Abstract:
A modular propellant assembly for fracturing wells has a propellant charge with a detonating cord extending along its length to ignite the propellant. Each propellant module also has end caps with male and female connectors that enable propellant modules to be connected in an end-to-end relationship to any desired length. These connectors align the ends of the detonating cords in adjacent propellant modules and are sealed with O-rings so that the detonating cords remains dry while submerged at high pressures. This enables the detonating cords to be ignited in series.

Description:
RELATED APPLICATION  
       [0001]    The present application is based on, and claims priority to U.S. Provisional Patent Application Ser. No. 60/347,442, filed on Jan. 11, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the field of fracturing the formation surrounding a well. More specifically, the present invention discloses a modular propellant assembly for fracturing the strata surrounding a well.  
           [0004]    2. Statement of the Problem  
           [0005]    Hydraulic fracturing has been used in the oil and gas industries for many years to stimulate production from wells. The prior art also includes several examples in which gases generated by combustion of propellants have been used for fracturing wells (e.g., U.S. Pat. Nos. 4,633,951 and 4,683,943 of Hill et al. and U.S. Pat. No. 5,295,545 of Passamaneck). The amount, type, and configuration of the propellant charge must be carefully selected for each well using sophisticated modeling techniques to optimize the effectiveness of the fracturing process.  
           [0006]    This creates a need for customized propellant charges for each well, which adds significantly to manufacturing and inventory costs for these assemblies. Therefore, a need exists for a propellant assembly that can be readily customized to meet the specific needs of a particular well.  
           [0007]    3. Solution to the Problem  
           [0008]    The present invention addresses the shortcomings associated with conventional propellant fracturing systems by providing a modular propellant assembly that can be readily customized and assembled in the field to meet the requirements for a specific well. This modular approach reduces costs and results in a safer, more reliable application.  
           [0009]    In addition, the modular propellant design allows easy assembly prior to transporting the propellant to the well site. A simple and quick procedure can be employed at the well site to ready the system to go down hole. Furthermore, reliability is dramatically improved because the system&#39;s integrated ignition components can be factory assembled and tested prior to application in the field.  
         SUMMARY OF THE INVENTION  
         [0010]    This invention provides a modular propellant assembly for use in fracturing wells. Each propellant module contains a propellant charge with a detonating cord extending along its length and end boosters to ignite the propellant. Subsequent propellant grains are ignited by an ignition booster-to-booster transfer. Each propellant module also has male and female connectors that are sealed with O-rings so that the ignition system remains dry while submerged at high pressures. These connectors enable propellant modules to be connected in an end-to-end relationship to any desired length, so that their detonating cords will be ignited in series. Each propellant module can also be provided with a steel rod extending between the end connectors for structural support and rigidity.  
           [0011]    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  
       [0012]    The present invention can be more readily understood in conjunction with the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 is a side cross-sectional view of a propellant module  20 .  
         [0014]    [0014]FIG. 2 is a side cross-sectional view of an assembly consisting of two propellant modules  20   a  and  20   b.    
         [0015]    [0015]FIG. 3 is a detail side cross-sectional view of the left end of the assembly in FIG. 2.  
         [0016]    [0016]FIG. 4 is a detail side cross-sectional view of middle portion of the assembly in FIG. 2 showing the male connector of the first propellant module  20   a  inserted into the female connector of the second propellant module  20   b.    
         [0017]    [0017]FIG. 5 is a detail side cross-sectional view of the right end of the assembly in FIG. 2.  
         [0018]    [0018]FIG. 6 is a cross-sectional view of the propellant module taken along lines  6 - 6  in FIG. 2.  
         [0019]    [0019]FIG. 7 is a cross-sectional view of an end of the propellant module taken along lines  7 - 7  in FIG. 2.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Turning to FIG. 1, a side cross-sectional view is provided showing one possible embodiment of the propellant module  20 . FIG. 6 is a cross-sectional view of the propellant module taken along lines  6 - 6  and FIG. 7 is a cross-sectional view of an end of the propellant module taken along lines  7 - 7  in FIG. 2. The major components of the propellant module  20  include a propellant charge  24 , a tube  23 , and a detonating cord  25  that is housed inside the tube  23 . Each propellant module  20  also includes a lower end connector  35  and an upper end connector  45  that can engage complementary end connectors on adjacent propellant modules in an end-to-end relationship. This enables any number of propellant modules to fastened together in series (male end connector  45  into female end connector  35 ) to form an assembly having the desired gas-generation characteristics tailored for a particular job.  
         [0021]    In the preferred embodiment of the present invention, the end connectors  35  and  45  are male and female connectors. However, other types of connectors could be readily substituted. The propellant modules can be secured together with set screws  28 , as shown in the drawings. Alternatively, the male and female end connectors  45 ,  35  can be threaded together, attached by a bayonet connection, bonded by an adhesive, or secured by a frictional fit between the end connectors  35 ,  45 .  
         [0022]    A mild detonating cord  25  within a tube  23  extends along the length of the propellant charge  24 . The tube  23  should be made of a high-strength, corrosion-resistant material, such as stainless steel, to protect the detonating cord  25  and to keep the detonating cord dry. For example, the tube  23  and detonating cords can be placed in a groove extended from end to end on the propellant charge  24 , as shown in FIG. 6.  
         [0023]    After the propellant module assembly has been fabricated and placed in the well, the detonating cord  25  is used to ignite the propellant charge  24 . The end connectors  35 ,  45  have openings that serve to align and maintain the water-tight seal between the ends of the tubes  23  in adjacent propellant modules  20   a  and  20   b , as illustrated in FIGS. 2 a  and  4 . This enables the detonating cords  25  in all of the propellant modules to be ignited in series. When ignited, the detonating cord  25  ruptures the tube  23  and ignites the main propellant charge  24  of each module. In addition, boosters can be included at the ends of the propellant cords  25  to help light the detonating cords  25  in series. O-rings  27  also help to keep the boosters  29  and detonating cord  25  dry prior to ignition.  
         [0024]    The propellant charge  24  can be any solid propellant having suitable burn-rate characteristics. In the preferred embodiment, each propellant charge  24  has a length of 60 inches, but this could be of any length that is practical. The diameter of the unit can be any value since it depends on the size of the well being treated.  
         [0025]    The propellant charge  24 , tube  23 , and detonating cord  25  can be placed in a carrier  26  having perforations to allow combustion gases to escape from the propellant charge  24 . The carrier provides structural support and helps to protect the assembly from damage in transit and while the propellant module is being lowered into the well. For example, the carrier  26  can be a perforated steel tubing similar to those used in propellant fracturing and perforation-gun systems.  
         [0026]    A threaded steel rod  22  extends between the upper and lower end caps  34  and  44  for structural support. Cap screws  21  are threaded into the ends of the steel rod  22  to tension the rod  22  and thereby pull the unit together creating a structurally stable unit.  
         [0027]    The following is a description of the assembly process for the propellant modules  20 . The detonating cord  25  is cut to the proper length and boosters  29  are placed on each end and crimped in place. The detonating cord  25  is placed into the tube  23  and bent to conform to the geometry of the male and female end connectors  45 ,  35 . The tube  23  with the detonating cord  25  and boosters  29  is placed in a groove in the propellant  24  that runs from end to end to receive the tube  23 . A second groove, placed 180 degrees from the previously mentioned groove, receives the steel rod  22 . Interior O-rings  27  are inserted into the O-ring grooves on both the male and female end connectors  45 ,  35  to keep well bore fluids from getting to the boosters  29  and detonating cord  25  from the propellant side of the end connectors  45 ,  35 . The male and female end caps  44 ,  34  are placed over the ends of the propellant  24 . Cap screws  21  are placed in the ends of the steel rod  22  to tension the rod  22 . External O-rings  27  are placed on the male end cap  44  to complete the module. With this arrangement, the system is totally well bore fluid proof. The male and female end caps  44 ,  34  have complementary male and female end connectors  45 ,  35  that enable a series of propellant modules  20  to connect together in an end-to-end manner.  
         [0028]    Additional propellant modules are prepared consistent with the number of modules needed for the job. These propellant modules can be identical to one another, or customized to meet the specific needs of a particular job. For example, FIG. 2 is a side cross-sectional view of an assembly consisting of two propellant modules  20   a  and  20   b . FIGS. 3 through 5 are corresponding detail side cross-sectional views of the left, middle, and right connections of this assembly, respectively. In particular, modules can be assembled by mating the male end connector  45  from one module  20   a  with the female end connector  35  from a second module  20   b  and placing a set screw  28  to hold them together. FIG. 4 is a detail side cross-sectional view showing the male connector  45  of a first propellant module  20   a  inserted into the female connector  35  of a second propellant module  20   b.    
         [0029]    The last or bottom module in the string of propellant modules has a plug  30 , which can be identical to the male connector  45  on the male end cap  44 . The plug  30  is secured to the bottom of the last propellant module with a set screw  28  to keep the lower end of the ignition system dry. The first or top module in the string inserts into a cross-over  40  which has the female geometry to complete the top seal for the ignition system. The cross-over  40  can be equipped to fire the system using either a conventional tubing-conveyed system, a coiled tubing system, or a wireline system.  
         [0030]    The above disclosure sets forth a number of embodiments of the present invention. Other arrangements or embodiments, not precisely set forth, could be practiced under the teachings of the present invention and as set forth in the following claims.