Abstract:
An apparatus for fracturing wells employs a propellant charge and an ignition cord wrapped around the outer surface of the propellant charge to rapidly ignite the outer surface of the propellant charge.

Description:
RELATED APPLICATION  
       [0001]     The present application is based on and claims priority to the Applicants&#39; U.S. Provisional Patent Application 60/607,929, entitled “Propellant for Fracturing Wells,” filed on Sep. 8, 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 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. If ignition of the propellant is limited to small areas of the outer surface of the propellant, then 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.  
         [0007]     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.  
         [0008]     The problem is further complicated by the following. When propellants are submerged in well fluids such as water (or water and KCI), 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 takes place over relatively short periods of time (i.e., 1 hour) to result not only in a reduction in the available energy to do work on the formation, but further, creation of 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.  
         [0009]     In summary, the problem consists of igniting sufficient surface area of propellant simultaneously to overcome the effects of not only a limited flame spread rate, but also to mitigate the effects of any sealing coating placed on the propellant. In addition, one must be able to accurately predict the amount of gas generation by burning of the exposed surface area at any given point in time for proper modeling.  
         [0010]     Solution to the Problem  
         [0011]     The solution to the problem is to rapidly ignite the entire outer surface of the propellant charge by wrapping the ignition cord around the propellant charge 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  
       [0012]     This invention provides an apparatus for fracturing wells that employs a propellant charge and an ignition cord wrapped around the outer surface of the propellant charge to rapidly ignite the outer surface of the propellant charge. For example, the ignition cord can be either a detonating cord or a deflagrating cord. The resulting rapid ignition of the outer surface of the propellant charge can be modeled more accurately and results in a more efficient use of the propellant charge in fracturing the well.  
         [0013]     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  
       [0014]     The present invention can be more readily understood in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  is a cross-sectional diagram of a well  10  with a packer  12  and a series of propellant charges  20 .  
         [0016]      FIG. 2  is a side elevational view of a propellant charge assembly embodying the present invention.  
         [0017]      FIG. 3  is a side elevational view of a propellant charge  20  with a helical groove to receive the ignition cord  30 .  
         [0018]      FIG. 4  is a cross-sectional view of an embodiment with a metal sheath  35  surrounding the ignition cord  30  and a protective coating  40  covering the entire assembly.  
         [0019]      FIG. 5  is a side elevational view of another embodiment with the ignition cord  30  wrapped longitudinally around the propellant charge  20 .  
         [0020]      FIG. 6  is a side elevational view of a propellant charge  20  with longitudinal grooves to receive the ignition cord.  
         [0021]      FIG. 7  is an end view of the propellant charge  20  corresponding to the  FIG. 6 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Turning to  FIG. 2 , a side elevational view of a first embodiment of the present invention is shown. The outer surface of the propellant charge  20  has a generally cylindrical shape. Ignition of the outer surface of the propellant charge  20  is accomplished by an ignition cord  30  wrapped around the propellant charge  20  in a helical pattern.  
         [0023]     Preferably, the ignition cord  30  is a high-speed mild detonating cord. The ignition cord  30  can be ignited conventionally (e.g., with an igniter patch  15 ). The detonating cord can either be enclosed in a metal sheath  35  (e.g., a mild steel tube designed to fail directionally toward the propellant charge  20 ), or placed directly in contact with the surface of the propellant  20 . Mild detonating cord is also commercially available with various metal sheathes, such as lead, silver, aluminum or tin, A grain size of approximately 2.5 to 15 gr/ft has been found to be satisfactory to reliably produce a speed of about 17,000 to 22,000 ft/sec.  
         [0024]     Alternatively a rapid deflagrating cord could be employed, 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 cords that could be used.  
         [0025]     The pitch and/or distance between each turn of the ignition cord  30  can be modified to reduce the spacing between each adjacent turns, to thus limit or substantially eliminate the reliance on the initial flame spread rate to achieve the desired surface burning area. Thus, the amount of time required for the flame to spread becomes insignificant, and the entire surface area of the propellant charge  20  is in effect ignited simultaneously.  
         [0026]      FIGS. 3 and 4  illustrate an embodiment in which the outer surface of the propellant charge  20  includes a helical groove  25  to receive the ignition cord  30  and substantially increase the burning surface area of the propellant charge  20 . The initial surface area burning can be modified by changing the depth and/or cross-sectional geometry of the groove  25  into which the cord  30  is placed. Thus, initial gas generation rates can also be modified by the design of the groove  25 . In addition, the groove  25  reduces the overall diameter of the assembly and helps to protect the cord  25  from damage.  
         [0027]     Optionally, because the ignition cord  30  is in contact with such a large percentage of the total surface area of the propellant charge  30  and flame spread is no longer an issue, the assembly can be coated and sealed from the well bore fluids, thus helping to preventing leaching. For example, the propellant charge  20  and ignition cord  30  can be wrapped or sealed in a protective coating or layer  40 , as depicted in the cross-section view depicted in  FIG. 4 . The protective layer  40  serves to protect both the propellant charge  20  and ignition cord  30  during transportation, handling, and insertion into the well bore, and also keeps them isolated from the well bore fluids. The assembly can be wrapped in a water tight aluminum scrim, heat shrink plastic, or other similar materials. For example, the propellant charge  20  and ignition cord  30  can be wrapped with a polymeric shrink-wrap material, such as the VITON 200 material marketed by the 3M Corporation of St. Paul, Minn.  
         [0028]      FIGS. 5 through 7  illustrate another embodiment with the ignition cord  30  wrapped longitudinally around the propellant charge  20 .  FIG. 5  is a side elevational view of this embodiment.  FIGS. 6 and 7  show a side elevational view and an end view, respectively, of a propellant charge  20  with longitudinal grooves to receive the ignition cord in this longitudinally-wrapped configuration. It should be understood that other wrapping configurations or combinations of wrapping configurations could be readily substituted.  
         [0029]     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.