Abstract:
An apparatus for fracturing wells employs a propellant charge with a metallic foil to rapidly ignite the surface of the propellant charge. The assembly can be covered with an protective coating to protect against fluids in the well bore.

Description:
RELATED APPLICATION  
       [0001]     The present application is based on, and claims priority to the Applicant&#39;s U.S. Provisional Patent Application No. 60/618,248, entitled “Propellant for Fracturing Wells,” filed on Oct. 13, 2004, and U.S. Provisional Patent Application No. 60/621,693, entitled “Propellant for Fracturing Wells,” filed on Oct. 25, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to the field of well fracturing. More specifically, the present invention discloses a propellant assembly for fracturing wells.  
         [0004]     2. Statement of the Problem  
         [0005]     Propellant charges have been used for many years to create fractures in oil, gas and water-bearing formations surrounding a well.  FIG. 1  is a cross-section diagram of a well  10  with a packer  12  and a series of propellant charges  20 . The propellant charges  20  are ignited to rapidly generate combustion gases that create sufficient pressure within the well bore to generate fractures in the surrounding strata.  
         [0006]     In order to achieve proper pressure-loading rates and adequate minimum pressures for sustained periods of time sufficient to extend fractures in the surrounding formations using gas-generating propellants, it is necessary that a sufficient surface area of propellant be burning to generate the volume of gas required to extend such fractures, as gas generation is a function of the surface area of the propellant burning at any given time.  
         [0007]     Typical ignition systems for propellant incorporate detonating or deflagrating materials in a cord-like format. Such ignition systems, however, ignite only small areas of the propellant immediately adjacent to the detonating or deflagrating cord. In addition, detonating cords tend to have two problems: (1) if too brisant, the cord tends to shatter the surrounding propellant, resulting in initial burn areas that are unknown and difficult to model; and (2) the cord only ignites small areas immediately adjacent to the cord, relying on flame spread to initiate adjacent surface areas. Deflagrating cords have limited burn rates (on the order of 1000 ft/sec) that are insufficient to ignite large areas of the propellant surface within a multi-millisecond time frame.  
         [0008]     If ignition of the propellant is limited to small areas of the propellant surface, the flame from the initial burning area of the propellant must spread across the face of the propellant to ignite the remaining surface area. This flame spread rate is a key limiting factor to achieving proper pressure loading rates and adequate minimum pressures for fracture propagation in the surrounding formations. If the flame spread from a localized ignition point is too slow, then the burning surface area at any given point in time will be limited, and the overall time that the propellant burns to completion may have to be extended sufficiently to compensate for the reduced amount of time that pressures exceed the minimum required fracture extension pressure, resulting in a longer but less efficient propellant burn.  
         [0009]     In addition, the propellant burn should be predictable and reproducible for the purpose of accurately modeling the fracturing process. It is difficult to accurately model a propellant burn unless the entire exposed surface of the propellant is ignited almost simultaneously. Modeling of propellants has been contemplated in the past, but with the assumption that ignition of the propellant surface over the entire exposed area of the propellant is simultaneous. Practically speaking, such simultaneous ignition is difficult to achieve.  
         [0010]     The problem is further complicated by the presence of well fluids. When propellants are submerged in well fluids such as water (or water and potassium chloride), flame spread rates tend to decrease. In addition, certain chemical coverings that are used as surface coatings on propellants to prevent leaching of the propellant fuel oxidizers into the surrounding well fluids also tend to inhibit the flame spread rate, thus exacerbating the problem. When such coatings are not applied to the surface of the propellant, sufficient leaching of the fuel oxidizer can take place over relatively short periods of time (i.e., about 1 hour) to result not only in a reduction in the available energy to do work on the formation, but also create an outer boundary layer absent of fuel oxidizer and comprised primarily of the propellant binder, which tends to inhibit the flame spread rate because the exposed fuel oxidizer in the binder has been leached away. Furthermore, because gas generation is a function of the area of propellant burning at any given time, it is also useful to engineer a propellant fracturing system that accounts for the required initial burning surface area to provide adequate pressure rise, in addition to taking into account the flame spread rate.  
         [0011]     In addition, it would be preferable to configure the propellant such that there is a rapid decrease in the burning surface area, rather than a slow regressive decrease in area to maintain the pressures above that of the fracture extension pressure as long as possible. This provides the most efficient use of the available bond energy of the propellant that is burned in the well.  
         [0012]     In summary, the prior art has the following shortcomings: 
        Detonating cord does not ignite sufficient surface area and relies on flame spread.     Detonating cord, if made too energetic to overcome the limited ignition area problem, can be too brisant and may shatter the propellant, resulting in an unknown burning surface area.     Flame spread is too slow to achieve adequate burning surface area of propellant for proper loading rate to cause multiple fractures.     Slow flame spread results in slow pressure rise, increasing heat loss by conduction into the surrounding well fluids, reducing the useful work available to extend fractures.     Insufficient burning surface areas do not result in generated pressures above that of fracture extension, limiting effectiveness.     Burning rate and flame spread are limited when the propellant is surrounded by well fluids.     Sealers tend to inhibit the flame spread.        
 
         [0020]     Solution to the Problem. One solution to address the problems discussed above is to rapidly ignite the entire surface of the propellant charge by means of a metallic foil (e.g., a bimetallic nickel-aluminum, nickel-palladium, or nickel-zirconium foil) in order to produce a burn that is reproducible, and can be accurately modeled to predict the resulting conditions in the well and surrounding strata during the fracturing process.  
       SUMMARY OF THE INVENTION  
       [0021]     This invention provides an apparatus for fracturing wells that employs a propellant charge with a metallic foil to rapidly ignite the surface of the propellant charge. The resulting rapid ignition of the propellant surface can be modeled more accurately and results in a more efficient use of the propellant charge in fracturing the well.  
         [0022]     These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The present invention can be more readily understood in conjunction with the accompanying drawings, in which:  
         [0024]      FIG. 1  is a cross-sectional diagram of a well  10  with a packer  12  and a series of propellant charges  20 .  
         [0025]      FIG. 2  is a cross-sectional view of a propellant assembly prior to ignition.  
         [0026]      FIG. 3  is a cross-sectional view of the propellant assembly in  FIG. 2  after ignition.  
         [0027]      FIG. 4  is a cross-sectional view of another embodiment of the propellant assembly prior to ignition.  
         [0028]      FIG. 5  is a cross-sectional view of the propellant assembly in  FIG. 4  after ignition.  
         [0029]      FIG. 6  is a side cross-sectional view of another embodiment of the propellant assembly.  
         [0030]      FIG. 7  is an orthogonal cross-sectional view of the embodiment of the propellant assembly shown in  FIG. 6 .  
         [0031]      FIG. 8  is a top view of another embodiment of the propellant assembly using a series of bimetallic ignition strip fuses  80  to ignite the metallic foil  30 . A portion of the outer protective layer  40  has been removed to show the ignition strip fuses  80  and foil  30 .  
         [0032]      FIG. 9  is an orthogonal cross-sectional view of the embodiment shown in  FIG. 8 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     Turning to  FIG. 2 , a cross-sectional view is shown of one embodiment of the propellant assembly prior to ignition. The propellant charge has been divided longitudinally into two segments  20   a  and  20   b  having opposing interior surfaces. Alternatively, the propellant charge could be divided into thirds, quarters, or any other desired fractional shape. The outer surface of the combined segments of the propellant charge  20   a ,  20   b  has a generally cylindrical shape with dimensions suitable for insertion into a well bore.  
         [0034]     A metallic foil  30  is sandwiched adjacent to the interior surfaces of propellant segments  20   a ,  20   b  and optionally around the circumference of the propellant to ignite these surfaces. Bimetallic foils  30  have been demonstrated to generate enough heat to ignite the surfaces of the propellant segments  20   a ,  20   b . For example, a bimetallic foil having a thickness of approximately 30 microns made of nickel-aluminum, nickel-palladium or nickel-zirconium has been found to be suitable, although other metallic foils could be substituted. It should be understood that a thin metallic mesh could also be substituted, and should be interpreted as falling with the scope of a “foil” for the purposes of this invention.  
         [0035]     An ignition element  50  is employed to initially ignite the metallic foil  30 . Preferably, an extremely mild detonating cord is used as the ignition element  50 . The detonating cord is sufficiently mild to not shatter the foil  30  or propellant segments  20   a ,  20   b , yet it ignites the metallic foil  30 , which in turn ignites the propellant segments  20   a ,  20   b . For example, a mild detonating cord having 2.5 grains per foot of HNS explosive sheathed in lead could be employed. The detonating cord  50  can be ignited conventionally (e.g., with an igniter patch). The detonating cord  50  can either be enclosed in a metal sheath (e.g., a lead or mild steel tube), or placed directly in contact with the foil  30 . Mild detonating cord is also commercially available with various metal sheathes, such as silver, aluminum or tin.  
         [0036]     The propellant  20   a ,  20   b  is configured to directly contact the metallic foil  30  such that it maximizes the exposure to the fuel oxidizer component of the propellant  20 . The mild detonating cord has a burn rate of approximately 17,000-20,000 ft per second, and thus the metallic foil  30  is ignited along the area adjacent to the mild detonating cord  50  within approximately 2.5 milliseconds for a practical-sized propellant treatment (less than 50 ft). Note that most propellant treatments are in the range of 10 to 20 ft., reducing this initiation time to less than 1 millisecond. The metallic foil  30  is then ignited. Because the foil  30  ignites all or nearly all of the exposed interior surfaces of the propellant segments  20   a ,  20   b , and because the distance that the foil  30  must ignite is limited to the approximate radius of the propellant charge  30 , the burn rate of the foil  30  is not as critical as the detonating cord  50 . Furthermore, the propellant area adjacent to the foil  30  can be roughened by cutting, rather than extruded, thus exposing more fuel oxidizer to facilitate ignition. After the propellant  20  is burning, combustion gases  55  generated from the burn are directed as shown in  FIG. 3 , thereby preventing any well fluid from entering the area of burn. This allows the propellant  20  to establish and maintain a rapid burn.  
         [0037]     Alternatively a rapid deflagrating cord could be employed in place of detonating cord, although rapid deflagrating cord has a much slower speed on the order of about 1000 ft/sec. Both detonating cord and deflagrating cord should be considered as examples of the types of the ignition elements that could be employed.  
         [0038]     The entire propellant assembly can be wrapped or sealed in a protective layer or coating  40  as depicted in the cross-section view provided in  FIG. 2 . The propellant assembly can be wrapped in a water-tight aluminum scrim, heat shrink plastic, or other similar materials. For example, the propellant assembly can be wrapped with a polymeric or fluoroelastomeric shrink-wrap material, such as the VTN-200 material marketed by the 3M Corporation of St. Paul, Minn.  
         [0039]     The protective layer  40  serves to protect the propellant assembly during transportation, handling, and insertion into the well bore. In particular, the protective layer  40  keeps all propellant and related ignition components dry, thus reducing leaching and eliminating the requirement to apply a sealer. It also compresses the propellant segments  20   a  and  20   b  against the foil  30 , facilitating heat transfer and ignition. Thus, there is little inhibition to flames spreading along the surfaces of the segments  20   a ,  20   b  of the propellant charge.  FIG. 3  is a cross-sectional view of the propellant assembly in  FIG. 2  after ignition. The sharp increase in pressure resulting from the combustion gases produced by the propellant charge  20  ruptures the protective layer  40 . As a result, sufficient surface area can be rapidly initiated as required to provide controlled pressure loading and sustained to assure fracture extensions which result in more efficient use of the propellant bond energy for improved production that would result from such multiple fractures and their extension.  
         [0040]     An alternative embodiment of the propellant assembly is shown in  FIGS. 4 and 5  with the detonating cord  50  in a groove on the outer surface of the propellant charge  20 . A protective coating  40  covers both the detonating cord  50  and propellant charge  20  to keep them dry.  FIG. 4  is a cross-sectional view of the propellant assembly prior to ignition and  FIG. 5  is a cross-sectional view of the propellant assembly after ignition.  
         [0041]     An additional embodiment of the propellant assembly is shown in  FIGS. 6 and 7  with metallic foil  30  covering the exterior surface of the propellant charge  20 .  FIG. 6  is a side cross-sectional view of this embodiment of the propellant assembly and  FIG. 7  is an orthogonal cross-sectional view. A protective layer  40  covers the foil  30  and propellant charge  20 . The metallic foil  30  adjacent to the exterior surface of the propellant charge  20  is ignited by a small piece of propellant  53  at one or more locations in the assembly. A number of channels or grooves  25  in the exterior surface of the propellant charge  20  can be used to facilitate the spread of hot combustion gases from the ignition element  53  over large areas of the metallic foil  30 .  
         [0042]     For example, a dowel or rod  53  of propellant could be used for this purpose as the ignition element for the foil  30 . The propellant dowel  53  is ignited by a shaped charge igniter  51  that fires through an isolating bulkhead  52  into the top of the propellant assembly. The propellant dowel  53  then ignites producing a burst of hot gas that is oriented directionally along the channels  25  down the longitude of the propellant charge  20 . This burst of hot gases produces temperatures over large areas of the foil  30  sufficient to ignite the foil  30  very rapidly. In turn, the foil  30  rapidly ignites the exterior surface of the propellant  20  beneath the protective layer  40 . The protective layer  40  is distended and then ruptured by the internal pressure created by these combustion gases.  
         [0043]     Alternatively, the metallic foil could be ignited electrically using capacitors in an electrical circuit to create the required power output to simultaneously ignite bimetallic ignition strip fuses  80  that in turn ignite the metallic foil  30  at multiple locations.  FIG. 8  is a top view of this embodiment with a portion of the outer protective layer  40  removed to show the ignition strip fuses  80  and metallic foil  30 .  FIG. 9  is an orthogonal cross-sectional view of the embodiment shown in  FIG. 8 . Positive and negative wire braid conductors  82  and  83  are connected to the electrical power source and run longitudinally along the propellant assembly. These leads  82 ,  83  are diametrically-opposed to one another and are shown at the top and bottom in  FIG. 9 . The metallic foil  30  does not completely cover the circumference of the propellant charge  20 , but rather leaves two narrow, diametrically-opposed gaps beneath the conductors  82 ,  83 . As shown in  FIG. 9 , this results in an electrical path running from the upper conductor  82  through the upper set of ignition strip fuses  80 , both sides of the metallic foil  30 , and the lower set of ignition strip fuses  80  to the lower conductor  83 . The electrical current through the ignition strip fuses  80  causes them to ignite, which in turn ignites the metallic foil  30  and the propellant  20 . The ignition strip fuses  80  can be located at selected intervals along the length of the propellant assembly, as shown in  FIG. 8 , to achieve a desired pressure rise. The capacitors can be charged by the wire line conveying device, or batteries in tubing conveyed applications.  
         [0044]     The above disclosure sets forth a number of embodiments of the present invention described in detail with respect to the accompanying drawings. Those skilled in this art will appreciate that various changes, modifications, other structural arrangements, and other embodiments could be practiced under the teachings of the present invention without departing from the scope of this invention as set forth in the following claims.