Abstract:
A hydraulic fracturing ball sealer used in fracturing of shale formations. A preferred embodiment is constructed of a generally spherical core and a pair of hemispherical shells positioned about the core. The shells are secured to each other along an equatorial seam. The multilayer frac-ball provides a strong but machinable structure with a pliable outer surface that is corrosion resistant, has a specific gravity that allows it to float in the fracturing fluid, and is relatively easy and inexpensive to manufacture. The frac-ball of the present invention is a two piece metal and polymer design. A two-part embodiment comprises a polymer core with a metal case or shell. A layer of epoxy resin may be used to secure the shell to the core. Alternate embodiments include multiple layers of different materials, generally arranged concentrically within the spherical shape. The surface of the frac-ball may be smooth, scored, or serrated.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under Title 35 United States Code §119(e) of U.S. Provisional Application 61/828,239, filed May 29, 2013 the full disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to systems for facilitating the extraction of natural gas deposits from underground geologic formations. The present invention relates more specifically to ball sealer devices commonly known as “frac-balls” that are generally spherical objects that are injected into a well to close off portions of the well to allow pressure to build up and cause fracturing in a target section of the geologic formation. 
         [0004]    2. Description of the Related Art 
         [0005]    Hydraulic fracturing, commonly referred to as “fracking”, is the process of creating small cracks, or fractures, in underground geological formations to allow natural gas to flow into the wellbore and to the surface where the gas is collected and distributed. Variables such as the permeability and porosity of the surrounding rock formations and the thickness of the targeted shale formations are studied by geoscientists before the fracking process is conducted. The result is a highly sophisticated and carefully engineered process that creates a network of fractures that are contained within the boundaries of the targeted deep shale natural gas formation. 
         [0006]    During the fracking process, a mixture of water, sand and other chemical additives designed to protect the integrity of the geological formation and enhance production is pumped under high pressure into the shale formation to create small fractures. The mixture is typically about 99.5% water and sand, along with small amounts of special-purpose additives. The newly created fractures are maintained in an open condition by the sand, which allows the natural gas to flow into the wellbore where it is collected at the surface and subsequently delivered to a wide ranging group of consumers. 
         [0007]    One of the tools used by some operators of hydraulic fracturing equipment are specially sized “frac-balls” that are injected into a well to block or close off portions of a well to allow pressure to build up and cause the fracturing in a target section of the well. Frac-balls may be made of various materials, including G-10 (or other related phenolic plastics), Torlon® (polyamide-imide or PAI), PEEK (polyether ether ketone), and other high-temperature thermosets or thermoplastics. Typically, the material selected is based upon the operators&#39; experience and the chemistry and temperatures within the well. 
         [0008]    Frac-ball sizes are selected specifically to fit within the well-bore or sliding sleeves which vary in diameter as the well sections progress from upper to lower (or end) sections. One popular method for creating multiple fractures in a wellbore is the use of fracturing ports &amp; sliding sleeves. Open hole packers isolate different sections of the horizontal well. A sliding sleeve is placed between each packer pair and is opened by injecting a properly sized frac-ball inside the borehole. Typically, a completion string is placed inside the well. The completion string includes frac ports and open hole packers spaced to specifications. The spacing between packers may be up to several hundred feet. The packers are actuated by mechanical, hydraulic or chemical mechanisms. In order to activate each sleeve, a properly sized frac-ball is pumped along with a fracturing fluid inside the well. Each ball is smaller than the opening of all of the previous sleeves, but larger than the sleeve it is intended to open. Seating of the frac-ball exerts pressure at the end of the sliding sleeve assembly, causing it to slide and open the frac ports. Once the port is opened, the fluid is diverted into the open hole space outside of the completion assembly, causing the formation to fracture. 
         [0009]    At the completion of each fracturing stage, the next larger frac-ball is injected into the well, which opens the next sleeve, and so on, until all of the sleeves are opened and multiple fractures are created in the well. The main advantage of this completion technique is the speed of operation (by activating multiple fractures with a single completion string) which also significantly reduces cost. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides an improved frac-ball structure used in the fracturing of shale formations. The frac-ball of the present invention is a unique two piece metal and polymer design. A first two-part structure comprises a polymer core with a metal case or shell. A second three-part structure comprises a fluid filled core surrounded by a polymer body, again enclosed within a metal case or shell. The surface of the frac-ball may be smooth, scored, or serrated. The hydraulic fracturing ball sealer structure of the present invention finds optimal use in sealing flow paths during the fracturing of shale formations. The generally spherical ball, is intended to be used alone or in combination with other similar balls, carried within a fracturing fluid, to seal off portions of a drilled well to facilitate the fracturing of formations surrounding the well. 
         [0011]    One basic embodiment of the ball is constructed of a generally spherical core and a pair of hemispherical shells positioned about the core. The hemispherical shells are secured to each other along an equatorial seam. The multilayer frac-ball provides a strong but machinable overall structure with a pliable outer surface that is corrosion resistant, has a specific gravity that allows it (and the material it is made from) to float on the fracturing fluid, and is relatively easy and inexpensive to manufacture. A layer of epoxy resin may be used to help secure the shell to the core. Further alternate embodiments may include multiple layers of differ materials, generally arranged concentrically within the spherical shape. The surface of the frac-ball may be smooth, scored, or serrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross-sectional view of the internal structure of a first preferred embodiment of the frac-ball device of the present invention. 
           [0013]      FIG. 2  is a cross-sectional view of the internal structure of a second preferred embodiment of the frac-ball device of the present invention. 
           [0014]      FIG. 3  is a detailed cross-sectional view of the internal structure of a third preferred embodiment of the frac-ball device of the present invention. 
           [0015]      FIG. 4  is a detailed cross-sectional view of the internal structure of a fourth preferred embodiment of the frac-ball device of the present invention. 
           [0016]      FIGS. 5A &amp; 5B  are perspective views of two variations of the outer casing or shell of the frac-ball devices of the present invention. 
           [0017]      FIG. 5C  is a perspective view of a number of frac-balls of the present invention of varying sizes. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The present invention provides an improved frac-ball structure used in the fracturing of shale formations. The frac-ball structures are generally described as two piece metal and polymer designs. A first, two-part structure (see  FIG. 1 ) comprises a polymer core  12  with a metal case or shell  14 . A second, three-part structure (see  FIG. 2 ) comprises a fluid filled core  28  surrounded by a polymer body  22 , again enclosed within a metal case or shell  24 . With either internal design the surface of the frac-ball may be smooth (see  FIG. 5A ), scored, or serrated (see  FIG. 5B ). 
         [0019]    The frac-ball inner design ( FIGS. 1 &amp; 2 ) primarily consists of a polymer stock material shape resin and/or mixture, and/or epoxy, epoxy glass or glass fibers, epoxy glass and/or fiber laminates, carbon fibers and/or windings, Kevlar® fibers and/or windings adhered to and/or bonded with an epoxy or polymer. All of the base materials may be various durometers and physical properties. 
         [0020]    The second preferred embodiment ( FIG. 2 ) of the present invention additionally comprises a fluid filled core  28  of cured or un-cured hydraulic based cement materials. The core may preferably consist of a variety of liquids, epoxies, water, synthetic and/or organic based oils. 
         [0021]    The inner core  28  is suspended with offsets  30   a  &amp;  30   b  by means of machined, inserted, bonded or otherwise incorporated onto the core from the base materials. The core centering offsets  30   a  &amp;  30   b  may also be incorporated on the outer shell  24  in any of the listed base materials. The offsets  30   a  &amp;  30   b  themselves may preferably comprise individual components of any of the listed base materials and or metals listed for the outer shell and the inner core. 
         [0022]    The outer case/cover/shell ( 14  in  FIG. 1 , and  24  in  FIG. 2 ) is preferably made by a machining process from solid rod, forging, stamping, deep drawing, casting or spray bonding. The cover may preferably be manufactured from one or any of the following metallic materials of various grades: aluminums, steels, stainless steels, Inconel®, titanium, and additional specialty blended steel and aluminum alloys. The design of the outer case/cover/shell is not restricted to the initial spherical shape and may be manufactured with or without outer serrations or additional external features (see  FIG. 5B ). The outer shell is (under appropriate manufacturing embodiments) joined either by welding, bonding, spin welding, forged, pressed, stamped or mechanically fastened as generally referenced ( 16  in  FIG. 1 , and  26  in  FIG. 2 ). 
         [0023]    The overall design of the frac-balls of the present invention is capable of performing in a variety of uses in the process of shale fracturing operations. Any of the listed combinations of designs will be capable of operating at pressures of 500 psi-25,000 psi. The described designs may be preferably sized from 1″ diameter incrementally up to 10″ in diameter (see represented generally in  FIG. 5C ), with various inner core sizes. The case/cover/shell may preferably be manufactured in a variety of wall thicknesses. 
         [0024]    Reference is next made to  FIGS. 3 &amp; 4  which are cross-sectional views of alternate preferred embodiments of the present invention. Reference to these embodiments includes additional specific details regarding preferred manufacturing processes for the multilayer configurations. The typical ball manufactured according to the present invention uses a shell thickness of 0.1875″-0.200″ regardless of the overall ball diameter. However, maintaining a consistent ratio or percentage between shell thickness and ball diameter is preferable as ball diameter grows. The added thickness will help overcome the increased force acting on the ball as its projected area increases. In the examples described herein, the shell thickness is preferably on the order of 8.3% of ball diameter. The outer shell, inside diameter is preferably slightly larger than the inner core, outside diameter. In the example described herein, the clearance between the inner core and the outer shell is typically between 0.0005″-0.0015″. In general, the shell (or shell halves) and core should fit as tightly as possible. 
         [0025]    In the example shown in  FIG. 4 , the G10 core is cylindrical. This allows more aluminum alloy in the area being welded. This also helps disperse heat during welding and adds additional strength in the seam area when under stress. 
         [0026]    The basic fracturing ball constructed according to the present invention is a single core, two layer design. However, the present invention anticipates ball construction comprising several cores and layers. These cores and layers can be made from, but are not limited to, plastics, rubbers, glass fibers, carbon fibers, zinc alloys, and aluminum alloys. Using dissimilar materials for construction facilitates the creation of a ball with ideal fracturing properties for a given borehole drilling environment. The target properties that make a fracturing ball function optimally are: (a) strong/resilient (able to withstand high pressures); (b) pliable (for sealing against the ball seat); (c) easily machinable (for removal from the pipe); (d) corrosion resistant; and (e) a specific gravity higher than the frac ball carrier or fracturing fluid (to insure that the ball and any ball debris will float). Additionally, the core and the layers should adhere well to one another to minimize the possibility that the ball may distort and fail when under stress. 
         [0027]    The basic multilayer designs of the frac-ball  40  of the present invention (according to the embodiment shown in  FIG. 3 ) are preferably made up of a G-10 (or other related phenolic plastics) core  42 , a two-part epoxy inner layer  48 , and a 6013-T8 aluminum (or similar) outer layer/shell (made up of hemispherical shell halves  44  &amp;  46  in the embodiment shown in  FIG. 3 ). The basic design contains a spherical G-10 core  42 , but a partially cylindrical core  62 , as in the two-piece design shown in  FIG. 4  may be preferable because of manufacturing efficiencies (discussed in more detail below). 
         [0028]    With all multilayer balls, it is important to keep the inner core  42  and outer layers  44  &amp;  46  centered during the manufacturing process. To facilitate this, rigid standoffs  50   a - 50   f  are positioned to create a space D g  between the core and the outer shell. These standoffs  50   a - 50   f  hold the core  42  in place as the interlayer epoxy  48  cures. A preferred minimum of six standoffs  50   a - 50   f  (orthogonally oriented) are inserted into the core  42  to take up the inner layer&#39;s cross section. These are preferably fixed (screwed down) using aluminum 4-40 button head screws. These standoffs  50   a - 50   f  are preferably located on the core top, bottom, left, right, front, and back (orthogonally oriented and angularly spaced). When fully inserted, the protruding head of the screw provides the required standoff. The head height that protrudes is approximately 0.062″. Alternately, dowel pins (similar to the structures shown in  FIG. 2 ) may be used, although it is important that they are positioned/inserted such that they are equally proud to one another. Ideally, whatever is used to center the core should be as small as possible so as to not interfere with the overall design intent related to strength, pliability, and specific gravity. 
         [0029]    After the standoffs  50   a - 50   f  are installed, the core  42  may be placed inside the shells  44  &amp;  46  in most any random orientation. However, it is preferable that none of the standoffs  50   a - 50   f  end up being located on the shell seam  56  &amp;  58 , as a weak area or void can develop during welding as a result. The epoxy material  48  is then injected through at least one 0.125″ tapered vent hole  52  located at the one or both of the shell&#39;s poles. The vent  52  provides both a place to inject the epoxy material  48  and additionally allows welding gasses to escape. Fracturing balls may be manufactured with either one or two vent holes, although in any case it is preferable to position these at the pole(s). 
         [0030]    After the epoxy cures, the two hemispheres  44  &amp;  46  can be carefully welded together (MIG welding as is typical for the preferred type of aluminum). The epoxy material  48  may actually be injected either before or after welding with similar results. Ideally, there should be full weld penetration, even though this may be difficult to achieve without affecting the epoxy and/or the G-10 as they do not typically hold up to welding temperatures. 
         [0031]    Examples of materials that meet the requirements of the manufacturing process described above include, but are not limited to, the following: 
         [0032]    Core  42 —G-10 Glass Based Phenolic. This type of glass-epoxy laminate material is specified for its extremely high strength and high dimensional stability over temperature. G-10 is often used for terminal boards, high humidity applications, electrical and electronic test equipment and electric rotor insulation. While the material is strong it may still be considered machinable under the conditions encountered within the present invention. 
         [0033]    Epoxy  48 —West Systems, G/flex Two Part Epoxy. A toughened, versatile, liquid epoxy typically used for permanent waterproof bonding of fiberglass, ceramics, metals, plastics, damp and difficult-to-bond woods. With a modulus of elasticity of 150,000 PSI, it is generally more flexible than standard epoxies and polyesters, but much stiffer than adhesive sealants. This type of epoxy provides structural bonds that can absorb the stress of expansion, contraction, shock and vibration, and make it ideal for bonding dissimilar materials. 
         [0034]    Standoffs  50   a - 50   f —304 Stainless Steel (SS) or Aluminum 4-40 Button Head Screws (BHS) ⅜ Long. These provide sufficient penetration into the core  42  for stability and offer a head thickness that creates an appropriate spacing to center the core  42  within the hemispherical shells  44  &amp;  46  and allow for the injection of the epoxy  48 . While other standoff devices may be used, these BHSs provide a consistent spacing without the need to accurately control profile height during the manufacturing process. 
         [0035]    Shell Hemispheres  44  &amp;  46 —Alcoa Excalibar® 6013-T8 Aluminum Round. Provides high strength and good corrosion resistance. This material is easily joined by most welding and brazing methods. The material has excellent compressive properties, good applied coating acceptance, and good machinability. 
         [0036]    Welding Rod (not shown)—Preferably 4043, 4047, or 4643. (5xxx series welding rods should not generally be used on 6013 aluminum.) 4043 is (for example) designed specifically for welding 6xxx series aluminum alloys. It has a lower melting point and more fluidity than the 5xxx series filler alloys, and is less sensitive to weld cracking with the 6xxx series base alloys. 4043 and similar generally give more weld penetration but may produce welds with less ductility. These welding rods (4043, 4047, and 4643) are also better suited to higher service temperatures exceeding 150° F. 
         [0037]    In the manufacturing process it is preferable to prepare the inside of the shells  44  &amp;  46  and the outside of the G-10 core  42  using 80/60 grit emery cloth or similar. All of the parts are assembled as shown in  FIG. 3 , again noting that the standoffs  50   a - 50   f  should not be positioned on the shell equator/seams  56  &amp;  58  or on a tapered vent  52 . The seams should be MIG welded with full penetration, taking care not to overheat G-10 core material  42  adjacent the seams  56  &amp;  58 . The epoxy  48  is injected into the tapered vent hole  52  until all air is expelled and epoxy comes out the lower vent. This will help assure that the ball inner layer is full of epoxy. After the epoxy  48  has cured, a 5/16″ hole is drilled (⅛″ deep) in the vent location(s)  52  with care taken to remove all loose debris. The plug  54  (preferably made of the same material as the shells  44  &amp;  46 ) is pressed into the drilled hole  52  to 1/16″ below the surface of the ball  40  and is TIG welded in place. The entire outer surface of the ball  40  is then machine finished and polished. 
         [0038]    The alternate embodiment shown in  FIG. 4  eliminates the epoxy layer and therefore the need for standoffs in the manufacturing process. In this embodiment, frac ball  60  is generally made up of partially cylindrical/spherical core  62  surrounded by hemispherical shells  64  &amp;  66 . The dimensions of the core  62  and the shells  64  &amp;  66  are such as to provide a tight interface  68  between the layers. A vent hole  70  is provided as shown but strictly for the purpose of venting welding gases during the manufacturing process. A plug (not shown) is secured in vent hole  70  after welding along seams  74  &amp;  76  has been completed. 
         [0039]    Use of a partially cylindrical spherical core  62  as shown in  FIG. 4  allows more aluminum alloy in the area  78  being welded which results in better heat dispersion during welding and adds strength to the seam area when the ball is in use under stress. The assembly process for the two piece ball embodiment, such as is shown in  FIG. 4 , is the same but without the standoffs and epoxy. At least one vent is still required to expel welding gases as described above. It may be beneficial to spray a thin layer of thermal insulating material between the layers (between the core  62  and the shells  64  &amp;  66 ) to help prevent incidental damage to the core during the welding process. 
         [0040]    Although the present invention has been described in conjunction with a number of preferred embodiments, those skilled in the art will recognize modifications to these embodiments that still fall within the scope of the present invention. While the basic structure of the frac-ball of the present invention is characterized by the preferred embodiments described above, various environments within which the frac-ball may be used may dictate variations in the material compositions of the various components in the multi-layer ball. In addition, variations in the size of the overall ball may dictate the selection of one of the specific internal structures described and defined in the above disclose to either improve the specific performance of the ball or to balance the geometry of the environmental requirements (the size of the ball) with its durability. The basic multilayer structure provide a means by which all of the desirable characteristics of a frac-ball may be optimized for a particular fracturing operation.