Abstract:
A well completion procedure and apparatus comprises a first assembly that includes production tubing combined with a production packer and an internal bore latching profile. A second assembly comprises an explosive perforating gun secured to a latching mechanism. The perforating gun and latching mechanism are dimensioned to freely traverse the flow bore of the production tubing for downhole retrieval and return after the packer is set. The latching mechanism may be released by discharge of the gun.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation-In-Part of U.S. patent application Ser. No. 10/002,791 filed Nov. 15, 2001 now U.S. Pat. No. 6,591,912. Said application Ser. No. 10/002,791 claims the filing priority date of Nov. 15, 2000 based upon U.S. Provisional Application Ser. No. 60/248,810. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the art of well drilling and earth boring. More particularly, the invention relates to methods and apparatus for perforating wellbore casing, casing liner and/or fracturing well production zones. 
   2. Description of Related Art 
   After the actual drilling of a borehole into the earth, the borehole shaft is often prepared for long term fluid production by a series of steps and procedures that are collectively characterized by the art as “completion.” Among these numerous procedures is the process of setting a casing, usually steel, within the borehole to line the shaft wall with a stable, permanent barrier. This casement is often secured by cement that is pumped into the annulus between the outside diameter of the casing and the inside diameter of the raw shaft wall. 
   While the casing stabilizes the shaft wall, it also seals the fluids within the earth strata that have been penetrated by the borehole from flowing into the borehole. The borehole inflow of some of the fluids is the desired objective of making the borehole in the first place. To selectively open the casing to such fluid flow, the casing wall is often penetrated in the region of a fluid production zone by shaped charge explosives or “bullets”. In the case of shaped charge explosives, the gaseous product of decomposing explosive material is focused linearly as a high temperature plasma to burn a perforation through the casing wall. Numerous of these charges are loaded into tubular “guns”, usually in a helical pattern along and around the gun tube axis for positioning within the wellbore at the desired location. The line of discharge from the gun is radial from the gun tube axis. 
   By traditional prior art procedure, the tubular gun may be releasably secured to the end of a wireline or coiled tube for running into the well. When the gun has been located at the desired depth, the gun is secured to the casing or casing liner bore wall by radially expandable slips, for example. This setting or anchoring procedure is essential to substantially center the gun within the casing bore for radially uniform penetration. In some cases, the slips are releasable from the casing to facilitate removal of the gun assembly from the casing bore in the event that need arises: either before of after firing. 
   Subsequent to the prior art perforation procedure, the production tubing is run into the well and set. Often, setting of the production tubing also includes a production packer around the production tubing to seal the well annulus around the tubing above the perforation zone. 
   The downhole environment of a deep earth boring is frequently hostile to the extreme. The borehole is usually filled with a mixture of drilling fluids, water and crude petroleum. At such depths, the bottom hole pressures may be in the order of tens of thousands of pounds per square inch and at hundreds of degrees Celsius temperature. Consequently, by the time the perforating gun arrives at the desired perforation location, the ignition system, the explosives or the propellant charges are sometimes compromised to the extent that discharge fails to occur on command. In anticipation of such contingencies, provision is often made for unrelated alternative firing systems. If all else fails, the defective gun must be withdrawn from the well and repaired or replaced and returned. 
   As a further consideration, many of the well completion steps require specific tools that are operatively secured within the length of a pipe or tubing work string and deposited into the wellbore from the surface. Placement of a completion tool on downhole location may require many hours of extremely expensive rig time and skilled labor. The full cycle of downhole tool placement and return is termed in the art as “a trip.” 
   At the present state of art, many of the necessary well completion tools are assembled collectively on a single work string and run into the wellbore together for the purpose of accomplishing as many of the several completion steps in as few “trips” as possible. There could be many advantages, therefore, for including the perforation gun at the end of a completion tube having a well production packer set above the gun prior to discharge. In a single trip, the well could be perforated, fractured, packed and produced. On the negative side, however, should the gun misfire, it would be necessary to disengage the production packer and withdraw the entire work string to repair or replace the perforation gun. 
   Comparatively, tools and instruments suspended from drum reeled “wirelines” are run into and out of a wellbore quickly and efficiently. There are advantages, therefore, in a well completion procedure that could position, secure, remove and/or replace a perforation gun or other such tool entirely by wireline. On the other hand, state-of-the-art wireline perforation is substantially a single purpose operation. The well is first perforated and, subsequently, the production packer is set. 
   Some completion assemblies connect the gun to the work string in such a manner that releases the spent gun tube to free fall further down the wellbore below the perforated production zone. In some cases, this gun release function may be desirable. In other cases, especially when additional drilling may be contemplated, the spent gun becomes downhole “junk” and must be extracted by a fishing operation. 
   It is, therefore, an object of the present invention to provide a means and method for securing a perforating gun to the end of a completion or production tube for alternative operational modes. In one mode, the gun may automatically disconnect from the work string when the gun is discharged and free fall from the perforation zone. In another operational mode, the gun may be tethered to a wireline and withdrawn from the well after discharge. 
   Another object of the invention is provision of a perforation gun assembly that may be lowered into a well along a work string tube bore at the end of a wire line, secured to the tube bore at the desired position and discharged. In the event of malfunction, the gun may, by wireline, be disconnected from the work string tube, withdrawn for repair, and returned by wireline. 
   SUMMARY OF THE INVENTION 
   A generalized description of the invention includes a perforation gun connection module, which is one element of a connecting linkage between a perforating gun and a string of production tubing or pipe. The perforating gun is firmly secured, by means of pipe threads, for example, to the lower end of the connection module. The lower end of the connection module, however, comprises an axially shifted trigger section that is temporarily secured for well run-in at an upper assembly position within the connection module by means of a first or lower set of latching dogs. 
   The upper end of the connection module is selectively secured to the tubing sub by means of a second or upper set of latching dogs. The tubing sub is provided with an internal connection profile into which connection module latching dogs may be engaged. The upper end of the tubing sub is traditionally secured, by pipe threads for example, to the lower end of a supporting tube string. 
   The gun outside diameter and that of the associated gun connection module is coordinated to the inside bore diameter of the production tubing whereby the gun and connection module may be drawn in either direction along the length of the production tubing bore. 
   Above the tubing connection sub is a completion packer joint. When deployed downhole, the completion packer joint secures and pressure seals the assembly to the wellbore. 
   A first or lower set of latching dogs temporarily secure a lower trigger section of the connection module to an upper section of the connection module. The perforating gun is connected directly to the trigger section. When the gun discharges, detonation gases generate a pressure surge within the bore of the perforating gun which are channeled to act upon one annular end face of a sleeve piston. The sleeve piston is thereby axially displaced by a resulting pressure differential to align a reduced radius release perimeter along the piston surface under the first dog set. When the release perimeter is axially aligned with the first latching dogs, the dogs radially retract from a position of meshed engagement with a circumferential ledge that is formed around the inside perimeter of a cylindrical counterbore in the connection module socket cylinder. Upon radial retraction of the first latching dogs, the spent gun is free to axially slide along the connection module socket cylinder for a limited distance. 
   The second or upper latching dog set is expanded into a circumferential latch channel formed around the inside bore of the work string connection sub. Radially shifting latch pins are caged by a setting piston and externally meshed with a latching cone. Internally, the latch pins are supported by a surface profiled latch tube. A connective relationship between the tubing connection sub and the upper latching dogs is maintained by shear pins and screws through the connection sub and the upper latch setting piston. Preferably, the gun and connection module are originally assembled in a fabrication shop and delivered to the well site as a pre-assembled unit. On the rig floor, for example, the assembled gun and connection module unit is secured by mating threads to the connection sub that is independently secured to the lower distal end of the production tubing 
   When the spent gun shifts downwardly, the profiled upper latch tube is pulled down to shear the respective retaining pin and remove the radial support structure under the upper latch pins. Without interior support, the upper latch pins retract radially inward to release the upper connecting dogs from the internal latching channel within the connecting sub. When the upper connecting dogs retract from the internal latching channel, the connection module and spent perforating gun are free to fall away from the end of the connector sub. 
   In an alternative operational mode, such as when the gun fails to discharge, the upper latching dogs may be retracted by a wireline pull on the upper latch profile tube. This releases the gun and connection module assembly as a unit from the work string tube. At any time, the unit may be drawn out of the wellbore at the end of the wireline along the work string internal bore, replaced or repaired and returned. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing. Briefly: 
       FIG. 1  is a downhole schematic of the invention. 
       FIG. 2  is a quarter section view of the invention assembly set for in-running down a work string tube at the end of a wireline. 
       FIG. 3  is the invention assembly in a set configuration of the upper latching dogs. 
       FIG. 4  is the invention assembly configured to the release of the lower latching dogs. 
       FIG. 5  is the invention assembly configured to the release of the upper latching dogs. 
       FIG. 6  is the invention assembly configured to the first step of the wireline release operational mode. 
       FIG. 7  is the invention assembly configured to the second step of the wireline release operational mode. 
       FIG. 8  is an enlarged view of the upper latching assembly within the detail delineation of FIG.  1 . 
       FIG. 9  is a detailed half section of the tubing connection module. 
       FIG. 10  is an enlarged view of the lower latching assembly. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   CONSTRUCTION AND ASSEMBLY 
   The invention is shown schematically by  FIG. 1  to include production tubing  10  suspended within a wellbore  11 . The production tubing may be secured to the wellbore wall by anchoring slip elements of a production packer joint  13 . An annular space between the packer joint outer perimeter and the inside wellbore wall is bridged by expansible packer seal elements  15 . This bridge across the wellbore annular space isolates the well production zone below the packer joint  13  from the wellbore space above the packer joint  13 . 
   Although the invention operating environment may include substantially horizontal wellbore orientation, references herein to “upper” and “lower” are generally related to the wellbore surface direction. Accordingly, the left end of the  FIGS. 2 through 10  illustrations normally represents the “upper” end direction of the assembly. Descriptive references to “up” and “down” hereafter will be consistent with this orientation. 
   Below the packer joint  13  is a tubing connection sub  30  that connectively links the perforating gun connection module  20  with the production tubing  10 . The upper end of the perforating gun  24  is secured to the lower end of the connection module  20 . 
     FIGS. 2 through 7  show the invention as a quarter-sectioned assembly within a half-sectioned connection sub  30 . Although the perforating gun  24  is not, per se, illustrated by  FIGS. 2-10 , the upper end of the gun is attached by screw threads to the trigger section  53  of the connection module  20 . The gun connection module  20  structurally links the tube connection sub  30  with the perforating gun assembly unit  24 . 
   Notably, the tubing connection sub  30  provides a latch channel  32  extending around the inside bore of the sub. Preferably, the connection sub  30  may be secured in a traditional manner such as by pipe threads, to a tubing extension below the packer joint  13 . 
   As an initial description of relative dimensions, it will be noted that the gun connection module  20  and perforating gun unit  24  preferably are cross-sectionally dimensioned to pass axially along the internal bores of the connection sub  30 , the packer joint  13  and the production tubing  10  entirely to the surface. 
   Referring to  FIGS. 2 through 10 , the gun release triggering mechanism  53  of the connection module  20  comprises a tubular case wall  21  having a plurality of latch dog windows  48  around the lower perimeter of the case. At the upper end of the outer case wall  21 , the inner bore is formed by internal profiles  16  to connect with a setting tool  17  (see FIG.  7 ). The setting tool  17  is typically run in by wireline, but may also be run in using coiled tubing or conventional tubing. 
   The  FIG. 10  enlargement of the connection module  20  illustrates the lower end of the case wall  21  as including a socket cylinder  22 . The internal bore of the socket cylinder  22  is threaded at its lower end to receive a latch collar  51 . The latch collar  51  profiles a structural support ledge for lower latching dogs  50 . 
   The gun assembly unit  24  is secured by assembly thread  60  to a caging sleeve  61 . The caging sleeve  61  is secured by assembly thread  62  to a stinger element  23 . A concentric cylinder lap between the lower end of the stinger element  23  and the caging sleeve  61  forms an annular cylinder space within which a lower latching piston  54  translates. A circumferential channel  58  in the outer perimeter of the lower latching piston  54  is sufficiently wide and deep to accommodate radial extraction of the lower latching dogs  50  from a radial engagement with the latch collar  51  when the channel  58  is axially aligned with the base of the latching dogs  50 . Under in-running conditions of gun placement, the latching dogs  50  are laterally and circumferentially confined within windows in the caging sleeve  61 . Radially, the latching dogs  50  are confined to the expanded position by a shoulder portion of the latching piston  54  when the latching piston is appropriately aligned. The latching piston shoulder portion has a greater diameter than the root diameter of channel  58 . In-running, the latching piston  54  support location for the radially expanded position of the latching dogs  50  is secured by shear pins  56 . 
   The upper end of the stinger element  23  is secured to an interventionless firing head (IFH)  27 . A detonation cord channel  14  extends from the IFH along the length of the stinger  23  to the gun  24  detonator not shown. Detonation cord ignition occurs in response to pressure pulse signals transmitted along the well fluid from the surface. The detonation cord channel  14  is vented at  66  against the lower ends of the latch piston  54 . When the perforating gun is discharged, combustion gas pressure is channeled through the vents  66  against the lower edge of the latch piston  54 . This combustion gas pressure displaces the piston  54  to align the channel  58  under the lower latching dogs  50  and allow retraction of the dogs  50  from a meshed engagement with the socket cylinder latch collar  52 . When the dogs  50  are retracted from the latch collar  52 , weight of the gun unit  24  axially pulls the stinger  23  down along the socket cylinder bore until the lower shoulder  31  of the IFH engages the annular step of a spacing collar  35 . 
   The spacing collar  35  joins a secondary release sleeve  25  to an upper latch profile tube  40 . The latch profile tube  40  has an axially sliding fit over the stinger tube  23 . The external surface of the latch tube  40  includes a profiled latching zone  41  having a greater outside diameter than the adjacent tube surface. The internal bore of the release sleeve  25  has a sliding fit over the IFH and a wireline latching profile  18  near its upper end. Proximate of the spacing collar  35 , the external surface of the release sleeve is channeled axially by a keyway  26 . A retaining pin  28  set in the outer case wall  21  is projected into the keyway  26  to limit axial displacement of the release sleeve  25  without shearing the pin  28 . 
   As best illustrated by the enlargement of  FIG. 8 , the latching zone  41  of the latch profile tube  40  cooperates with upper latch pins  46  to secure an axially firm connection with an upper latch cone  44 . Axial displacement of the latch cone  44  is limited by one or more guide pins  45  confined within an axially slotted guide window  47 . The upper latch pins  46  are laterally confined within caging windows  43  in an upper setting piston  36 . The axial position of the setting piston is secured to the outer case  21  by shear pins  38  for run-in. The setting piston  36  is responsive to wellbore pressure admitted by the opening of a calibrated rupture disc  34 . When the wellbore pressure is sufficient, rupture of the disc  34  allows a fluid pressure bias to bear upon the piston  36 . Nevertheless, the piston  36  may remain immobile due to the shear strength of the pins  38 . However, as the tool continues its descent into a well, the hydrostatic pressure increases proportionally. When the pressure bias on the piston  36  is sufficient, retention pins  38  are sheared thereby allowing the wellbore pressure bias to drive the piston  36  against the latch pins  46 . Since the latch pins  46  have a meshed engagement with the latch cone  44 , the piston  36  force is translated by the latch pins  46  to the latch cone  44  and finally, to the shear pins  59 . 
   Shear pins  59  secure the relative run-in alignment positions between the latch cone  44  and the upper latching dogs  42 . When the pins  59  fail under the wellbore pressure generated force, the latch cone  44  slip face  49  is axially pulled under the upper latching dogs  42  by the setting piston  36  to radially translate the latching dogs  42  out through the latch dog windows  48  and against the inside bore wall of the production tubing  10 . The latching dogs  42  may drag against the inside bore wall as the assembly descends into the well until the upper latching dogs  42  align with the latch channel  32  whereupon the latching dogs  42  engage the channel and anchor the assembly to the production tubing  10  at this precise point of operation. 
   The stinger  23  is also connected to an electronic firing head (IFH)  29 . The IFH is operative to ignite the detonation cord  14  in response to sonic signals transmitted along the well fluid from the surface. Conveniently, the electronic firing head  29  may be removed and replaced from a downhole location by an appropriate wireline tool. If desired, the IFH may be replaced by a more traditional percussion head for igniting the detonation cord  14  by such means as a falling rod that impacts a detonation hammer. 
   Operation 
   With respect to  FIG. 2 , the gun assembly unit  24  may be unitized with the gun connection module  20  and the tubing connection sub  30  at a convenient remote location such as a shop or manufacturing facility and transported as a unit to the utility well site. To do this, the calibrated rupture disc  34  is removed and replaced by a temporary pressure plug (not shown). The module  20  is attached to the tubing connection sub  30  by preloading the latching mechanism with fluid under a shop pressure so that the latching mechanism remains secured. The upper latching dogs  42  are aligned with the latch channel  32 , and a hydraulic hose (not shown) is operably secured to the temporary pressure plug to provide a sufficient hydraulic fluid pressure to bear upon piston  36  to that the latching dogs  42  can be engaged. 
   The upper end of the tubing connection sub  30  may be easily secured to the bottom end of the production tubing  10  on a rig floor while the tubing is suspended from the derrick crown in the same manner as connecting a bit or other well tool. 
   When the gun assembly unit  24  is secured to the connection module  20 , the lower latching dogs  50  are extended radially to engage the end of the lower latching collar  51 . This radially extended position is temporarily secured by the subjacent support of the cylindrical surface profile of the lower latch piston  54 . This position of the axially translated lower latch piston is secured by one or more shear pins  56 . As the assembly is lowered into the well, the weight of the gun assembly unit is directly carried by the latching dogs  50  bearing upon the latching collar  51 . 
   The weight of the gun assembly and the connection module  20  is transferred to the production tubing  10  by the upper latching dogs  42  in meshed engagement with the latching channel  32  of the tubing connection sub  30  as shown by FIG.  3 . The latching dogs  42  are confined between opposing ram faces respective to the upper latch cone  44  and the fixed base cone  55 . Upper latch pins  46  secure the axially mobile position of the upper latch cone  44   
   In this disposition, the gun assembly is lowered into the well down to the bottom end of the production tubing string  10  and positioned for perforation. 
   Upon discharge of the perforating gun  24 , combustion gas produced by the decomposing explosive is channeled through conduits  66  against the end face of the latch piston  54  to translate the reduced diameter channel zone  58  of the latch piston surface into radial alignment with the lower latching dogs  50 . This change in radial support under the lower latching dogs  50  permits radial contraction of the latching dogs  50  inside of the inner bore of the latch collar  51 . Release of the latch dog bearing on the latch collar  51  allows the gun weight to axially shift the gun  24  and stinger  23  relative to the connection module  20 . 
   This axial shift of the stinger  23  draws the lower shoulder  31  of the IFH into engagement with the spacing collar  35  as illustrated by FIG.  4 . 
   As a further consequence of the axial shift within the connection module  20 , the gun weight  24 , applied by the IFH shoulder  31  against the spacing collar  35 , translates the stinger latching profile  41  from subjacent support of the upper latch pins  46 . As illustrated by  FIG. 5 , loss of subjacent support by the latching profile  41  allows the upper latch pin  46  to withdraw from engagement with the upper latch cone  44 . Without the latch pin  46  engagement, the latch cone  44  is allowed to translate axially from support of the upper latching dog  42 . Retraction of the latching dog  42  from the completion tube latching channel  32  resultantly releases the gun  24  and connection module  20  from the connection sub  30 . 
   Unless a wireline is connected, the assembly is now free to fall from the production tubing bore. If the assembly is wireline connected to the surface, the spent gun assembly may alternatively be removed along the production tubing bore to the surface. 
   The manual mode for mechanically disconnecting and removing a gun and connection module assembly from a connection sub tube is illustrated by  FIGS. 6 and 7 . With respect to  FIG. 6 , a running tool  17  is aligned in the tool bore and secured to the release sleeve  25  by the connection profile  18 . 
   Tension is drawn on the running tool  17  by manipulation of the wireline, coiled tubing or other system used to suspend the running tool  17  within the wellbore, in order to axially translate the sleeve  25  toward the surface direction. Uphole translation of the release sleeve  25  is normally limited by the meshed cooperation of the shear pins  28  and key slot  26 . However, with the upper latch dogs  42  meshed with the completion tube latch channel  32 , sufficient tension may be drawn on the release sleeve  25  to shear the pins  28  and displace the latch pin support profile  41  portion of the integral latch profile tube  40  from support alignment with the upper latch pin  46 . Retraction of the latch pin  46  releases the latch cone  44  from support of the latch dogs  42 . As previously described, release of the upper latch dogs  42  has the consequence of releasing the connection module  20  from the connection sub  30 . 
     FIG. 7  illustrates the downhole extraction of the gun and connection module  20  from the connection sub  30 , which is an option after a wireline disconnect. Tension is drawn on the running tool  17  to release the upper latching dogs  42  from the latching windows  48 . Once released, the tool line may be displaced in either direction. Consequently, the gun and connecting module assembly may be released by the running tool  17  and allowed to fall from the completion tube bore as indicated by FIG.  6 . Conversely, the entire assembly may be drawn to the surface. If the gun has malfunctioned, the defect may be repaired or replaced and the assembly returned to the firing position without disturbing the remainder of the completion tube or any of the tools therein. 
   Return of the gun and connection module to the bottomhole location following complete removal of the assembly from the wellbore requires a few minor modifications to the connection module  20 . Essentially, such modifications include installation of a rupture disc  34  suitably calibrated for the depth of the latch channel  32 . Additionally, the upper latching dog mechanism is expanded to radially retract the upper latching dogs  42 . This expanded setting of the mechanism is temporarily secured by shear pins  59  between the latching dog elements  42  and the upper latch cone  44 . 
   At the end of a wireline, the repaired or replaced perforating gun  24  and connection module  20  is lowered into the wellbore with the latching dogs  42  retracted as illustrated by  FIGS. 2 and 8 . At the predetermined depth (pressure), the pressure differential across the rupture disc  34  will exceed the disc capacity. This may occur as the hydrostatic head of the wellbore or as a consequence of external pressure from surface sources. 
   When the rupture disc  34  fails, wellbore pressure is admitted against the setting piston  36 . This pressure on the piston  36  imposes shear stress on the calibrated pins  38  (FIG.  3 ). When the pins  38  fail, the resulting translation of the setting piston  36  defeats the pins  59  and allows the setting piston  36  to draw the upper latch cone  46  against the latching dogs  42 . Such shear pin failure is followed by a translation of the setting piston  36 . 
   Translation of the setting piston from the run-in position pulls the latch cone  44  against the shear pins  59 . Failure of the shear pins  59  allows the slip face  49  of the latch cone  44  to be drawn under and radially displace the upper latch dogs  42 . This hydrostatic pressure induced force on the dogs  42  is a standing bias that holds the latch dogs  42  against the inside borewall of this completion tube. When the assembly aligns with the latch channel  32  in the connection sub  30 , the latching dogs  42  will mesh with the channel and secure the gun at the exact downhole location from which it was removed. 
   Although our invention has been described in terms of specified embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.