Patent Publication Number: US-2020284104-A1

Title: Flexible Tubular Sub, and Method of Running a Tool String Into a Wellbore

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Ser. No. 62/814,129 filed Mar. 5, 2019. That application is entitled “Flexible Tubular Sub, and Method of Running a Tool String Into a Wellbore.” 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art. 
     FIELD OF THE INVENTION 
     The present disclosure relates to the field of hydrocarbon recovery operations. More specifically, the invention relates to a flexible tubular body that may be used for running a tool string into a deviated wellbore. Further still, the invention relates to a perforating gun assembly having a flexible tubular sub, enabling the perforating gun assembly to traverse the transition section of a horizontally-completed wellbore. 
     TECHNOLOGY IN THE FIELD OF THE INVENTION 
     In the drilling of an oil and gas well, a near-vertical wellbore is formed through the earth using a drill bit urged downwardly at a lower end of a drill string. After drilling to a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular region is thus formed between the string of casing and the formation penetrated by the wellbore. 
     A cementing operation is conducted in order to fill or “squeeze” the annular region with cement along part or all of the length of the wellbore. The combination of cement and casing strengthens the wellbore and facilitates the zonal isolation of aquitards and hydrocarbon-producing zones behind the casing. 
     In connection with the completion of the wellbore, several strings of casing having progressively smaller outer diameters will be cemented into the wellbore. These will include a string of surface casing, one or more strings of intermediate casing, and finally a production casing. The process of drilling and then cementing progressively smaller strings of casing is repeated until the well has reached total depth. In some instances, the final string of casing is a liner, that is, a string of casing that is not tied back to the surface. 
     Within the last two decades, advances in drilling technology have enabled oil and gas operators to “kick-off” and steer wellbore trajectories from a vertical orientation to a horizontal orientation. The horizontal “leg” of each of these wellbores now often exceeds a length of one mile, and sometimes two or even three miles. This significantly multiplies the wellbore exposure to a target hydrocarbon-bearing formation. The horizontal leg will typically include production casing. 
       FIG. 1  is a side, cross-sectional view of a wellbore  100 , in one embodiment. The wellbore  100  has been completed horizontally, that is, it has a horizontal leg  156 . The wellbore  100  defines a bore  10  that has been drilled from an earth surface  105  into a subsurface  110 . The wellbore  100  is formed using any known drilling mechanism, but preferably using a land-based rig or an offshore drilling platform (not shown). 
     The wellbore  100  is completed with a first string of casing  120 , sometimes referred to as surface casing. The wellbore  100  is further completed with a second string of casing  130 , typically referred to as an intermediate casing. In deeper wells, that is wells completed below 7,500 feet, at least two intermediate strings of casing will be used. In  FIG. 1A , a second intermediate string of casing is shown at  140 . 
     The wellbore  100  is finally completed with a string of production casing  150 . In the view of  FIG. 1 , the production casing  150  extends from the surface  105  down to a subsurface formation, or “pay zone”  115 . Where the wellbore is completed horizontally, meaning that the horizontal “leg”  156  is formed. The leg  156  includes a heel  153  and a toe  154 . The heel  153  may be referred to as a transition section, while the toe  154  defines the end (or “TD”) of the wellbore  100 . The production casing  150  will also extend along the horizontal leg  156 . 
     It is observed that the annular region around the surface casing  120  is filled with cement  125 . The cement (or cement matrix)  125  serves to isolate the wellbore  100  from freshwater zones and potentially porous formations around the casing string  120 . 
     The annular regions around the intermediate casing strings  130 ,  140  are also filled with cement  135 ,  145 . Similarly, the annular region around the production casing  150  is filled with cement  155 . However, the cement  135 ,  145 ,  155  is optionally only placed behind the respective casing strings  130 ,  140 ,  150  up to the lowest joints of the immediately surrounding casing strings. Thus, for example, a non-cemented annular area  132  is typically preserved above the cement matrix  135 , and a non-cemented annular area  152  is frequently preserved above the cement matrix  155 . 
     In order to enhance the recovery of hydrocarbons, particularly in low-permeability formations  115 , the casing  150  along the horizontal section  156  undergoes a process of perforating and fracturing (or in some cases perforating and acidizing). Due to the very long lengths of new horizontal wells, the perforating and formation treatment process is typically carried out in stages. 
     In one method, a perforating gun assembly (shown schematically at  114 ) is pumped down towards the end of the horizontal leg  156  at the end of a wireline  118 . The perforating gun assembly  114  will include a series of perforating guns, with each gun having sets of charges ready for detonation. 
     In operation, the perforating gun assembly  200  is pumped down towards the end  154  of the wellbore  100 . The charges associated with one of the perforating guns are detonated and perforations are “shot” into the casing  150 . Those of ordinary skill in the art will understand that a perforating gun has explosive charges, typically shaped, hollow or projectile charges, which are ignited to create holes in the casing (and, if present, the surrounding cement)  150  and to pass at least a few inches and possibly several feet into the formation  115 . The perforations (not shown) create fluid communication with the surrounding formation  115  so that hydrocarbon fluids can flow into the casing  150  and up to the surface  105 . 
     After perforating, the operator will fracture (or otherwise stimulate) the formation  115  through the perforations (not shown). This is done by pumping treatment fluids into the formation  115  at a pressure above a formation parting pressure. 
     After the fracturing operation is complete, the wireline  118  will be raised and the perforating gun assembly  114  will be positioned at a new location (or “depth”) along the horizontal leg  156 . A plug  112  is set below the perforating gun assembly  114  and new shots are fired in order to create a new set of perforations (not shown). Thereafter, treatment fluid is again pumped into the wellbore  100  and into the formation  115  at a pressure above the formation parting pressure. In this way, a second set of fractures is formed away from the wellbore. 
     The process of setting a plug, perforating the casing, and fracturing the formation is repeated in multiple stages until the wellbore  100  has been completed. 
     In order to provide perforations for the multiple stages without having to pull the perforating gun after every detonation, the perforating gun assembly employs multiple guns in series.  FIG. 2  is a side view of an illustrative perforating gun assembly  200 , or at least a portion of an assembly. The perforating gun assembly  200  comprises a string of perforating guns  210 . 
     Each perforating gun  210  represents various components. These typically include a “gun barrel”  212  which serves as an outer tubular housing. An uppermost gun barrel  212  is supported by an electric wire (or “e-line”)  240  that extends from the surface and that delivers electrical energy down to the tool string  200 . Each perforating gun  210  also includes an explosive initiator, or “detonator” (not shown) that receives electrical energy. In addition, each perforating gun  210  comprises a detonating cord (also not shown). The detonating cord contains an explosive compound that is detonated by the detonator. The detonator, in turn, initiates one or more shots, or “shaped charges.” The charges are held in an inner tube, referred to as a carrier tube, for security and discharge through openings  215  in the selected perforating gun  210 . 
     The perforating gun assembly  200  also optionally includes short centralizer subs  220 . In addition, tandem subs  225  are used to connect the gun barrels end-to-end. Each tandem sub  225  comprises a metal threaded connector placed between the gun barrels  210 . Typically, the gun barrels  210  will have female-by-female threaded ends while the tandem sub  225  has opposing male threaded ends. 
     An insulated connection member  230  connects the e-line  240  to the uppermost perforating gun  210 . The perforating gun assembly  200  with its long string of gun barrels (the housings  212  of the perforating guns  210 ) is carefully assembled at the surface  105 , and then lowered into the wellbore  10  at the end of the e-line  240  and connection member  230 . The e-line  240  extends upward to a control interface (not shown) located at the surface  105 . An operator of the control interface may send electrical signals to the perforating gun assembly  200  for detonating the shaped charges through the openings and for creating the perforations in the casing  150 . 
     After the casing  150  has been perforated and at least one plug  112  has been set, the setting tool  160  and the perforating gun assembly  200  are taken out of the well  100  and a ball (not shown) is dropped into the wellbore  100  to close the plug  112 . When the plug  112  is closed, a fluid, (e.g., water, water and sand, fracturing fluid, etc.) is pumped by a pumping system (not shown), down the wellbore  100  for fracturing purposes. 
     The above operations may be repeated multiple times for perforating and/or fracturing the casing  150  at multiple locations, corresponding to different stages of the well. Note that in this case, multiple plugs may be used for isolating the respective stages from each other during the perforating phase and/or fracturing phase. When all stages are completed, the plugs are drilled out and the wellbore is cleaned using a circulating tool. 
     Those of ordinary skill in the art will appreciate that the perforating gun assembly  200  and its long string of gun barrels (the housings  212  of the perforating guns  210 ) is not a particularly flexible tool string. This creates a problem for the operator when trying to pump the perforating gun assembly  200  through the heel  153  of a horizontally formed wellbore (or across any deviated section). This problem is becoming more severe as drilling companies form wells having tighter transition sections. Not only that, but the horizontal leg  156  can itself sometimes undulate and cork screw, creating somewhat tight areas for a gun barrel string. 
     Therefore, a need exists for a perforating gun assembly having flexible subs spaced between the gun barrels to reduce bending stress and drag in the wellbore. Further, a need exists for a flexible tubular sub that may be threadedly connected between gun barrels (or other rigid wellbore tools), such as by connection to tandem subs. Still further, a need exists for a method of running a perforating gun assembly into a wellbore using one or more flexible tubular subs. 
     BRIEF SUMMARY OF THE INVENTION 
     A flexible tubular sub is provided herein. In one aspect, the tubular sub comprises a cylindrical body having a first end and an opposing second end. Each of the first and second ends comprises threads. In addition, each of the first and second ends has a first outer diameter. 
     The flexible tubular sub also comprises an elongated shaft that is part of the cylindrical body. The shaft resides between the first and second ends, and has a second outer diameter that is smaller than the first outer diameter. This serves to reduce a moment of inertia for the flexible sub. 
     The tubular sub also has an elongated bore. The bore extends between the first and second ends and is dimensioned to receive an electrical wire or data cable. 
     The flexible tubular sub further includes a pair of transition sections. The transition sections reside between the shaft and each of the opposing first and second ends. Thus, a pair of transition sections is actually provided. 
     In one aspect, the shaft of the tubular sub is fabricated from high strength steel, titanium, beryllium copper, or a metal alloy thereof. Preferably, the shaft represents between 40% and 70% of the end-to-end length of the tubular sub. Preferably, the shaft, or more specifically the material comprising the shaft, has a modulus of elasticity that allows the shaft to deform as it is pumped across or pulled out of the heel of a wellbore, and allowing the shaft to return to its original shape. 
     Preferably, either or both of the threads at the first and second ends comprises female threads, with the female threads being configured to threadedly connect to an end of a perforating gun through tandem subs. 
     A flexible tool string is also provided herein. The tool string comprises a first cylindrical wellbore tool having a rigid housing and a second cylindrical wellbore tool also having a rigid housing. 
     The tool string also includes a flexible tubular sub. The tubular sub is designed in accordance with the flexible tubular sub described above in its various embodiments. The tubular sub resides between the first cylindrical wellbore tool and the second cylindrical wellbore tool. 
     In one embodiment the tubular sub comprises a cylindrical body having:
         a first end and an opposing second end, wherein each of the first and second ends defines a coupling having a first outer diameter that approximates an outer diameter of an adjoining wellbore tool;   an elongated shaft between the first and second ends, wherein the shaft has a second outer diameter that is smaller than the first outer diameter, thereby reducing a moment of inertia for the flexible tubular sub;   a bore extending between the first and second ends, with the bore being dimensioned to closely receive an electrical wire or data cable; and   a transition section residing between the shaft and each of the opposing first and second ends.       

     Preferably, each of the first and second wellbore tools is a perforating gun. In this instance, the tool string is a perforating gun assembly. The outer diameter of each perforating gun is formed by a respective gun barrel housing. The threads at each of the first and second ends of the tubular sub comprises female threads configured to threadedly connect to an end of a perforating gun. Optionally, the connection is made through opposing tandem subs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the present inventions can be better understood, certain illustrations, charts and/or flow charts are appended hereto. It is to be noted, however, that the drawings illustrate only selected embodiments of the inventions and are therefore not to be considered limiting of scope, for the inventions may admit to other equally effective embodiments and applications. 
         FIG. 1  is a side, cross-sectional view of a wellbore, in one embodiment. The wellbore has been completed with an elongated horizontal section. 
         FIG. 2  is a side view of an illustrative string of gun barrels forming a perforating gun assembly. The perforating gun assembly represents an illustrative rigid tool string. 
         FIG. 3A  is a perspective view of a flexible tubular sub of the present invention, in one embodiment. 
         FIG. 3B  is a cut-away view of the flexible tubular sub of  FIG. 3A . Here, an inner bore of the tubular sub is shown. 
         FIG. 4  is a side view of the flexible tubular sub of  FIG. 3A . Here, the tubular sub is shown between opposing tandem subs of a perforating gun assembly. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     Definitions 
     For purposes of the present application, it will be understood that the term “hydrocarbon” refers to an organic compound that includes primarily, if not exclusively, the elements hydrogen and carbon. Hydrocarbons may also include other elements, such as, but not limited to, halogens, metallic elements, nitrogen, carbon dioxide, and/or sulfuric components such as hydrogen sulfide. 
     As used herein, the terms “produced fluids,” “reservoir fluids” and “production fluids” refer to liquids and/or gases removed from a subsurface formation, including, for example, an organic-rich rock formation. Produced fluids may include both hydrocarbon fluids and non-hydrocarbon fluids. Production fluids may include, but are not limited to, oil, natural gas, pyrolyzed shale oil, synthesis gas, a pyrolysis product of coal, nitrogen, carbon dioxide, hydrogen sulfide and water. 
     As used herein, the term “fluid” refers to gases, liquids, and combinations of gases and liquids, as well as to combinations of gases and solids, combinations of liquids and solids, and combinations of gases, liquids, and solids. 
     As used herein, the term “subsurface” refers to geologic strata occurring below the earth&#39;s surface. 
     As used herein, the term “formation” refers to any definable subsurface region regardless of size. The formation may contain one or more hydrocarbon-containing layers, one or more non-hydrocarbon containing layers, an overburden, and/or an underburden of any geologic formation. A formation can refer to a single set of related geologic strata of a specific rock type, or to a set of geologic strata of different rock types that contribute to or are encountered in, for example, without limitation, (i) the creation, generation and/or entrapment of hydrocarbons or minerals, and (ii) the execution of processes used to extract hydrocarbons or minerals from the subsurface region. 
     As used herein, the term “wellbore” refers to a hole in the subsurface made by drilling or insertion of a conduit into the subsurface. A wellbore may have a substantially circular cross section, or other cross-sectional shapes. The term “well,” when referring to an opening in the formation, may be used interchangeably with the term “wellbore.” 
     DESCRIPTION OF SELECTED SPECIFIC EMBODIMENTS 
       FIG. 3A  is a cross-sectional view of a flexible tubular sub  300  of the present invention, in one embodiment.  FIG. 3B  is a cross-sectional view of the flexible tubular sub  300  of  FIG. 3A . The tubular sub  300  will be discussed with reference to  FIGS. 3A and 3B  together. 
     The tubular sub  300  is designed to reside along an otherwise rigid string of tools, such as the gun barrel assembly  200  of  FIG. 2 . The tubular sub  300  operates to reduce bending stress and drag on the tool string  200  while it is being pumped into or pulled out of a wellbore  100 . This is of particular benefit when the string of tools is being passed across the transition section, or “heel,”  153  of a horizontally completed wellbore. 
     The tubular sub  300  defines a generally cylindrical body having a first end  312  and an opposing second end  314 . Each end  312 ,  314  represents a threaded connector. Threads are shown at  311 . Preferably, each end  312 ,  314  defines female threads  311  for receiving male threads from an adjoining wellbore tool or connector sub. In addition, each of the first and second ends  312 ,  314  has a first outer diameter. 
     The first  312  and second  314  ends, with their threads  311 , form couplings  317 . In one aspect, each coupling  317  extends inward from the threads about 1 to 4 inches. Flat surfaces (or “flats”)  313  may be placed around the couplings  317  for use in torqueing the sub  300  onto an adjoining tandem sub (as shown at  225  of  FIG. 4 ). 
     The tubular sub  300  also comprises an elongated shaft  316 . The shaft  316  extends between the first  312  and the second ends  314  forming a part of the cylindrical body. Of interest, the shaft  316  has a second outer diameter that is smaller than the first outer diameter. This serves to reduce a moment of inertia for the flexible tubular sub  300 . It is understood that the smaller the outer diameter of the shaft  316 , the better it will flex when passing across the deviated section of a wellbore  100 . The O.D. minimum is limited by the bending stress applied when picking up the tool string  200  from horizontal to vertical. 
     The shaft  316  and opposing couplings  317  may be fabricated from a high strength steel. Alternatively, the shaft  316  and couplings  317  may comprise titanium, beryllium or copper. Combinations of these materials, forming an alloy, may also be used. Preferably, the shaft  316 , or more specifically the material comprising the shaft  316 , has a modulus of elasticity that allows the shaft  316  to deform as it is pumped across or pulled out of the heel  153  of a wellbore, and allowing the shaft  316  to return to its original shape. 
     The tubular sub  300  also comprises a pair of transition sections  318 . Each transition section  318  resides between the shaft  316  and an opposing first  312  or second end  314 . Each transition section  318  is about 1 to 4 inches in length. Preferably, the shaft  316  represents 40% to 70% inclusive of the end-to-end length of the tubular sub  300 . 
     The tubular sub  300  is configured to slidably receive a data cable or an electrical wire (not shown) for the transmission of signals, data or power. In this regard, the cylindrical body making up the sub  300  has a bore  315  configured to receive the cable or wires. A smaller I.D. is preferred to closely hold the cable or wires. 
     In a preferred embodiment, the flexible tubular sub  300  is threadedly placed between two perforating guns, such as perforating guns  210  of  FIG. 2 . Traditional gun barrels (the rigid housings  212  of the perforating guns  210 ) are female-by-female, with the connecting tandem subs  225  being male-by-male, meaning that each end has male threads. With this in mind, the flexible sub  300  may be used as an in-line replacement for any typical gun barrel by using the same female-by-female threaded ends. 
     Of course, the same ends  312 ,  314  of the flexible sub  300  may be made with any combination of threads, such as male-by-female. Instead of replacing just a gun barrel, the operator may replace both a gun barrel and a sub together and then use a flexible sub  300  having male-by-female threaded ends. However, with the female-by-female design, no additional insulators, conductors, contact pins, or springs are required as the design may utilize the existing gun wire and bulkheads to pass electrical continuity through the bore  315  downhole as the replaced gun barrel would. 
       FIG. 4  is a side view of the flexible tubular sub  300  of  FIG. 3A . Here, the tubular sub  300  is shown between opposing tandem subs  225  of a perforating gun assembly (such as assembly  200  of  FIG. 2 ). In this view, the tandem subs  225  are shown in greater illustrative detail, and are exploded away from the tubular sub  300 , revealing male threaded ends  227 . The male threaded ends  227  thread directly into respective female threaded ends  312 ,  314  (or couplings  317 ) of the flexible sub  300 . 
     In addition to the tandem subs  225 , perforating guns  210  are shown exploded away from opposing ends of the tubular sub  300 . In this arrangement, the tubular sub  300  is connected to the perforating guns  210  by means of the tandem subs  225 . 
     Each tandem sub  225  has a male threaded end  227 . One male end  227  of a tandem sub  225  connects to a female end, e.g., end  312 , of the tubular sub  300 , while the other male end  227  of the tandem sub  225  connects to a female end  217  of the perforating gun  210 . In essence, the tubular sub  300  serves as a flexible, “blank” perforating gun in a perforating gun assembly. 
     The flexible sub  300  preferably has a length that is between one and five times the value of the first outer diameter In another aspect, the flexible sub  300  has an overall length that matches or approximates the length of the gun barrels  210  used in the tool string  200 . For example, if there are 16-inch long gun barrels being used, the flexible sub  300  will also be 16 inches from end  312  to end  314 . Of course, the length of the tubular sub  300  may be longer or shorter than the gun barrels  210 . However, the longer the length of the flexible sub  300  the more flex/deviation the sub  300  will offer, allowing the operator to navigate through more highly deviated wellbores. In one aspect, the flexible tubular sub  300  is between 2 and 12 inches in length, and more preferably between 5 and 10 inches in length. 
     As noted above, the internal bore  315  of the flexible sub  300  serves as an internal chamber for holding wires and/or data cables en route to a next perforating gun downhole. The wires or data cables extend through the perforating gun assembly, transmitting detonation signals one gun at a time, from the bottom up. When a detonation signal is received from the wireline  318 , the electronics inside the tandem sub  225  initiate the detonation of the upstream perforating gun  210 . 
     In one unique embodiment, certain of the electronics are stored in the tandem sub  225  rather than in the perforating gun housing  212 . The adjoining tandem sub  225  holds a seal mechanism (e.g., adapter and dart or dart puck and dart) (not shown) that is designed to pressure seal the downstream end of the bore of the sub  225 . In this way, detonation of the shaped charges of a downstream perforating gun  210  does not damage the electronics inside the tandem sub  225 . Such seal mechanisms are provided in U.S. Ser. No. 15/808,290 entitled “Switch Sub With Two-Way Sealing Features and Method,” the entirety of which is incorporated herein by reference. 
     In U.S. Ser. No. 15/808,290, the term “puck” is used to mean an element having a certain surface that is used to cover an opening in a switch sub. The puck may have any shape and/or size as long as the features discussed later can be implemented in such element. The puck may be made of any appropriate material. For example, the puck may be a slab of metal. The term “dart” is used to mean an element that can partially enter inside a conduit formed in the puck. Under normal conditions, the dart can enter only partially inside the conduit. However, under increased pressure, the dart can deform and enter more inside the conduit. The dart may have any shape and/or size as long as it fulfils the features noted above. For example, the dart may be a projectile. After the shaped charged are detonated, the debris from the gun assembly, the wellbore fluid, and/or pressure wave produced by the detonation will not enter the internal chamber  315  of the upstream flexible sub  300  and damage the wires and/or data cables upstream. However, this arrangement is optional. 
     A method of running a tool string into a wellbore is also provided herein. In one aspect, the method first includes providing a wellbore. The wellbore will have a deviated section, such as a horizontal section having a heel and a toe. 
     The method further comprises running a tool string into the wellbore. The tool string has a first cylindrical wellbore tool comprising a rigid housing, and a second cylindrical wellbore tool also comprising a rigid housing. Preferably, each wellbore tool is a perforating gun and the rigid housings are gun barrels. 
     The tool string also includes a flexible tubular sub defining a cylindrical body. The flexible sub may be in accordance with the sub  300  described above in its various embodiments. For example, the flexible tubular sub may comprise:
         a cylindrical body having a first end and an opposing second end, wherein each of the first and second ends has a first outer diameter that approximates an outer diameter of an adjoining wellbore tool;   an elongated shaft between the first and second ends, wherein the shaft has a second outer diameter that is smaller than the first outer diameter, thereby reducing a moment of inertia for the flexible tubular sub;   a bore extending between the first and second ends, with the bore being dimensioned to closely receive an electrical wire or data cable; and   a transition section residing between the shaft and each of the opposing first and second ends.       

     The method also comprises passing the tool string through the deviated section. In one aspect, running the tool string into the wellbore and passing the tool string through the deviated section comprises pumping the tool string downhole at the end of a wireline, using hydraulic pressure. 
     Preferably, each of the first and second wellbore tools is a perforating gun  210 . In this instance, the tool string is a perforating gun assembly that is run into the wellbore  100  at the end of an electric line  240 . The outer diameter of each perforating gun is formed by a respective gun barrel housing and represents the O.D. of the “adjacent wellbore tool.” 
     In one aspect, the threads at each of the first and second ends of the flexible tubular sub comprises female threads configured to threadedly connect to an end of a tandem sub. Each tandem sub comprises male-by-male ends, with a first male end being threadedly connected to an end of the flexible tubular sub, and a second male end being threadedly connected to an end of a perforating gun (or, more particularly, the housing of the perforating gun). Thus, the tandem subs  225  become a means for threadedly connecting the flexible sub  300  to adjoining perforating guns  210 . In this instance, each tandem sub  225  is threaded to a perforating gun  210  and encases a bulkhead assembly. The bulkhead assembly includes a contact pin that transmits electrical signals from gun barrel  210  to gun barrel  210 . 
     If a single flexible sub  300  is run in with the tool string  200 , the flexible sub  300  will ideally be placed central to the tool string  200 . Alternatively, multiple flexible subs  300  may be used in the tool string  200 , in which case one flexible sub  300  would be placed at or near the top of the tool string  200  and one would be placed near the middle of the tool string  200 . An additional flexible sub  300  may be added near the bottom of the tool string. 
     As can be seen, a flexible joint is provided herein. The flexible joint is an improvement over known knuckle joints which can be expensive, complex and prone to failure. Further, variations of the tool and of methods for using the tool within a wellbore may fall within the spirit of the claims, below. It will be appreciated that the inventions are susceptible to other modifications, variations and changes without departing from the spirit thereof.