Abstract:
A shaped charge for generating a large hole in material such as well casing downhole in a wellbore. A shaped charge liner is oriented about a longitudinal axis, and a disk is positioned at the liner apex. When an explosive material is initiated the liner collapses into a perforating jet. The disk alters the jet formation process and changes the shape and location of a bulge within the perforating jet. Consequently, the shape of the perforating jet retains a larger diameter for generating a larger hole in the material to be perforated or for controlling the penetration depth. The disk surfaces can be flat, concave, convex or other shapes, and the disk composition can be varied to accomplish different design criteria.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the field of lined explosive charges for perforating targets. More particularly, the present invention relates to a disk shaped component in a shaped charge liner for producing a material penetrating jet to produce a large target perforation downhole in a wellbore.  
           [0002]    The invention is particularly useful in the field of downhole well casing perforations. Well casing is typically installed in boreholes drilled into subsurface geologic formations. The well casing prevents uncontrolled migration of subsurface fluids between different well zones and provides a conduit for production tubing in the wellbore. The well casing also facilitates the running and installation of production tools in the wellborfe. Well tubing can be installed within well casing to convey fluids to the well surface.  
           [0003]    To produce reservoir fluids such as hydrocarbons from a subsurface geologic formation, the well casing is perforated by multiple high velocity jets from perforating gun shaped charges. A firing head in the perforating gun detonates a primary explosive and ignites a booster charge connected to a primer or detonator cord. The detonator cord transmits a detonation wave to each shaped charge.  
           [0004]    In a conventional shaped charge, booster charges within each shaped charge activate explosive material which collapse a shaped liner toward the center of a cavity formed by the shaped charge liner. The collapsing liner generates a centered high velocity jet for penetrating the well casing and the surrounding geologic formations. The jet properties depend on the charge case and liner shape, released energy, and the liner mass and composition. Shaped charge jets perforate the well casing and establish a flow path for the reservoir fluids from the subsurface geologic formation to the interior of the well casing. This flow path can also permit solid particles and chemicals to be pumped from the casing interior into the geologic formation during gravel packing operations.  
           [0005]    Various efforts have been made to modify the performance of shaped charges. Barriers and voids have been placed within the explosive material to modify the detonation wave shape collapsing the liner. Examples of detonation wave shaping techniques are described in U.S. Pat. No. 4,594,947 to Aubry et al. (1986), U.S. Pat. No. 4,729,318 to Marsh (1988), and U.S. Pat. No. 5,322,020 to Bernard et al. (1984). In each of these patents, detonation wave shapers are positioned in the explosive material between the detonator cord and the liner. In U.S. Pat. No. 5,753,850 to Chawla et al. (1998), a spoiler was positioned within the liner cavity to modify the perforating jet shape.  
           [0006]    Other efforts have been made to modify perforating jet performance by changing the liner shape. In U.S. Pat. No. 3,268,016 to Bell (1964), a disk-like appendage in a liner was provided to peen the rough perforation burr after the leading perforating jet portion penetrated through the target. The disk-like appendage was configured to form a slug portion with a diameter larger than the perforating jet entry hole diameter. In U.S. Pat. No. 5,559,304 to Schweiger et al. (1996), a liner having a flattened outer surface for the purpose of stretching and flattening the perforating jet shape. The flattened central region of the liner apex reduced the thickness of the liner between 10-15 percent. The velocity of the perforating jet was reduced to enhance stable flight and end-ballistic performance. In U.S. Pat. No. 4,702,171 to Tal et al. (1987), the liner apex was hollowed, and in U.S. Pat. No. 3,137,233 to Lipinski (1962), a conical liner represented a squared liner apex in one view for the purpose of facilitating the liner manufacture.  
           [0007]    One technique for generating a large diameter perforation uses a mandrel to shape the perforating jet shape. In U.S. Pat. No. 4,841,864 to Grace (1989), a mandrel was placed along the liner longitudinal axis to control the perforating jet shape. In U.S. Pat. No. 5,155,297 to Lindstadt et al. (1992), a solid weight member was centrally positioned in the liner to stabilize the deformation of the perforating jet. The weight member extended into the explosive charge and through the liner material.  
           [0008]    Another technique for generating a larger perforating hole incorporates a liner having a hemispherical portion attached to a conical skirt. Because the hemispherical portion has a discontinuity in the liner slope, a negative velocity gradient creates a bulge in the material perforating jet which leads to a larger hole in the target material. Although a larger hole is created, the size of the hole is limited by the configuration of the composite liner surfaces.  
           [0009]    In certain well completion activities such as gravel packing operations, large diameter well perforations are desirable to facilitate the rapid placement of solid particles into the well. To accomplish this objective, a perforating gun should remove a large target surface area from the casing before the energy of the perforating jet is expended. Conventional shaped charge techniques are limited in their ability to generate large casing holes without significantly increasing the shaped charge size. Accordingly, a need exists for an apparatus that can efficiently create large diameter perforations or minimum penetration in well casing and other selected targets.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides an apparatus actuatable by a detonator to perforate a material. The apparatus comprises an explosive material which can be initiated by the detonator to create a detonation wave, a shaped liner proximate to said explosive material and having a first end facing the detonator and having a second end formed about a longitudinal axis through a hollow space, wherein said shaped liner is collapsible about said hollow space when impacted by said detonation wave to form a material penetrating jet, and a disk proximate to said liner first end and deformable by said detonation wave to modify the material penetrating jet by resisting axial movement of said collapsing liner toward said liner longitudinal axis.  
           [0011]    In other embodiments of the invention, the explosive material can be positioned within a housing recess, the disk can be attached to the liner, and the disk can be formed with different materials in different configurations. The disk surfaces can be flat, concave, convex, or other shapes, and the disk can be integrated into the liner. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 illustrates a liner and disk proximate to the explosive material in a charge case.  
         [0013]    [0013]FIG. 2 illustrates a disk integrated within a shaped charge liner.  
         [0014]    [0014]FIG. 3 illustrates a disk having a greater thickness than the liner.  
         [0015]    [0015]FIG. 4 illustrates a disk having less thickness than the liner.  
         [0016]    FIGS.  5 - 9  illustrate different configurations for disks having flat, concave, or convex surfaces.  
         [0017]    [0017]FIG. 10 illustrates a multiple material disk having axially positioned layers.  
         [0018]    [0018]FIG. 11 illustrates a multiple material disk having radially positioned layers.  
         [0019]    [0019]FIG. 12 illustrates a disk having an aperture through the disk middle section. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    As previously described, conventional shaped charges initiate an explosive material to collapse a liner material about a cavity defined by the liner. The collapsing liner material moves axially inwardly toward the longitudinal axis and simultaneously moves outwardly in the direction of the detonation wave to generate a high velocity, perforating jet. Energy from the detonation wave is transferred to the individual particles of the collapsing liner material. The penetration hole diameter of the conventional perforating jet depends on the target composition, the perforating jet diameter, and the energy dissipated radially as the perforating jet penetrates the target material.  
         [0021]    The present invention significantly improves conventional large hole penetration capability by creating a substantially larger hole in a target. The invention accomplishes this function by resisting collapse of the liner toward the longitudinal axis, and by maintaining a perforating jet diameter greater than conventional jets.  
         [0022]    Referring to FIG. 1, charge case or housing  10  defines a recessed cavity  12  having open end  14 , housing wall  16 , and closed end  18 . If the cavity  12  of housing  10  has a parabolic or elliptical shape, wall  16  and closed end  18  are collectively defined by a continuous shaped surface. Liner  20  forms a geometric figure having liner apex  22  and liner base  24  formed about longitudinal axis  26 . Liner  20  can be symmetrical about longitudinal axis  26 , or can be offset. Liner  20  is positioned within cavity  12  so that liner apex  22  faces housing closed end  18 . Liner base end  24  faces toward open end  14 . Liner  20  defines an interior volume or hollow space  28  between liner base  24  and liner apex  22 .  
         [0023]    High explosive material  29  is positioned between housing wall  16  and liner  20 . Detonator  30  comprises a primer or detonator cord suitable for igniting high explosive material  29  to generate a detonation wave. Such detonation wave focuses liner  20  to collapse toward longitudinal axis  26  and to form a material perforating jet. As collapsing liner moves  20  towards open end  14  in the same direction as the detonation wave travel, the perforating jet also moves in such direction consistent with the laws of mass momentum and energy conservation. The perforating jet exits housing  10  at high velocity and is directed toward the selected target. Although liner  20  is preferably metallic, liner  20  can be formed with any material suitable for forming a high velocity perforating jet.  
         [0024]    Disk  32  is shown in FIG. 1 as a thin, flat circular plate. Disk  32  is located proximate to liner  20  near liner apex  22  and has disk edge  34  and disk surfaces  36  and  38 . Disk edge  34  can be circular, oval, rectilinear, or irregular in shape. Disk  32  is positioned within aperture  40  through liner apex  22 . As shown in FIG. 1, disk surfaces  36  and  38  are substantially flat and are substantially perpendicular to longitudinal axis  26 . In other embodiments of the invention, disk edge  34  can have an oval, irregular, or other shape, and disk surfaces  36  and  38  can be concave, convex, irregular, or another shape.  
         [0025]    The mechanism of the perforating jet resulting from disk  32  generally performs as follows. Disk  32  is accelerated by the detonation wave along longitudinal axis  26 . Because of the curvature of liner  20 , each element of liner  20  is accelerated toward longitudinal axis  26  and forward in a direction parallel to longitudinal axis  26 . By being pushed toward longitudinal axis  26  the elements of liner  20  will create a fast moving perforating jet followed by a slug component.  
         [0026]    The resulting jet creates a larger hole in the target than conventional jets formed in the absence of a disk. Disk  32  interrupts the normal formation of the perforating jet by interrupting or resisting the inner collapse of liner  20  toward longitudinal axis  26 . This change in collapse flow significantly alters the conditions forming the perforating jet component and the slug component. The mass and velocity of the perforating jet do not change materially by altering the final position of the collapse process, but the resulting to perforating jet diameter is increased because the jet flow is formed away from longitudinal axis  26  as the residue from disk  32  is accelerated along longitudinal axis  26 . The jet hole size, penetration, and other factors can be controlled by the size, mass, thickness, composition, orientation, and other characteristics of disk  32 .  
         [0027]    [0027]FIG. 2 illustrates another embodiment of the invention wherein disk  40  is integrated into liner  42 . Liner  42  is formed with hemispherical section  44  and conical section  46 . The discontinuity in the slope between hemispherical section  44  and conical section  46  creates a bulge in the resulting perforating jet, and this bulge is enhanced by the operation of disk  40  in response to a detonation wave. By having a discontinuity in the second (or higher) derivative of the liner  42  contour, a negative velocity gradient is generated to form the perforating jet bulge. Disk  40  interferes with the perforating jet to increase the size of the hole generated by the resulting perforating jet. The bulge formation can be controlled to modify the shape and location of the bulge relative to the other portions of the perforating jet.  
         [0028]    [0028]FIG. 3 illustrates another embodiment of the invention wherein disk  48  has a thickness t D  greater than the thickness t L  of liner  50 . As illustrated, surfaces  52  and  54  of disk  48  are offset from liner  50  with dimensions “a” and “b”, so that t D =t L +a+b. In different embodiments of the invention, surfaces  52  or  54  can be flush with the respective surfaces of liner  50 , or can be disposed in other positions relative to the respective surfaces along longitudinal axis  26 . The position of liner  50  along longitudinal axis  26  can be adjusted to time the movement of disk  48  relative to the collapse of liner  50  following initiation of explosive material  29 . By moving the initial position of disk  48  along longitudinal axis  26  toward the direction of the perforating jet, the impact of moving disk  48  on the perforating jet can be slowed. In another embodiment of the invention as shown in FIG. 4, the thickness of disk  56  can be less than that of liner  50 .  
         [0029]    FIGS.  5 - 9  illustrate other embodiment of a disk suitable to use in cooperation with a shaped charge liner. In FIG. 5, disk  52  has concave surface  54  and flat surface  56 . In FIG. 6, disk  58  has concave surface  60  and concave surface  62 . In FIG. 7, disk  64  has concave surface  66  and convex surface  68 . In FIG. 8, disk  70  has convex surface  72  and flat surface  74 . In FIG. 9, disk  76  has convex surface  78  and convex surface  80 .  
         [0030]    Disks such as disk  32  can be made with materials such as copper, from other metallic materials, from non-metallic materials, from solids or from pressed powders, or other components or combinations of components. The density of disk  32  can be greater or less than the liner density. The type of material forming disk  32  will affect the thickness and diameter of the optimal shape of the disk  32  and the desired location of disk  32  relative to the liner. Various combinations of materials are useful to accomplish different functions. FIG. 10 illustrates disk  82  having axially positioned layers  84  and  86 , and FIG. 11 illustrates disk  88  having radially positioned layers  90  and  92 . Other configurations and orientations of two or more materials are possible. Longitudinal axis  26  can bisect disk  32  or can be placed offset from the center of disk  32 . As shown in FIG. 12, disk  90  can have aperture  92  through the interior of disk  90  to modify the shape and location of the perforating jet bulge.  
         [0031]    Although the invention has been described in terms of certain preferred embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.