Method and apparatus for perforating subsurface earth formations
BACKGROUND OF THE INVENTION 
     This invention relates generally to subsurface well apparatus and more specifically, to methods and apparatus for perforating subsurface earth formations utilizing a tubing conveyed perforating apparatus. 
     It has become common practice in the completion of oil and gas wells to perforate the well casings and the surrounding formations to bring a well into production. One method of providing this capability has a perforating apparatus attached to the end of a tubing string which is lowered and set in place when the perforating apparatus is opposite the formation to be produced. The perforating apparatus may then be detonated and the well placed into production through the tubing strings. 
     The systems for firing the perforating apparatus have typically been either an electrical firing system or a non-electric percussion firing system. Neither method has been entirely satisfactory in the past since electrical firing systems require care in connection and running because these systems can be activated from stray electrical currents. In addition, electrical connections can be short-circuited by moisture. Percussion firing systems commonly have some primary explosives in the perforating apparatus while it is affixed to the tubing and lowered into position. As a result of the deficiencies of these firing systems, accidental and premature firings are a possibility. Further, in the event of a malfunction, making removal of the perforating apparatus necessary, the chance of accidental ignition of the perforating apparatus could prove dangerous to personnel. 
     These and other disadvantages are overcome with the present invention by providing a method and an apparatus for perforating well casing and the surrounding earth formations using a primary percussion firing system which is installed in the perforating assembly only after the perforating apparatus has been set and additionally by providing for removal of the percussion firing system in case of malfunction and installation of another percussion firing system or a secondary electrical firing system which likewise can be removed in case of malfunction. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment of the invention, a perforating system is provided which, in its overall concept, includes a perforating gun and a firing head assembly which are coupled to a tubing string and positioned thereby within a well adjacent a formation to be perforated. A percussion firing system includes a grapple end portion and an outwardly flared end portion for detachable coupling to a setting tool. The setting tool and the attached percussion firing system are lowered through the tubing string, into the firing head assembly where the grapple portion latches onto a detonator stem. The setting tool and the wireline are removed leaving an armed firing system. To initiate the firing system an impact bar is dropped through the tubing string impacting a firing pin in the percussion firing system. The firing pin in turn impacts a plurality of explosive primer cartridges. The explosive force of the primer cartridges sets off a booster cartridge which detonates a shaped charge having an axis of perforation aligned substantially along the longitudinal axis of the firing head assembly. The jet from the shaped charge penetrates an explosively loaded screw port mounted in the detonator stem. The jet formed thereby detonates a loaded screw port located in a detonator stem. The detonation of the screw port is coupled by a booster charge, onto a length of detonator cord terminating at another booster charge in juxtoposition to a shaped charge. The jet from the shaped charge detonates a loaded screw port which transfers a detonation wave to a length of detonator cord which traverses the perforating gun thereby detonating the shaped charges therein to perforate the adjacent formations. 
     In the event ignition of the perforating gun is not caused by the impact bar hitting the firing pin an overshot grapple is lowered through the tubing string and engages the an upper flared portion of the percussion firing system. Upward tension detaches the percussion firing system which is removed from the well. A second percussion firing system can be lowered into place in the above-described manner. In an alternative, should it be desirable to electrically detonate the system, an electrical firing head is lowered through the tubing string into the firing head assembly. A grapple end portion of the electrical firing head latches onto an end flared portion of the detonator stem. An electrical signal detonates a shaped charge the jet which detonates the loaded screw port in the detonator stem thereby causing the detonation of the perforating gun in the above-described manner. Should once again detonation not occur the electrical firing head is removed leaving a perforating system having no primary explosive therein. Now unarmed, the perforating gun and the firing head assembly can be removed from the well by pulling the tubing string. 
     These and other features and advantages of the present invention will be more readily understood by those skilled in the art from a reading of the following detailed description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic elevational view of a perforating operation within a cased wellbore in accordance with the present invention. 
     FIGS. 2A and 2B are a longitudinal sectional view of the firing head assembly and a portion of the perforating gun of FIG. 1. 
     FIG. 3 is a more detailed view of the percussion firing assembly of the firing head assembly of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings in more detail, particularly to FIG. 1, there is illustrated a tubing conveyed perforating system in accordance with the present invention. A wellhead 10 has a tubing string 12 extending through the interior of a casing string 14. A suitable packer assembly 16, which can be of any number of commonly used forms, is attached to tubing string 12 and sealingly engages casing 14 dividing the casing annulus into a lower and an upper annulus 18 and 20, respectively. A fluid vent assembly 22, which typically is a perforated nipple or a tubing activated sub, is attached to tubing string 12 below packer assembly 16 and includes a plurality of ports 24 which in the open position fluidly communicate lower annulus 20 with the interior of tubing string 12. A firing head assembly 26 is attached to and underlies vent assembly 22. Mechanically coupled to firing head assembly 26 and disposed adjacent a potential hydrocarbon containing formation is perforating gun 28. Perforating gun 28 can be of any suitable type of the style commonly referred to as a &#34;shaped charge&#34; perforating gun. 
     Referring now to FIG. 2 there is illustrated partly in longitudinal section firing head assembly 26 attached to the upper end of perforating gun 28. Connector sub 30 is a generally cylindrical member having a screw threaded proximal end for attachment to vent assembly 22 or tubing string 12. The second end section of connector sub 30 forms a reduced diameter externally threaded section. Tubular firing head assembly housing 32 has a screw threaded socket concentric therewith for receiving the reduced diameter threaded end of connector sub 30. Seal members 34 and 36 provide a fluid occlusive seal isolating wellbore fluids from the interior of housing 32. 
     Mounted within connector sub 30 and extending into housing 32 is guide housing 38. Guide housing 38 is a generally cylindrical member having an enlarged outer diameter upper segment 40 engaging flange 42. The internal bore 44 of upper segment 40 is flared with the reduced diameter of the flare projecting into the internal passage 46 of the lower section of guide housing 38. Attached to guide housing 38, by suitable means such as screws 48 and 50, are centralizers 52 and 54, respectively. Centralizers 52 and 54 serve to keep guide housing 38 located substantially within the center of the internal bore 56 of firing head assembly housing 32. 
     Centralizers 52 and 54 additionally provide for the centering of the percussion firing assembly 58 within the internal passage 46 of guide housing 38 when such firing assembly 58 is installed within firing head assembly 26 in a manner to be more fully described later herein. Percussion firing assembly 58 includes firing pin housing 60 threadably coupled to firing sub 62 which is further threadably coupled to grapple sub 64 terminating with dogs 66 having beveled ends. Firing pin housing 60 threadably coupled to sub 62 which is further a generally cylindrical cavity 70 formed therein. Upper section 68 has outwardly projecting flange 72 which allows percussion firing assembly 58 to be detachably coupled to a delivery tool, such as a setting tool affixed to a gamma ray instrument. This allows percussion firing assembly 58 to be lowered from the surface and latched into firing head assembly 26 after firing head assembly 26 and perforating gun 28 are located within the well. 
     A better understanding of the firing system can be had by reference to FIGS. 2 and 3 where there is illustrated in detail the percussion firing system of percussion firing assembly 58. Identical reference numbers are used for identical elements in these figures. Firing pin 74 is retained within cavity 70 with one end extending outside firing pin housing 60. A plurality of percussion ignition pins 76 are attached to the other end of firing pin 74. In the preferred embodiment three elongated ignition pins having hemispherically tapered ends are utilized. Ignition pins 76 extend through passages within retainer ring 78. Frangible barrier 80 isolates ignition pins 76 from explosive primer cartridges 82 which are retained within three cavities 84 within cartridge retainer 86. Alignment of ignition pins 76 with primer cartridges 82 is provided by alignment pin 88 which inserts into bore 90 of retainer ring 78 and bore 92 of cartridge retainer 86. Additionally, seal member 94 fits within circular groove 96 in cartridge retainer 86. 
     Returning now to FIG. 2, sub 62 has a first elongated bore 100 located along the longitudinal axis with a diameter of approximately one-half inch in diameter located at the lower end thereof. Cavity 100 opens into a reduced diameter cavity 114, aligned on the longitudinal axis. Cavity 114 is sealed by frangible barrier 102. Located within bore 100 is a booster charge retainer 98 housing explosive booster charge 106 which may be, for example a DuPont booster. The booster charges are relatively insensitive and not readily detonated other than by a force of an explosive nature as provided by primer cartridges 82. Shaped charge 108 is installed within cavity 100 in juxtoposition with booster 106 and has an axis of perforation aligned substantially along the longitudinal axis of firing head assembly 26. 
     To assemble this portion of the percussion firing assembly 58 shaped charge 108 is inserted into bore 100 of sub 62. Booster charge retainer 98 and booster charge 106 are inserted into bore 100 in juxtoposition with shaped charge 108. Firing pin housing 60, containing cartridge retainer 86 and the other associated members of the percussion firing pin assembly, is threadably connected to sub 62. 
     Grapple sub 64 is threadably attached to the other end of sub 62. Grapple sub 62 includes a generally cylindrical section 120 having a plurality of elongated fingers 122 extending by means of screws 124 are dogs 66. In the preferred embodiment screws 124 are 4-40×0.19&#34; long stainless steel screws. Attached to grapple sub 64 is the upper end portion of explosive stem assembly. Explosive assembly includes an elongated tubular member 140 having a first end with cavity 142 formed therein and a second threaded end 144. Installed within cavity 142 is a loaded screw port 146. Loaded screw port 146, includes a quantity of explosive material 148 retained within a threaded member 150 commonly referred to as a screw port. Extending into cavity 142 is explosive booster charge 152, which in the preferred embodiment is a DuPont Model P-3, which is crimp-connected to a detonator cord 154 which traverses the longitudinal bore of tubular member 140. Detonator cord 154 is preferably, but not limited to, the type known commercially as R.D.X. plastic covered Primacord. The first end of elongated tubular member 140 includes a tapered outer section 156, a reduced outer diameter section 158 returning to an end portion 164 having an outer diameter equal to that of the main section of tubular member 140. 
     Threadably attached to second end 144 of tubular member 140 is bushing member 168 having a central bore therethrough traversed by detonator cord 154. Bushing member 168 is mounted within a central bore 170 of coupling sub 172 which is threadably coupled to firing head assembly housing 32. Occlusive seals are provided by seal members 174 between coupling sub 172 and firing head assembly housing 32, by seal members 176 between coupling sub 172 and bushing member 168 and by seal member 178 between coupling sub 172 and tubular member 140. 
     Mounted within coupling sub 172 is donor assembly 180 of the perforating gun detonating system. Donor assembly 180 includes an outer housing or bushing 182 sized for insertion within bore 170 of coupling sub 172 and has a central bore therethrough. A pair of retainer rings 184 and 186 constrain outer housing 182 within coupling sub 172 and a pair of seal members 188 provides an occlusive fluid seal therebetween. Retained within the central bore of bushing 182 and extending rearwardly therefrom is internal member 190 having a pair of seal members 192 thereabout. Mounted within an internal bore of internal member 190 is shaped charge 194. Shaped charge 194 may be of various designs known in the art, in the preferred embodiment is approximately one inch in length and one-half inch in outer diameter and having the type of explosive commonly referred to as cyclonite. Shaped charges 194 and 108 are of a common design. Screw port 196 is threadably installed within bushing 182 substantially in line with the axis of perforation of the &#34;jet&#34; produced by shaped charge 194. Retained within the rearward portion of internal member 190 and placed in juxtaposition with shaped charge 194 is booster charge 198 which is connected to detonator cord 154 and is preferably a model P-3 booster available from DuPont Corporation. 
     Coupling sub 172 is threadably coupled to perforating gun 28. The threaded joint is provided with a fluid-tight seal by seal members 200. Perforating gun 28 includes a carrier member 202 retained therein. Mounted along the length of carrier member 202 are a plurality of shaped charges, illustrated at 204 and 206 having their axis of perforation directed generally in the surrounding formations. Mounted within the central bore of perforating gun 28 is acceptor assembly 208 of the detonation system. Acceptor assembly 208 includes a housing or holder member 210 having a cavity formed generally centrally therein. Mounted within the cavity is a generally cup shaped pellet 212 of explosive material. Explosive pellet 212 can be from approximately 2-6 grams of cyclonite or other suitable explosive material. The cavity is covered with a frangible barrier 214, such as a relatively thin piece of aluminum. Explosive booster charge 216 is connected by suitable means, such as a crimped-connection, to detonator cord 218. Detonator cord 218 extends the length of the perforating gun 28 and provides the detonation of any shaped charges mounted therein. The second end of detonator cord 218 may be terminated at a donor assembly identical with the one described herein thereby allowing for the serial explosive coupling of additional perforating gun assemblies. 
     In the operation of the perforating system described in the Figures, perforating gun 28 and firing head assembly 26 are attached to tubing string 12 and portioned within the casing string 14 at a location below packer assembly 16. In accordance with the present invention no primary explosives are present in the perforating gun/firing head assembly during this operation. Firing sub 58 is lowered through tubing string 12 by means of a setting tool attached to a wireline (not shown). The setting tool couples over flange 72 of sub 60. By means of the wireline, the setting tool and firing sub are lowered through internal bore 44 of guide housing 38 until dogs 66 of grapple sub 64 engage the shoulder formed by the upper terminus of reduced diameter section 158 of stem housings 140. Once sub 60 is latched in place the setting tool and the wireline are removed from tubing string 12. To instigate ignition of perforating gun 28 an impact bar, commonly referred to as a &#34;go devil&#34; is dropped from the surface through tubing string 12. The impact bar passes through internal bores 44 and 46 of guide housing 38 hitting firing pin 74 driving ignition pins 76 through frangible barrier 80 onto primer cartridges 82. The explosion of primer cartridges 82 detonates booster charge 106 further detonating shaped charge 108. 
     Upon ignition, shaped charge 108 forms a &#34;jet&#34; which penetrates frangible barrier 102 and screw port 150 igniting explosive material 148 thereby igniting explosive booster charge 152. The detonation of booster charge 152 causes detonation wave to travel through detonator cord 154 to booster charge 198. Booster charge 198 transfers the detonation wave into shaped charge 194 causing a jet to be formed. The jet in turn detonates explosive pellet 212 thereby causing ignition of explosive booster charge 216. The detonation of booster charge 216 is transferred onto detonator cord 218 further detonating shaped charges 204, 206 and the other shaped charges in perforating gun 28, or subsequent perforating guns attached thereto. 
     In the event of the perforating gun shaped charges is not caused by the impact bar, the present system provides a back-up percussion and/or electrical ignition method. An overshot grapple is lowered into the tubing string by means of a slick line or wireline. The grapple is lowered into guide housing 38 until the grapple engages flange 72 of percussion firing assembly 58. Upward tension is exerted causing screws 124 to shear allowing dogs 66 to drop off fingers 122 and allowing percussion firing assembly 58 to be removed from firing head assembly housing 32. In the preferred embodiment screws 124 are chosen to shear in a range from between 1,200 to 1,600 pounds, most preferably 1,400 pounds. With percussion firing assembly 58 removed, another percussion firing assembly 58 or an electrical firing head can be attached to a wireline and lowered through tubing string 12 into firing head assembly 26. The substitute percussion firing assembly is detonated and/or is removable in the manner above-described. 
     The electrical firing head is equipped with a grapple sub identical to grapple sub 64. The electrical firing head is lowered into firing head assembly 32 until the dogs, which are identical to dogs 66 of grapple sub 64, clamp over end portion 164 onto the shoulder of elongated tubular member 140 of the explosive stem assembly. An electrical signal can then be transmitted from the surface to a detonator located in the electrical firing head, thereby igniting a shaped charge which forms a jet further igniting explosive material 148 and booster charge 152 which detonates the system in the manner hereinbefore described. Should the electrical firing head fail to detonate the perforating gun the electrical firing head is pulled off by tension from the wireline and is removed from the well. Perforating gun 28 and firing head assembly 26 can then be removed by pulling tubing string 12. Since no primary explosives are present the danger of accidental ignition during the removal process are all but eleminated. 
     Many modifications and variations besides those specifically mentioned may be made in the techniques and structures described herein and depicted in the accompanying drawing without departing substantially from the concept of the present invention. Accordingly, it should be clearly understood that the form of the invention described and illustrated herein is exemplary only, and is not intended as a limitation on the scope of the present invention.