Abstract:
A perforating system for creating perforations that azimuthally circumscribe an inner wall of a wellbore, and that are at substantially the same depth in the wellbore. The perforating system includes perforating assemblies that are housed in a gun body and spaced axially apart. The perforating assemblies have shaped charges positioned at selective angles around an axis of the gun body and at substantially the same axial location in the gun body. Bulkheads are provided between adjacent shaped charges, so that initiating the shaped charges forms angularly spaced apart perforations in a tubular in which the perforating system is inserted. Pressurizing the wellbore with fracturing fluid extends the perforations into fractures, where the fractures are normal to an axis of the wellbore and in a plane of minimum stress.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present disclosure relates in general to a system for fracturing a subterranean formation by creating perforations in a wellbore that intersects the formation, where the perforations extend along the circumference of the wellbore and at substantially the same depth. 
         [0003]    2. Description of Prior Art 
         [0004]    Perforating systems are typically used for forming hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically lined with a string of casing that is cemented to the wellbore wall. Reasons for cementing the casing against the wellbore wall includes retaining the casing in the wellbore and hydraulically isolating various earth formations penetrated by the wellbore. Without the perforations oil/gas from the formation surrounding the wellbore cannot make its way to production tubing inserted into the wellbore within the casing. 
         [0005]    Perforating systems typically include one or more perforating guns connected together in series to form a perforating gun string, which can sometimes surpass a thousand feet of perforating length. The gun strings are usually lowered into a wellbore on a wireline or tubing, where the individual perforating guns are generally coupled together by connector subs. Included with the perforating gun are shaped charges that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge at very high velocity in a pattern called a jet that perforates the casing and the cement and creates a perforation that extends into the surrounding formation. Each shaped charge is typically attached to a detonation cord that runs axially within each of the guns. 
         [0006]    The perforations are sometimes elongated into subterranean fractures by adding a pressurized fracturing fluid to the wellbore. Elongating the perforations increases the surface area of the formation that is in communication with the wellbore, therefore increasing fluid flow from the formation, which in turn increases hydrocarbon production. Sometimes a particulate, referred to as a proppant, is introduced into the perforations and fractures for structural support and to maintain an open passageway for connate fluid into the wellbore. 
       SUMMARY OF THE INVENTION 
       [0007]    Disclosed herein is an example of a perforating system for use in fracturing a subterranean formation adjacent a wellbore, and which includes a gun body, and a perforating assembly in the gun body. In this example the perforating assembly includes shaped charge assemblies that each have an amount of explosive with a rearward side facing an axis of the perforating assembly, a forward side facing away from the axis of the perforating assembly, and lateral sides that extend between the rearward and forward sides and that are substantially planar, and. Bulkheads are also included with this example that are between each of the adjacent shaped charge assemblies, and that define barriers, so that when the amount of explosive in each shaped charge assembly is detonated, each amount of explosive that is detonated forms a jet that forms a perforation in a sidewall of the gun body that is angularly spaced away from an adjacent perforation in the sidewall of the gun body. Optionally included is a housing having a cavity on its outer periphery, and wherein the shaped charge assemblies are disposed in the cavity. This example can further include passages that extend radially through the housing and provide communication between the amounts of explosive and a detonating cord that axially intersects the housing. A liner may optionally be included on a surface of the explosive. The explosive may include a mixture having one or more of cyclotetramethylene-tetranitramine, hexanitrostilbene, cyclotrimethylenetrinitramine, 2,6-pyridinediamine, 1,1,3 trinitroazetidine, and combinations thereof. The shaped charge assemblies may each have a V-shaped cross section with an apex that is directed towards the axis of the perforating assembly, and wherein the V-shaped cross section fully extends between the lateral sides. The perforating system can further include a plurality of perforating assemblies that are axially spaced apart from one another in the gun body to define a first perforating gun. A plurality of gun bodies may optionally be included that are connected end to end and coupled with the first perforating gun to define a downhole string. 
         [0008]    Also provided herein is an example method of fracturing a subterranean formation which involves providing a downhole string, where the downhole string includes a gun body and a perforating assembly. The perforating assembly of this example includes shaped charges at substantially the same axial location in the gun body and that are directed radially outward from an axis of the gun body, the shaped charges each having an explosive and planar lateral sides. In this embodiment bulkheads are between adjacent shaped charges. The example method also includes inserting the downhole string in a wellbore that intersects the formation, forming a series of perforations into the formation, so that perforations in each series are angularly spaced from one another along an inner surface of the wellbore and at substantially the same depth in the wellbore, and creating fractures in the formation that propagate from the perforations by pressurizing the wellbore. The method may further include removing the downhole string from the wellbore, inserting a line into the wellbore, and directing pressurized fluid into the line that discharges from the line into the wellbore and is for pressurizing the wellbore. The fractures formed in the method may be in a minimum plane of stress in the formation. 
         [0009]    Also disclosed herein is an example of a perforating system for use in fracturing a subterranean formation adjacent a wellbore which includes a gun body and a perforating assembly in the gun body. The example perforating assembly includes an axis, a midsection, and an outer surface that angles radially outward from the axis with distance from the midsection. Shaped charge assemblies are included in this example of the perforating system and that each have an amount of explosive with a rearward side facing an axis of the perforating assembly, a forward side facing away from the axis of the perforating assembly, and lateral sides that extend between the rearward and forward sides and that are substantially planar. Bulkheads are included that are between each of the adjacent shaped charge assemblies that define barriers, so that when the amount of explosive in each shaped charge assembly is detonated, each amount of explosive that is detonated forms a jet that forms a perforation in a sidewall of the gun body that is angularly spaced away from an adjacent perforation in the sidewall of the gun body. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]    Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a side partial sectional view of an example of a perforating system deployed in a wellbore and in accordance with the present invention. 
           [0012]      FIG. 2  is a perspective view of an example of a perforating assembly for use with the perforating system of claim  1  and in accordance with the present invention. 
           [0013]      FIG. 3  is a side sectional view of an example of a portion of the perforating system of  FIG. 1  and in accordance with the present invention. 
           [0014]      FIG. 4  is a side partial sectional view of an example of the perforating system of  FIG. 1  creating perforations in the formation that surrounds the wellbore, and in accordance with the present invention. 
           [0015]      FIG. 5  is an axial sectional view of an example of a portion of the perforating system of  FIG. 4  during a perforating step and in accordance with the present invention. 
           [0016]      FIG. 6  is a partial sectional and perspective view of an example of a stack of perforating assemblies in a housing and in accordance with the present invention. 
           [0017]      FIG. 7  is a partial sectional view of an example of forming fractures in the wellbore of  FIG. 2  and in accordance with the present invention. 
       
    
    
       [0018]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF INVENTION 
       [0019]    The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. In an embodiment, usage of the term “about” includes +/−5% of the cited magnitude. In an embodiment, usage of the term “substantially” includes +/−5% of the cited magnitude. 
         [0020]    It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described; as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
         [0021]      FIG. 1  shows in a partial side sectional view one example of a downhole string  10  inserted into a wellbore  12  that is lined with casing  13 . The wellbore  12  intersects a subterranean formation  14 , and is capped on its upper end by a wellhead assembly  16 . A wireline  18  is used for deploying the downhole string  10 , where the wireline  18  is threaded through the wellhead assembly  16  for pressure control, and has an upper end that connects to a surface truck  20 . Wireline  18  provides one technique for deploying string  10  in the wellbore  12 , and in an embodiment includes a medium for transmitting signals and/or power. In one example, provided within truck  20  are mechanical means for raising and lowering the wireline  18 , such as a motorized reel (not shown), as well as communications systems (not shown) for transmitting and receiving signals via the wireline  18  to and from downhole string  10 . String  10  includes a gun body  22 , which is generally elongate and has a curved outer surface and resembles a tubular member. A connector sub  24  is provided on a lower end of gun body  22  for attaching additional gun bodies  22  that make up string  10 . Each gun body  22  is equipped with an outer housing  26 ; shown in dashed outline in the housing  26  are sets of perforating assemblies  28 . 
         [0022]      FIG. 2  is a perspective view of an example of a perforating assembly  28 . In the illustrated embodiment, the perforating assembly  28  has a midsection  29 , and a diameter that that increases with distance away from the midsection  29 . Thus in one example, perforating assembly  28  has a configuration that approximates an hourglass like shape. Perforating assembly  28  is made up of a series of segments, wherein each segment extends along an axial length of the perforating assembly  28 , and has an inner portion adjacent an axis A X  of the perforating assembly  28 , and an outer radial portion that makes up a portion of the outer surface of the perforating assembly  28 . Thus each segment extends along a portion of the circumference of the perforating assembly  28 . The segments include a shaped charge assembly  30 , and bulkhead  32 , wherein a bulkhead  32  is provided within each adjacent shaped charge assembly  30 . In an example, bulkhead  32  is formed from a non-explosive material. Optionally, the bulkhead  32  remains substantially solid after detonation of shaped charge assembly  30 . 
         [0023]    An example of a sectional view of perforating assembly  28  is provided in  FIG. 3  and which is taken along line  3 - 3  of  FIG. 6 . In the example of  FIG. 3 , shaped charge assemblies  28   1 ,  28   2  are shown stacked axially on top of one another. Further shown in  FIG. 3  is a detonating cord  34  which extends along a path that generally follows axis A X . Shape charge assemblies  28   1 ,  28   2  each include a case  36  that provides a structure for mounting the shape charge assemblies  30  and bulkheads  32 . Case  36  includes a generally planer and disc-like mid portion  38  that extends radially outward a distance from axis A X . Case  36  has an axial thickness that increases with distance away from the outer edge of the middle portion  38  and in which a cavity  40  is formed that defines an open and outward facing space on the outer periphery of case  36 . An explosive  42  is shown set within the cavity  40  and having a generally V shaped cross section on the axial view. An optional liner  44 , also having a V shaped cross section, is disposed on an outer surface of explosive  42 . A booster assembly  46  is shown on an upper terminal end of the detonating cord  34 ; booster explosive  48  is shown provided in passages  50  that extend radially outward within the case  36  and from axis A X  into the apex of the cavity  40 . Initiating booster assembly  46  can create a detonation wave in detonating cord  34  that initiates detonation of booster explosive  48  and explosive  42  for forming jets  51  ( FIG. 5 ) that extend into the formation  14  ( FIG. 1 ). Optionally included with the gun body  22  is a spacer  52  which is a cylindrically shaped member shown set approximate to the upper terminal end of gun  22 . In the example of  FIG. 3 , spacer  52  has a cylindrical configuration with a radius that exceeds its axial thickness. A filler material  53  is shown in voids between the adjacently stacked perforating assemblies  28   1 ,  28   2 . The filler material  53  can be any particular matter as well as a cement or other matrix-like material for taking up space and providing structural support. 
         [0024]    Shown in partial side sectional view in  FIG. 4  is an example of the downhole string  10  having formed perforations  54  in the formation  14 . As discussed above, directing a signal to booster assembly  46  ( FIG. 3 ) via wireline from surface can initiate a detonation chain that detonates the shaped charges  28  ( FIG. 3 ) form aforementioned jets  51  that project radially outward and form the perforations  54 . An advantage of the perforating assemblies  28  described herein is that the shaped charge assemblies  30  ( FIG. 2 ) in each individual perforating assembly  28  are at substantially the same axial location within the gun body  26  ( FIG. 3 ). Thus the ensuing perforations  54  formed by detonating these shaped charge assemblies  30  are at substantially the same depth within the wellbore  10 . As explained in more detail below, an advantage of creating these perforations  54  at the same depth is that they are created in generally the same plane. Further shown in  FIG. 4  are apertures  56  that are formed in the side wall of the gun bodies  26  and further illustrating how the strategic axial positioning of the shaped perforating assemblies  28  ( FIG. 2 ) creates the apertures  56  at discrete axial locations on the gun body  26 . 
         [0025]      FIG. 5  is an axial sectional view of a portion of the downhole string  10  and taken along lines  5 - 5  of  FIG. 4 . Further, in the example of  FIG. 5  the shaped charge assemblies  30  ( FIG. 2 ) have been detonated to generate the jets  51  that project radially outward and from the apertures  56  in the side wall of the gun body  26 . Jets  51  extend further outward and past the casing  13  which lines wellbore  12 . Detonating the shape charge assemblies  30  removes the explosive  42  and liner  44  that makes up the assemblies  30  and leaves voids  68  between the adjacent bulkheads  32 . 
         [0026]      FIG. 6  is a side partial sectional view that illustrates a series of shape charge assemblies  28   1 - 28   n  that are axial disposed within the gun body  26  to form a stack  62  within gun body  26 . Optionally, spacers (not shown) may be included between axially adjacent perforating assemblies  28  for strategically forming perforations within a subterranean formation. Further shown is the detonating cord  34  projecting into an upper end of the upper most perforating assembly  28   n . 
         [0027]    Referring now to  FIG. 7 , an example of the wellbore  12  is shown in side sectional view, where the downhole string  12  ( FIG. 1 ) has been removed from within the wellbore  10  and replaced with a fracturing system  64 . In this example, fracturing system  64  includes a pressurized fluid source  68  that is in communication with the wellhead assembly  16  via line  68 . Fluid from within the pressurized fluid source  66  makes its way into the wellbore  12  by way of a schematically illustrated tubular  70 . Tubular  70  depends downward from a lower end of wellhead assembly  16  and has an open end within wellbore  12  below a packer  72 ; where packer  72  provides a fluid barrier between tubular  70  and walls of wellbore  12 . In an example of fracturing, pressurized fluid from pressurized fluid source  66  is introduced into the wellbore  12  and adjacent the area where the perforations  54  ( FIG. 4 ) were formed. The addition of the pressurized fluid extends the perforations  54  and creates fractures  74  that extend radially outward from the wellbore  12 , and at a distance that is greater than that of the perforations  54 . The advantage of creating the perforations at substantially the same depth in the wellbore  12  is that the perforations  54  at each discrete depth adjacent wellbore  12  are within a plane of minimum stress. Therefore, the fracture  74  is also in this plane and will be substantially perpendicular to wellbore  12 . A drawback of known perforating systems, is that size constraints dictate that the shaped charges are arranged in a general helical formation down the axis of the perforating gun, which in turn creates perforations extending into the wellbore wall that follow a helical path by having adjacent perforations that are axially and angularly offset from one another. Accordingly, a fracture may be created in the formation  12  that is not in a plane of minimum stress and at an oblique angle with respect to the axis of the wellbore  12 . An advantage of fractures along the plane to minimum stress is that a greater amount of connate fluid within the formation  14  can then make its way into the wellbore  12  and be produced at surface. 
         [0028]    The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.