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CROSS-REFERENCE TO RELATED APPLICATIONS 
     “This is a continuation of application Ser. No. 09/304,653 filed on May 4, 1999, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an apparatus and process for forming an opening through conduit located in a borehole formed in the earth. 
     2. Description of the Related Art 
     U.S. Pat. Nos. 4,298,063, 4,598,769, and 5,435,394 disclose apparatus for cutting conduit located in a borehole formed in the earth. U.S. Pat. Nos. 4,598,769 and 5,435,394 are incorporated into this application by reference. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a new and useful apparatus and process for cutting an opening in a conduit located in a borehole formed in the earth. 
     The apparatus of the invention comprises a body adapted to be lowered into the casing located in the borehole. The body comprises a surrounding wall defining an elongated chamber having a combustible charge receiving portion and an ignition system, or ignition means portion located close to the combustible charge receiving portion. A portion of the wall surrounding the combustible charge receiving portion has a plurality of spaced apart apertures formed therethrough in a given pattern. A combustible charge is located in the combustible charge receiving portion of the chamber. An ignition means is located in the ignition means portion of the chamber for igniting the combustible charge for creating a flame and hot combustion products for passage through the apertures for cutting an opening in the surrounding conduit. 
     The opening may be formed by burning an enlarged area through the conduit or by burning a slot through the conduit around a conduit wall portion which wall portion then is removed. 
     In the preferred embodiment, the combustible charges are located above, at the level of and below the apertures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1 is an exploded cross-sectional view of the apparatus of the invention. 
     FIG. 2 is a cross-sectional view of FIG. 1 taken along lines  2 — 2  thereof. 
     FIG. 3 is an isometric view of a portion of apparatus of FIG.  1 . 
     FIG. 4 is an isometric view of a portion of a borehole casing having a window cut through one side of its wall. 
     FIG. 5 is a schematic view of the apparatus of FIG. 1 in a cased borehole. 
     FIG. 6 is an isometric view of a portion of the apparatus of FIG. 1 showing nozzles apertures located in a given pattern. 
     FIG. 7 is an isometric view of a portion of a borehole casing showing a window cut through one side of its wall with the resulting metal plugs still in place. 
     FIGS. 8 and 9 illustrate a different process of forming a window in the casing. 
     FIG. 10 is a schematic view of a borehole tool for removing the loose plugs of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, the apparatus of the invention is identified at  421 . It comprises an anchor subassembly  16 , an ignition means subassembly  30  comprising members  32  and  34 , an upper combustible charge holding subassembly  431 , a nozzle and intermediate combustible charge holding subassembly  433  and a lower combustible charge holding subassembly  435 . Members  32 ,  34 ,  431 ,  433 , and  435  are formed of suitable metal. 
     The anchor  16 , and subassemblies  30 ,  431 ,  433 ,  435  are assembled as shown in FIG.  5  and lowered to a desired level into a borehole  601  that has been cased with metal casing  603  and operated to form an elongated window  605  or opening through the casing  603  as shown in FIG. 4 to allow access to the formation from the existing borehole for example to allow recovery of petroleum in the formations through the opening  605 . The length of the opening  605  may be generally parallel to the central axis of the casing  603 . Its length and width can vary. 
     The anchor subassembly  16  and the ignition subassembly  30  are similar to those disclosed in U.S. Pat. No. 4,598,769. The anchor subassembly  16  has a wireline cable  18  coupled to its upper end and has its lower end coupled to the ignition means subassembly  30 . The ignition subassembly comprise metal members  32  and  34  screwed together with an electrode plug  46  coupled to member  32 . The electrode  46  has a prong  48  which engages an electrical conductor  50  supported by the lower end of member  32 . A metal spring  52  is disposed between the conductor  50  and an electrically actuated ignition means or squib  44  which is located in a small aperture  383  extending through the lower end  34 E of member  34 . Members  37 A,  37 B, and  37 C are O-ring seals. The members  46 ,  48 ,  50  and  52  are electrically insulated to prevent a short. This ignition system may be defined as an electric line firing system. 
     Member  431  has annular wall  432  with an enlarged opening  435  at its upper end  436  with threads  437  leading to a smaller opening  439 . Members  444  are O-ring seals. The lower end  441  member  431  has exterior threads  443  end O-ring seals  445 . 
     The nozzle subassembly  433  comprises an annular wall  447  with a cylindrical opening  451  formed therethrough with interior threads  453  and  455  at its upper and lower ends  457  and  459 . The wall  447  comprises a nozzle section  471  having a smaller outside diameter than the ends  447  and  459 . A plurality of rows of apertures  473  extend through the wall section  471  on one side thereof as shown in FIGS. 3,  6 , and  8 . In FIG. 3 there is shown three spaced apart rows A, B, C of apertures  473  with each row comprising a plurality of spaced apart apertures  473  and with each row being generally parallel to the axis  475  of the member  433 . In FIG. 2, there is shown nine rows A-I of apertures  473 . The number of rows and the length of the rows of apertures depend on the width and length of the window  605  to be formed in the casing  603  of the borehole  601 . 
     Located on the inside of the wall section  471  is a hollow cylindrical shield  481  having apertures  483  formed therethrough which are aligned with the apertures  473 . A thin metal sleeve  485  is secured around the outer wall  447  to prevent water from entering the apertures  473  and  483 . Members  487  and  489  are O-ring seals. 
     The lower subassembly  435  comprises an annular wall  501  having an upper end  503  with O-ring seals  505  and exterior threads  507 . A cylindrical aperture  509  extends into the member  435  to a larger diameter opening  511  having interior threads  513 . A metal plug  515  with O-ring seals  517  and exterior threads  519  is inserted into the opening  511  and screwed into the lower end  521  of the member  435 . 
     Also provided are a plurality of combustible pyrotechnic charges  578  made of conventional material which is compressed into donut shaped pellets. Each of the charges has a cylindrical outer surface and a central aperture  578 A extending therethrough. The charges  578  are stacked on top of each other within the annular inside chamber portions  43   1 C,  433 C (inside of the carbon sleeve  481 ) and  435 C with their apertures  578 A in alignment. Loosely packed combustible material  580  preferably of the same material used in forming the charges  578  is disposed with the apertures  578 A of the charges  578  such that each charge  578  is ignited from the loosely packed combustible material upon ignition by the ignition means  44 . 
     In assembling the components  30 ,  431 ,  433 , and  435 , the threads  507  of end  501  of member  435  are screwed into threads  455  of the open end  459  of member  433 ; the threads  443  of end  441  of member  431  are screwed to the threads  453  of the open end  457  of member  433 . During the assembly process, the charges  478  are stacked into the chamber portions  435 C,  433 C, and  431 C of members  435 ,  433 , and  431 . The threads  34 T of end  34 E of assembled member  30  are screwed to the threads  437  of the open end  436  of the member  431 . During the assembly process the charges  578  are stacked on each other from the top end  51 ST of the plug  515  and the material  580  placed in their apertures  578 A. 
     The apparatus then is lowered into the borehole  601  and into the casing  603  by way of the cable  18  and uphole equipment illustrated at  611  in FIG.  5 . The equipment  611  includes a reel around which the cable  18  is wound and unwound to raise and lower the apparatus  421 . The cable  18  includes an electrically insulated electrical lead  615  which is coupled to the ignition means  44  by way of members  46 ,  48 ,  50  and  52  and an electrically insulated ground or return lead  617  coupled to the ignition means  44 . An electrical power source  621  and a switch  623  are provided for applying electrical power to the ignition means  44  when the switch  623  is closed. The ignition means  44  includes an electrical resistor which generates heat when electrical current is applied thereto. Thus when switch  623  is closed, current is applied to the resistor of the ignition means  44 , which generates enough heat to ignite the material  580  and hence the charges  578  to generate a very high temperature flame with other hot combustion products which pass through the heat shield apertures  483  and the nozzle apertures  473  and through the thin sleeve  485  to cut or burn a window or opening  605  through the casing  603 . After the window is formed, the apparatus  421  is removed from the borehole  601  and casing  603  by winding the cable  18  around the reel of uphole equipment  611 . 
     Instead of forming the window  605  by burning through the wall of casing  603  in all areas within the window  605 , the window  605  may be formed by forming the nozzle apertures  473  and  483  in a rectangular pattern to define the window as shown in FIGS. 8 and 9. In this embodiment, the nozzle apertures  473  and  483  will be located along two spaced apart vertical lines  673 A and  673 B and along two spaced apart horizontal lines  673 C and  673 D on one side of the axis  475 . If the length of the window  605  to be formed is long, the nozzle apertures also  473  and  483  may be located along spaced apart intermediate horizontal lines  673 E- 673 H as shown in FIG.  8 . Referring to FIGS. 8 and 9, upon firing of the charges, slits or grooves will be formed through the wall of the casing  603  along lines  603 A- 603 D surrounding a loose casing plug  603 P or a plurality of plugs  603 P if the nozzle apertures  673 E- 673 H are used which then is/are removed by removing the apparatus  421  from the borehole and lowering a tool down the borehole with a junk basket or an electro-magnet. Referring to FIG. 10, a tool  731  having an electro-magnet  733  coupled to the uphole electrical source  621  is lowered in the borehole  601  to the level of the window  605 . The switch  623  is closed to energize the electro-magnet  733  to attach and pull the plug  603 P or plugs  603 P from the window  605  and which is/are removed from the borehole by removing the tool  731 . 
     Instead of forming a rectangular window  605 , the nozzle apertures  473  and  483  may be arranged for example in a circle to form a circular window  605 . 
     As mentioned above, the window  605  may be formed through the wall of the casing  603  to provide access to the formation from the borehole  601  for the recovery of petroleum through the window  605 . 
     The apparatus of the invention also may be used to cut a window through metal production tubing, coiled metal tubing or metal drill pipe in a borehole. 
     The invention may be used as a window cutter for casing or casing liner where the initial cement job was poorly performed such that little or, no cement exists between the casing outside diameter and the formation wall. A poor cement job allows for migration of well fluids in the annular cavity behind the casing. The invention allows for the formulation of a large defined opening in the casing wall for transport of cement through the window and into the annular cavity filling the void. 
     The invention also may be used as a window cutter for drill pipe/drill collars to form a large window so that fluid circulation may be established above the drill bit and at or below a stuck point. The large window will allow the operator to pump a high volume of fluid through the window and up the annular side of the drill pipe flushing the annulus between the drill pipe and the formation wall. Once circulation is established, the drill pipe can be freed and removed from the well, thus eliminating an expensive fishing job. 
     In another embodiment, a slickline battery firing system may be employed in lieu of the electric line firing system to energize the ignition means  44 . This system comprises a slickline cable connection for supporting the modified apparatus  421  and which is connected to a pressure firing head. The pressure firing head comprises a metal piston having a larger diameter head with a smaller diameter metal rod extending downward from the bottom of the larger diameter head. The piston is slidably located in a hollow cylinder. A spring surrounding the rod is employed to provide upward pressure against the under side of the larger diameter head. The spring is adjustable to allow for hydrostatic compensation of well fluids so that the system does not fire at bottom hole pressure. When the piston is moved downward the lower end of the rod will make contact with an electrical lead from the battery pack and an electrical lead coupled to one side of the ignition means (the minus terminal of the battery pack and the other side of the ignition means  44  are grounded) to discharge current to the ignition means to ignite the material  580  and fire the combustible charges  578 . Fluid ports extend through the wall of the cylinder above the larger diameter piston head. When the borehole apparatus is in place in the borehole ready to cut the metal conduit to form an opening therethrough, a pump at the surface increases the fluid pressure in the conduit and moves the piston downward against the pressure of the spring to allow the rod to make electrical contact with the leads to fire the combustible charges  578 . 
     In still another embodiment, a slickline percussion firing system may be employed in lieu of the electric line firing system to ignite the charges  578 . This system comprises a slickline cable head connection connected for supporting the modified apparatus  421  and which is connected is to a pressure firing subassembly. The pressure firing subassembly comprises a cylinder having the piston and spring described in connection with the battery firing system. Ports are formed through the cylinder wall above the piston. Fluid pressure is increased to force the piston rod (firing pin) against a lower percussion firing cap which ignites upon impact to ignite the charges  578 . 
     Also a coiled tubing percussion firing system may be employed in lieu of the electric line firing system to ignite the charges  578 . This system comprises coiled tubing for supporting the modified apparatus  421  connected to a connector subassembly which connects to a pressure firing head which comprises a hollow cylinder which supports an interior piston by shear pins. The coiled tubing is coupled to the interior of the cylinder at its upper end. The piston has a central flow path extending axially downward from its upper end and then radially outward through the cylinder wall. A firing pin extends from the lower end of the piston. The flow path allows the coiled tubing to fill with water as the assembly is lowered in a downhole and also allows for circulation of fluid in running of the assembly. When the apparatus is at the desired cutting depth, a ball is dropped into the coiled tubing which passes to the piston, plugging the flow path allowing an increase in fluid pressure to be achieved in the coiled tubing and cylinder which shears the shear pins driving the firing pin into a percussion cap to ignite the charges  578 . 
     Preferably the charges  578  are located above the nozzle apertures  473 ; at the level of the nozzle apertures  473 ; and below the nozzle apertures  473  to provide a balanced force when the charges  578  are ignited. This arrangement also provides more heat and force for forming a wide and long window  605  in the casing  603 . 
     In the event that the window  605  to be formed is relatively narrow and short, the charges  578  may be located, above the apertures  473  only, above and at the level of the apertures  473  only, at the level of the apertures  473  only, at the level of and below the apertures  473  only, or below the apertures  473  only. For forming a narrow window, only one or two rows A or A plus B of apertures  473  and  483  may be formed through the wall  471  of the nozzle subassembly  433  and through the heat shield  481 . The charges  578  may be located above the apertures  473  only by forming an annular shoulder on the inside wall  432  of the member  431  at its lower end  411  and stacking the charges upward from the shoulder to the ignition means  44 . The charges  580  may be located above and at the level of the apertures  473  only by forming an annular shoulder on the inside of the wall  471  just below the apertures  473  and stacking the charges  578  upward from the shoulder to the ignition means  44 . The charges  578  may be located at the level of the apertures  473  only by forming an annular shoulder on the inside of the wall  471  below the apertures  473  and dispensing with the use of the member  431 ; stacking the charges  578  from the shoulder to the upper level of the apertures  473 ; and screwing the end  34 E of the member  34  into the upper open end of the member  433 . The charges  578  may be located at the level of and below the level of the apertures  473  only by dispensing with the use of the member  431 ; stacking the charges from the top  515 T of the plug  515  up to the upper level of the apertures  473 ; and screwing the end  34 E of the member  34  into the upper open end of member  433 . When the electric line firing system is used, the charges may be located below the apertures  473  only by dispensing with the use of the member  431 ; stacking the charges upward from the top  515 T of the plug  515  to the lower level of the apertures  473 ; and locating the ignition system  46 ,  48 ,  50 ,  42 ,  44  at the bottom of the chamber  435  and with the ignition means  44  facing upward next to the lower end of the charges  578 ; and coupling the anchor subassembly  16  to the upper end of member  433 . The leads  18 ( 11 ) and  18 (R) will extend through the apertures  578 A of the charges  578  to the ignition means  44 . 
     In one embodiment, the member  431 ,  433 , and  435  each may have an outside diameter of 3½ inches. The nozzle apertures  473  and  483  each may have a diameter of ¼ inches. For the embodiment of FIG. 2, for nine rows A-1, adjacent apertures  473  in each row may be spaced apart 0.400 of an inch and adjacent rows spaced apart 15° (400 of an inch). The length of the rows A-1 may be 8 feet. It is to be understood that these specifications may vary. The apertures  473  each may have a diameter of 0.200″. 
     For the embodiment of FIGS. 8 and 9, the two rows  673 A and  673 B of apertures  473  forming the long side of the rectangle may be spaced apart 8″.

Summary:
The apparatus is used for cutting an opening through the wall of a conduit located in a borehole traversing the subsurface formations. The apparatus includes a body adapted to be lowered into the conduit to a desired level. The body is formed by a cylindrical wall defining an elongated chamber having a combustible charge receiving portion and an ignition system portion located close to the combustible charge receiving portion. A plurality of spaced apart apertures are formed through the wall defining a given pattern which may be at least one elongated row of apertures generally parallel with the axis of the cylindrical wall or a line which encloses a given configuration on one side of the axis. A combustible charge is located in the combustible charge receiving portion of the chamber, and an ignition system is located in the ignition portion of the chamber for igniting the combustible charge for creating a flame and hot combustion products for passage through the apertures for cutting the wall of the conduit to form an opening through the wall of the conduit. In the preferred embodiment, the combustible charge is located in the chamber above, at the level and below the apertures.