Abstract:
A perforating system having a perforating gun with a pressurizable gun body. The gun body can be pressurized prior to deployment in a wellbore, or while in the wellbore. Pressurizing the gun body can include adding fluid into the gun body, such as a pressurized gas, a liquid, or combustion products. A seal diaphragm can be used to transfer wellbore pressure into the gun body.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of co-pending U.S. Provisional Application Ser. No. 61/163,705, filed Mar. 26, 2009, the full disclosure of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a perforating system having a system for compensating pressure inside a perforating gun body with wellbore pressure. 
     2. Description of Prior Art 
     Perforating systems are used for the purpose, among others, of making 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 completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore. 
     Perforating systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length. In  FIG. 1  an example of a perforating system  4  is shown. For the sake of clarity, the perforating system  4  depicted comprises a single perforating gun  6  instead of the typical multitude of guns. The perforating gun  6  is shown disposed within a wellbore  1  on a wireline  5 . The perforating system  4  as shown also includes a service truck  7  on the surface  9 , where in addition to providing a raising and lowering means, the wireline  5  also provides communication and control connectivity between the truck  7  and the perforating gun  6 . The wireline  5  is threaded through pulleys  3  supported above the wellbore  1 . As is known, derricks, slips and other similar systems may be used in lieu of a surface truck for inserting and retrieving the perforating system into and from a wellbore. Moreover, perforating systems may also be disposed into a wellbore via tubing, drill pipe, slick line, coiled tubing, to mention a few. 
     Included with the perforating gun  6  are shaped charges  8  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  8  at very high velocity in a pattern called a “jet”  12 . The jet  12  perforates the casing and the cement and creates a perforation  10  that extends into the surrounding formation  2 . 
       FIG. 2  illustrates in side partial sectional view an example of a prior art perforating gun  6 . The perforating gun  6  includes an annular gun tube  16  in which the shaped charges  8  are arranged in a phased pattern. The gun tube  16  is coaxially disposed within an annular gun body  14 . On an end of the perforating gun  6  is an end cap  20  shown threadingly attached to the gun body  14 . On the end of the perforating gun  6  opposite the end cap  20  is a lower sub  22 , also threadingly attached to the gun body  14 . The lower sub  22  includes a chamber shown having an electrical cord  24  attached to a detonator  26 . A detonating cord  28  is included shown having an end connected to the detonator  26  and wound around the gun tube  16  for connection to the lower end of each shaped charge  8 . As is known, an associated firing head (not shown) can emit an electrical signal that transferred through the electrical cord  24  and to the detonator  26  for igniting the detonating cord  28  to then detonate the shaped charge  8 . 
     An annulus  18  is formed between the gun body  14  and gun tube  16  that typically is at a pressure substantially the atmospheric pressure of the location where the perforating gun  6  is assembled—which is generally about 0 pounds per square inch gauge (psig). Thus at surface  9 , no differential pressure is exerted on the gun body  14 . However, wellbore fluids in a wellbore  1  can generate static head pressure that often exceeds 5,000 psig. Thus when the perforating gun  6  is deployed at depth within the wellbore  1 , the gun body  14  will experience a significant differential pressure. The large pressure difference across the gun body  14  wall requires thicker and stronger walls to enhance their strength, as well as robust seals in a perforating gun  6 . 
     SUMMARY OF INVENTION 
     Disclosed herein is a perforating system having a perforating gun with an equalized pressure. The space within the perforating gun body can be pressurized to reduce or eliminate the pressure differential caused by downhole fluid static pressure. The gun body can be pressurized prior to being deployed within a wellbore or can be activated downhole. Optionally, a sealing system can translate downhole pressure to within the gun body for equalizing purposes. Equalizing can occur through a sliding piston or a bladder that transmits pressure. 
     Also disclosed is an example of a method of perforating that includes pressurizing within a gun body of a perforating system. The perforating system is deployed into a wellbore and shaped charges within the gun body are detonated to create perforations in a side of the wellbore. The step of pressurizing can occur before or after the gun body is inserted into the wellbore. Example methods of pressurizing include: injecting fluid into the gun body to increase pressure therein as well as equalizing pressure in the gun body with ambient pressure to minimize pressure differential across the wall of the gun body. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       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: 
         FIG. 1  is partial cutaway side view of a prior art perforating system in a wellbore. 
         FIG. 2  is a side sectional view of a prior art perforating gun. 
         FIG. 3  is a side sectional view of an embodiment of a perforating gun having an equalizing bladder. 
         FIG. 4  is a side sectional view of an embodiment of a perforating gun having a combustible material. 
         FIG. 5A  is a side sectional view of an embodiment of a perforating gun having a slidable piston. 
         FIG. 5B  is a side sectional view of an embodiment of a perforating gun having an expandable bladder. 
         FIG. 6  is an axial sectional view of an embodiment of a perforating gun in accordance with the present disclosure. 
         FIG. 7  is a side partial sectional view of a perforating system as described herein deployed in a wellbore. 
     
    
    
     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 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. This invention may, however, be embodied 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 the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For the convenience in referring to the accompanying figures, directional terms are used for reference and illustration only. For example, the directional terms such as “upper”, “lower”, “above”, “below”, and the like are being used to illustrate a relational location. 
     It is to be understood that the invention 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 of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the invention is therefore to be limited only by the scope of the appended claims. 
     With reference now to  FIG. 3  an example of a perforating gun  40  is provided in a side partial sectional view. As shown, the perforating gun  40  includes an annular gun body  44  having an upper end cap  42  coaxially attached at one end and lower end cap  55  on an opposite end. A lower sub  54  is coaxially defined within an end of the gun body  44  opposite the upper end cap  42 . In the example of  FIG. 3 , the lower sub  54  is a tubular segment coaxial with the gun body  44  and capped with the lower end cap  55 . Coaxially secured within a portion of the gun body  44  is a gun tube  46  thereby defining an open space annulus  48  (also referred to herein as a plenum) between the gun tube  46  and gun body  44 . The gun tube  46  is an annular member with apertures formed through the side wall and shaped charges  50  inserted within the apertures; a detonating cord  52  is shown connecting to each of the shaped charges  50 . In the embodiment shown, a bladder  64  encases the gun tube  46  on its outer surface providing a sealing barrier between the gun tube  46  and the annulus  48 . The bladder  64  can be a flexible member made from an elastomer or other polymer material, or can also be a foil-like metal. In the example of  FIG. 3 , the bladder  64  is a sleevelike member having ends attachable to either the outer surface of the gun tube  46  or the end cap  42 /bulkhead  61 . 
     A solid bulkhead  61  is shown mounted in the gun body  44  and in a plane transverse to an axis A X  of the perforating gun  40 . In an example, the bulkhead  61  defines the lower end of the gun body  44  and upper end of the lower sub  54 . Bulkhead  61  spans the entire space within the gun body  44 . A lower bulkhead  60  is shown provided within the lower sub  54  in a plane substantially parallel to that of the first bulkhead  61  and defining a chamber  58  between the bulkheads  60 ,  61 . An orifice  56  formed through a lateral wall of the gun body  44  provides fluid communication between the chamber  58  and the space surrounding the perforating gun  40 . For example, prior to deployment the chamber  58  would freely communicate air at atmospheric pressure through the orifice  56 . Similarly, when deployed in a fluid filled wellbore, wellbore fluid can flow into the chamber  58  through the orifice  56  driven by the higher pressure in the wellbore. Eventually, as the wellbore fluid enters the chamber  58 , the pressure in the chamber  58  equalizes with wellbore pressure. A passage  62  axially formed through the bulkhead  61  provides fluid communication from the chamber  58  into the annulus  48  in the space between the gun body  44  and the bladder  64 . The fluid communication from the space ambient the perforating gun  40  into the annulus  48  pressurizes the annulus  48  to substantially ambient pressure thereby minimizing pressure differential across the wall of the gun body  44 . The bladder  64  prevents fluid migration into the gun tube  46 , thus avoiding damaging or fouling the shaped charge  50  by wellbore fluid. 
     Shown in  FIG. 4  is a side sectional view of an embodiment of a perforating gun  40 A that includes an oxidizing material for pressurizing within the gun body  44 . In this example embodiment, the bulkheads  61 ,  60  are shown substantially the same as the embodiment of  FIG. 3 ; including the passage  62  formed through the first bulkhead  61 . Added in this embodiment is an oxidizing agent  68  within the chamber  58  between the gun tube  46  and lower sub  54 A. An example oxidizing agent  68  is combustible, and can also combust in the absence of oxygen or when exposed to wellbore fluid. In the example of  FIG. 4 , the oxidizing agent  68  is in the process of being combusted and producing off gases. Arrows illustrate flow of the off gases from within the chamber  58 , through the passage  62 , and into the annulus  48 . The combustion off gas pressurizes the annulus  48  to substantially reduce or eliminate stresses on the gun body  44  from an applied pressure differential. Other alternatives for use in the chamber  58  to produce pressure within the gun body  44  include chemical reactions, gas generators or slow burn elements. 
     With reference now to  FIG. 5A , an alternative example of a perforating gun  40 B is shown in a side partially sectional view. In this embodiment, the perforating gun  40 B includes a gun body  44 , an end cap  42  on the end of the gun body  44 , and a lower sub  54 B on the gun body  44  end opposite the end cap  42 . The gun tube  46  is shown axially anchored within the gun body  44  defining an annulus  48  between the gun body  44  and gun tube  46 . In this example, a bulkhead  61 A is at the lower terminal end of the gun tube  46  to form a boundary between the gun body  44  and lower sub  54 B. The lower sub  54 B is shown as a largely annular member having an open space with a pressure chamber  70 . A piston  72  is coaxially provided in the pressure chamber  70  and having seals  73  optionally provided on the outer radial periphery of the piston  72 . The piston  72  is axially moveable within the pressure chamber  70 ; a pressure differential axially applied across the piston  72  can urge the piston  72  within the pressure chamber  70  in a direction along the axis A X . A port  76  is shown formed on through a lateral wall of the lower sub  54 B allowing fluid and pressure communication into the pressure chamber  70  on a side of the piston  72  opposite from the bulkhead  61 A. When the perforating gun  40 B is in a wellbore, higher pressure wellbore fluid can flow through the port  76  and into the pressure chamber  70  and urge the piston  72  upwards towards the bulkhead  61 A. Passages  74  are axially formed through the bulkhead  61 A allowing fluid communication between the chamber  70  and the annulus  48 . A fluid such as hydraulic fluid, air, an inert gas, nitrogen, combinations thereof and the like, can be in the annulus  48  and in pressure chamber  70  between the bulkhead  61 A and the piston  72 . The fluid can be at atmospheric pressure, or pressurized above atmospheric. Urging the piston  72  towards the bulkhead  61 A pressurizes the fluid in the annulus  48  and chamber  70  thereby to equalize pressure in the annulus  48  with ambient pressure to minimize gun body  44  wall differential pressure. Alternatively, the piston  72  can be replaced with an expandable bladder  75  shown having ends sealed within the chamber  70  and along an inner circumference of the chamber  70 . The bladder  75  can include folds so that when fluid enters the chamber  70  through the port  76 , the bladder  75  “unfolds” towards the gun tube  46  and pressurizes the pressurizing fluid in the annulus  48  and side of the bladder  75  facing the gun tube  46 . 
     Referring now to  FIG. 6 , an example of a perforating gun  40 C is shown in a partially sectional axially view. In this embodiment, a valve  78  is provided through an opening  80  formed in the wall of the gun body  44 A. A pressurized gas, such as nitrogen or air, can be injected through the valve  78  and into the annulus  48  between the gun body  44   a  and gun tube  46 . Deploying a relatively inert gas, such as nitrogen, reduces chances of harm to the shaped charge  50 , detonating cord  52 , or associated electronics (not shown). In this example, the shaped charge  50  includes a case  49 , a liner  51  in the case,  49 , and high explosive  53  between the liner  51  and case  49 . Pressurizing the space in the annulus  48  increases the pressure within the gun body  44 A which in turn can minimize pressure differentials across the wall of the gun body  44  as the gun  40 C is disposed in a pressurized wellbore. As is known, detonating the high explosive  53 , produces a force to expel the liner  51  from the case  49 . The liner  51  is further inverted by the explosive force into a metal jet used to perforate a formation adjacent a wellbore. 
     Illustrated in a side partial sectional view in  FIG. 7  is an example of use of a perforating system as described herein deployed within a wellbore  96  on a wireline  94 . In this example, a perforating system  82  is shown having multiple perforating guns  84  that can be the same or similar to the perforating guns  40 ,  40 A,  40 B,  40 C described in  FIGS. 3-6 . While deployed in the wellbore  96 , shaped charges  86  in the perforating system  82  can be detonated to emit metal jets  88  that form perforations  90  within the adjoining subterranean formation  92 . A surface truck  102  is shown at surface  98  for raising/lowering, and communicating with the gun string. The wireline  94  attaches the string with the surface truck  102  and is wound through pulleys  10  in a derrick structure. Advantages of reducing the pressure differential across the wall of the gun body  44  are reduced size and weight of the gun body  44 , that can result in more and/or larger shaped charges  50  included with a perforating gun and a perforating gun system. 
     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.