You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to systems for unloading fluids from generally upright tubular members of a wellbore. In. another aspect, the invention concerns a method and apparatus for efficiently and effectively unloading fluids from production tubing of a subterranean hydrocarbon well by utilizing a fuel cell to generate content-lifting gases in the tubing. 
     2. Discussion of Prior Art 
     In preparation for producing, and during the production of, hydrocarbons from subterranean formations, it is often necessary to unload the fluid contents of a fluid-filled production tubing string before hydrocarbon production can begin or continue. For example, certain processes either involve introducing fluid into the production tubing that must later be removed prior to production (e.g., drilling fluids, fracturing fluids, completion fluids, production fluids, etc.) or require fluids already in the production tubing to be removed (e.g., water, oil, condensate, etc.). 
     There are systems known in the art for unloading contents from well tubing. For example, well operators typically run coiled tubing into the well and pump the contents out of the production tubing. It is also known in the art to introduce certain gases (e.g., nitrogen) into a liquid-filled tubular to create a “bubbling” lifting force to assist in removing liquid from the tubing string. 
     These prior art systems are problematic and suffer from several limitations. For example, the coiled tubing, while effective, is inefficient as it is considerably time-consuming and requires significant additional equipment and materials to operate. Prior methods of unloading a well using gases are ineffective and may expose the formation or casing annulus to undesired elevated pressure in order to introduce sufficient gases into the well. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved system for unloading fluids from tubular members (e.g., tubing or casing) of a wellbore that does not suffer from the problems and limitations of the prior art systems as set forth above. The inventive system provides a way to effectively and efficiently unload fluids from a well tubing or casing without exposing the formation or casing to undesired elevated pressure. 
     In accordance with one embodiment of the present invention, a method of unloading a fluid from a generally upright tubular member of a wellbore is provided. The method comprises the steps of: (a) placing a combustible fuel cell inside the tubular member under the fluid to be unloaded; (b) burning the fuel cell to generate a gas in the tubular member; and (c) using the gas to force at least a portion of the fluid upward out of the tubular member. 
     In accordance with another embodiment of the present invention, an apparatus for unloading a fluid contained in a generally upright tubular member of a wellbore is provided. The apparatus generally comprises a combustible and expandable fuel cell, a cap coupled to the fuel cell, and an ignition device coupled to the fuel cell. The fuel cell is adapted to be received in the tubular member. The fuel cell defines an internal chamber which is adapted to be filled with a gas generated by the fuel cell when the fuel cell is burned. The cap is operable to hold the gas in the internal chamber until the pressure of the gas in the internal chamber causes the fuel cell to expand sufficiently to form a seal with the tubular member. The ignition device is operable to initiate burning of the fuel cell when the ignition device is actuated. 
     In accordance with a still further embodiment of the present invention, a wellbore extending into a subterranean formation is provided. The wellbore comprises a generally upright tubular member, a fluid disposed in the tubular member, a combustible fuel cell disposed in the tubular member generally below at least a portion of the fluid, and an ignition device coupled to the fuel cell. The fuel cell is operable to generate a gas when burned. The ignition device is operable to initiate burning of the fuel cell when the ignition device is actuated. 
     Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Preferred embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein: 
     FIG. 1 is a partial sectional view of an apparatus for unloading well tubing constructed in accordance with the principles of the present invention and shown schematically in a plugged production tubing inside a perforated well casing; 
     FIG. 2 is a sectional view of the apparatus taken substantially along line  2 — 2  of FIG. 1; and 
     FIG. 3 is a partial sectional view of an alternative apparatus for unloading well tubing constructed in accordance with the principles of the present invention and shown schematically in a plugged production tubing inside a perforated well casing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning initially to FIG. 1, the apparatus  10  for unloading well tubing selected for illustration is shown submerged at the bottom of a production tubing T for producing hydrocarbons from a subterranean well. The production tubing T is filled with fluid (e.g., water, oil, condensate, drilling fluids, fracturing fluids, completion fluids, production fluids, etc.). The production tubing is plugged by a plug P. The tubing T is located in a perforated well casing C. However, the principles of the present invention are applicable to unload the contents of virtually any kind of tubular in any type of well. For example, the present invention could be utilized to unload fluid contents from a well casing. The apparatus  10  broadly includes a combustible fuel cell  12  operable to generate content-lifting gases as it combusts, a cap  14  cooperating with the cell  12  to provide a desired seal, and an ignition assembly  16  operable to ignite the cell  12 . 
     The fuel cell  12  is adapted to be inserted into the tubing T and positioned below the fluid to be unloaded therefrom. In particular, the illustrated fuel cell  12  is generally cylindrical in shape and includes an upper end  18  and a lower end  20  axially spaced from the upper end  18 . The fuel cell  12  preferably has a generally annular cylindrical configuration and defines an outer diameter (prior to ignition) that is less than the inner diameter of the tubing T (e.g., typical production tubing has an inner diameter between 2 and 7 inches). Because the fuel cell  12  is inserted into a fluid-filled well tubing, it is important that the outer diameter of the fuel cell  12  provide sufficient clearance between the inner wall of the tubing T to allow the fuel cell  12  to be submerged to the desired position—i.e., below the fluid to be unloaded (e.g., typically this will be towards or at the bottom of the well tubing). However, for purposes that will subsequently be described, it is further important that the outer diameter of the illustrated fuel cell  12  be as large as possible and still allow the desired insertion clearance. 
     As shown in FIGS. 1 and 2, the illustrated fuel cell  12  defines an internal burn hole  22 . The burn hole  22  is generally located around the central longitudinal axis of the fuel cell  12  and extends between the upper and lower ends  18 , 20 . The burn hole  22  is configured to influence the rate at which the fuel cell  12  burns once it is ignited (e.g., accelerating over time). It is believed the rate at which the fuel cell  12  burns will be proportional to the surface area of fuel exposed to the hot gases created by the burn. For purposes that will subsequently be described, it is important for the illustrated fuel cell  12  to burn slowly at first (e.g., just after ignition) so that the corresponding pressure created by the gases formed in the burn also builds slowly at first. In this regard, the surface area of fuel exposed to the burn will, upon ignition, initially be limited to the surface area of fuel defining the internal burn hole  22 . However, as the fuel burns, the diameter of the burn hole  22  will expand, thereby exposing more surface area of fuel to the burn. It is believed that as the surface area of fuel exposed to the burn increases, the rate of combustion of the fuel cell also increases. 
     As previously indicated, the fuel cell  12  is combustible and operable to generate content-lifting gases as it combusts. In this regard, the illustrated fuel cell  12  is preferably formed from an expandable propellent-type material that burns at a relatively slow rate of combustion, produces relatively large amounts of gases as it bums, burns without utilizing heat or oxygen external to the cell  12 , and substantially incinerates when burned. As will subsequently be described in detail, the fuel cell  12  may operate as a “rocket assist” to create a gas bubble that drives the fluid up and out of the tubing string T. Depending on the application, a tubing string could be as much as 10,000 feet deep and filled with fluid that must be unloaded. It is therefore important that the fuel cell  12  burn at a slow enough rate of combustion to prevent damaging the tubing string T and create a sufficient and sustained lifting force to unload the vast amount of fluid out of the top of the tubing string T. For example, an explosion-type burn is undesired because it could both damage the tubing string and may not sufficiently expel the fluid out of the top of the tubing string. In an exemplary application involving a 7 inch diameter tubing filled with fluid to a depth of 5000 feet, a representative burn rate would be 2-3 minutes for a fuel cell having an axial length of around thirty feet. It is also important that the fuel cell  12  produce large amounts of gases as it burns in order to create the necessary lifting force to drive the fluids up the tubing string T and expel them out of the top of the string T. Because the fuel cell  12  is ignited once it has been submerged beneath the fluids to be unloaded, it is further important that the cell  12  burn without the need to utilize heat or oxygen from a source external to the cell  12 . To facilitate cleaning the tubing string after it has been unloaded, it is preferred that the fuel cell  12  substantially incinerate upon burning. In this regard, it is preferred that the fuel cell  12  be formed from a propellant-type material that exists in a solid form (e.g., gel-like, etc.) to eliminate the need for any casing structure. An exemplary material suitable for the construction of the fuel cell  12  is available from Atlantic Research Corporation, Gainesville, Va., under the trade name ARCITE 479. However, it is within the ambit of the present invention to utilize virtually any material having the desired burn characteristics. For example, a liquid fuel cell packed in a solid casing could be utilized. 
     The cap  14  cooperates with the fuel cell  12  to provide a seal below the fluid to be unloaded from the well tubing T after the fuel cell  12  is positioned therein. In the illustrated apparatus  10 , the cap  14  is integrally formed with the fuel cell  12  proximate the upper end  18  and is formed from a similar combustible propellant-type material. The cap  14  is configured to control the pressure within the fuel cell  12  after the cell  12  is ignited until a threshold pressure is achieved. In particular, the fuel cell  12 , once ignited, begins to burn thereby creating gases. During these early stages of the burn, the cap  14  prevents the gases from exiting the upper end  18  of the cell  12  thereby causing pressure to build within the cell  12 . As the pressure builds, it causes the fuel cell  12  to radially expand until the circumferential surface thereof seals against the inside wall of the tubing T. Once the cell  12  seals against the tubing T, the cell  12  continues to burn (at a faster rate) producing more gases and thereby building further pressure. During these middle stages of the burn, the cap  14  continues to prevent the gases from exiting the upper end  18  of the cell  12 . During these middle stages of the burn, the pressure eventually overcomes the friction forces between the circumferential surface of the cell  12  and the inner wall of the tubing T causing the unburned portion of the cell  12  to begin to shift upwards. Although the unburned portion of the cell  12  loses its frictional grip on the internal wall of the tubing T, it maintains the seal between the contents above the cell  12  and the gases therebelow. As the cell  12  shifts up the tubing T, fluid above the cell  12  is driven up the tubing T. Once the cell  12  has begun to drive the fluid up the tubing T, the cell  12  continues to burn (at still a faster rate) producing even more gases and thereby building even further pressure. During these late stages of the burn, the cap  14  ruptures allowing the gases within the cell  12  to expand out of the cell  12 . As the gases expand out of the cell  12 , they force the fluid inside the tubing T upward until the fluid is expelled therefrom. Once the fluid is expelled from the tubing T, the gases vent to the surface atmosphere out of the tubing T. During these final stages of the burn, any fuel remaining in the tubing T continues to burn until it is consumed leaving the tubing T unloaded and clean. 
     The illustrated apparatus  10  includes a bottom cap  24  integrally formed with the fuel cell  12  proximate the lower end  20 . The bottom cap  24  is formed from a similar combustible material so that it is consumed during the final stages of the burn. However, the bottom cap  24  is configured to withstand pressure in excess of the pressure at which the cap  14  ruptures. In this manner, the bottom cap  24  protects anything down-hole of the cell  12  from the gases generated thereby. For example, the bottom cap  24  prevents the gases generated by the cell  12  from penetrating the well, the perforations, the formation, etc. It is within the ambit of the present invention to utilize alternative fuel cell configurations that do not utilize a bottom cap. It is also within the ambit of the present invention to utilize a plug (e.g., the plug P) to prevent the cell-generated gases from escaping down-hole. If a plug is used, it must be placed in the well tubing prior to inserting the fuel cell (e.g., in any manner commonly known in the art) and can be removed once the well is unloaded (e.g., using a slick line, etc.). It is preferred to utilize a plug to protect the sand face of the formation when the well has already been perforated. 
     As previously indicated, the ignition assembly  16  is operable to ignite the fuel cell  12  after the cell  12  is positioned in the tubing T. The illustrated assembly  16  includes an electric triggering device  26 , a communication wire  28 , and a fuse  30 . The triggering device  26  is located on top of the cap  14  and includes a connecting element (not shown) adapted to electrically, mechanically, and removably connect the triggering device  26  to a well line. For example, the illustrated apparatus  10  is preferably coupled to a wire-line  31  for inserting the apparatus  10  into the well tubing T and setting it in its submerged position therein. The wire-line  31 , in a manner known in the art, also carries electric current from a source external to the well tubing T. The wire-line  31  conveys the electric current to the triggering device  26 . The triggering device  26  is electrically coupled to the communication wire  28 . The wire  28  is in firing communication with the fuse  30 . The triggering device  26  generates a firing signal that is conveyed through the communication wire  28  to the fuse  30  where the firing signal causes the fuse  30  to light. The fuse  30 , once lit, starts the fuel cell  12  burning. The fuse  30  is positioned in the burn hole  22  adjacent the lower end  20  of the cell  12  so that the cell  12  begins burning at the lower end  20  and burns radially outward from the burn hole  22 . It is preferred that the wire-line  31  be removed once the firing signal has been generated. It is within the ambit of the present invention to utilize alternative ignition assemblies. For example, the triggering device  26  could be a time trigger or a pressure trigger that do not require the use of a wire-line to either set the apparatus  10  or deliver electric current thereto. However, it is important that the ignition assembly be able to ignite the cell  12  at a desired location after the cell  12  is submerged in the desired position in the well tubing T. 
     It is within the ambit of the present invention to utilize various alternative configurations, designs, materials, etc. for the apparatus for unloading well tubing. However, it is important that the apparatus is configured to be submerged in the fluid in the well tubing, ignited therein, and operable to generate content-lifting gases which can be used to drive the fluid contents up the tubing and expel them therefrom. An alternative embodiment is the apparatus  100  for unloading well tubing as illustrated in FIG.  3 . The apparatus  100  is illustrated in an environment similar to the environment previously discussed above with respect to the apparatus  10 . That is, the apparatus  100  is illustrated submerged toward the bottom of a plugged fluid-filled well tubing T that is incased in a perforated well casing C. The apparatus  100  broadly includes a combustible fuel cell  102  operable to generate content-lifting gases as it combusts, a plunger  104  operable to displace fluids in the tubing T when shifted within tubing T, and an ignition assembly  106  operable to ignite the cell  102 . 
     The fuel cell  102  is similar to the previously discussed fuel cell  12 , however, for reasons that will subsequently become clear, the fuel cell  102  need not be expandable and therefore does not include either a burn hole or a bottom cap. The fuel cell  102  is generally cylindrically shaped and includes axially spaced upper and lower ends  108  and  110 , respectively. Other than the need to be expandable, the fuel cell  102  includes all of the burn qualities previously detailed with respect to the fuel cell  12  and is preferably formed of the same or a similar material. 
     The plunger  104  may provide a slidable seal below the fluid to be unloaded from the well tubing T after the fuel cell  102  is positioned in the well tubing T. The plunger  104  is positioned proximate the upper end  108  of the fuel cell  102 . The illustrated plunger  104  is shown in contact with and coupled to the cell  102 , however, it is within the ambit of the present invention for the plunger to be spaced from the upper end  108  of the fuel cell  102 . The plunger  104  may be adapted to create and maintain a mechanical seal with the inside wall of the well tubing T. In this regard, the plunger  104  is preferably not formed from a propellant-type material. There are several ways known in the art to create a down-hole mechanical slidable seal inside a well tubing (e.g., elastomeric seals, spring-biased sealing pads, etc., that are typically used in artificial plunger lift systems) and any of these can be utilized to seal the plunger  104 . It is also within the ambit of the present invention to utilize non-mechanical seals, such as a grooved cap sealed by upward gas flow. However, it is important that the seal be adapted to slide up the well tubing T while maintaining a relationship with the inner wall of the well tubing T sufficient to displace at least a portion of the fluid contents to be unloaded. In this regard, the plunger  104  may not completely seal against the inner wall of the well tubing T. It is within the ambit of the present invention to utilize a plunger  104  having an outer diameter that provides sufficient clearance from the inner diameter of the well tubing T to allow the apparatus to “drift” down the fluid-filled tubing T during insertion into the desired unloading position in the tubing T. However, the clearance between the outer diameter of the plunger  104  and the inner diameter of the tubing T should be sufficiently minimal so that when the fuel cell  102  is ignited and begins to generate content-lifting gases, the plunger  104  isolates at least a substantial portion of the fluid contents to be unloaded from the fuel cell  102 . 
     As the fuel cell  102  begins to burn, the pressure of the generated gas builds below the plunger  104  until the gases drive the plunger  104  up the well tubing T. As the plunger  104  slides up the tubing T, the fluid above the plunger  104  is also driven up the tubing string T until it is expelled out of the top of the tubing T. 
     The ignition assembly  106 , unlike the previously described assembly  16 , preferably comprises a pressure triggering device. The ignition assembly  106  is coupled to the lower end  110  of the cell  102 . In one manner known in the art, the pressure trigger of the assembly  106  ignites once the trigger is exposed to a threshold pressure (e.g., ignition instigated by the weight of the cell  102  and the fluids thereabove compressing the assembly  106  against the plug toward the bottom of the tubing T). Once the trigger is ignited, it burns through the lower end  110  of the cell  102  thereby igniting the cell  102 . An exemplary pressure trigger device suitable for use in the ignition assembly  106  is available from Pacific Scientific Energetic Materials Co., Chandler, Ariz. as model no. PS-190 CP/HNS. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.

Summary:
A system for unloading fluids from a generally upright tubular member of a wellbore wherein a combustible fuel cell can be placed in the tubular member below the fluid and ignited to generate gasses. The gasses generated by the burning of the fuel cell can be used to force at least a portion of the fluid upward out of the tubular member.