Patent Publication Number: US-6702009-B1

Title: Select-fire pressure relief subassembly for a chemical cutter

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to a pressure relief subassembly for a chemical cutting tool used in oil and gas wells, and in particular to a selectively fired, pressure relief subassembly for a downhole chemical cutting tool. 
     BACKGROUND OF THE INVENTION 
     Downhole, chemical cutting tools, often called chemical cutters, have been used to sever, or cut into two separate sections, steel tubular members within oil and gas wells. Typically, a chemical cutter is used when a lower section of a steel pipe string, such as a tubing string, a casing string, or a drill string, is stuck within a well, and it is desired to sever the lower section of the pipe string to allow retrieval of an upper portion of the pipe string from the well. A chemical cutter may be lowered within the stuck pipe string on a wireline to a position adjacent to the portion of the pipe string which is to be severed. Then, a flammable solid is ignited within the chemical cutter to force a liquid cutting chemical to flow over a chemical activator, and then outward of the chemical cutter through flow ports. The flow ports are arrayed for directing the activated cutting chemical to discharge in a pattern which extends circumferentially around the chemical cutter and into an annular-shaped section of the pipe string surrounding the chemical cutter. The cutting chemical and the activator are selected to provide high temperatures and pressures, such that the cutting chemical will cut through the adjacent section of the steel pipe string, severing the section into two halves. Activation of the cutting chemical and downhole well pressures expose the interior of the chemical cutter to high pressures, which should be released from being contained within the chemical cutter prior to the cutter being removed from within a well. 
     Prior art chemical cutting tools have interior chambers connected by flow passages within which high pressures may become trapped, causing safety concerns if high pressure fluids are unexpectedly released on the surface after chemical cutters are retrieved from within wells. Pressure bleed-off ports have been provided which are manually operated at the surface after chemical cutters are retrieved from wells, such as by providing a threaded plug which blocks a bleed-off port when the tool is downhole, and which is manually removed from blocking the bleed-off port after a chemical cutter is removed from a well. Bleed-off ports are often of a small diameter, and may become sealed by debris from the well. Unexpected discharges of trapped pressures and chemical cutting fluid at the surface after retrieval from wells have caused injuries to persons and damage to equipment. 
     SUMMARY OF THE INVENTION 
     A chemical cutter is provided having a pressure relief feature, such that after the chemical cutter is operated for dispensing a cutting chemical in a well to severe a tubular member, the pressure within the chemical cutter is equalized with the pressure which is exterior of the chemical cutter. An interior passage extends through a central portion of a tool housing of the chemical cutter. A propellant disposed in the interior passage, and is ignited for creating pressure to push a cutting chemical from within the tool housing and into the well. A first ignitor passage extends parallel to a longitudinal axis of the tool housing, and in fluid communication with the interior passage. A first ignitor is disposed in the first ignitor passage, such that ignition of the first ignitor ignites the propellant. A second ignitor passage extends in the tool housing, and has a first portion which extends parallel to the longitudinal axis, spaced apart from the first ignitor passage. An interior opening is provided in an end of the first portion of the ignitor passage which is adjacent to the interior passage. The second ignitor passage also has a second portion which extends transverse to the longitudinal axis of the tool housing, from an exterior of the tool housing to the first portion of the second ignitor passage. An exterior opening is provided in the outward end of the second portion of the second ignitor passage. A second ignitor is disposed in the second ignitor passage. 
     A first seal member is disposed in the interior opening, sealing the second ignitor passage from the interior passage of the tool housing. A second seal member disposed in the exterior opening, sealing the second ignitor passage from the exterior of the tool housing. The first and second seal members seal the second ignitor from the interior passage and from the exterior of the tool housing after the first ignitor is ignited and the propellant is combusted to dispense the cutting chemical from the cutting tool. Igniting the second ignitor pushes the first seal member from the interior opening and the second seal member from the exterior opening, such that the second ignitor passage is in fluid communication with the interior passage and the exterior of the tool housing. A control circuit is provided having two diodes connected in parallel, a first diode is configured for passing current of a first polarity to the first ignitor and a second diode is configured for passing current of a second polarity to the second ignitor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying Drawings in which: 
     FIG. 1 is a partial, longitudinal section view of a downhole tool string having a chemical cutter which includes a selectively fired pressure relief subassembly; 
     FIG. 2 is a longitudinal section view of a control section of the chemical cutter; 
     FIG. 3 is a longitudinal section view of an ignitor section of the chemical cutter; 
     FIG. 4 is a side view of an upper seal member for use in the ignitor section to provide pressure relief for the chemical cutter; 
     FIG. 5 is a side view of a lower seal member for use in the ignitor section to provide pressure relief for the chemical cutter; 
     FIG. 6 is a is a schematic diagram of electrical components used in the control section of the chemical cutter; 
     FIG. 7 is a partial, longitudinal section view of a strainer section of the chemical cutter; 
     FIG. 8 is a partial, side view of an anchor section of the chemical cutter; and 
     FIG. 9 is a partial cutaway and exploded view of the anchor section of the chemical cutter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a partial, longitudinal section view of a downhole tool string  10  having a chemical cutter  12  which includes a selectively fired, pressure relief and ignitor subassembly  22 . The chemical cutter  12  is used in oil and gas wells for lowering into a tubular member  14 , such as a portion of a drill string, tubing or casing, to sever the tubular member  14  into two sections. The tool string  10  and the chemical cutter  12  have a generally cylindrical shape with a centrally disposed, longitudinal axis  16 . The tool string  10  includes a firing head  18 , which is mounted to the end of a wireline  20 . The chemical cutter  12  is preferably secured to the wireline  20  by the firing head  18 , and is preferably lowered into a well on the wireline  20 . 
     The subassembly  22  includes a control section  24  and an ignitor section  26 , with a pressure relief feature. The control section  24  is threadingly secured to the firing head  18 . An upper end of the ignitor section  26  is secured to the control section  24 . A propellant section  28  is secured to the lower end of the ignitor section  26 . The propellant section  28  has a centrally disposed pressure chamber  32  defined within the tool housing  30 , coaxial with the longitudinal axis  16  of the tool string  10 . A propellant  34 , preferably ammonium perchlorate, is disposed within the pressure chamber  32 . A strainer section  36  is disposed at the lower end of the pressure chamber  32  for trapping debris from ignition of the propellant  34 . A pressure relief subassembly  38  is connected to the lower end of the strainer section  36 , and connected to the upper end of an anchor section  40 . A flow control section  42  is connected beneath the anchor section  40  and to the upper end of a chemical section  44 . A cutting chemical  46  is disposed within the chemical section  44  when the chemical cutter  12  is ready for use. The cutting chemical  46  is preferably provided by bromine trifluoride (BrF3). The lower end of the chemical section  44  is connected to a combustion section  48  which houses a catalyst  50 . The catalyst  50  is preferably provided by a steel wool, over which the cutting chemical  46  is passed to activate the cutting chemical  46 . A cutting head  52  is connected to the lower end of the combustion section  48 , and has flow ports  54 . The flow ports  54  have central axes which extend radially outward relative to the longitudinal axis  16  in an angularly spaced apart pattern around the longitudinal axis  16  for dispensing the cutting chemical  46  in a desired phasing pattern for cutting the tubular member  14 . A piston  56  is disposed in the cutting head  52  for sealing the flow ports  54  until the cutting tool  12  is fired. A bull plug  58  is mounted to the lower end of the cutting head  52 , and has centralizer arms  60  for centering the cutting tool  12  within the tubular member  14 . 
     FIG. 2 is a longitudinal section view of the control section  24  of the chemical cutter  12 , which includes an exploded view of a contact rod  76  and wiring harness  86 . The control section  24  has a cylindrical housing  62  which has an open, upper chamber  64 , an aperture  66  and a lower, open chamber  68 , which are coaxially disposed with the longitudinal axis  16 . The aperture  66  connects the upper chamber  64  to the lower chamber  68 . The upper chamber  64  has an open upper end for receiving the lower end of the firing head  18 , an upper portion which provides a seal surface  70 , and an intermediate threaded portion  72  for securing to the threaded end of the firing head  18  (shown in FIG.  1 ). A lower portion of the chamber  64  receives the upward end of the contact rod  76 . 
     The contact rod  76  is secured in the aperture  66 , as shown in FIG.  1 . The contact rod  76  has an insulating sleeve provided by a teflon tube, which insulates the contact rod  76  from making electrical contact with the housing  62 . The upper, end face of the contact rod  76  has a socket  78  which preferably extends coaxial with the longitudinal axis  16 , for receiving a contact spring  74  of the firing head  18  (shown in FIG.  1 ), to electrically connect the contact rod  76  to the firing head  18 . The exterior of the upper end of the contact rod  76  is threaded for securing to a nut  80  for retaining the contact rod  76  in the aperture  66 . The lower end portion of the contact rod  76  has an enlarged portion to define an annular-shaped shoulder  82  for retaining the contact rod  76  within the aperture  66 . The lower end face of the contact rod  76  has a threaded hole  83  for receiving a threaded fastener  84  to secure the wiring a harness  86  to the contact rod  76 , in electrical contact with the contact rod  76 . An upper end of the lower chamber  68  defines a cavity  88  within which the wiring harness  86  extends, with ample room to prevent binding or crimping of the wiring harness  86  when the control section  24  is threadingly secured to the ignitor section  26  (shown in FIG.  1 ). A lower portion of the chamber  68  has a threaded portion  90  for securing to the ignitor section  26  (shown in FIG.  1 ), and a seal surface  92 . 
     The wiring harness  86  includes a first conductor wire  104  having a first diode  106  and a second conductor wire  108  having a second diode  110 . The conductors  104  and  108  are connected together at upper ends to a contact  112 , which is secured in electrical contact with the contact rod  72  by the threaded fastener  84 . The conductor wires  104  and  108  extend from the contact  112  to provide two parallel circuits, with the two diodes  106  and  110  configured in each circuit for passing current of opposite polarity, respectively. The diode  106  is connected for passing current from the contact rod  72  of positive polarity, and the diode  110  is configured for passing current from the contact rod  72  of negative polarity. Two contacts  116  are mounted to the terminal ends of the wires  104  and  108 , respectively. Two rubber seal boots  114  are mounted on respective ones of the wires  104  and  108 , for: sealingly securing to upper contacts  238  of sealed contact connectors  196  which are included in the ignitor section  26  (shown in FIG.  3 ). The two conductors  104  and  108  are of sufficient length such that the terminal ends thereof will extend outward of the housing  64  for connecting to the upper contacts of the ignitor section  26 . 
     FIG. 3 is a longitudinal section view of the ignitor section  26  of the chemical cutter  12 , and includes an exploded view of the electric components of the ignitor section  26 . The ignitor section  26  includes a housing  122  of generally cylindrical shape, having two flow passages  124  and  126  which are provided by bores that extend in parallel, longitudinally through the housing  122 . The flow passage  124  provides a first ignitor flow passage. The exterior of the housing  122  has a threaded upper end  128  and a threaded lower end  130 . Seal sections  132  and  134  are provided on respective ends  128  and  130 , having seal glands  136  and  138 , respectively. An annular shaped recess  142  is provided in an intermediate portion of the exterior of the housing  122 . A port  144  is formed into the annular shaped recess  142 , and extends from the exterior of the housing  122  directly into the flow passage  126  to connect the flow passage  126  to the exterior of the housing  122 , at the annular shaped recess  142 . The port  144  preferably, has a three-eighths inch diameter. The port  144  and the flow passage  126  together provide a second ignitor flow passage which extends from the interior passage defined by the pressure chamber  32 , to the exterior of the tool housing  30  in the annular shaped recess  142 . The port  144  preferably has a longitudinal axis  146  which is disposed transverse to the longitudinal axis  16 . The two flow passages  124  and  126  preferably have longitudinal axes  148  and  150 , respectively, which extend parallel to the longitudinal axis  16 . The longitudinal axis  146  of the port  144  preferably extends perpendicular to the longitudinal axis  16  and the longitudinal axes  148  and  150 . 
     The outward end of the port  144  has an enlarged portion  152 , which preferably defines an exterior opening for the flow passage  126 . A lower end of the flow passage  126  has an enlarged portion  172  which preferably defines an interior opening for the flow passage  126 . The enlarged portion  152  which has a seal surface  154  for sealingly with a seal element  160  of a seal member  156 , and has an annular-shaped shoulder  158  to provide a stop for engaging the seal member  156 . The seal member  156  is preferably a round-shaped disk. The seal element  160  is preferably provided by an elastomeric O-ring which is disposed in a seal gland formed into the edge of the seal member  156 . The lower end of the flow passage  126  has an enlarged portion  172  which has a seal surface  174  for engaging with a seal element  180  of a seal member  176 , and has an annular shaped shoulder  178  which provides a stop for engaging the seal member  176 . The seal member  176  is preferably a round-shaped disk. The seal element  180  is preferably provided by an elastomeric O-ring which is disposed in a seal gland formed into the edge of the seal member  176 . The seal members  156  and  176  are preferably held in place within respective ones of the enlarged portions  152  and  172  of the port  144  and the flow passage  126  by friction of the seal elements  160  and  180  being squeezed between the respective ones of the seal members  156  and  176 , and the seal surfaces  154  and  174 . When lowered into a well, the seal member  156  is also held in place against the shoulder  158  by well pressures, until the second ignitor  218  is fired. Firing of the second ignitor  218  causes the pressure inside of the flow passage  126  to exceed the well pressure exterior of the tool  12 , and the seal member  156  is pushed outward from sealing the interior opening defined by the enlarged portion of the flow passage  126 . Firing of the second ignitor  218  also pushes the seal member outward from sealing the interior opening of the flow passage  126 , which is defined by the enlarged portion  172 . 
     FIG. 4 is a side view of the upper seal member  156  for use in the pressure relief subassembly  12  of the chemical cutter  12 , and FIG. 5 is a side view of the lower seal member  176  for use in the pressure relief subassembly  26  of the chemical cutter  12 . An O-ring disposed in a seal gland to provide the seal element  160 . An O-ring is disposed in a seal gland to provide the seal element  160 . Preferably, the seal member  176  has a thickness which is greater than the thickness of the upper seal member  156 . 
     Referring again to FIG. 3, upper portions of the flow passages  124  and  126  have enlarged diameter portions defining sockets  192  and  194 , respectively, for receiving the two sealed contact connectors  196 . The lower ends of the sockets  192  and  194  define annular shaped shoulders  198  and  200 , which define stops for the connectors  196 . A lower end portion  204  of the flow passage  124  has a reduced diameter from the diameter of an adjacent intermediate portion  206  to define an annular shaped shoulder  208 , which provides a stop for an ignitor  210 . A lower intermediate section  214  of the flow passage  126  has a reduced diameter from the diameter of an upper intermediate section  212  of the flow passage  126  do define an annular shaped shoulder  216  which defines a stop for an ignitor  218 . The diameter of the intermediate section  214  of the flow passage  126  is preferably three-eighths of an inch. 
     FIG. 3 also shows side elevation views of the electric contact components of the ignitor section  26 , which include the two sealed contact connectors  196 , a contact rod  222 , a contact rod  224  and two spring contacts  226  and  228 . A contact assembly  182  is an electrical conductor which is provided by the connector  196 , the contact rod  222  and the contact spring  226 , which electrically connects between the ignitor  210  and the wire  104 . A contact assembly  184  is an electrical conductor which is provided by the connector  196 , the contact rod  224  and the contact spring  228 , which electrically connects between the ignitor  218  and the wire  108 . The sealed contact connectors  196  are available from KEMLON PRODUCTS, of Pearland, Tex. Each of the sealed contact connectors  196  have two seal glands  232 , preferably for receiving O-ring type seals. Shoulders  234  extend radially outward of the bodies  236  of the connectors  196 . Upper contacts  238  and lower contacts  240  are insulated by ceramic enclosures, which include annular-shaped ceramic beads  242  and  244 . The ceramic beads  242  align the contacts  240  within the flow passages  124  and  126 , to prevent electrical contact between the housing  122  and the contacts  240 . The annular-shaped ceramic beads  244  provide an enlarged portion for the seal boots  114  (shown in FIG. 2) to engage. 
     The contact rods  222  and  224  engage between respective ones of the contacts  240  and the contact springs  226  and  228 . The outer diametrical surfaces of the rods  222  and  224  are insulated by outer non-conductive, plastic sleeves  246  and  250 , respectively, to prevent direct electrical contact between the rods  222  and  224 , and the housing  122 . Rod end tips  248  are disposed on opposite, longitudinal ends of the contact rods  222  and  224 . The rod end tips  248  have a smaller diameter than the outer diameter of intermediate portions of the rods  222  and  224 , and are sized such that the end tips  248  will fit within the springs  226  and  228 , centering the springs  226  and  228  with respect to the longitudinal azis  148  and  150  of the flow passages  124  and  126 , respectively. The springs  226  and  228  and the end tips  248  are sized in relation to the interior diameters of the flow passages  124  and  126 , such that the springs  226  and  228  will remain centered within the flow passages  124  and  127  and not make direct electrical contact with the sidewalls of the flow passages  124  and  126 , and the housing  122 . The contact springs  226  and  228  electrically engage the tops of the ignitors  210  and  218 . Contact wires  230  are provided on the sides of each of the ignitors  210  and  218  for making contacting the sidewall of the flow passages  124  and  126 , respective, to electrically connect to the housing  122  and complete the firing circuit for the ignitors  210  and  218 . 
     FIG. 6 is a is a schematic diagram of an electrical control circuit  252  of the control section  14  of the chemical cutter  12 . The control circuit  252  includes the contact  112  connected to a node  254 , and two parallel circuits  256  and  258  connected to the node  254 . The first circuit  256  includes the conductor  104 , which has an upper end connected to the node  254  and the contact  112 . The diode  106  is connected in series between two sections of the conductor  104 , with the diode  106  aligned in a configuration for passing negative current through from the node  54  to the ignitor  210 , and preventing positive current from passing in the same direction. The lower end of the conductor is connected to the contact assembly  182 , which provides an electrical conductor which connects between the wire  104  and the upper end of the ignitor  210 . The circuit  256  is completed by the contact wire  230  of the ignitor  210  contacting the conductive housing  122 , which provides a ground for the circuit  256 . The second circuit  258  includes the conductor  108 , which has an upper end connected to the node  254  and the contact  112 . The diode  110  is connected in series between two sections of the conductor  104 , with the diode  110  aligned in a configuration for passing positive electric current from the node  54  to the ignitor  218 , and preventing negative current from passing in the same direction. The lower end of the conductor  108  is connected to the contact assembly  184 , which provides an electrical conductor which connects between the wire  108  and the upper end of the ignitor  218 . The circuit  258  is completed by the contact wire  230  of the ignitor  218  contacting the conductive housing  122 , which provides a ground for the circuit  258 . 
     FIG. 7 is a partial, longitudinal section view of a strainer section  36  located in the lower end of the pressure chamber  32 . The strainer section  36  has a strainer body  262  which is centrally disposed within the lower end of the pressure chamber  32  to define an annular flow passage  264  which extends between the tool housing  30  and the outer diameter of the strainer body  262 , preferably coaxial with the longitudinal axis  16 . The annular flow passage  264  extends within the pressure chamber  32 , exteriorly of the strainer body  262 . A central strainer flow passage  266  is defined within the interior of the strainer body  262 , and preferably extends coaxially with the longitudinal axis  16  and the annular flow passage  264 . The upper end  268  of the strainer body  262  is solid to seal the uppermost end of the central strainer flow passage  266 . The lower end of the central strainer flow passage  266  extends directly into a flow passage  272  of the pressure relief subassembly  38 . Flow ports  268  are defined by a plurality of holes which preferably have central axes  270  that are perpendicular to the longitudinal axis  16 , and which provide flow passages that extend between the annular flow passage  264  and the central flow passage  266 . 
     When the propellant  34  is ignited, debris will become trapped in the annular flow passage  264  as high pressure gases provided by combustion of the propellant  34  pass from the pressure chamber  32 , into the annular flow passage  264 , and then will change from a first flow direction which is generally parallel to the longitudinal axis  16  within the annular flow passage  264  to a second flow direction which generally transverse to the longitudinal axis  16  in passing from the annular flow passage  264  and into the flow ports  268  in the sidewall of the strainer body  262 . After passing through the flow ports  268 , the high pressure gases will again change flow direction from the second flow direction which is generally transverse to the longitudinal axis  16  when passing through the flow ports  268 , to a third flow direction which is generally parallel to the longitudinal axis  16  in the central flow passage  266 . The high pressure gas then passes from the central flow passage  266  and into the flow passage  272  of the pressure relief subassembly  38 . 
     Referring again to FIG. 1, the pressure relief subassembly  38  has the flow passage  272  which connects between the strainer  36  in the lower end of the propellant section  28  and the upper end of the anchor section  40 . In the preferred embodiment, the flow passage  272  is sized to have approximately a diameter of three-eighths of an inch, which is of a size for restricting the flow of gases from the propellant section  28  into the anchor section  40 . In other embodiments, a plate may be used having an orifice of a selected size to provide a desired flow rate of propellant gases from the propellant section  28  to the anchor section  40 . A pressure bleed port  274  is connected to the flow passage  272  and extends transversely from the flow passage  272  to the exterior of the pressure relief subassembly  38 . A seal member  276  is preferably provided by a threaded plug, which seals the pressure bleed port  274 . Preferably, the seal member  276  has a seal element, such as an elastomeric O-ring. The seal member  276  is removed from sealing the pressure bleed port  274  after the tool  12  is removed from a well to bleed off pressure which may be trapped within the flow passage  272  after the cutting tool is run to sever a tubular member  14 . 
     FIG. 8 is a partial, side elevation view and FIG. 9 is a partial cutaway, and exploded view of an anchor section  40  of the chemical cutter  12 . The anchor section  40  has a housing  282  and slidably extendable anchor members  284 , which are retained in holes  286  in the anchor section  40  housing  282  by retainer bars  288 . The retainer bars  288  are fixedly secured to the housing  282  with threaded fasteners  290 . There are six holes  286 , with vertically adjacent pairs of the holes  286  being offset, or angularly spaced apart, one-hundred and twenty degrees around the longitudinal axis  16 . The vertically adjacent pairs of holes each extend from respective ones of three central flow passages  292 . The three flow passages  292  have preferably each have an internal diameter 0.187 inches, and extend longitudinally through the anchor section  40 . The central flow passages  292  are preferably coaxial with the longitudinal axis  16 , and the holes  286  have axes  287  which extend transverse, preferably perpendicular, to the longitudinal axis  16 . The outward ends of the anchor members  284  have teeth  294  for grippingly engaging a tubular member  14  (shown in FIG. 1) being severed with the cutting tool  12 , to secure the chemical cutter  12  in a fixed position within the tubular member  14 . Slots  296  extend into the outward end of the anchor members  284  for receiving the retainer bar  288 , for a depth which provides sufficient travel of the anchor members  284  to move outward from within the holes  286  for grippingly engaging the interior surface of the tubular member  14  being cut by the cutting tool  12 . The slots  296  are formed into the outward end of the anchor members  284  to define shoulders  300  which engage the inwardly disposed sides of the retainer bars  288  when the anchor members are fully extended within a well, such that the shoulders  300  define stops which engage against the retainer bars  288  to retain the anchor members within the holes  286 . Blind holes  298  are formed into the outward faces of respective ones of the anchor members  284  for receiving bias springs  302 . The bias springs  302  urge the anchor members  284  into the holes  286 , except when the biasing forces of the springs  302  are overcome by the high pressure of propellant gasses within the central flow passages  292  when the propellant  34  is ignited. After the propellant  34  is expended and the pressure is relieved within the central flow passages  292 , the bias springs  302  will push the anchor members  284  back into respective ones of the holes  286  to release the cutting tool  12  from the tubular member  14  being severed so that the cutting tool  12  may be retrieved from the well. Seals  304  are provided on the inward ends of the anchor members  284 , preferably by two O-rings for each of the anchor members  284 . 
     Referring again to FIG. 1, a flow control section  42  has a central flow passage  312  which preferably extends parallel to the longitudinal axis  16 . The flow passage  312  has an interior diameter which restricts flow through the flow control section  42  to an exit portion  314 . In the preferred embodiment, the inside diameter of the flow passage  213  is one-quarter inch. In other embodiments, an orifice of a particular size may be used, such as a disk-shaped plate having an orifice hole for disposing in the exit portion  314 , for controlling the rate at which gas provided by the propellant will pass from the anchor section  40 , and through the flow passage  312  and into the chemical section  44 . 
     A chemical section  44  has an interior chemical chamber  316 , within which the chemical  46  providing the cutting fluid is disposed. Rupture discs  320  are provided on opposite ends of the chemical chamber  316  to contain the chemical cutting fluid  46  within the chemical chamber  316  until the propellant  34  is ignited. The rupture discs  320  are sized such that pressures within the cutting chemical tool  12  achieved by ignition of the propellant  34  will rupture both the upper and the lower discs  320 , and the cutting fluid will be pushed downward and from within the chemical section  44  into the combustion section  48 . 
     The combustion section  48  has a combustion chamber  324  defined in within the tool housing  122 . The tool housing  122  is part of the housing  30  of the chemical cutter  12 . The combustion chamber  324  defines a central passage within which is disposed a catalyst  50 . The catalyst  50  is preferably provided by steel wool, which reacts with the cutting chemical  46  to activate the cutting fluid to reach high temperatures and pressures, to overcome well pressures and cause activated cutting fluid to pass through the flow ports  54  of the cutting head  52  at high velocity. The cutting chemical  46  will flow from within the combustion section  48  and into the cutting head  52 . 
     The cutting head  52  has a central flow passage  328  and flow ports  54 . Prior to igniting the propellant  34  to operate the tool, a piston  56  is disposed within the central flow passage  328  of the cutting head  52 . Seals  332  are disposed on opposite ends of the piston  56 , such that the piston will straddle the flow ports  54 , with the seals  332  preventing flow through the flow ports  54 . An upper piston latch  334  is provided for securing the piston in the sealing position (shown in FIG. 1) such that the central flow passage  328  is sealed to prevent fluid flow between the flow passage  328  and the flow ports  54 . After the propellant  34  is ignited, the piston  56  will be moved downward within the flow passage  328 , into a downward position located beneath the flow parts  54 . The piston  56  will not move downward until the pressure of the cutting chemical fluid  46  exceeds the well pressures exterior of the tool, which are in communication with a central passage  342  of the bull nose  58  and the bottom of the piston  56 . Once the well pressures exterior of the cutting tool  12  are overcome, the piston  56  will move downward within the cutting head  52 , until a lower piston latch  336  secures the piston  56  in the downward position, to allow the activated cutting chemical  46  to pass through the flow ports  54  and from within the chemical cutter  12 . 
     The flow ports  54  of the cutting head  52  are arranged in a phasing pattern, such that the cutting chemical  46 , once activated, will preferably be evenly dispersed in a desired pattern to evenly sever the tubular member  14  being cut with the cutter  12 . The flow ports  54  are preferably angularly spaced apart around the central axis  16 , in an evenly spaced pattern along a circumference of the tool housing  30  of the chemical cutter  12 . Central axes of the flow ports are preferably disposed at right angles to the longitudinal axis  16  of the chemical cutter  12 , equally spaced around a circumference of the tool housing  30 . 
     A bull plug  58  is provided on the lower end of the cutting tool  12 . Centralizer arms  60  are mounted to extend downward from the bull plug  58  to provide a centralizer for centering the lower end of the cutting tool  21  within a tubular member  14  in a well. A central passage  342  is provided through the bull plug  58  to apply well fluid pressures to the lower end of the piston  56 , so that the piston  56  will not move downward from sealing the flow ports  54  from communicating with the central flow passage  328  until after the pressure within the cutting head  52  exceeds the pressure of the well fluid exterior of the tool  12 . This prevents flow of well fluids through the flow ports  54  and into the tool housing  30  prior to the cutting chemical  46  being activated to pressures which exceed well pressures. The lower end face of the tool housing  48  provides an annular-shaped stop to prevent well pressures acting on the lower end of the piston  56  from pushing the piston  56  upwards from sealing the flow ports  54 . The lower end of the combustion section  48  provides an annular-shaped stop  336  for a lower position of the piston  56 . 
     In operation, the chemical cutter  12  is lowered into a well and located relative to a tubular member  14  which is to be severed, such that the flow ports  54  of the cutting head  52  are aligned with a desired cutting plane. Then, current of negative polarity is applied to the chemical cutter  12 , which is passed through the diode  106  and to the ignitor  210 . Firing of the ignitor  210  ignites the propellant  34 , which provide high pressure gasses. The gasses pass through the strainer  36 , the subassembly  38  and into the anchor section  40 . The pressure of the gasses pushes anchor members  284  outward from within the tool housing  30  against the force of the bias springs  302 , and engages the teeth  294  of the anchor members  284  with the interior of the tubular member  14 , to secure the cutting tool  12  in a fixed position within the well as the cutting chemical  46  is dispensed from within the tool housing  30 . The pressure of the gasses will also rupture the plates  320 , allowing the cutting chemical  46  to flow from within chemical chamber  316  of the chemical section  46 , and through the catalyst  50  in the combustion section  48 . The pressure of the gasses will also push the piston  56  downward, to allow the activated cutting chemical  46  to flow from within the combustion section  48 , through the cutting head  52  and outward from the tool housing  30  through the flow ports  54 . The activated cutting chemical will come into contact with the section of the tubular member  14  adjacent the flow ports  54 , cutting through the tubular member  14 . 
     The pressure within the chemical cutter  12  caused by ignition of the propellant  34  will then bleed off, and the bias springs  302  push the anchor members  284  back into the tool housing  30 , releasing the teeth  294  of the anchor members  284  from gripping the tubular member  14 . The tool may then be retrieved, to a location just beneath the surface of the well, or into a riser above the wellhead. Preferably, current of positive polarity is applied to the chemical cutter  12 , which is passed through the diode  110  to the ignitor  218 . Firing of the ignitor  218  pushes the seal members  156  and  176  from within the sockets  152  and  172 , respectively. This creates a flow passage between the interior passage  32  and the exterior of the tool housing  30 , so that pressure may be equalized prior to removing the chemical cutter  12  from the well. If necessary to equalize pressure between well and the interior of the anchor section  40  when the chemical cutter  12  is downhole, such as to release the teeth  294  of the anchor members  284  from gripping the tubular member  14 , the ignitor  218  may be fired when the chemical cutter  12  is downhole. 
     The present invention provides various advantages over the prior art. A chemical cutter is provided which has a pressure relief feature for equalizing pressure between an interior passage and an exterior of the tool housing. A flow passage is selectively opened by selectively firing an ignitor, which removes two seal members from sealing the flow passage. The ignitor may be selectively fired downhole if necessary to release the tool from within a tubular member being cut by the chemical cutter, or the ignitor may be selectively fired close to the surface of the well to relieve pressures trapped within the interior of the chemical cutting tool. 
     Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.