Abstract:
A torch apparatus for consuming a material having a fuel load that produces heat and a source of oxygen when burned, and a plurality of slots having interstitial spaces therebetween for allowing longitudinal flow of fluid along the torch apparatus without interfering with the flow of fluid through the slots. The slots are oriented such that the heat and source of oxygen are provided to a material that is at least partially consumed when exposed to heat and oxygen, to thereby cause destruction of an object containing the material or disengagement of the object such that it falls into the wellbore.

Description:
This application is a continuation-in-part of application Ser. No. 12/055,428, filed Mar. 26, 2008 now U.S. Pat. No. 7,726,392. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to consumable downhole tools and methods of removing such tools from well bores. More particularly, the present invention relates to downhole tools comprising materials that are burned and/or consumed when exposed to heat and an oxygen source and methods and systems for consuming such downhole tools in situ. 
     BACKGROUND OF THE INVENTION 
     A wide variety of downhole tools may be used within a well bore in connection with producing hydrocarbons or reworking a well that extends into a hydrocarbon formation. Downhole tools such as frac plugs, bridge plugs, and packers, for example, may be used to seal a component against casing along the well bore wall or to isolate one pressure zone of the formation from another. Such downhole tools are well known in the art. 
     After production or reworking is complete, these downhole tools must be removed from the well bore. Tool removal has conventionally been accomplished by complex retrieval operations, or by milling or drilling the tool out of the well bore mechanically. Thus, downhole tools are either retrievable or disposable. Disposable downhole tools have traditionally been formed of drillable metal materials such as cast iron, brass or aluminum. To reduce the milling or drilling time, the next generation of downhole tools comprises composites and other non-metallic materials, such as engineering grade plastics. Nevertheless, milling and drilling continues to be a time consuming and expensive operation. To eliminate the need for milling and drilling, other methods of removing disposable downhole tools have been developed, such as using explosives downhole to fragment the tool, and allowing the debris to fall down into the bottom of the well bore. This method, however, sometimes yields inconsistent results. Therefore, a need exists for disposable downhole tools that are reliably removable without being milled or drilled out, and for methods of removing such disposable downhole tools without tripping a significant quantity of equipment into the well bore. 
     Furthermore, in oil and gas wells, a drill string is used to drill a well bore into the earth. The drill string is typically a length of drill pipe extending from the surface into the well bore. The bottom end of the drill string has a drill bit. 
     In order to increase the effectiveness of drilling, weight in the form of one or more drill collars is included in the drill string. A string of drill collars is typically located just above the drill bit and its sub. The string of drill collars contains a number of drill collars. A drill collar is similar to drill pipe in that it has a passage extending from one end to the other for the flow of drilling mud. The drill collar has a wall thickness around the passage; the wall of a drill collar is typically much thicker than the wall of comparable drill pipe. This increased wall thickness enables the drill collar to have a higher weight per foot of length than comparable drill pipe. 
     During drilling operations, the drill string may become stuck in the hole. If the string cannot be removed, then the drill string is cut. Cutting involves lowering a torch into the drill string and physically severing the drill string in two, wherein the upper part can be removed for reuse in another well bore. The part of the drill string located below the cut is left in the well bore and typically cannot be retrieved or reused. Cutting is a salvage operation. A particularly effective cutting tool is my radial cutting torch described in U.S. Pat. No. 6,598,679. 
     The radial cutting torch produces combustion fluids that are directed radially out to the pipe. The combustion fluids are directed out in a complete circumference so as to cut the pipe all around the pipe circumference. 
     It is desired to cut the drill string as close as possible to the stuck point, in order to salvage as much of the drill string as possible. Cutting the drill string far above the stuck point leaves a section of retrievable pipe in the hole. 
     If, for example, the drill bit or its sub is stuck, then in theory one of the drill collars can be cut to retrieve at least part of the drill collar string. Unfortunately, cutting a drill collar, with its thick wall, is difficult. It is much easier to cut the thinner wall drill pipe located above the drill collars. Consequently, the drill collar string may be left in the hole, as the drill string is cut above the drill collar. 
     It is desired to cut a drill collar for retrieval purposes. 
     SUMMARY OF THE INVENTION 
     Disclosed herein is a downhole tool having a body or structural component comprising a material that is at least partially consumed when exposed to heat and a source of oxygen. In an embodiment, the material comprises a metal, and the metal may comprise magnesium, such that the magnesium metal is converted to magnesium oxide when exposed to heat and a source of oxygen. The downhole tool may further comprise an enclosure for storing an accelerant. In various embodiments, the downhole tool is a frac plug, a bridge plug, or a packer. 
     The downhole tool may further comprise a torch with a fuel load that produces the heat and source of oxygen when burned. In various embodiments, the fuel load comprises a flammable, non-explosive solid, or the fuel load comprises thermite. The torch may further comprise a torch body with a plurality of nozzles distributed along its length, and the nozzles may distribute molten plasma produced when the fuel load is burned. In an embodiment, the torch further comprises a firing mechanism with heat source to ignite the fuel load, and the firing mechanism may further comprise a device to activate the heat source. In an embodiment, the firing mechanism is an electronic igniter. The device that activates the heat source may comprise an electronic timer, a mechanical timer, a spring-wound timer, a volume timer, or a measured flow timer, and the timer may be programmable to activate the heat source when the pre-defined conditions are met. The pre-defined conditions comprise elapsed time, temperature, pressure, volume, or any combination thereof. In another embodiment, the device that activates the heat source comprises a pressure-actuated firing head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, cross-sectional view of an exemplary operating environment depicting a consumable downhole tool being lowered into a well bore extending into a subterranean hydrocarbon formation. 
         FIG. 2  is an enlarged cross-sectional side view of one embodiment of a consumable downhole tool comprising a frac plug being lowered into a well bore. 
         FIG. 3  in an enlarged cross-sectional side view of a well bore with a representative consumable downhole tool with an internal firing mechanism sealed therein. 
         FIG. 4  is an enlarged cross-sectional side view of a well bore with a consumable downhole tool sealed therein, and with a line lowering an alternative firing mechanism towards the tool. 
         FIG. 5  is a cross-sectional view of the torch, in accordance with a preferred embodiment. 
         FIG. 6  is a side view of the openings of the torch nozzle. 
         FIG. 6A  is a cross-sectional view of the nozzle section of the torch, taken through lines VIA-VIA of  FIG. 6 . 
         FIG. 7  is a schematic cross-sectional view of a well showing the use of plural isolation tools. 
         FIG. 8  is a cross-sectional view of a borehole with an uncut drill collar and a torch. 
         FIG. 9  is the same as  FIG. 8 , but the torch has been ignited. 
         FIG. 10  shows the drill collar of  FIG. 8 , having been cut and separated. 
         FIG. 11  is a cross-sectional view of  FIG. 8 , taken along lines XI-XI. 
         FIG. 12  is a cross-sectional view of  FIG. 10 , taken along lines XII-XII. 
         FIG. 13  is a longitudinal cross-sectional view of the torch. 
         FIG. 14  is a side elevational view of the nozzle pattern of the torch, taken along lines XIV-XIV of  FIG. 13 . 
         FIGS. 15A-15C  show the dressing of a cut end of a drill collar to form a new pin joint. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the description that follows,  FIGS. 1-7  will be discussed first. These figures show a downhole tool such as a plug, which tool contains a torch. The torch is used after the plug is no longer needed, to remove the plug from operation.  FIGS. 5-7  show the torch in more detail. Then,  FIGS. 8-15C  will be discussed. These figures show removal of a downhole tool, such as a drill collar, from a borehole using a torch. 
     Certain terms are used throughout the following description and claims to refer to particular assembly components. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. 
     Reference to up or down will be made for purposes of description with “up”, “upper”, “upwardly” or “upstream” meaning toward the surface of the well and with “down”, “lower”, “downwardly” or “downstream” meaning toward the lower end of the well, regardless of the well bore orientation. Reference to a body or a structural component refers to components that provide rigidity, load bearing ability and/or structural integrity to a device or tool. 
       FIG. 1  schematically depicts an exemplary operating environment for a consumable downhole tool  100 . As depicted, a drilling rig  110  is positioned on the earth&#39;s surface  105  and extends over and around a well bore  120  that penetrates a subterranean formation F for the purpose of recovering hydrocarbons. At least the upper portion of the well bore  120  may be lined with casing  125  that is cemented  127  into position against the formation F in a conventional manner. The drilling rig  110  includes a derrick  112  with a rig floor  114  through which a work string  118 , such as a cable, wireline, E-line, Z-line, jointed pipe, or coiled tubing, for example, extends downwardly from the drilling rig  110  into the well bore  120 . The work string  118  suspends a representative consumable downhole tool  100 , which may comprise a frac plug, a bridge plug, a packer, or another type of well bore zonal isolation device, for example, as it is being lowered to a predetermined depth within the well bore  120  to perform a specific operation. The drilling rig  110  is conventional and therefore includes a motor driven winch and other associated equipment for extending the work string  118  into the well bore  120  to position the consumable downhole tool  100  at the desired depth. 
     While the exemplary operating environment depicted in  FIG. 1  refers to a stationary drilling rig  110  for lowering and setting the consumable downhole tool  100  within a land-based well bore  120 , one of ordinary skill in the art will readily appreciate that mobile workover rigs, well servicing units, such as slick lines and e-lines, and the like, could also be used to lower the tool  100  into the well bore  120 . It should be understood that the consumable downhole tool  100  may also be used in other operational environments, such as within an offshore well bore. 
     The consumable downhole tool  100  may take a variety of different forms. In an embodiment, the tool  100  comprises a plug that is used in a well stimulation/fracturing operation, commonly known as a “frac plug”.  FIG. 2  depicts an exemplary consumable frac plug, generally designated as  200 , as it is being lowered into a well bore  120  on a work string  118  (not shown). The frac plug  200  comprises an elongated tubular body member  210  with an axial flowbore  205  extending therethrough. A ball  225  acts as a one-way check valve. The ball  225 , when seated on an upper surface  207  of the flowbore  205 , acts to seal off the flowbore  205  and prevent flow downwardly therethrough, but permits flow upwardly through the flowbore  205 . In some embodiments, an optional cage, although not included in  FIG. 2 , may be formed at the upper end of the tubular body member  210  to retain ball  225 . A packer element assembly  230  extends around the tubular body member  210 . One or more slips  240  are mounted around the body member  210 , above and below the packer assembly  230 . The slips  240  are guided by mechanical slip bodies  245 . A cylindrical torch  257  is shown inserted into the axial flowbore  205  at the lower end of the body member  210  in the frac plug  200 . The torch  257  comprises a fuel load  251 , a firing mechanism  253 , and a torch body  252  with a plurality of nozzles  255  distributed along the length of the torch body  252 . The nozzles  255  are angled to direct flow exiting the nozzles  255  towards the inner surface  211  of the tubular body member  210 . The firing mechanism  253  is attached near the base of the torch body  252 . An annulus  254  is provided between the torch body  252  and the inner surface  211  of the tubular body member  210 , and the annulus  254  is enclosed by the ball  225  above and by the fuel load  251  below. 
     At least some of the components comprising the frac plug  200  may be formed from consumable materials, such as metals, for example, that burn away and/or lose structural integrity when exposed to heat and an oxygen source. Such consumable components may be formed of any consumable material that is suitable for service in a downhole environment and that provides adequate strength to enable proper operation of the frac plug  200 . By way of example only, one such material is magnesium metal. In operation, these components may be exposed to heat and oxygen via flow exiting the nozzles  255  of the torch body  252 . As such, consumable components nearest these nozzles  255  will burn first, and then the burning extends outwardly to other consumable components. 
     Any number of combination of frac plug  200  components may be made of consumable materials. In an embodiment, the load bearing components of the frac plug  200 , including the tubular body member  210 , the slips  240 , the mechanical slip bodies  245 , or a combination thereof, may comprise consumable material, such as magnesium metal. These load bearing components  210 ,  240 ,  245  hold the frac plug  200  in place during well stimulation/fracturing operations. If these components  210 ,  240 ,  245  are burned and/or consumed due to exposure to heat and oxygen, they will lose structural integrity and crumble under the weight of the remaining plug  200  components, or when subjected to other well bore forces, thereby causing the frac plug  200  to fall away into the well bore  120 . In another embodiment, only the tubular body member  210  is made of consumable material, and consumption of that body member  210  sufficiently comprises the structural integrity of the frac plug  200  to cause it to fall away into the well bore  120  when the frac plug  200  is exposed to heat and oxygen. 
     The fuel load  251  of the torch  257  may be formed from materials that, when ignited and burned, produce heat and an oxygen source, which in turn may act as the catalysts for initiating burning of the consumable components of the frac plug  200 . By way of example only, one material that produces heat and oxygen when burned is thermite, which comprises iron oxide, or rust (Fe 2 O 3 ), and aluminum metal powder (Al). When ignited and burned, thermite reacts to produce aluminum oxide (Al 2 O 3 ) and liquid iron (Fe), which is a molten plasma-like substance. The chemical reaction is:
 
Fe 2 O 3 +2Al( s )→Al 2 O 3 ( s )+2Fe( l )
 
The nozzles  255  located along the torch body  252  are constructed of carbon and are therefore capable of withstanding the high temperatures of the molten plasma substance without melting. However, when the consumable components of the frac plug  200  are exposed to the molten plasma, the components formed of magnesium metal will react with the oxygen in the aluminum oxide (Al 2 O 3 ), causing the magnesium metal to be consumed or converted into magnesium oxide (MgO), as illustrated by the chemical reaction below:
 
3Mg+Al 2 O 3 →3MgO+2Al
 
When the magnesium metal is converted to magnesium oxide, a slag is produced such that the component no longer has structural integrity and thus cannot carry load. Application of a slight load, such as a pressure fluctuation or pressure pulse, for example, may cause a component made of magnesium oxide slag to crumble. In an embodiment, such loads are applied to the well bore and controlled in such a manner so as to cause structural failure of the frac plug  200 .
 
     In one embodiment, the torch  257  may comprise the “Radial Cutting Torch”, developed and sold by MCR Oil Tools Corporation. The Radial Cutting Torch includes a fuel load  251  constructed of thermite and classified as a flammable, nonexplosive solid. Using a nonexplosive material like thermite provides several advantages. Numerous federal regulations regarding the safety, handling and transportation of explosive add complexity when conveying explosive to an operational job site. In contrast, thermite is nonexplosive and thus does not fall under these federal constraints. Torches  257  constructed of thermite, including the Radial Cutting Torch, may be transported easily, even by commercial aircraft. 
     In order to ignite the fuel load  251 , a firing mechanism  253  is employed that may be activated in a variety of ways. In one embodiment, a timer, such as an electronic timer, a mechanical timer, or a spring-wound timer, a volume timer, or a measured flow timer, for example, may be used to activate a heating source within the firing mechanism  253 . In one embodiment, an electronic timer may activate a heating source when pre-defined conditions, such as time, pressure and/or temperature are met. In another embodiment, the electronic timer may activate the heat source purely as a function of time, such as after several hours or days. In still another embodiment, the electronic timer may activate when pre-defined temperature and pressure conditions are met, and after a specified time period has elapsed. In an alternate embodiment, the firing mechanism  253  may not employ time at all. Instead, a pressure actuated firing head that is actuated by differential pressure or by a pressure pulse may be used. It is contemplated that other types of devices may also be used. Regardless of the means for activating the firing mechanism  253 , once activated, the firing mechanism  253  generates enough heat to ignite the fuel load  251  of the torch  257 . In one embodiment, the firing mechanism  253  comprises the “Thermal Generator”, developed and sold by MCR Oil Tools Corporation, which utilizes an electronic timer. When the electronic timer senses that pre-defined conditions have been met, such as a specified time has elapsed since setting the timer, a single AA battery activates a heating filament capable of generating enough heat to ignite the fuel load  251 , causing it to burn. To accelerate consumption of the frac plug  200 , a liquid or powder-based accelerant may be provided inside the annulus  254 . In various embodiments, the accelerant may be liquid manganese acetate, nitromethane, or a combination thereof. 
     In operation, the frac plug  200  of  FIG. 2  may be used in a well stimulation/fracturing operation to isolate the zone of the formation F below the plug  200 . Referring now to  FIG. 3 , the frac plug  200  of  FIG. 2  is shown disposed between producing zone A and producing zone B in the formation F. As depicted, the frac plug  200  comprises a torch  257  with a fuel load  251  and a firing mechanism  253 , and at least one consumable material component such as the tubular body member  210 . The slips  240  and the mechanical slip bodies  245  may also be made of consumable material, such as magnesium metal. In a conventional well stimulation/fracturing operation, before setting the frac plug  200  to isolate zone A from zone B, a plurality of perforations  300  are made by a perforating tool (not shown) through the casing  125  and cement  127  to extend into producing zone A. Then a well stimulation fluid is introduced into the well bore  120 , such as by lowering a tool (not shown) into the well bore  120  for discharging the fluid at a relatively high pressure or by pumping the fluid directly from the surface  105  into the well bore  120 . The well stimulation fluid passes through the perforations  300  into producing zone A of the formation F for stimulating the recovery of fluids in the form of oil and gas containing hydrocarbons. These production fluids pass from zone A, through the perforations  300 , and up the well bore  120  for recovery at the surface  105 . 
     Prior to running the frac plug  200  downhole, the firing mechanism  253  is set to activate a heating filament when predefined conditions are met. In various embodiments, such predefined conditions may include a predetermined period of time elapsing, a specific temperature, a specific pressure, or any combination thereof. The amount of time set may depend on the length of time required to perform the well stimulation/fracturing operation. For example, if the operation is estimated to be performed in 12 hours, then a timer may be set to activate the heating filament after 12 hours have lapsed. Once the firing mechanism  253  is set, the frac plug  200  is then lowered by the work string  118  to the desired depth within the well bore  120 , and the packer element assembly  230  is set against the casing  125  in a conventional manner, thereby isolating zone A as depicted in  FIG. 3 . Due to the design of the frac plug  200 , the ball  225  will unseal the flowbore  205 , such as by unseating from the surface  207  of the flowbore  205 , for example, to allow fluid from isolated zone A to flow upwardly through the frac plug  200 . However, the ball  225  will seal off the flowbore  205 , such as by seating against the surface  207  of the flowbore  205 , for example, to prevent flow downwardly into the isolated zone A. Accordingly, the production fluids from zone A continue to pass through the perforations  300 , into the well bore  120 , and upwardly through the flowbore  205  of the frac plug  200 , before flowing into the well bore  120  above the frac plug  200  for recovery at the surface  105 . 
     After the frac plug  200  is set into position as shown in  FIG. 3 , a second set of perforations  310  may then be formed through the casing  125  and cement  127  adjacent intermediate producing zone B of the formation F. Zone B is then treated with well stimulation fluid, causing the recovered fluids from zone B to pass through the perforations  310  into the well bore  120 . In this area of the well bore  120  above the frac plug  200 , the recovered fluids from zone B will mix with the recovered fluids from zone A before flowing upwardly within the well bore  120  for recovery at the surface  105 . 
     If additional well stimulation/fracturing operations will be performed, such as recovering hydrocarbons from zone C, additional frac plugs  200  may be installed within the well bore  120  to isolate each zone of the formation F. Each frac plug  200  allows fluid to flow upwardly therethrough from the lowermost zone A to the uppermost zone C of the formation F, but pressurized fluid cannot flow downwardly through the frac plug  200 . 
     After the fluid recovery operations are complete, the frac plug  200  must be removed from the well bore  120 . In this context, as stated above, at least some of the components of the frac plug  200  are consumable when exposed to heat and an oxygen source, thereby eliminating the need to mill or drill the frac plug  200  from the well bore  120 . Thus, by exposing the frac plug  200  to heat and an oxygen source, at least some of its components will be consumed, causing the frac plug  200  to release from the casing  125 , and the unconsumed components of the plug  200  to fall to the bottom of the well bore  120 . 
     In order to expose the consumable components of the frac plug  200  to heat and an oxygen source, the fuel load  251  of the torch  257  may be ignited to burn. Ignition of the fuel load  251  occurs when the firing mechanism  253  powers the heating filament. The heating filament, in turn, produces enough heat to ignite the fuel load  251 . Once ignited, the fuel load  251  burns, producing high-pressure molten plasma that is emitted from the nozzles  255  and directed at the inner surface  211  of the tubular body member  210 . Through contact of the molten plasma with the inner surface  211 , the tubular body member  210  is burned and/or consumed. In an embodiment, the body member  210  comprises magnesium metal that is converted to magnesium oxide through contact with the molten plasma. Any other consumable components, such as the slips  240  and the mechanical slip bodies  245 , may be consumed in a similar fashion. Once the structural integrity of the frac plug  200  is compromised due to consumption of its load carrying components, the frac plug  200  falls away into the well bore  120 , and in some embodiments, the frac plug  200  may further be pumped out of the well bore  120 , if desired. 
     In the method described above, removal of the frac plug  200  was accomplished without surface intervention. However, surface intervention may occur should the frac plug  200  fail to disengage and, under its own weight, fall away into the well bore  120  after exposure to the molten plasma produced by the burning torch  257 . In that event, another tool, such as work string  118 , may be run downhole to push against the frac plug  200  until it disengages and falls away into the well bore  120 . Alternatively, a load may be applied to the frac plug  200  by pumping fluid or by pumping another tool into the well bore  120 , thereby dislodging the frac plug  200  and/or aiding the structural failure thereof. 
     Surface intervention may also occur in the event that the firing mechanism  253  fails to activate the heat source. Referring now to  FIG. 4 , in that scenario, an alternate firing mechanism  510  may be tripped into the well bore  120 . A slick line  500  or other type of work string may be employed to lower the alternate firing mechanism  510  near the frac plug  200 . In an embodiment, using its own internal timer, this alternate firing mechanism  510  may activate to ignite the torch  257  contained within the frac plug  200 . In another embodiment, the frac plug  200  may include a fuse running from the upper end of the tubular body member  210 , for example, down to the fuel load  251 , and the alternate firing mechanism  510  may ignite the fuse, which in turn ignites the torch  257 . 
     In still other embodiments, the torch  257  may be unnecessary. As an alternative, a thermite load may be positioned on top of the frac plug  200  and ignited using a firing mechanism  253 . Molten plasma produced by the burning thermite may then burn down through the frac plug  200  until the structural integrity of the plug  200  is compromised and the plug  200  falls away downhole. 
     Removing a consumable downhole tool  100 , such as the frac plug  200  described above, from the well bore  120  is expected to be more cost effective and less time consuming than removing conventional downhole tools, which requires making one or more trips into the well bore  120  with a mill or drill to gradually grind or cut the tool away. The foregoing descriptions of specific embodiments of the consumable downhole tool  100 , and the systems and methods for removing the consumable downhole tool  100  from the well bore  120  have been presented for purposes of illustration and description and are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many other modifications and variations are possible. In particular, the type of consumable downhole tool  100 , or the particular components that make up the downhole tool  100  could be varied. For example, instead of a frac plug  200 , the consumable downhole tool  100  could comprise a bridge plug, which is designed to seal the well bore  120  and isolate the zones above and below the bridge plug, allowing no fluid communication in either direction, Alternatively, the consumable downhole tool  100  could comprise a packer that includes a shiftable valve such that the packer may perform like a bridge plug to isolate two formation zones, or the shiftable valve may be opened to enable fluid communication therethrough. 
     In addition to an isolation tool, such as a frac plug, bridge plug or packer, the downhole tool  100  can be drill collars, as discussed more fully below with respect to  FIGS. 8-15C . 
     The plug shown in  FIG. 2  has a valve  225  at its upper end. When the valve  225  is closed the flowbore, or cavity,  205  is closed or plugged. The body member, or mandrel,  210  has apertures  261  located at or near the lower end of the body member. When the torch  257  is coupled to the plug  200 , the lower end of the body member  210  is closed. The apertures  261  allow flow into and out of the annulus  254  and the axial flowbore  205 . 
     The plug shown in  FIG. 2  has retainers, or holding components, in the form of slips  240  and slip bodies  245 . When setting the plug, the slips  240  move radially out to engage the wall of the well, which is shown in  FIG. 2  as casing  125 . Other holding components can be used, such as arms, etc. The mandrel  210  that holds the slips  240  and slip bodies  245  is also a holding component, because structural failure of the mandrel results in the plug  200  releasing from the secured position in the well. 
     The torch  257  produces a hot plasma, or cutting fluids, that can cut through, dissolve, melt, ignite or otherwise disrupt the structural integrity of a variety of materials. For example, the torch  257  can cut through composite materials. The torch can also cut through metals, such as steel, aluminum and magnesium. When cutting through metals such as steel or aluminum, the cutting fluids melt and erode the metal. Of metals, magnesium has particular attributes that make it useful for fabricating tool component parts. Magnesium is easily machined so that component parts can be fabricated with ease. Also, magnesium has high strength so that component parts will operate under adverse environments such as downhole. Furthermore, magnesium is highly flammable for metals, igniting with relative ease. Once ignited, it will burn, even if submerged. In a downhole environment, the plug or other isolation tool is submerged in well fluids. Thus, a downhole tool having holding components made of magnesium is easier to disable and release, or remove, than the same downhole tool having the same holding components made of non-magnesium materials. The cutting fluids of the torch ignite the magnesium components. Once ignited, the magnesium components combust. When holding components, such as slips  240 , are burned away, these components can no longer hold the tool and the tool falls away. Still other materials that can be used are combinations of magnesium and aluminum. Aluminum imparts strength to the part and burns easier than steel, while magnesium burns easier than aluminum. 
     Still other materials that can be used for the tool, and in particular the components that hold the tool in place in the well, include lead and lead derivatives. Lead can be used as a binder. A component made with lead as a material can be melted or dissolved by the heat of the cutting fluids. Fraccing wells are typically in the temperature range of 150-200° F., which is cool enough not to melt many lead alloys. Thus, lead alloys can be used as structural components of the tool, which components have a relatively low melting point suitable for the torch. 
       FIGS. 5-7  show the torch  257  in more detail. The torch has an elongated tubular body  610  which body has an ignition section  612 , a nozzle section  616  and a fuel section  614  intermediate the ignition and fuel sections. In the preferred embodiment, the tubular body is made of two components coupled together by threads. One component  620  is external to the downhole tool  200  and contains the ignition section  612  and part of the fuel section  614 . The external component  620  has a coupling  624 , such as threads, which allow the external component to couple to the tool  200 . In the embodiment shown in  FIG. 2 , the external component  620  is coupled to the lower end of the plug body member  210 . The other component  622  is received by, and is interior to, the tool  200 . The internal component  622  contains the remainder of the fuel section  614  and the nozzle section  616 . 
     The ignition section  612  contains an ignition source  625 . In the preferred embodiment, the ignition source is a thermal generator, previously described in my U.S. Pat. No. 6,925,937. The body of the thermal generator is incorporated into the body of the torch. The thermal generator is provided with a battery that provides electrical power for ignition. The firing mechanism  253  is connected to the thermal generator  625  so as to trigger ignition. As previously discussed, the firing mechanism  253  can trigger ignition by the ignition source  625  after a period of time has elapsed, after the temperature downhole has reached a pre-defined or threshold temperature, after the pressure downhole has reached a pre-defined threshold of pressure, etc. 
     The fuel section  614  contains the fuel  626 . The fuel can be made up of a stack of pellets which are donut or toroidal shaped. When stacked, the holes in the center of the pellets are aligned together; these holes are filled with loose fuel. When the fuel combusts, it generates hot combustion fluids that are sufficient to cut through a pipe wall, if properly directed. The combustion fluids comprise gasses and liquids and form cutting fluids. 
     The fuel  626 ,  251 , is a thermite, or modified thermite, mixture. The mixture includes a powered (or finely divided) metal and a powdered metal oxide. The powdered metal includes aluminum, magnesium, etc. The metal oxide includes cupric oxide, iron oxide, etc. In the preferred embodiment, the thermite mixture is cupric oxide and aluminum. When ignited, the flammable material produces an exothermic reaction. The flammable material has a high ignition point and is thermally conductive. The ignition point of cupric oxide and aluminum is about 1200 degrees Fahrenheit. Thus, to ignite the flammable material, the temperature must be brought up to at least the ignition point and preferably higher. It is believed that the ignition point of some thermite mixtures is as low as 900 degrees Fahrenheit. 
     The nozzle section  616  has a hollow interior cavity  628 . An end plug  630  is located at the free end of the nozzle section, which closes the cavity  628 . The cavity  628  contains fuel  626 . The fuel  626  extends in a continuous manner from one section to the next  612 ,  614 ,  616 . 
     The side wall  632  of the nozzle section  616  has openings  255  (see  FIGS. 5-7 ) that allow communication between the cavity  628  and the exterior of the nozzle section  616 . In the preferred embodiment, the openings  255  are slots. Each individual slot  255  extends in a longitudinal direction along the nozzle section. The slots are arranged in rows, with each row of slots being located around the circumference of the nozzle section at a particular longitudinal location. For example, each row has six slots, with the slots spaced sixty degrees apart from one another. The slots are relatively narrow, so as to have interstitial spaces  256  between adjacent slots. In addition, there are several rows of slots. For example, as shown in  FIG. 6 , there are six rows of slots. The rows are grouped into sets, namely an upper set  255 U and a lower set  255 L. The upper set  255 U is located next to the upper holding components  240 U (such as slips, slip bodies, etc.) (see  FIG. 2 ) of the downhole tool, while the lower set  255 L is located next to the lower holding components  240 L of the downhole tool. 
     The nozzle section  616  can be made of a material that is able to withstand the heat of the cutting fluids and remain intact long enough to cut the tool  200 . For example, the nozzle section can be made of a high carbon steel such as cast iron, can be made of tungsten or can be made of ceramic. Alternatively, the nozzle section can be made of some other material, such as low carbon steel, and is provided with a heat resistant liner  634  and a heat resistant plug  636 , which plug is adjacent to the end plug  630 . The liner  634  and plug  636  can withstand the temperatures of the ignited fuel and may be carbon based. The outside of the nozzle section  616  receives a sleeve  640 , which prevents fluid from entering through the openings  255 . O-rings  642  are located around the nozzle section on each side of the openings  255  and provide a seal between the nozzle section  616  and the sleeve  640 . 
     To assemble the tool, the torch  257  is inserted into the plug  200 , typically through the bottom end so as not to interfere with any valving or line connection at the upper end. The coupling  624  on the torch is used to connect the torch to the tool. When the torch is fully coupled to the tool, the slots  255  are aligned with and next to the holding components  240 ,  245  of the tool. The nozzle section  616  is located inside of the tool  200 , while the remainder of the torch depends from the lower end of the tool. 
     The length of the torch depends on the amount of fuel needed. If the cutting requires a relatively large amount of energy, then more fuel is needed. Because the outside diameter of the nozzle section  616  is limited by the inside diameter of the tool, to increase the fuel load, the torch can be lengthened (for example at  251  in  FIG. 2 ) so as to depend further below the tool. 
     Once the tool  100  is assembled, it can be lowered into the well by the work string  118 . Unlike my radial cutting torch in U.S. Pat. No. 6,598,679 and other torches, where the nozzle section is located below or downhole of the fuel section and igniter, this torch  257  is upside down, wherein the nozzle section is located above or uphole of the fuel section and igniter. Nevertheless, the torch works well. The fuel section depends from, or is located below, the nozzle section. 
     Because the torch  257  extends from the lower end of the plug  200 , and because the work string  118  couples to the upper end of the plug, the torch does not interfere with the lowering, placing or operation of the plug in the well. The plug is lowered to its desired location in the well. Once properly located, the plug is manipulated to engage the holding components and secure the tool in position in the well. For example, the slips  240  are manipulated to move along the slips bodies  245  and extend radially out to engage the casing. Engaging the slips also expands the packer element assembly  230 , wherein the well is plugged. The plug effectively isolates flow from one formation into another formation along the well. For example, in fraccing, high pressure is developed above the plug  200 . The plug prevents fraccing fluids from flowing into formations that are located below the plug. The plug can withstand differential pressures, such as are found in fraccing operations. If pressure below the plug is sufficiently greater than the pressure above the plug, then the valve  225  opens and allows fluid to flow. 
     Once the formation of interest has been fracced, the plug is no longer needed and can be removed by operating the torch  257 . 
     As discussed above, the torch is initiated by the igniter  253 . Suppose, for example, the igniter  253  contains a timer; after an elapsed period of time, the timer causes the igniter  253  to operate. The timer can be started when the tool is lowered into the well, when the tool reaches a threshold or pre-defined pressure (depth), when the tool encounters a threshold of pre-defined temperature, etc. The period of time is selected to allow proper use of the tool, plus some additional time. After the period of time elapsed, the igniter  253  ignites the fuel. 
     The fuel produces cutting fluids, which cutting fluids exit the torch at the nozzle slots  255 . The cutting fluids are directed radially out. Preferably, when the tool was assembled on the surface, the slots  255  were placed adjacent to the holding components  240 ,  245 . One advantage to the nozzle design shown in  FIG. 6  is that a single nozzle design and size can be used for a variety of tools. The provision of sets of slots, with each set having a number of rows of slots that extend along a longitudinal distance allows the tool to be used for a variety of spacings between upper and lower holding components. For example, in one tool, the holding components may align with the longitudinal center row of slots in each set, while in another tool, the holding components may align with the longitudinal lower row of slots in each set. 
     In addition to radial flow of the cutting fluids, there may be some longitudinal flow. For example, as shown in  FIG. 2 , the upper end of the flow chamber  205  is plugged by a valve  225 . The valve  225  is a one-way valve, but the pressure developed by the cutting fluids may be insufficient to overcome the head pressure acting on the valve from above, wherein the valve remains closed. As the cutting fluids flow from the nozzle section, a back pressure will build up above the torch. If the slots in the upper set  255 U of nozzles are incorrectly spaced, then the radial flow of cutting fluids exiting these slots will be counter-acted by the back pressure and these slots will in essence be plugged. Plugged slots no longer produce cutting fluids and the holding components located adjacent to the plugged slots will not be cut. However, with the nozzle design of the present invention, the cutting fluids can flow longitudinally through the interstitial spaces  256  between the slots. The radial elements of the cutting fluids are thus spaced sufficiently far apart to create interstitial spaces. These spaces allow longitudinal flow of cutting fluids. Thus, the nozzles in the upper set of slots do not become plugged and continue to produce cutting fluids cutting into the tool. As the longitudinal elements of the cutting fluids flow, these longitudinal elements will cut the mandrel at locations other than at the slips and the slip bodies. Thus, the tool is cut not only at the slips  240 , slip bodies  245 , but a length of the mandrel  210  is also cut as well. Cutting the slips, slip bodies and a length of the mandrel provides a high reliability in cutting the tool  200 . The apertures  261  near the lower end of the tool serve as vent ports and prevent back pressure at the lower end of the nozzle section. The result is the tool is released and falls to the bottom of the well. 
     Frequently a well has more than one formation of interest. As shown in  FIG. 7 , a typical well may have between 2-12 formations F 1 , F 2 , etc., which formations are fracced one at a time and separately from each other. When fraccing in a well with plural formations, the work begins with the bottommost formation and proceeds uphole one formation at a time. For example, the bottom formation is fracced first. Next, the second to bottom formation is fracced, and so on. A frac plug is placed or set in the well below the formation that is to be fracced. 
     The well has a rat hole  651 , which is the length of well that extends below the bottommost formation F 1 . During completion operations, such as fraccing, the rate hole may fill up, particularly in a well with many formations. The rat hole can fill with sand from fraccing operations and from the isolation tools that have been released and allowed to drop to the bottom of the well. When the rat hole fills up, the casing perforations of the bottommost formation F 1  may become plugged, wherein production from this bottommost formation is interrupted. Fishing debris from the bottom of the well adds to the overall cost of the well and may not be successful. 
     To prevent the rat hole from filling up, a bottommost isolation tool  100 B, such as a frac plug is set above the rat hole, which tool is equipped with a torch  257 . The isolation tool  100 B may be used to frac the bottommost formation. After the bottommost formation is fracced, the other formations are fracced or otherwise completed; the isolation tool  100 B is left in place above the rat hole. Thus, the well may have two or more isolation tools  100 B,  100 N in place at any given time. 
     In the prior art, using two or more isolation tools in a well at the same time is seen as creating problems because the isolation tools have to be removed by drilling out each tool. The uppermost tool  100 N, once released, falls on top of lower tool  100 B, thereby blocking access to the lower tool  100 B and making releasing the lower tool difficult if not impossible. 
     With the present invention, the bottommost isolation tool  100 B is left in place covering the rat hole  651  until all of the formations F 1 , F 2 , etc. are fracced or otherwise completed. Any sand that is above the bottommost tool  100 B can be removed by production fluids from the formations. The torch  257  is then used to release the bottommost tool, wherein the tool debris is allowed to fall to the bottom of the well. Because the sand has been removed, the debris falling into the rat hole is less in quantity than it would otherwise be. Thus, the rat hole is less likely to fill up, thereby preserving the production of the bottommost formation. The torch timer is set to ignite for a period of time that is the total time of fraccing operations in the well plus some additional time, such as an extra day or week. When the period of time elapses, the torch ignites and the bottommost torch is released and allowed to fall, along with any debris from other released tools that may be on top of the bottommost tool. 
     Turning now to drill collars, the present invention cuts a drill collar  11  (see  FIGS. 8 and 11 ) in a well  12 , thereby enabling the retrieval and future reuse of some or most of the drill collar string. The present invention utilizes a cutting torch  15  lowered down inside of the drill string  17 . A torch is positioned at one of the joints  21  of one of the drill collars. The joints are high torque couplings. 
     When the torch  15  is ignited (see  FIG. 9 ), it produces combustion fluids  81 . The combustion fluids form a longitudinal slice or cut  23  through the coupling  21 . This is different than conventional cutting techniques that cut a pipe all around its circumference. The longitudinal cut effectively splits the coupling (see  FIGS. 10 and 12 ). Because the coupling is under high torque before being cut, after being cut it unwinds and decouples. Thus, a relatively small amount of cutting energy can effectively cut a thick walled drill collar  11 . The portion of the drill collar string that is decoupled is retrieved. 
     The present invention will be discussed now in more detail. First, a drill collar  11  will be discussed, followed by a description of the torch  15  and then the cutting operation will be discussed. 
     Referring to  FIG. 8 , the drill collar  11  is part of a drill string  13  that is located in a well  12  or borehole. The drill string  13  typically has a bottom hole assembly made up of a drill bit  25  and its sub and one or more drill collars  11 . There may be other components such as logging while drilling (LWD) tools, measuring while drilling (MWD) tools and mud motors. Drill pipe  27  extends from the bottom hole assembly up to the surface. The drill string may have transition pipe, in the form of heavy weight drill pipe between the drill collars and the drill pipe. The drill string forms a long pipe, through which fluids, such as drilling mud, can flow. 
     The various components of the drill string are coupled together by joints. Each component or length of pipe has a coupling or joint at each end. Typically, a pin joint is provided at the bottom end, which has a male component, while a box joint is provided at the upper end, which has a female component. For example, as shown in  FIG. 8 , the lower joint of a drill collar  11  is a pin joint  21 A, while the upper joint  21 B is a box joint. 
     As illustrated in  FIG. 8 , the drill collar  11  is a heavy or thick walled pipe. The thickness of the drill collar wall  31  is greater than the thickness of the drill pipe wall  33 . A passage  35  extends along the length of the drill collar, between the two ends. 
     The wall thickness of the pin joint  21 A is less than the thickness of the wall  31  of the drill collar portion that is located between the two ends. Typical dimensions of the pin joint are 4 inches in length and ½ to 1 inch in wall thickness. The pin joint is tapered to fit into the similarly tapered box joint  21 B. 
     The joints or couplings in the drill string and particularly in the drill collars are tight due to drilling. During drilling, the drill string  13  is rotated. This rotation serves to tighten any loose couplings. Consequently, the joints are under high torque. 
     The cutting torch  15  is shown in  FIG. 13 . The torch  15  has an elongated tubular body  41  which body has an ignition section  43 , a nozzle section  45  and a fuel section  47  intermediate the ignition and fuel sections. In the preferred embodiment, the tubular body is made of three components coupled together by threads. Thus, the fuel section  47  is made from an elongated tube or body member, the ignition section  43  is made from a shorter extension member and the nozzle section  45  is made from a shorter head member. 
     The ignition section  43  contains an ignition source  49 . In the preferred embodiment, the ignition source  49  is a thermal generator, previously described in my U.S. Pat. No. 6,925,937. The thermal generator  49  is a self-contained unit that can be inserted into the extension member. The thermal generator  49  has a body  51 , flammable material  53  and a resistor  55 . The ends of the tubular body  51  are closed with an upper end plug  57 , and a lower end plug  59 . The flammable material  53  is located in the body between the end plugs. The upper end plug  57  has an electrical plug  61  or contact that connects to an electrical cable (not shown). The upper plug  57  is electrically insulated from the body  51 . The resistor  55  is connected between the contact  61  and the body  51 . 
     The flammable material  53  is a thermite, or modified thermite, mixture. The mixture includes a powered (or finely divided) metal and a powdered metal oxide. The powdered metal includes aluminum, magnesium, etc. The metal oxide includes cupric oxide, iron oxide, etc. In the preferred embodiment, the thermite mixture is cupric oxide and aluminum. When ignited, the flammable material produces an exothermic reaction. The flammable material has a high ignition point and is thermally conductive. The ignition point of cupric oxide and aluminum is about 1200 degrees Fahrenheit. Thus, to ignite the flammable material, the temperature must be brought up to at least the ignition point and preferably higher. It is believed that the ignition point of some thermite mixtures is as low as 900 degrees Fahrenheit. 
     The fuel section  47  contains the fuel. In the preferred embodiment, the fuel is made up of a stack of pellets  63  which are donut or toroidal shaped. The pellets are made of a combustible pyrotechnic material. When stacked, the holes in the center of the pellets are aligned together; these holes are filled with loose combustible material  65 , which may be of the same material as the pellets. When the combustible material combusts, it generates hot combustion fluids that are sufficient to cut through a pipe wall, if properly directed. The combustion fluids comprise gasses and liquids and form cutting fluids. 
     The pellets  65  are adjacent to and abut a piston  67  at the lower end of the fuel section  47 . The piston  67  can move into the nozzle section  45 . 
     The nozzle section  45  has a hollow interior cavity  69 . An end plug  71  is located opposite of the piston  67 . The end plug  71  has a passage  73  therethrough to the exterior of the tool. The side wall in the nozzle section  45  has one or more openings  77  that allow communication between the interior and exterior of the nozzle section. The nozzle section  45  has a carbon sleeve  79  liner, which protects the tubular metal body. The liner  75  is perforated at the openings  77 . 
     The openings are arranged so as to direct the combustion fluids in a longitudinal manner. In the embodiment shown in  FIG. 14 , the openings  77  are arranged in a vertical alignment. The openings  77  can be rectangular in shape, having a height greater than a width. Alternatively, the openings can be square or circular (as shown). In another embodiment, the nozzle section  45  can have a single, elongated, vertical, slot-type opening. 
     The piston  67  initially is located so as to isolate the fuel  63  from the openings  77 . However, under the pressure of combustion fluids generated by the ignited fuel  63 , the piston  67  moves into the nozzle section  45  and exposes the openings  77  to the combustion fluids. This allows the hot combustion fluids to exit the tool through the openings  77 . 
     The method will now be described. Referring to  FIG. 8 , the torch  15  is lowered into the drill string  13 , which drill string is stuck. Before the torch is lowered, the decision has been made to cut the drill string and salvage as much of the drill string as possible. Also, the drill string is stuck at a point along the drill collar string or below the drill collar string. 
     The torch  15  can be lowered on a wireline, such as an electric wireline. The torch is positioned inside of the drill collar  11  which is to be cut. Specifically, the openings  77  are located at the same depth of the pin coupling  21 A which is to be cut. The length of the arrangement of openings is longer than the pin joint. The longer the arrangement of openings, the less precision is required when positioning the torch relative to the pin joint  21 A. Then, the torch is ignited. An electrical signal is provided to the igniter  49  (see  FIG. 13 ), which ignites the fuel  65 ,  63 . The ignited fuel produces hot combustion fluids. The combustion fluids  81  produced by the fuel force the piston  67  down and expose the openings  77 . The combustion fluids  81  are directed out of the openings  77  and into the pin coupling  21 A (see  FIG. 9 ). The combustion fluids are directed in a pattern that is longitudinal, rather than circumferential. The combustion fluid pattern is at least as long as the pin joint, and in practice extends both above and below the pin joint. 
     The torch creates a cut  23  along the longitudinal axis in the pin joint  21 A (see  FIGS. 10 and 12 ). The pin  21 A is severed. The portions of drill collar above and below the pin joint have longitudinal cuts therein, but due to the wall thickness, these cuts do not extend all the way to the outside.  FIG. 12  shows the cut extending part way into the corresponding box joint. Thus, the box joint and the portions of the drill collar above and below the pin joint are not cut completely through and are unsevered. Nevertheless, when the pin joint is cut, it unwinds or springs open. The joint decouples and the drill string becomes severed at the joint. Thus, only the pin joint need be cut to sever the drill collar. That portion of the drill string that is unstuck, the upper portion, is retrieved to the surface. 
     The drill collar  11  that was cut at its pin joint can be reused. Referring to  FIG. 15A , the pin joint  21 A has a longitudinal cut  23  therein. The pin joint  21 A is cut off of the drill collar, as well as any damaged portions of the collar to form a clean end  83  (see  FIG. 15B ). The end  83  is remachined to form a new pin joint (see  FIG. 15C ). The drill collar can now be reused. 
     Each of the torches can be provided with ancillary equipment such as an isolation sub and a pressure balance anchor. The isolation sub typically is located on the upper end of the torch and protects tools located above the torch from the cutting fluids. Certain well conditions can cause the cutting fluids, which can be molten plasma, to move upward in the tubing and damage subs, sinker bars, collar locators and other tools attached to the torch. The isolation sub serves as a check valve to prevent the cutting fluids from entering the tool string above the torch. 
     The pressure balance anchor is typically located below the torch and serves to stabilize the torch during cutting operations. The torch has a tendency to move uphole due to the forces of the cutting fluids. The pressure balance anchor prevents such uphole movement and centralizes the torch within the tubing. The pressure balance anchor has either mechanical bow spring type centralizers or rubber finger type centralizers. 
     While various embodiments of the invention have been shown and described herein, modifications may be made by one skilled in the art without departing from the spirit and the teachings of the invention. The embodiments described here are exemplary only, and are not intended to be limiting. Many variations, combinations, and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, the scope including all equivalents of the subject matter of the claims.