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CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of and priority to a U.S. Provisional Patent Application No. 60/681,553 filed May 16, 2005, the technical disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field of the Invention  
         [0003]     The present invention relates generally to perforation guns that are used in the oil and gas industry to explosively perforate well casing and underground hydrocarbon bearing formations, and more particularly to an improved method and an improved apparatus for explosively perforating a well casing and its surrounding underground hydrocarbon bearing formation while limiting the amount of explosion debris in the well bore and hydrocarbon bearing formation following perforation.  
         [0004]     2. Description of the Related Art  
         [0005]     During the completion of an oil and/or gas well, it is common to perforate the hydrocarbon containing formation with explosive charges to allow inflow of hydrocarbons to the well bore. These charges are loaded in a perforation gun and are typically shaped charges that produce an explosive formed penetrating jet in a chosen direction.  
         [0006]      FIG. 1  illustrates a perforation gun consisting of a cylindrical carrier  14  hanging from a cable  20 . At the well site, the explosive charges  16  are placed into the charge carrier  14 , and the charge carrier  14  is then lowered into oil and gas well casing to the depth of the hydrocarbon bearing formation  12 . The exploding charges  16  fire outward from the charge carrier  14  and the force from each charge punctures holes  24  in the wall  18  of the casing and the hydrocarbon bearing formation  12 , which allows oil, gas, water and/or minerals to flow into the casing from the hydrocarbon bearing formation  12 .  
         [0007]     While perforation guns do increase fluid production from hydrocarbon bearing formations, the effectiveness of traditional perforation guns is limited by the fact that the firing of a perforation gun can leave behind “debris” inside the casing and the hydrocarbon bearing formation  12 . This debris can cause significant operational difficulties for the well operator and has to be cleaned out of the well at a significant cost.  FIG. 2  shows a traditional hollow carrier perforation gun  14 B, positioned adjacent to a hydrocarbon bearing formation  12  as shown in  FIG. 1 , after it has been fired and the explosive charge receiving areas  16 B have been damaged. The debris  22  left behind is essentially blast shrapnel, which are pieces of the charge carrier  14 B, the explosive charges, and the explosive charge receiving areas  16 B that obstruct the production of oil and gas from the well.  
         [0008]     Prior art has proposed an apparatus used to trap this debris before it enters the well casing and hydrocarbon bearing formation, which is disclosed in Rouse et. al. PCT Application WO 2005/033472.  FIG. 3  is a depiction of the perforation gun described in Rouse &#39;472. It is composed of an outer gun barrel  210  with a coaxial interior hollow charge carrier  212 . The explosive charges  214  are inside the charge carrier  212 .  FIG. 4  shows the perforation gun of  FIG. 3  as it is being fired. When the explosive charges  214 B contained inside the charge carrier  212  are detonated, the explosions  226  create holes in both the interior charge carrier  228  and the outer gun barrel  230 . The perforation gun then theoretically traps the debris  22  from the detonation within the charge carrier  212  by moving the entire charge carrier  212  (including the portion of the gun that originally housed the explosive charges  232 ) along the axis it shares with the outer gun barrel  210  until the holes created by the charges  228  and  230  are no longer aligned. The Rouse &#39;472 application discloses that the movement of the interior charge carrier  212  can theoretically be actuated using explosives, a strained spring, or the force from the explosive charges  214 B. Such actuating force must be great enough to break the shear pin  216  and move the charge carrier  212  the distance X  222  (which distance must be large enough to allow movement sufficient to seal the holes created by the charges  228  and  230 ) until the charge carrier  212  impacts the endplate  220 . The Rouse &#39;472 application also teaches that the walls of both the outer gun barrel  210  and the charge carrier  212  are solid before the perforation charges  214  are detonated.  
         [0009]     A need exists for an improved and more comprehensive and more efficient design for a debris trapping perforation gun. A further need exists for an improved, more comprehensive and more efficient method of operation of the debris trapping perforation gun.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention is thus directed to an improved perforation gun, which traps debris created by the explosion inside the gun. One embodiment overcomes many of the disadvantages of the Rouse &#39;472 prior art by pre-drilling holes in the interior charge carrier. The holes in the charge carrier allow the explosive charges to easily pass through the charge carrier. This reduces or eliminates the damage done to the charge carrier by the explosive charges, which in turn allows the charge carrier to shift inside the gun with less resistance than the charge carrier in the Rouse &#39;472 device. This also prevents reduced shaped charge performance as would happen in the Rouse &#39;472 device.  
         [0011]     The present invention is also an improvement over the Rouse &#39;472 prior art through another embodiment, which seals the holes in the outer gun barrel using a movable inner sleeve. The inner sleeve has pre-drilled holes and shifts to close the holes created in the outer gun barrel by the explosive charges while holding the charge carrier portion of the gun in place. The shifting inner sleeve with pre-drilled holes also moves with less resistance and more success than the charge carrier in Rouse &#39;472.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein:  
         [0013]      FIG. 1  is a cross-sectional view of a perforation gun inside a well casing;  
         [0014]      FIG. 2  is a cross-sectional close-up view of a prior art conventional perforation gun right after it has been detonated inside a well casing;  
         [0015]      FIG. 3  is a cross-sectional view of the Rouse &#39;472 prior art perforation gun before firing;  
         [0016]      FIG. 4  is a cross-sectional view of the Rouse &#39;472 prior art perforation gun shown in  FIG. 3  as it is firing;  
         [0017]      FIG. 5  is a cross-sectional view of the Rouse &#39;472 prior art perforation gun shown in  FIG. 3  immediately after firing;  
         [0018]      FIG. 6  is a cross-sectional view of the Rouse &#39;472 prior art perforation gun shown in  FIG. 3  after the inner tube has shifted to trap the debris;  
         [0019]      FIG. 7  is a cross-sectional view of one embodiment of the debris trapping perforation gun of the present invention before it has been fired;  
         [0020]      FIG. 8  is a cross-sectional view of the embodiment of the debris trapping perforation gun shown in  FIG. 7  of the present invention after it has been fired and the charge carrier has shifted to trap the debris;  
         [0021]      FIG. 9  is a cross-sectional view of another embodiment of the debris trapping perforation gun of the present invention before it has been fired;  
         [0022]      FIG. 10  is a cross-sectional view of the embodiment of the debris trapping perforation gun shown in  FIG. 9  of the present invention after it has been fired and the inner sleeve has shifted to trap the debris; 
     
    
       [0023]     Where used in the various figures of the drawing, the same numerals designate the same or similar parts. Furthermore, when the terms “top,” “bottom,” “first,” “second,” “upper,” “lower,” “height,” “width,” “length,” “end,” “side,” “horizontal,” “vertical,” and similar terms are used herein, it should be understood that these terms have reference only to the structure shown in the drawing and are utilized only to facilitate describing the invention.  
         [0024]     All figures are drawn for ease of explanation of the basic teachings of the present invention only; the extensions of the figures with respect to number, position, relationship, and dimensions of the parts to form the preferred embodiment will be explained or will be within the skill of the art after the following teachings of the present invention have been read and understood. Further, the exact dimensions and dimensional proportions to conform to specific force, weight, strength, and similar requirements will likewise be within the skill of the art after the following teachings of the present invention have been read and understood.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The present invention involves an improved debris trapping perforation gun and the unique charge carrier or inner sleeve it incorporates. The invention produces superior debris trapping results because the pre-drilled holes in the charge carrier or inner sleeve, as appropriate, limits or eliminates deformations caused by the explosive charges which allows the charge carrier or inner sleeve to shift with more ease and success.  
         [0026]     Referring initially to  FIG. 1 , the reference numeral  14  refers in general to a perforation gun (of which the present invention is one type), which has been lowered into a well bore to the depth of a hydrocarbon bearing formation  12 .  
         [0027]     Even though FIG. I shows a vertical well, one skilled in the art knows that the perforation gun of the present invention is equally well-suited for use in wells having other geometries such as deviated wells, inclined wells, or horizontal wells. Accordingly, use of directional terms such as above, below, up, down, upper, and lower and the like are used with reference to the embodiments illustrated in the figures and should not be construed as limitations on the invention. Also, even though  FIG. 1  depicts an onshore operation, one skilled in the art will recognize that the present invention is equally well suited for use in offshore operations. In addition, although  FIG. 1  depicts a single perforation gun, the principles of the present invention are applicable to perforation operations which utilize a series of perforation guns inside the same well casing. Finally, the number of shaped charges contained in any figure should not be viewed as a limitation on the invention. One skilled in the art knows that the number of shaped charges used in the present invention will vary according to the requirements of the specific application.  
         [0028]     In the first preferred embodiment, referring to  FIG. 7 , a charge carrier  36  is contained inside an outer gun barrel  30 . The wall of the charge carrier  36  is geometrically similar to the wall of the gun barrel  30 , with the outside diameter of the charge carrier  36  being slightly smaller than the inside diameter of outer gun barrel  30 . The charge carrier  36  has a plurality of explosive charges  16 , with each explosive charge  16  being aligned with a hole  34  in the wall of the charge carrier  36 . Each hole  34  prevents any reduced performance of the adjacent explosive charge  16 . Each hole  34  in the wall of the charge carrier  36  is also aligned with the scalloped sections  32  of the outer gun barrel  30 . The scalloped sections  32  of the outer gun barrel  30  are sections of the outer gun barrel  30  wall that are thinner than other parts of the outer gun barrel  30  to allow the force from the explosive charge to pass through the outer gun barrel  30  more easily. The charge carrier  36  is held in place near or against the upper endplate  46  before firing by a stress failing connector, which is a connector designed to fail under a specific amount of stress (for example, a shear pin or pins  38 ) and allow the charge carrier  36  to shift axially along the axis it shares with the outer gun barrel  30 . The charge carrier is initially held in place by the shear pin  38  a distance “Y”  40  between the lower end of the charge carrier  36  and the lower endplate  44 . An optional propellant disk  42  can be placed between the charge carrier  36  and the upper endplate  46  to facilitate shifting of the charge carrier  36  after firing of the explosive charges.  
         [0029]     Referring now to  FIG. 8 , therein is depicted the first preferred embodiment of  FIG. 7  of the present invention after the explosive charges  16  have been fired exposing the explosive charge receiving areas  16 B and the charge carrier  36  has axially shifted. The force from the explosive charges have passed through the holes  34  in the charge carrier wall  36  and created holes in the scalloped sections  32 B of the outer gun barrel  30 . The shear pin  38 B has been broken by force exerted on it by the charge carrier  36 , said force being created either by the optional propellant disk  42  depicted in  FIG. 7 , or by ballistic pressure and shock created inside the carrier  36  by the firing of the explosive charges. The broken shear pin or pins  38 B allows the charge carrier  36  to move axially along the axis it shares with the outer gun barrel  30 . The distance the charge carrier moves is determined by the distance Y  40  depicted in  FIG. 7 . The distance Y  40  should be such that after the charge carrier  36  shifts, the holes  34  in the charge carrier wall  36  are not aligned with the holes in the scalloped sections  32 B of the outer gun barrel  30 , but not such that the holes  34  in the charge carrier  36  re-align with different holes in the scalloped sections  32 B of the outer gun barrel  30  after the charge carrier  36  shifts, thereby sealing off the interior of the perforation gun from its surroundings. The debris  22  created by the explosive charges is now trapped inside the charge carrier  36 .  
         [0030]     In the second preferred embodiment, referring to  FIG. 9 , a charge mount  68  with explosive charges  16  is fixed in position between the upper endplate  66  and the lower endplate  52  by means of a mounting plate  56 . The upper endplate  66  and the lower endplate  52  are held in place by alignment screws  64  and  62 , respectively. Unlike the charge carrier  36  of the first preferred embodiment depicted in  FIG. 7  and  FIG. 8 , the charge mount  68  of the second preferred embodiment does not shift axially after the explosive charges have been fired. Instead, located immediately inside the outer gun barrel  30  is an inner sleeve  70 , the wall of which is geometrically similar to the wall of the outer gun barrel  30 , and which fits closely inside the outer gun barrel  30  (preferably about ⅛ th  inch clearance between the outer wall of the inner sleeve  70  and the inner wall of the outer gun barrel  30 ). The outer gun barrel  30  has scalloped sections  32  (thin sections of the outer gun barrel  30  which allow the force from the explosive charge to pass through the outer gun barrel  30  more easily) which are initially aligned with the explosive charges  16  located on the charge mount  68 . The inner sleeve  70  contains holes  34  that are initially aligned with the explosive charges  16  and the scalloped sections  32  of the outer gun barrel  30 . The inner sleeve  70  is permanently affixed to a guideplate  50  by means known to those skilled in the art (for example, welding). The guideplate  50  and the lower endplate  52  are geometrically shaped such that the guideplate  50  is the male/female counterpart of the lower endplate  52 . The inner sleeve  70  and guideplate  50  are held in place initially by a stress failing connector (for example, a shear pin  38 ) which is anchored to the mounting plate  56 , and two O-rings  58  and  60 . The lower surface of the guideplate  50  is initially located a distance Y  40  from the corresponding upper surface of the lower endplate  52  leaving empty space  54  between the lower surface of the guideplate  50  and the upper surface of the lower endplate  52 .  
         [0031]     Referring now to  FIG. 10 , therein is depicted the second preferred embodiment of  
         [0032]      FIG. 9  of the present invention after explosive charges  16  have been fired and the inner sleeve  70  and guideplate  50  have axially shifted. The force from the explosive charges have passed through the holes  34  in the inner sleeve  70  and created holes in the scalloped sections  32 B of the outer gun barrel  30 . The shear pin  38 B has been broken by force exerted on it by the inner sleeve  70  and guideplate  50 , said force being created by the increased hydraulic pressure created in the well bore by the firing of the explosive charges. The air chamber that exists between the O-rings  58  and  60  allows the explosive pressure from the explosive charges and the hydrostatic pressure in the well bore to shift the guideplate  50  (which is connected to the inner sleeve  70 ). The broken shear pin or pins  38 B allow the inner sleeve  70  and guideplate  50  to move axially along the axis they share with the outer gun barrel  30 . The force required for the shear pin or pins  32  to support the carrier assembly until the explosive charges have been fired is selected by those skilled in the art. The distance the inner sleeve  70  and guideplate  50  moves is determined by the distance Y  40  depicted in  FIG. 9 . The distance Y  40  should be such that when the inner sleeve  70  and guideplate  50  shifts, the holes  34  in the inner sleeve  70  are no longer aligned with the holes in the scalloped sections  32 B of the outer gun barrel  30 , but not such that the holes  34  in the inner sleeve  70  re-align with different holes in the scalloped sections  32 B of the outer gun barrel  30  after the inner sleeve  70  and guideplate  50  shifts, thereby sealing off the interior of the perforation gun from its surroundings. The debris  22  created by the explosive charges is now trapped inside the inner sleeve  70 .  
         [0033]     It should be understood by one skilled in the art that in order for the present invention to be used in practice, explosive charges  16  must be placed in the explosive charge receiving areas  16 B before the perforation gun is placed into the well bore. Explosive charges used in the industry vary widely and it is understood by one skilled in the art that a plurality of different explosive charges is within the scope of the present invention.  
         [0034]     Even though the figures described above have depicted all of the explosive charge receiving areas as having uniform size, it is understood by those skilled in the art that, depending on the specific application, it may be desirable to have different sized explosive charges in the perforation gun. Also, even though the above described figures have depicted a uniform axial distance between each of the explosive charge receiving areas, it is understood by those skilled in the art that, depending on the specific application, it may be desirable to have varied axial spacing between the explosive charges.  
         [0035]     It is also understood by those skilled in the art that several variations can be made in the foregoing without departing from the scope of the invention. For example, the particular number and location of the explosive charges can be varied within the scope of the invention. Also, the particular techniques that can be used to fire the explosive charges within the scope of the invention are conventional in the industry and understood by those skilled in the art.  
         [0036]     It will now be evident to those skilled in the art that there has been described herein an improved perforation gun that reduces the amount of debris left in the well bore and perforations in the hydrocarbon bearing formation after the perforation gun is fired.  
         [0037]     Although the invention hereof has been described by way of preferred embodiments, it will be evident that other adaptations and modifications can be employed without departing from the spirit and scope thereof. The terms and expressions employed herein have been used as terms of description and not of limitation; and thus, there is no intent of excluding equivalents, but on the contrary it is intended to cover any and all equivalents that may be employed without departing from the spirit and scope of the invention

Summary:
The improved perforation gun of the present invention includes an outer gun barrel, which is used in conjunction with an inner movable charge carrier or an inner movable sleeve to trap virtually all of the debris created by the firing of the perforation gun. This elimination of debris reduces costly operational problems in both gravel pack and horizontal well completions. It also improves the production from a perforated underground hydrocarbon bearing formation since there is no debris to potentially cause plugging in the well or subsequent production lines.