual layers, or the manner in which layers are assembled or constructed. Further, the invention is not limited to the use of a binder or laminating agent between material layers; for example the outer surface  218 A on the inner most layer  210 A and the inner surface of the next outer layer (not shown).  
         [0063]    It will be appreciated that lamination of multiple layers of the same or differing materials may be used to enhance the performance over a single layer of material without increasing thickness. Use of fibrous materials, such as high strength carbon, graphite, silica based fibers and coated fibers are included within the scope of this invention. Although some embodiments may utilize one or more binding elements between one or more layers of material, the invention is not limited to the use of such binders. Plywood is an example of enhancing material properties by layering wood to produce a material that is superior to a solid wood board of equal thickness. Applications of multi-layered lamination can be subdivided into primary and complex designs. Additional embodiments of the invention are described below.  
         [0064]    [0064]FIG. 8 illustrates the primary “tube-within-a-tube” design, similar to the embodiment of the invention illustrated in FIG. 7 and having a longitudinal axis  115 . The outer layer  210 D is a cylinder or tube in which holes  230 A  230 B have been cut through the thickness of the cylinder wall  231 D. The diameter of the outer cylinder  210 D is approximately equal to the outer diameter of the next inner cylinder  210 C. In the embodiment illustrated in FIG. 8, there are no holes cut through the walls of the next inner cylinder  210 C. Therefore, the combined cylinder, comprising the “tube within a tube” of  210 D and  210 C, has the approximate physical shape of the prior art single walled gun having recesses or scallops machined into the outer surface of the wall. In a preferred embodiment of the invention, holes  230 A  230 B are cut through the outer cylinder wall  210 D prior to assembly of the two cylinders  210 C and  210 D. The line VIII-VIII designates the location of the cross sectional view illustrated in FIG. 8A. FIG. 8A shows a portion of the inner cylinder wall  210 C and its relationship with the outer wall  210 D and annulus  215 . The illustration does not; however depict the radial curvature of each layer. The diameter of the hole  288  may be varied. The axis  119  of the resulting hole  230  may be orthogonal to the longitudinal axis ( 115  of FIG. 8). It will be appreciated that the resulting recess  225  depicted in  8 A is comparable to the recess or scallop  220  machined into the gun wall  210  illustrated in FIG. 2A. In the structure of the invention shown in FIG. 8A, the thickness  231 D of outer cylinder wall  230 D forms the side wall ( 228  in FIG. 8) of the recess  225 . The outer surface  218 C of the next inner cylinder  230 C forms the bottom ( 229  in FIG. 8) of the recess or scallop  225 .  
         [0065]    It will be readily appreciated that the composition of the several layers or cylinders might differ. Also the thickness and number of layers might be varied, depending upon the requirements of the specific application. The cutting of holes can be accomplished before assembly, thereby eliminating the need for machining.  
         [0066]    [0066]FIG. 8 also illustrates the ability to perform machining or other fabrication on the individual cylinder components prior to assembly into the completed unit. For example, machining of connector structures can be performed on the inner cylinders individually prior to being inserted or pulled into the larger cylinders. These structural components may be machined threads, seal bores, etc. FIG. 8 illustrates a design that incorporates a machined connection end components  591   592  on the innermost tube  210 C of a multi-layer tube construction.  
         [0067]    As discussed above, it is not necessary that the interface ( 212  in FIG. 8A) of the surfaces of the inner and outer of tubes or cylinders be bound or otherwise mechanically attached together. An advantage to this design is its simplicity and ease of manufacture. Each of the tubes may have different chemical and mechanical characteristics, depending on the performance needs of the perforation work. Alternatively, each tube can be made of the same material. In another variation, layers of tubing can be made of the same material but oriented differently to achieve the desired properties (similar to the mutually orthogonal layering of plywood). One further variation can be implemented by offsetting a seam of each cylinder or tube layer created in the manufacturing process by rolling flat material into a tube.  
         [0068]    One variation of the embodiment illustrated in FIG. 8 might include an inner tube of high-strength material (such as the high-strength, alloy metals currently used for guns) and an outer tube of mild steel.  
         [0069]    [0069]FIG. 9 illustrates an embodiment of the invention in which the gun has four material layers ( 210 D  210 C  210 B  210 A). The invention, however, is not limited to four layers. The multi-layer design might consist of tube-in-a-tube fabrication or the wrapping of material around the outer surface of an inner tube maintaining a relative uniform radius about a central axis  115 . The inner tube defines the area of the tube annulus  215 . The tubing layers may be seamless or rolled. It will be readily appreciated that layering material can be wrapped in various orientations  285   286  to provide enhanced strength. Two layers  210 C and  210 B are shown helically wrapped  285  at a radius around the longitudinal axis  115 . The next inner layer  210 A is shown comprised a rolled tube having a seam parallel to the longitudinal axis. It will also be appreciated that the wrapping might include braiding or similar woven construction of material. FIG. 9 also illustrates that any given layer  210 C  210 B might consist of a material “tape” wrapped around an inner tube or cylinder  210 A. The inner most layer  210 A may also be formed around a removable mandrel (not shown). The laminations can consist of other metals or non-metals to obtain desirable characteristics. For example, aluminum is a good energy absorber, as is magnesium or lead. This invention does not limit the material choices for the lamination layers or the manufacturing method in obtaining a layer; it specifies only that layers exist and provide advantages over single-wall, monolithic gun designs.  
         [0070]    Also illustrated in FIG. 9 are one or more layers  210 D  210 C containing holes  230 D  230 C having diameters cut prior to assembly. The hole  230 D cut into the outer tube  210 D has a diameter  288 . The axis of the holes can be orthogonal to the longitudinal axis  115  of the gun  200 . The tube layer thickness  231 D  231 C of the cut  230 D  230 C forms the wall of the recess  225  and the outer surface  218 B of the next underlying layer  210 B forms the bottom of the recess  225 . The architecture of the resulting recess is comparable, but advantageous to, the prior art machined scallops.  
         [0071]    Wrapping designs and fabrication techniques allow far greater numbers of metals and non-metallic materials to be used as lamination layers, thereby achieving cost savings and reducing production and fabrication times. Improved rupture protection can be achieved without increasing the weight or cost. FIGS. 9 and 9A illustrate two examples of this embodiment.  
         [0072]    [0072]FIG. 9A illustrates how a perforated or non-continuous material can produce a lamination layer, even though voids may exist within that layer. The layers might consist of continuous sheets with regular perforations, woven sheets of wire, bonded composites, etc. An energy absorption layer  210 C contains numerous perforations  226  each having small diameter  289 . In another embodiment, not shown, the voids might contain material contributing to material strength at ambient temperature and pressure, but that is readily vaporized by the explosive high-temperature and high-pressure energy pulse, thereby providing minimal energy impedance proximate to the explosive charge, recess and well casing, but maximum shock absorption in other portions of the gun not immediately subjected to the directed high temperature explosive gas jet.  
         [0073]    The energy absorption layer  210 C illustrated in FIG. 9A has mechanical properties permitting the inner layers  210 B  210 A to expand into the volume occupied by the absorption layer in response to the high impact outward traveling explosive energy pulse occurring upon charge detonation. This mechanical action will consume energy that might otherwise contribute to a catastrophic failure of the outer layer  210 D. As already discussed, such failure can hinder the intended perforation of the well casing and the surrounding geologic formation (not shown) or hinder the removal of the gun from the well. These mechanical property enhancements allow higher strength, thinner wall perforating guns with high impact resistance and energy absorption.  
         [0074]    In addition to the specific energy absorbing layer shown in FIG. 9A, it will be appreciated that each layer could provide strength or other properties specifically selected by the design engineer to meet conditions of an individual well bore. Therefore, this invention allows wall thickness and composition to become design variables without needing mill runs or large quantities of material.  
         [0075]    [0075]FIG. 9A also illustrates a recess  225  in the gun wall  210  fabricated from hole  230 D cut through selected layers  210 D prior to assembly of the combined tubes. The outer surface  218 C forms the bottom of the precut recess  230 D.  
         [0076]    [0076]FIG. 9B illustrates an embodiment using helically wound fiber or wire  397   398  around an inner layer  210 A. The wrapping can also be performed utilizing a removable mandrel. The wrapped layers  210 B  210 C can be combined with tubes or cylindrical layers  210 A  210 D. The tube layers can incorporate precut holes  230 . In the embodiment illustrated in FIG. 9B, the outer surface  218 C of layer  210 C is exposed by the precut hole  230  in the outer layer  210 D. The winding may be performed prior to placement of the next outer layer. The fiber or wire can be high strength, high modulus material. This material can provide strength against the explosive pulse. The diameter of fiber or thickness of wrapping can be varied for specific job requirements. The geometry of the winding (or braiding) can be varied, particularly in regard to the orientation to the longitudinal axis  115 .  
         [0077]    [0077]FIG. 10 illustrates a complex gun  200  formed from multiple layers or tubes radially aligned around a longitudinal axis  115 . The wall  210  of the gun  200  forms a housing around an annulus  215 . The explosive charges, detonator cord, and carrier tube can be placed within this annulus  215 . Also illustrated is a recess  225  formed in the manner described previously. The center axis  119  of the illustrated recess  225  is orthogonally oriented  910  to center axis of the gun  115 . FIG. 10A illustrates an embodiment of the invention wherein the outer three layers  210 D  210 C  210 B of the gun wall  210  contain holes cut prior to assembly of the tubes into a single cylinder. Although the diameter  288 D  288 C  288 B of each hole is different, the center axis  119  of the combined holes  230  are aligned. The inner layer  210 A is not cut, and the outer surface  218 A of that tube forms the bottom  229  of the resulting recess  225 . The thickness of each precut layer creates a stepped wall  228  of the recess. An explosive charge as depicted in FIG. 2A may be installed proximate to the inner surface of the innermost layer  210 A and aligned with the recess center axis  119 . FIG. 10B illustrates another embodiment wherein the inner tube layer  210 A is cut through prior to assembly, a next outer layer  210 B is not cut at the location, but the next outermost layers  210 C  210 D are cut through and the center axes of the precut holes are aligned  119 . This architecture achieves an inner recess  226  within the gun wall  210  aligned with an outer recess  225 . This architecture or structure can be readily achieved by this invention. This structure cannot be practically achieved by the prior technology.  
         [0078]    [0078]FIG. 10C illustrates another embodiment readily achieved by the invention, but that is not practicable by prior technology. It will be appreciated that the shape of the interior recess  226  can be varied in the same manner as the outer recesses may be formed. Accordingly, the recess diameter can be varied within the interior of the gun wall  210 .  
         [0079]    [0079]FIG. 10D illustrates a structure that has not been possible prior to the invention. The gun wall  210  can contain an interior recess or cavity  235 . The radial axis  119  of the cavity can be aligned with an explosive charge as illustrated in FIGS.  2 A and  6 . At the time of assembly, the cavity may be filled with a eutectic material or other material selected to provide strength at ambient conditions but disperse, vaporize or otherwise degrade with the rapid explosive energy pulse. FIG. 10E illustrates a combination interior recess  236  with an internal cavity  235 . The interior recess diameter  288 A and the internal cavity diameter  288 C may be varied as selected by the gun designer.  
         [0080]    It will be readily appreciated that the dimensions of each precut hole can be specified. This ability can achieve recesses within multiple layers that, when assembled into the composite gun, the recess walls may possess a desired geometry that may enhance the efficiency of the explosive charge or otherwise impact the directionality of the charge. Further, it will be appreciated that interior recesses may be filled with materials that, when subjected to high temperature, rapidly vaporize or undergo a chemical reaction enhancing or contributing to the original energy pulse.  
         [0081]    [0081]FIG. 10F is a detail of a complex recess  225  comprised of precut holes of varying diameters and aligned in relationship to the same radial axis  119 . It will be appreciated that the illustrated recess may comprise part of an internal wall cavity (similar to that depicted in FIG. 10D) or a recess on the interior gun wall (similar to that depicted in FIG. 10C). It will be appreciated that the recess illustrated in FIG. 10F contains stepped walls  228   231 B  231 C  231 D having increasing diameter outward along the axis  219 . The outer gun wall is comprised of the surface  218 D of the outer layer  210 D. The bottom of the recess is formed by the outer surface  218 A of inner layer  210 A.  
         [0082]    [0082]FIG. 11 illustrates precut holes forming recesses  225  in the outer layer  210 D of the multi-layered gun wall ( 210 D  210 C) having predefined complex outside wall shapes alternative to the circular shaped precut hole. The layer thickness  231 D and surface  218 D  218 C as well as the annulus  215  and longitudinal axis  115  are also shown. Actual shape design is unlimited since design is no longer restricted by conventional machining methods. Any combination between layers (such as the example shown in FIGS. 10, 10A thru  10 F) and any shape (such as the example shown in FIG. 11) can be easily produced by laser cutting, tube assembly or layer lamination, and any required material wrapping.  
         [0083]    An additional advantage of the invention is fewer “off-center” shot problems and better charge performance due to scallop wall orientation (comparing FIGS. 12A and 12B) since the outer tube&#39;s recess  229  can achieve a constant underlying wall thickness  210 B regardless of the explosive jet  420  exit point. In comparison, FIG. 12B illustrates the prior art machined scallop  220  having a constant diameter  288 X. The bottom of the scallop  229 X is flat and of non uniform thickness. It will be appreciated that if the explosive pulse of the detonated charge is not oriented perpendicular to the outside gun wall, the brief explosive jet pulse will encounter a non uniform gun wall, thereby creating a disruption or turbulence in the flow with resulting dissipation of energy. The invention subject of this disclosure results in a uniform wall thickness, thereby minimizing energy dissipation.  
         [0084]    [0084]FIG. 12A illustrates the constant angle  289 D  289 C of the recess side wall  228 D  288 C oriented to the centerline  119  achieved by this invention. Unlike the prior art technology of milling scallops into solid monolithic tube wall, the radial orientation of the recess side wall formed by the invention can be maintained constant to a point on the longitudinal axis. The cut hole results in a removal of an arc segment  289 D  289 C from the circumference of the cylinder or tube wall  210 D  210 C. The angle can be varied by the length of the arc segment  289 D  289 C cut relative to the diameter of the tube layer (or radial distance from the center axis of the gun). It will be appreciated by persons skilled in the technology that the angle can facilitate the accuracy or efficiency of the explosive charge. This angle may minimize interference or disruption of the explosive gas jet  420  through the gun toward the casing and strata. The prior art scallops generally have a fixed orientation to the center axis of the scallop  119 . However, this fixed dimension creates a non uniform orientation to the center axis of the gun (not shown) or the explosive charge positioned within the annulus  215  and proximate to the center axis. (See FIG. 2A and FIG. 6.)  
         [0085]    [0085]FIG. 12C illustrates the gun wall recess  225  of the present invention may also achieve variable side wall angles θ  289 D. The relationship of the precut hole diameter  288 D to the side wall angle and to the center axis  115  of the gun, as well as the annulus  215  is also shown. The curvature of the bottom surface  218 C of the recess  225  is also illustrated.  
         [0086]    [0086]FIG. 13A illustrates a weld seam  268  connecting components  265  to multiple layers of a gun wall  210  requiring less machining. This weld can be performed by laser welding, similar to techniques available for the precutting of holes  225  within the gun wall  210 . The weld seam  268  illustrated in FIG. 13B depicts the size achieved by conventional well technology.  
         [0087]    In some embodiments, it may be advantageous to weld or mechanically attach machine threaded connection ends to at least one tube layer. FIG. 13 illustrates use of laser welding gun connection fittings for designs utilizing multiple layers. Laser welding involves a low-heat input process, thereby allowing completed machined connection end turnings to be welded directly. Conventional multi-pass welds may require machining after welding to eliminate the effects of distortion.  
         [0088]    Other advantages of the invention include more choices of tube supply, especially domestic supplies with far shorter lead times. Lower manufacturing costs are achieved by laser cutting scallops in the outer lamination instead of machining solid, heavy-walled tubes, which is the practice of current technology.  
         [0089]    Specific benefits from the construction of guns utilizing multi-layering of differing materials and material orientations as specified by this invention include, but are not limited to lower material costs, reduction of material weight and thickness, decreased dependence upon expensive high strength materials having long lead-time production requirements, and greater flexibility in gun designs including tailoring the properties of the gun wall to accommodate varying field conditions to achieve enhanced performance. In addition, better gun performance is achieved by precut tube scallops having uniform thickness, increased flexibility to create modified scallop walls and shapes, and increased impulse shock absorption by the multiple tube layer interface. Also an inner tube can have higher strength without the adverse effects of brittleness since an outer ductile layer may contain the inner tube.  
         [0090]    Since recesses (scallops) can be cut individually into each tube layer before being assembled into a gun tube, many different recess designs are available. One benefit of this recess capability is to produce internal and inner diameter (inner wall) recesses that would be virtually impossible to produce in conventional gun manufacture. It is not the intent of this invention to specifically describe the benefits of all recess designs, but rather to indicate that the advantages will be apparent to persons skilled in the technology of this invention.  
         [0091]    It will be appreciated that other modifications or variations may be made to the invention disclosed herein without departing from the scope of this invention.