Patent Publication Number: US-2013228066-A1

Title: Resistor-based Ignition System for a Core Gun

Description:
FIELD OF THE INVENTION 
     The field of the invention is igniter systems for explosively operated subterranean tools and more particularly core guns. 
     BACKGROUND OF THE INVENTION 
     As shown in  FIG. 1   a , core gun cartridges have been manufactured for many years using Nichrome wire  10  coated with TK1 ignition mix  12  to ignite the main load of the propellant  14 . However there are several negatives and safety issues with the current system. TK1 mix  12  is hazardous to handle when dry and is sensitive to electrostatic discharge, friction and impact. The TK1 mix  12  is a fine titanium power which could be hazardous to handle in certain stages of manufacturing process. The manufacturing process of TK1  12  is relatively a ill defined process and is left up to operator judgment to ensure a consistent product. Solder process sometimes fails do to cold solder joints. 
       FIG. 1   a  shows the core barrel  16  tethered with a wire rope  18 , a portion of which remains attached to the barrel  16  for retrieval after the barrel  16  is propelled into the formation  20 . The gun body  22  may also contain additional barrels such as  24  that can be independently fired. 
     The ignition mix  12  has in the past caused small fires and presented a personnel safety hazard when assembled into a cartridge  26 . Despite such risks core guns have been assembled with this dangerous ignition mix for many years. 
       FIG. 2  represents a cased hole chemical cutter where an igniter  30  comprises a 30 ohm resister that when fed current gets sufficiently hot to ignite the propellant  32  which in turn sets the slips  34  against the casing to be cut below  36 . The pressure generated by the propellant  32  also drives out the chemical  38  from a cylinder  40  by breaking the rupture disc  42  and forcing the chemical  38  out nozzles  44  to create the cut  46 . This tool has been used with the above described ignition system for many years. 
     Yet despite the existence of the chemical cutter of  FIG. 2  the core guns that have been in use for years have featured the use of the ignition mix  12  and its associated hazards. 
     The present invention removes the ignition mix and replaces the nichrome wire  10  with the ignition mix  12  combination with a precision resistor preferably under 50 ohms in the core gun to remove the stated disadvantages in the context of a core gun where the core is directly driven by the ignited propellant into the surrounding formation. Those skilled in the art will further appreciate these and other aspects of the invention from a review of the detailed description of the preferred embodiment and the associated drawing while appreciating that the full scope of the invention is to be found in the appended claims. 
     SUMMARY OF THE INVENTION 
     A core gun igniter assembly features a resistor under 50 ohms that is welded rather than soldered to a circuit. Upon energizing the circuit, the resistor creates the heat needed to directly set off the propellant to directly drive the core barrel into the surrounding formation to obtain the desired core sample. The use of a titanium powder ignition mix on a nichrome wire with its attendant safety risks is eliminated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  and  1   b  represent a prior art core gun in the run in and shot positions respectively where the gun is set off with an ignition mix coated nichrome wire; 
         FIG. 2  is a chemical cutter used to cut and remove casing that is triggered with a resistor that gets hot to ignite the propellant to then push out a chemical through nozzles to cut the tubular. 
         FIGS. 3   a  and  3   b  are the core gun of the present invention shown in the run in and the fired conditions. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3   a  shows a resistor  48  that when energized creates heat to burn the propellant  50  in the cartridge  52 . The resistor is preferably under 50 ohms and is secured at opposed ends with welds  54  and  56 . The barrel  58  is secured by the tether  60  that operates in the same manner as in  FIG. 1   a . Additional barrels such as  62  can be used in the core gun body  64 . The core sample of the formation  66  is captured in the passage  64 . 
     Those skilled in the art will appreciate that the elimination of the nichrome wire coated with the ignition mix that had been in persistent use in the past for years for core guns posed safety issues that caused fires and threatened personal injury for the assembly workers. The soldering process for attaching the wire also created reliability issues with the possibility that the barrel would not be propelled into the formation. 
     Now the core gun with a welded resistor of preferably under 50 ohms to set off the propellant to launch the barrel, the assembly safety issues are eliminated as are the reliability issues with the soldering using the welded connections. The preferred resistance is under 25 ohms. The design meets API safety standard API RP 67 IME SLP-20. 
     The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below: