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TECHNICAL FIELD OF THE INVENTION 
     This invention relates, in general, to an apparatus for perforating a subterranean wellbore using shaped charges and, in particular, to a low debris shaped charge perforating apparatus that utilizes contoured recesses in the charge carrier that reduce the size of the holes made in the charge carrier upon detonation of the shaped charges thus enhancing debris containment. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the present invention, its background will be described with reference to perforating a subterranean formation with a shape charge perforating apparatus, as an example. 
     After drilling the section of a subterranean wellbore that traverses a formation, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic opening or perforation must be made through the casing string, the cement and a short distance into the formation. 
     Typically, these perforations are created by detonating a series of shaped charges located within the casing string that are positioned adjacent to the formation. Specifically, one or more charge carriers are loaded with shaped charges that are connected with a detonating device, such as detonating cord. The charge carriers are then connected within a tool string that is lowered into the cased wellbore at the end of a tubing string, wireline, slick line, coil tubing or the like. Once the charge carriers are properly positioned in the wellbore such that shaped charges are adjacent to the formation to be perforated, the shaped charges are detonated. Upon detonation, each shaped charge creates a Jet that blasts through a scallop or recess in the carrier. Each jet creates a hydraulic opening through the casing and the cement and enters the formation forming a perforation. 
     When the shaped charges are detonated, numerous metal fragments are created due to, among other things, the disintegration of the metal casings of the shaped charges. These fragments often fall out or are blown out of the holes created in the carrier. As such, these fragments become debris that is left behind in the wellbore. It has been found that this debris can obstruct the passage of tools through the casing during subsequent operations. This is particularly problematic in the long production zones that are perforated in horizontal wells as the debris simply piles up on the lower side of such wells. 
     A need has therefore arisen for an apparatus and method that reduce the likelihood that debris will be left in the well following perforation of the formation. A need has also arisen for such an apparatus and method that will contain the fragments created when the shaped charges are detonated. Further, a need has arisen for such an apparatus and method that will enhance the performance of the shaped charges in perforating the formation. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a shaped charge perforating apparatus and a method for perforating a subterranean formation using a shaped charge perforating apparatus that reduce the likelihood that debris will be left in a well following perforation of a formation. The shaped charge perforating apparatus of the present invention achieves this result by containing the fragments created when the shaped charges are detonated. In addition, the shaped charge perforating apparatus of the present invention enhances the performance of the shaped charges in perforating the formation. 
     The shaped charge perforating apparatus of the present invention comprises a support member having a plurality of shaped charge mounting locations each of which receive a shaped charge therein. The shaped charges each have an initiation end and a discharge end. The initiation ends of the shaped charges are each coupled to a detonator cord. The shaped charges are placed within an elongated and generally tubular carrier. The carrier has a plurality of recesses that are spaced on the exterior surface thereof and have contoured bottom surfaces. Each of the recesses is axially and radially aligned with the discharge end of one of the shaped charges such that the jet formed upon the initiation of each shaped charge will penetrate the carrier through a recess. 
     In one embodiment of the present invention, the contoured bottom surface of the recesses is formed such that the center depth of the recess in the carrier is greater than a perimeter depth of the recess. For example, the center depth of the recess may be greater than the perimeter depth of the recess at a point on the perimeter of the recess that is displaced from the center of the recess in a longitudinal direction. Alternatively or additionally, the center depth of the recess may be greater than the perimeter depth of the recess at a point on the perimeter of the recess that is displaced from the center of the recess in a circumferential direction. 
     In another embodiment of the present invention, the contoured bottom surface of the recesses is formed such that the recesses have a flat bottom portion proximate the center of the recesses and an angular bottom portion extending from the flat bottom portion to the perimeter of the recesses. In this embodiment, the angular bottom portion may have an angle of between about 10 and 40 degrees and may preferably have an angle of between about 15 and 25 degrees. 
     In one embodiment of the present invention, the contoured bottom surface of the recesses is formed such that the angular bottom portion extends from the flat bottom portion of the recess to the exterior surface of the carrier. In another embodiment of the present invention, the contoured bottom surface of the recesses is formed such that the angular bottom portion of the recess extends from the flat bottom portion of the recess to a sidewall section of the recess at a location offset from the exterior surface of the carrier by a depth. 
     The method for perforating a subterranean well of the present invention comprises running a shaped charge perforating apparatus of the present invention downhole, operating the shaped charge perforating apparatus and discharging jets formed from the shaped charges through respective contoured bottom surfaces of recesses in the shaped charge carrier. In such a method, the recesses have a center depth that is greater than a perimeter depth, either in the longitudinal direction from the center, the circumferential direction from the center or both. The recesses may have a flat bottom portion proximate the center of the recesses and an angular bottom portion extending from the flat bottom portion to the perimeter of the recesses. The angular bottom portion of the recesses may extend from the flat bottom portion of the recesses to the exterior surface of the carrier or may intersect the sidewall of the recesses at the perimeter of the recesses. In all such configurations, using the method of the present invention to discharge jets formed from the shaped charges through the respective contoured bottom surfaces minimizes the size of openings created through the carrier by the discharge of the jets and prevents peeling of the recesses in the longitudinal direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is a schematic illustration of an offshore oil and gas platform operating a shaped charge perforating apparatus of the present invention; 
     FIG. 2 is partial cut away view of a shaped charge perforating apparatus of the present invention; 
     FIG. 3 is a cross sectional view taken in the circumferential direction of a contoured recess of a charge carrier of the present invention taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is a prior art drawing of a recess of a charge carrier show in a circumferential cross sectional view; 
     FIG. 5 is a cross sectional view taken in the longitudinal direction of a contoured recess of a charge carrier of the present invention taken along line  5 — 5  of FIG. 2; 
     FIG. 6 is a prior art drawing of a recess of a charge carrier show in a longitudinal cross sectional view; 
     FIG. 7 is a longitudinal cross sectional view of a contoured recess of a charge carrier of the present invention; 
     FIG. 8 is a longitudinal cross sectional view of a contoured recess of a charge carrier of the present invention; and 
     FIG. 9 is a longitudinal cross sectional view of a contoured recess of a charge carrier of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
     Referring initially to FIG. 1, a low debris shaped charge perforating apparatus operating from an offshore oil and gas platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to wellhead installation  22  including blowout preventers  24 . Platform  12  has a hoisting apparatus  26  and a derrick  28  for raising and lowering pipe strings such as work sting  30 . 
     A wellbore  32  extends through the various earth strata including formation  14 . A casing  34  is cemented within wellbore  32  by cement  36 . Work string  30  include various tools including shaped charge perforating apparatus  38 . When it is desired to perforate formation  14 , work string  30  is lowered through casing  34  until shaped charge perforating apparatus  38  is positioned adjacent to formation  14 . Thereafter, shaped charge perforating apparatus  38  is fired by detonating the shaped charges that are disposed within charge carrier  40  and aligned with recesses  42  of charge carrier  40 . Upon detonation, the liners of the shaped charges form jets that pass through recesses  42  of charge carrier  40  and form a spaced series of perforations extending outwardly through casing  34 , cement  36  and into formation  14 . 
     Even though FIG. 1 depicts a vertical well, it should be noted by one skilled in the art that the low debris shaped charge perforating apparatus of the present invention is equally well-suited for use in deviated wells, inclined wells or horizontal wells. Also, even though FIG. 1 depicts an offshore operation, it should be noted by one skilled in the art that the low debris shaped charge perforating apparatus of the present invention is equally well-suited for use in onshore operations. 
     Referring now to FIG. 2, therein is depicted a low debris shaped charge perforating apparatus of the present invention that is generally designated  50 . Perforating apparatus  50  includes a carrier  52  made of a cylindrical sleeve having a plurality of recesses, such as recess  54 , defined therein. Radially aligned with each of the recesses is a respective one of a plurality of shaped charges, such as shaped charge  56 . Each of the shaped charges includes an outer housing, such as housing  58  of shaped charge  56 , and a liner, such as liner  60  of shaped charge  56 . Disposed between each housing and liner is a quantity of high explosive. 
     The shaped charges are retained within carrier  52  by a support member  62  which includes an outer charge holder sleeve  64 , an inner charge holder sleeve  66 . In this configuration, outer tube  64  supports the discharge ends of the shaped charges, while inner tube  66  supports the initiation ends of the shaped charges. Disposed within inner tube  66  is a detonator cord  70 , such as a Primacord, which is used to detonate the shaped charges. In the illustrated embodiment, the initiation ends of the shaped charges extend across the cental longitudinal axis of perforating apparatus  50  allowing detonator cord  70  to connect to the high explosive within the shaped charges through an aperture defined at the apex of the housings of the shaped charges. 
     Each of the shaped charges is longitudinally and radially aligned with a recess in carrier  52  when perforating apparatus  50  is fully assembled. In the illustrated embodiment, the shaped charges are arranged in a spiral pattern such that each shaped charge is disposed on its own level or height and is to be individually detonated so that only one shaped charge is fired at a time. It should be noted, however, by those skilled in the art that alternate arrangements of shaped charges may be used, including cluster type designs wherein more than one shaped charge is at the same level and is detonated at the same time, without departing from the principles of the present invention. 
     Referring now to FIG. 3, therein is depicted a circumferential cross sectional view of recess  54  of carrier  52  taken along line  3 — 3  of FIG.  2 . Recess  54  has a contoured bottom surface  80 . In the illustrated embodiment, recess  54  has a flat bottom center portion  82 . Radially outwardly extending from flat bottom center portion  82  are angular bottom portions  84  and  86 . Angular bottom portion  84  extends radially outwardly toward sidewall  88  of recess  54  while angular bottom portion  86  extends radially outwardly toward sidewall  90  of recess  54 . As such, the depth of recess  54  at the center is greater than the depth of recess  54  near the circumferential perimeters of recess  54 , i.e., near sidewalls  88  and  90 . 
     In the illustrated embodiment, the angle of angular bottom portions  84  and  86  relative to flat bottom potion  82  is angle θ. Angle θ may be any angle greater than zero but is preferably between 10 degrees and 40 degrees and most preferably between 15 degrees and 25 degrees. The exact angle θ will depend upon the desired performance characteristics of recess  54  as well as the relative diameters of recess  54  and flat bottom portion  82 . For example, if recess  54  has a diameter of 1.25 inches, the diameter of flat bottom portion  82  may be between about 0.3 inches and 0.7 inches and is preferable between about 0.4 inches and 0.6 inches. As should be understood by those skilled in the art, other diameter recesses having other diameter flat bottom portions are also desirable and considered within the scope of the present invention. 
     Utilizing a charge carrier having recesses, such as recess  54  with contoured bottom surface  80 , enhances the performance of a shaped charge for which recess  54  is the first target. Specifically, as compared with conventional recesses having a flat bottom surface, see FIG. 4, the thickness of the first target is reduced as the depth of recess  54  near the center of recess  54  is greater than the allowable depth for a conventional flat bottom recess due to the required pressure rating for the charge carrier. There must be a sufficient amount of metal remaining behind a recess to withstand the high downhole pressures seen by the charge carrier. Using recess  54  with contoured bottom surface  80 , the required pressure rating can be achieved even though the metal behind flat bottom center portion  82  is thinner than previous allowable due to the extra metal behind angular bottom portions  84  and  86 . As such, since the first target seen by a shaped charge disposed behind recess  54  having contoured bottom surface  80  is thinner than with conventional flat bottom recesses, the performance of such a shaped charge is improved as the depth of penetration into a formation is increased. 
     Referring now to FIG. 5, therein is depicted a longitudinal cross sectional view of recess  54  of carrier  52  taken along line  5 — 5  of FIG.  2 . Recess  54  has a contoured bottom surface  80 . In the illustrated embodiment, recess  54  has a flat bottom center portion  82 . Radially outwardly extending from flat bottom center portion  82  are angular bottom portions  92  and  94 . Angular bottom portion  92  extends radially outwardly toward sidewall  96  of recess  54  while angular bottom portion  94  extends radially outwardly toward sidewall  98  of recess  54 . As such, the depth of recess  54  at the center is greater than the depth of recess  54  near the longitudinal perimeters of recess  54 , i.e., near sidewalls  96  and  98 . In the illustrated embodiment, the angle of angular bottom portions  92  and  94  relative to flat bottom center portion  82  is angle θ, which is the same angle of angular bottom portions  84  and  86  relative to flat bottom center portion  82  in FIG. 3 as recess  54  is symmetric about its central axis. 
     Utilizing a charge carrier having recesses, such as recess  54  with contoured bottom surface  80 , not only enhances the performance of shaped charges for which recess  54  is the first target, but also, reduces the likelihood that debris will be left in the well following perforation as such a charge carrier will contain the fragments created when the shaped charges are detonated. Specifically, a smaller opening is made when a jet passes through recess  54  with contoured bottom surface  80  than when a jet passes through conventional recesses. With recess  54 , not only does the jet pass through a thinner metal section, contoured bottom surface  80  is not susceptibly to the longitudinal pealing effect as the thickness of the metal behind recess  54  becomes progressive thicker in angular bottom portions  92  and  94 . 
     Unlike the present invention, with conventional flat bottom recesses, as seen in FIG. 6, the thickness of the metal in the longitudinal direction behind the recess is constant. As such, a jet penetrating a conventional flat bottom recess tends to create a large opening as the metal behind the recess peals back toward the sidewalls of the recess. In some cases, this longitudinal pealing effect results in an opening that is substantially as wide as the diameter of the conventional flat bottom recess in the longitudinal direction. As such, the fragments created when the shaped charges are detonated tend to fall or be blasted out of these large openings to become debris in the well. In addition, the pealed section of metal from behind the conventional flat bottom recess sometimes protrudes outwardly beyond the exterior of a charge carrier having conventional flat bottom recesses and scores the casing as such a charge carrier is retrieved uphole. 
     Referring now to FIG. 7, therein is depicted a longitudinal cross sectional view of a recess  100  of a carrier  102 . Recess  100  has a contoured bottom surface  104 . In the illustrated embodiment, recess  100  has a flat bottom center portion  106 . Radially outwardly extending from flat bottom center portion  106  are angular bottom portions  108  and  110 . Angular bottom portions  108  and  110  extend radially outwardly to the exterior surface of carrier  102 . As such, the depth of recess  100  at the center is greater than depth of recess  100  near the longitudinal perimeters of recess  100 , i.e., at the intersection with the exterior surface of carrier  102 . 
     Referring next to FIG. 8, therein is depicted a longitudinal cross sectional view of a recess  120  of a carrier  122 . Recess  120  has a contoured bottom surface  124 . In the illustrated embodiment, recess  120  has an apex  126 . Radially outwardly extending from apex  126  are angular bottom portions  128  and  130 . Angular bottom portion  128  extends radially outwardly toward sidewall  132  of recess  120  while angular bottom portion  130  extends radially outwardly toward sidewall  134  of recess  120 . As such, the depth of recess  120  at the center is greater than the depth of recess  120  near the longitudinal perimeters of recess  120 , i.e., near sidewalls  132  and  134 . 
     Referring to FIG. 9, therein is depicted a longitudinal cross sectional view of a recess  140  of a carrier  142 . Recess  140  has a contoured bottom surface  144 . In the illustrated embodiment, recess  140  has an arcuate contour that extends from a maximum at location  146  to minimums at sidewalls  148  and  150 . As such, the depth of recess  140  at the center is greater than the depth of recess  140  near the longitudinal perimeters of recess  140 . 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
A shaped charge perforating apparatus ( 50 ) for perforating a subterranean well is disclosed. The shaped charge perforating apparatus ( 50 ) comprises a support member ( 62 ) having a plurality of shaped charge mounting locations each having shaped charge ( 56 ) positioned therein. Each of the shaped charges ( 56 ) has an initiation end and a discharge end. The initiation end of each shaped charge ( 56 ) is coupled to a detonator cord ( 70 ) The shaped charges ( 56 ) are enclosed in a carrier ( 52 ) having a plurality of recesses ( 54 ) spaced on an exterior surface thereof and having contoured bottom surfaces ( 80 ). Each of the recesses ( 54 ) is longitudinally and radially aligned with the discharge end of one of the shaped charges ( 56 ).