Patent Publication Number: US-6668726-B2

Title: Shaped charge liner and process

Description:
FIELD OF INVENTION 
     This invention relates generally to liners for shaped charges and more particularly to liners for shaped charges of the type used in perforating gun systems. 
     BACKGROUND OF THE INVENTION 
     Development of an oil or gas well often involves fixing a tubular steel casing in cement in an underground well borehole. Holes, or perforations, are subsequently created in the steel well casing and surrounding cement in order to gain access to the surrounding formation, e.g., an oil or gas bearing stratum. Such holes are generally created through a process known also as perforation using a perforating gun. 
     Perforation is a process of piercing the well casing, the surrounding cement, and rocks in the surrounding formation to provide fluid communication between the oil or gas deposit and the interior of the well. Explosive charges are typically used to pierce the well casing. Perforation involves introducing a firing device, commonly termed a perforating gun, into the well, positioning the perforating gun at a desired depth, and detonating the gun. The process of locating a perforation gun in position is sometimes referred to as “delivering” the gun. After detonation, the gun may be retracted or released and dropped to the bottom of the well. If discarded, the size of the gun is limited by the depth of the bottom hole available. 
     Several perforating methods have been used to deliver and detonate a perforating gun. For example, the “wireline” process involves attaching a perforating gun, or string of guns, to a long, flexible steel cable paid out from a truck-mounted drum at the surface. An electrical conductor, paid out with the cable and connected to the gun, or the gun in the string nearest to the surface, carries an electrical signal to energize the perforating gun detonator. Alternatively, tube conveyed perforating (TCP) employs straight production tubing to carry or convey the perforating gun to a desired position in the well. The gun may be activated by a drop bar. The gun may also be activated by hydraulic means. For example, fluid in the tubing may be pressurized sufficiently to activate a hammer and firing pin of a percussion detonator on the gun. 
     An explosive charge is typically contained in a charge assembly that may include a casing and a metal liner. The casing may typically have a recess having an inner wall structure and an opening. The explosive charge is packed against the inner wall of the charge casing. The metal liner may line the explosive charge opposite the casting. As such the explosive charge is contained between the liner and the casing. The shape of the explosive charge is defined by the space between the inner wall of the casing and the metal liner and is thus referred to as a “shaped charge”. This space often has a concave, typically generally conical, shape. The term “shaped charge” may also refer to a charge assembly. The shape of such a charge may be varied, depending on the pattern of perforations desired, the size and number of perforations desired, and the depth of the perforations in the surrounding mineral bearing stratum. 
     A perforating gun may typically include an elongate member in the nature of a hollow tube having openings, or seats, into which the shaped changes are mounted. Several charge assemblies are generally arranged along the length of the elongate member. Typically, a detonation cord runs along the perforation gun between, and is connected to, the charges. Typically, the shaped changes are mounted such that the wide part of the conical shape faces radially outwardly, i.e., away from the gun, and toward the wall of the well bore, generally having a central axis aimed in a plane transverse to the length of the elongate member. Different explosive charges may face radially outwardly in different angular directions in the plane or in spaced, parallel planes to produce a helical perforation pattern. 
     When detonated, each shaped charge produces a compressive shock wave. This may collapse the liner and propel the central portion of the liner at a very high speed, possibly of the order of about 10,000 m/sec, thereby forming an explosive central jet. This jet pierces the well casing and the surrounding cement, and penetrates some distance into the oil bearing formation. The differently facing charges explode radially outwardly in different angular directions into the oil-bearing formation. The result is a perforated wall, like a colander, that facilitates entry of oil or gas into the well. 
     The outer, slower moving portion of the liner may have a tendency to form a slug, sometimes called a “carrot” due to its shape. The slug can cause numerous problems. The slug may embed itself in the exit hole of the perforating gun and cause mechanical difficulties in removing the perforating gun from the well borehole. The slug, when embedded in the perforation pierced by the explosive jet, may tend to impede the outflow of oil or gas, thus reducing the performance of the well. Sometimes, the slug may be carried by the gas or oil flow to the surface and cause malfunction of surface devices. The slug may also fall from the perforation gun down the well borehole into other downhole devices, possibly causing these devices to malfunction. 
     These problems caused by slugs have long been recognized. Efforts continue to be made to minimize or eliminate the formation of slugs. For example, U.S. Pat. No. 5,098,487, issued to Brauer et al. on Mar. 24, 1992 (“Brauer”), gives an account of various solutions directed at minimizing slug formation. 
     The majority (perhaps up to 90%) of liners used in the field are constructed of compacted metal powders. Metal powder liners tend to pulverize upon striking the well casing, and thus do not tend to cause undue formation of slugs. However, this type of liner may tend to have other disadvantages. They tend to be used in a green (i.e., unsintered) state, and as such may tend also to be relatively fragile. Care must be taken in their handling and assembly. Sintering, such as the process disclosed in U.S. Pat. No. 6,012,392, issued to Norman et al. on Jan. 11, 2000 (“Norman”) may reduce this fragility, but is sometimes considered an unnecessary manufacturing step, particularly when it is often desirable for the device to fragment upon detonation. 
     Compacted metal powder liners in the green state also tend to be porous. There may be water at a depth in the well at which the shaped charge is to perforate holes in the well casing. Water may leak more easily through a porous liner and dampen the explosive charge lined by such a liner. This may cause detonation difficulties. 
     Often, liners made of compacted metal powders tend to be formed individually. Compared with liners formed in batches, individually formed liners may tend to have increased cost, and their product quality may tend to be less consistent and reliable. Additionally, because liners made of unsintered metal powders often pulverize upon striking the well casing, they may tend to be less effective for perforating large holes. 
     “Large holes” in this context may be holes of diameter up to about 1 inch. “Large holes” are often required for wells of heavy oil. Heavy oil, having higher viscosity, may tend to flow more easily from the surrounding oil bearing formation through these large holes and into the well. To obtain good production performance from such a heavy oil well, deep penetration with a depth of up to 30 inches may often be desired as well. Solid liners made of relatively heavy material may tend to be more effective for producing holes satisfying those requirements. This type of liner typically accounts for most, if not all, of the remaining 10% of liners in use. However, this type of liner has the tendency of forming relatively large slugs. 
     Zinc and zinc alloys have been used as a material for the outer casings of shaped charges. A casing made of zinc or a zinc alloy may tend to disintegrate without forming significant debris upon explosion of the explosive charge contained inside such a casing. The long held belief has been that some other material is required for the liner. Efforts have been made in searching for a better liner material. This may involve the development of special alloy mixes of copper. 
     For example, Brauer (quoted above) discloses a specially chosen copper alloy for making metal liners. The alloy, when heated to a temperature in the liner following detonation, is said to have a ductile matrix with a molten second phase dispersed within the matrix. Brauer states that the molten second phase reduces the tensile strength of the matrix so that the liner slug pulverizes on striking a well casing. 
     It has been observed by the present inventor that it would be advantageous to employ a liner that is made of a material of which the largest component is zinc. Advantageously, the liner may be made from the same material for making the casing. This material does not tend to require special alloy combinations, tends to be readily available, and tends to be relatively inexpensive. It tends not to require careful mixing of special expensive or exotic copper alloys. A number of methods are suitable for manufacturing zinc-based casings and liners. The present invention is part of the continuing efforts directed at overcoming the foregoing and other attendant difficulties. The liner disclosed herein may tend to be particularly useful in situations where it is required to perforate large holes, with reduced tendency to form slugs, although its application is not limited to such. 
     SUMMARY OF THE INVENTION 
     In an aspect of the invention there is a shaped charge comprising a casing, a liner, and an explosive charge. The casing has an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth. The liner has a central region and a divergent skirt extending from the central region. The central region lies closer to the apex than to the mouth, and the skirt is mounted to the divergent wall structure closer to the mouth than to the apex. A charge cavity is defined between the casing and the liner. The explosive charge is contained within the charge cavity. The liner is formed from a metal material whose greatest component, by weight, is zinc. 
     In an additional feature of that aspect of the invention, the metal material of the liner is predominantly zinc. In another additional feature, the metal material of the liner is more than 70% zinc by weight. In yet another additional feature, the metal material of the liner is more than 90% zinc by weight. 
     In still another additional feature, the metal material of the liner is made of a metal material that is essentially zinc. In a further additional feature, the liner is made from zinc, aluminium and magnesium. In yet a further additional feature, the liner and the casing are made of the same material. In still a further additional feature, the casing is formed from a metal material whose greatest component, by weight, is zinc. In another additional feature, the liner and the casing are made of a metal material that is at least 50% zinc by weight. 
     In yet another additional feature, the shaped charge has an axis of symmetry, and the casing has the form of a body of revolution relative to the axis of symmetry. In still another additional feature, the charge cavity has a first region lying on the axis of symmetry, and a second region lying radially away from the axis of symmetry. The second region has a diminished thickness relative to the first region. In still yet another additional feature, the liner has a conic region lying at a conic angle relative to the axis. In a further additional feature, the conic angle is in the range of 15 to 40 degrees. In yet a further additional feature, the liner is of constant thickness. In still a further additional feature, the central region of the liner forms a bottom region of a valley and the divergent skirt forms sloped sides of the valley such that the liner is generally V-shaped. In another additional feature, the liner is a casting. 
     In another aspect of the invention there is a method of manufacturing a shaped charge. The method comprises the steps of providing a casing having an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth, forming a liner from a metal material whose greatest component, by weight, is zinc. The liner has a central region and a divergent skirt extending from the central region, providing an explosive charge and locating the explosive charge in the casing, locating the liner in position relative to the casing to form a charge cavity containing the explosive charge. The step of locating the liner in position includes the step of locating the central region of the liner closer to the apex of the casing than to the mouth and locating the skirt adjacent to the divergent wall structure closer to the mouth than to the apex. 
     In an additional feature of that aspect of the invention, the step of forming the liner includes the step of providing a material whose greatest component, by weight, is zinc. In another additional feature, the step of forming the liner includes the step of providing a liner material that is predominantly zinc. In yet another additional feature, the step of forming the liner includes the step of providing a liner material that is more than 70% zinc by weight. In still another additional feature, the step of forming the liner includes the step of providing a liner material that is more than 90% zinc by weight. In still yet another additional feature, the step of forming the liner includes the step of providing liner material that is essentially zinc. 
     In a further additional feature, the step of forming the liner includes the step of making the liner and the casing of the same material. In yet a further additional feature, the step of forming the liner includes the step of making both the liner and the casing of materials that are at least 50% zinc by weight. In still a further additional feature, the step of forming the liner from a powder metal. In another additional feature, the step of locating the liner includes the step of pressing the powder metal liner in a green state against the explosive charge. In yet another additional feature, the step of forming the liner includes sintering the powder metal. 
     In still another additional feature, the step of forming the liner includes the step of using a metal material of constant thickness. In a further additional feature, the step of forming the liner includes casting the liner. In yet a further additional feature, the step of forming the liner includes machining the liner. 
     In another aspect of the invention there is a kit for assembling into a shaped charge for receiving and retaining an explosive charge for use with perforating guns, comprising a casing and a liner. The casing has an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth. The liner has a central region and a divergent skirt extending from the central region. The skirt is mountable to the divergent wall structure to form a charge cavity between the liner and the casing for containing the explosive charge. The liner is mountable to the casing, and the skirt is closer to the mouth than to the apex and the central region lies closer to the apex than to the mouth upon mounting the skirt to the divergent wall. The liner is formed from a metal material whose greatest component, by weight, is zinc. 
     In an additional feature of that aspect of the invention, the metal material of the liner is predominantly zinc. In another additional feature, the metal material of the liner is more than 70% zinc by weight. In yet another additional feature, the metal material of the liner is more than 90% zinc by weight. In still another additional feature, the metal material of the liner is essentially zinc. In still yet another additional feature, the casing is formed from a metal material whose greatest component, by weight, is zinc. In a further additional feature, the liner and the casing are made of the same material. In yet a further additional feature, the liner is a casting. 
     In another aspect of the invention there is a rail method of manufacturing a shaped charge liner for use with a casing for containing an explosive charge. The casing has an apex at which to connect a detonation source, a mouth, and a divergent wall structure extending between the apex and the mouth. The divergent wall structure is narrower at the apex of the casing than at the mouth, comprising the steps of forming a liner. The liner has a central region and a divergent skirt extending from the central region. The skirt is mountable to the divergent wall structure to form a charge cavity between the liner and the casing for containing the explosive charge. The skirt is closer to the mouth than to the apex and the central region lies closer to the apex than to the mouth upon mounting the skirt to the divergent wall. The liner is formed from a metal material whose greatest component, by weight, is zinc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example, and not of limitation, to the accompanying drawings, which show an apparatus according to the principles of the present invention and in which: 
     FIG. 1 a  shows a perforating gun suspended in an oil well by a production tubing; 
     FIG. 1 b  shows an alternative method of delivering a perforating gun of FIG. 1 a , i.e., suspending the perforating gun in an oil well by a wireline; 
     FIG. 2 is a partially sectional view of the perforating gun of FIG. 1 a;    
     FIG. 3 a  shows, in cross-sectional view, a shaped charge of the perforating gun of FIG. 2; 
     FIG. 3 b  shows a detail view of a liner for the shaped charge of FIG. 3 a;    
     FIG. 3 c  shows an isometric exploded view of the shaped charge of FIG. 3 a;    
     FIG. 3 d  shows a detail view of a casing for the shaped charge of FIG. 3 a;    
     FIG. 4 shows a shaped charge liner of a substantially conical shape, as an alternative to a liner of bowl-like shape as shown in FIG. 3 c;    
     FIG. 5 shows a shaped charge liner having a cross-sectional “V” shape and a shaped charge casing for receiving the V-shaped charge liner, as an alternative to the casing and liner shown in FIG. 3 a;    
     FIG. 6 shows a V-shaped rectangular shaped charge liner and casing of FIG. 5 in an exploded configuration; 
     FIG. 7 shows another alternative shaped charge liner having a cross-sectional “W” shape as a further alternative to the liner of FIG. 3 a.    
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The description which follows, and the embodiments described therein, are provided by way of illustration of an example, or examples of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the invention. In the description that follows, like parts are marked throughout the specification and the drawings with the same respective reference numerals. The drawings are not necessarily to scale and in some instances proportions may have been exaggerated in order more clearly to depict certain features of the invention. 
     By way of a general overview, an oil or gas well is indicated generally in FIG. 1 a  as  20 . Well  20  has a well casing  22  surrounded by cement  24 . Cement  24  is cast in the bore to fill the generally annular space between casing  22  and oil- or gas-bearing formation  26 . Well  20  may extend to a depth of several kilometers from the surface, i.e., ground level. A perforating gun  30  is suspended in well  20  by production tubing  32  and is delivered at a desired depth. At such a depth, perforations produced in well casing  22 , surrounding cement  24 , and surrounding oil-bearing formation  26  may tend to allow oil to flow from oil-bearing formation  26  into well  20 . Shaped charges  40  are arranged along the length of gun  30 , as indicated schematically in FIG. 1 a . FIG. 1 b  shows an alternative delivery method, where gun  30  is suspended and delivered at a desired depth by wireline  36 . 
     For the purposes of this description, the bore of well  20 , and the perforating gun for location in the well, can be considered in a polar co-ordinate system in which the axial direction of the bore (i.e., the direction in which well bore is drilled) is a first, or longitudinal axis. In such a frame of reference the radial direction extends perpendicularly outward from the axial direction. A third, circumferential angular co-ordinate establishes the angular orientation, relative to some specified datum angle, along a radius lying in a plane perpendicular to the axial direction. Although the well bore may be drilled at an angle, or vertically, or horizontally, for the purposes of this description, in the context of a well bore, upward means toward the well head, and downward means away from the well head, whatever the local orientation of the well bore may be. Local reference to “upward” and “downward” is then the direction parallel to the longitudinal axis, i.e., the axial direction. Similarly, radial means the direction toward or away from the longitudinal axis. 
     FIG. 2 shows perforating gun  30  having an elongate member in the nature of outer housing  42 . Outer housing  42  has the general shape of a hollow circular tube, or cylinder. A threaded connection  44  is formed on the inner wall of the tube toward one end of housing  42  for mounting gun  30  to production tubing  32 . A charge holder  46  is mounted inside outer housing  42 . Charge holder  46  has a number of longitudinally spaced recesses  48  in which to mount shaped charges  40 . Recesses  48  are generally formed radially into charge holder  46 , with one side being open. The wall thickness of housing  42  is locally reduced as at  49  opposite each recess  48  to facilitate perforation upon detonation, yet to tend to keep water out of gun  30  during delivery. 
     A set of shaped charges  40  form an array mounted along charge holder  46 , as indicated by dotted lines in FIG.  2 . Most typically the charges are arrayed in a spiral, or helical fashion, in that each charge is turned relative to its neighbours on an angular pitch, while the charges are separated along the gun at a longitudinal pitch. The combination of angular and longitudinal incremental pitches may tend to result in a helical discharge pattern. FIG. 2 illustrates  6  shaped charges  40  mounted on charge holder  46 . A different charge holder that can hold a different number of shaped charges may also be used, depending on the desired density of perforation at a given depth of the oil or gas well. 
     FIG. 3 a  shows a shaped charge  40  in a cross-sectional view. Shaped charge  40  has a casing  50 , a liner  52 , and an explosive charge  54 . Liner  52  cooperates with casing  50  to hold explosive charge  54  in casing  50  between casing  50  and liner  52 . In this specification, the term “shaped charge” hereinafter refers to an assembly of a casing, a liner and an explosive charge, such as the one illustrated, for example, in FIG. 3 a . Casing  50  has a leg  56 . When shaped charge  40  is mounted in recess  48 , leg  56  is proximate to the longitudinal axis of charge holder  46  and is located in the interior of recess  48 . 
     A second frame of reference can be defined for the purposes of this description in terms of the geometry of the shaped charge. Where a shaped charge is formed as a body of revolution, the axis of the resultant jet of the shaped charge can be defined as the primary, or central axis of the charge. Typically, the shaped charge is installed in a detonator at an orientation such that the central axis of the charge is perpendicular to the longitudinal axis of the perforating gun, such that the jet of the shaped charge, when fired, will discharge radially (relative to the longitudinal axis of the well bore) into the well liner and casing, and into the surrounding mineral bearing stratum. The shaped charge is usually installed such that a second axis can be defined parallel to the longitudinal axis of the perforating gun, and a third axis can be defined lying in a plane (typically a horizontal plane) perpendicular to the longitudinal axis of the perforating gun. The proximal end of the shaped charge is located near the longitudinal axis  31  of gun  30 , and the distal end of the shaped charge is located remote from the longitudinal axis  31  of gun  30 . 
     Casing  50  as illustrated in FIG. 3 a  is a body of revolution, the axis of rotation, or axis of symmetry, being the central axis  41  of shaped charge  40 . Casing  50  has a divergent or bowl-like wall structure, in the nature of interior wall  70 , a recess formed along the central axis  41 , and a mouth, or opening  58 . Interior wall  70  has an apex  72  located toward leg  56 . Opposite apex  72  is opening  58 . In general, interior wall  70  widens gradually from apex  72  toward opening  58 . Interior wall  70  shown in FIG. 3 a  is made up of a series of truncated conical sections catenated together, each conical section having its smaller end lying closer to leg  56  and its larger end lying closer to opening  58 . 
     Leg  56  protrudes along the central axis, and, on assembly seats at the inner end of recess  48  of charge holder  46 . A circular groove  60  is formed on leg  56 . A fastener in the nature of clip  62 , is received in groove  60  and co-operates with recess  48 , to secure leg  56 , and thereby shaped charge  40 , inside recess  48 . In this position, opening  58  is remote from longitudinal axis  31  of change holder  46  and generally faces radially outwardly away from longitudinal axis  31 . Leg  56  also has an aperture  64  in the nature of a through hole formed along the central axis. Initiator material  66  fills aperture  64 . A detonation cord (not illustrated) runs into aperture  64  for detonating initiator material  66 . 
     Liner  52  shown in FIG. 3 a  is also a body of revolution and also has a divergent or bowl-like wall structure  53 . Liner  52  has a first portion shown as a generally round, central region in the nature of bowl end  80 . A second portion of liner  52  namely skirt, or sidewall  82 , extends smoothly away from bowl end  80 , gradually widens as it extends along the central axis, and terminates at distal lip or brim  84 . The shape of sidewall  82  is generally that of the sidewall of a truncated cone. The truncated cone has a conical angle, ψ as measured between axis  41  and tangent portion  82 . 
     In a preferred embodiment, liner  52  has a substantially uniform thickness of about 0.064 inches. The conical angle ψ is about 26°. The truncated cone has a large end brim  84  of about 1.8 inches in diameter, matching the size of opening  58 , and a height of about 0.5 inches. Bowl end  80  is approximately a portion of a sphere of about 0.7 inches in diameter and joins smoothly with sidewall  82  at the small end of the truncated cone. 
     FIG.3 a  shows casing  50  and liner  52  in an assembled configuration. In this configuration, the central axes of casing  50  and liner  52  tend to coincide with each other. Bowl end  80 , i.e., the central region of liner  52 , lies closer to apex  72  than to opening  58 . The skirt, or sidewall  82 , is mounted to casing  50  at a location near opening  58 . Distal brim  84  encircles an area that is comparable to opening  58  in size. In the assembled position, liner  52  tends to block opening  58 , thereby containing, and retaining, charge  54 . 
     As described above, liner  52  has a first profile  86  defined by the inner faces of sidewall  82  and bowl end  80 . Casing  50  has a second profile  88  defined by interior wall  70 . When liner  52  is mounted to casing  50 , the first and second profiles cooperate to define an enclosed space in the nature of charge cavity  90  therebetween. Explosive charge  54  is packed against interior wall  70  and liner  52 . As such, explosive charge  54  is retained in charge cavity  90  and may acquire a shape defined by charge cavity  90 . Aperture  64  provides a path of communication between initiator material  66  and explosive charge  54  retained in charge cavity  90 . Detonation of initiator material  66  tends to subsequently detonate explosive charge  54 . As will be described below, the shape of explosive charge  54  may be varied by varying first profile  86  of liner  52  and second profile  88  of casing  50  so that different perforation results may be achieved. 
     In assembling shaped charge  40 , an appropriate amount of explosive powder is added to casing  50 . Leg  56  may be tapped to level the powder. A die, having a shape substantially conforming with first profile  86  of liner  52 , may press down through opening  58  and compact the explosive charge powder against interior wall  70 . Liner  52  may then be placed against the concave side of explosive charge  54  and mounted to opening  58 . Any stray powder may then be cleaned away. 
     According to the present invention, liner  52  is formed from a metal material whose greatest component, by weight, is zinc. Notwithstanding previous assumptions to the contrary, the inventor has found that such a zinc-based liner may tend to be effective for perforating steel well casings. Using zinc or zinc alloy as a liner material may tend to have a number of advantages. Zinc is readily available and tends to be cheaper than some other liner materials such as special alloy mixes of copper. Unlike a lead-based liner, a zinc-based liner may tend to be more environmentally benign. Forming liners from sheets of a zinc alloy by stamping, or from bars of a zinc alloy by machining may tend to be relatively easy because of the relatively good formability and machinability of zinc alloys. The relatively low melting temperature of zinc or zinc alloys may also tend to permit direct casting of liners in relatively large quantities. Casting tends to be faster and more cost-effective than other production methods such as machining. Additionally, as casings are often made of a zinc alloy, liners may advantageously be made of the same material as is presently used for making casings. This may provide an additional advantage of reducing inventory costs. 
     It is advantageous to use an alloy that is predominantly zinc by weight, i.e., an alloy that contains more than 50% of zinc by weight. For example, the zinc alloy may have at least 70% of zinc by weight. Pure zinc, i.e. a metal having more than 99% of zinc by weight, may also be used. Or, the zinc alloy may have a proportion of zinc, by weight, lying in the range of about 70% to about 97% zinc. In particular, the zinc alloy may be formed from zinc, aluminium, and magnesium. 
     In a preferred embodiment, liners are formed from a zinc alloy containing about 95.96% zinc, about 4% aluminium, and about 0.04% magnesium by weight. More particularly, the practice of the present invention may advantageously be accomplished utilizing an alloy available from Zincaloy Inc., 1565 Bonhill Road, Mississauga, Ontario, Canada, L5T 1M1 and sold under the trade name of ZAMAC #3. In a preferred embodiment, both casing  50  and liner  52  are made by casting from the zinc alloy identified as ZAMAC #3. 
     Other methods may be employed to form zinc-based casings and liners. For example, both casings and liners may be formed by machining a bar. However, because of the generally bowl-like shape, forming casings and liners by machining may tend to entail the discard of more raw material than may be desirable. Stamping or pressing a sheet of liner material upon a form that has a desirable surface shape for the liner may be another method of forming a liner. Cold pressing metal powders of the liner material may be yet another method of forming a liner. Although the density of a liner formed by cold pressing may potentially reach about 96% of the density of the powder material or higher, an unsintered liner, i.e., a liner in a green state, may lack a great strength against any crushing force. Sintering may tend to encourage coherence of cold pressed powders and thus may tend to increase the strength of the liner. 
     In a preferred embodiment, the explosive powder is RDX, a commercially available explosive sold by, for example, Goex International, 423 Vaughn Road West, Cleburne, Tex. 76031. Any other suitable explosive may be used. Some examples include CH-6, HMX, PETN, HNS, PYX, all of which are trade names of commercially available explosive products. 
     Liner shapes may affect the perforation results. For example, liner  52  illustrated in FIG. 3 a  is more bowl-like than conical given the relative proportions of bowl end  80 , conical wall portion  82  and distal brim  84 . Such a liner may tend to be advantageous in perforating large holes of a penetration depth of as deep as 10 inches. Large holes of such a penetration depth have the tendency of providing good communication between the bore of the well and the adjacent formation containing heavy oils. A more sharply conical, less bowl-like shape may yield different penetration characteristics. 
     FIG. 4 illustrates an alternative embodiment of a shaped charge liner. Liner  100  has a relatively sharply conical sidewall  102  joined to bowl end  104 . Side wall  102  extends from bowl end  104  to brim  106 . Bowl end  104  is significantly smaller in relative proportion to wall  102  and brim  106  than in the example of liner  52  of FIG. 3 a . As such, liner  100  is substantially conical. Substantially conical liners may tend to produce more focused high-speed jet. Thus, these liners tend to produce smaller perforations, but deeper perforations perhaps as deep as 60 inches. Perforations like these may be more suitable for wells of gas or lighter oil. Shaped changes with intermediate properties can be produced using liners of proportions intermediate these of liners  52  and  100 . 
     Shaped charges may take some other shapes. Descriptions of shaped charges of various other shapes and corresponding shaped charge liners may be found, for example, on pages 737-8 of  High Velocity Impact Dynamics  (Ed. Jonas A. Zukas, John Wiley &amp; Sons, Inc., New York 1990). 
     FIG.  5  and FIG. 6 show yet another alternative embodiment. A generally linear liner  120  is mounted to a generally linear casing  122 . In cross-section, liner  120  has the shape of a long “V” having an elongate valley portion  124  flanked by two generally flat sidewalls  126 . Sidewalls  126  terminate at edges  128 . Casing  122  has an interior wall structure  130  in the nature of interior walls, i.e., two sloped flat sides  132  and four side walls  134 . Two generally sloped flat sides  132  intersect each other along a line lying toward the innermost end of casing  122  and form a cross-sectional “V”. Four side walls  134 , two of which enclose each side of the long V formed by sloped flat sides  132 , and two of which intercept each of sloped flat sides  132 , terminate at a rectangular lip, or brim  136 . Brim  136 , having a rectangular shape, defines opening  140  of casing  122 . Liner  120  is mounted in casing  122 , as indicated in FIG. 5, with its valley portion  124  lying closer to the interior of casing  122  and its sidewall edges  128  near casing brim  136 . In the mounted position, liner  120  tends to block opening  140 . 
     FIG. 7 shows, in cross-sectional view, another generally linear liner  150  having a cross-sectional “W” shape. Liner  150  has a valley portion  152  having a slightly raised ridge  154 . Divergent and flat sidewalls  156  of liner  150  extend from valley portion  152  and terminate at distal edges  158 . Linear liners may tend to produce large holes of penetrations perhaps as deep as up to 30 inches. 
     Various embodiments of the invention have now been described in detail. Since changes in or additions to the above-described best mode may be made without departing from the nature, spirit or scope of the invention, the invention is not to be limited to those details, but only by the appended claims.