Abstract:
A latching system permits a firing head to be lowered into a wellbore and reliably coupled to a perforating gun previously lowered into the wellbore. In a disclosed embodiment thereof, the latching system includes a tubular collar associated with the firing head, and a stinger associated with the perforating gun. As the firing head approaches the previously lowered perforating gun, shearable studs projecting into the interior of the collar are received in external side surface J-slots formed on the stinger. The latching of the collar studs in the stinger J-slots permits the firing head/perforating gun connection to be verified simply by pulling up on and creating increased tension in the structure, such as a slick line, used to lower the firing head to the perforating gun. After the firing head is used to detonate the perforating gun, the spent firing head may be retrieved by pulling it uphole with sufficient force to shear its collar studs. Cooperating auxiliary attachment structures are formed on the firing head and perforating gun to facilitate their interconnection and simultaneous lowering into the wellbore if desired.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to apparatus operably positionable in the wellbore of a subterranean well and, in a preferred embodiment thereof, more particularly provides specially designed latching apparatus and associated methods for operatively coupling a firing head structure to a perforating gun. 
     In subterranean wells, such as oil and gas wells, it is common practice to facilitate the flow of production fluid by perforating a fluid bearing subterranean formation using a device commonly referred to as a perforating gun which is lowered into the wellbore to the depth of the formation and then detonated to form perforations in the formation surrounding the gun. A firing head assembly is operatively coupled to the gun and detonated to fire the gun. While the firing head assembly may be coupled to the perforating gun before the gun is lowered into the wellbore, it is often preferred, for safety and other reasons, to couple the firing head to the gun after the gun is positioned downhole in the wellbore. 
     For the lowered gun to function, it must be properly coupled to the subsequently lowered firing head. This downhole coupling, or “latching”, of the firing head to the associated perforating gun has heretofore been subject to several problems, limitations and disadvantages. For example, one previously proposed firing head/perforating gun latching system utilizes flexible collet fingers on the firing head that are designed to be outwardly deflected over an upper end of an associated stinger portion of the perforating gun, and then snap into a circumferential groove in the stinger to operatively latch the firing head to the perforating gun. 
     The collet fingers, as they approach the stinger, pass though a centering restriction in the tubing on which the perforating gun has been previously lowered into the wellbore, and through which the firing head passes on its way to the perforating gun. This centering restriction is designed to laterally align the collet fingers with the upper end of the stinger, but can easily be struck by and inwardly bend one or more of the collet fingers, thereby preventing the proper latching between the firing head and the perforating gun. This same undesirable bending of the collet fingers could also result from the collet structure striking some other obstruction or irregularity in the tubing as the collet structure passes through it in a downhole direction toward the previously lowered perforating gun. 
     A potential solution to this downhole firing head/perforating gun latching problem is simply to attach the firing head to the perforating gun at the surface, and then lower the coupled firing head and perforating head into the wellbore together. However, as previously mentioned, in many instances this is considered undesirable from safety and other standpoints. Additionally, if for some reason the firing head malfunctions, both the firing head and the perforating gun must be pulled from the wellbore, as opposed to simply pulling and replacing the malfunctioning firing head. 
     As can readily be seen from the foregoing, a need exists for improved apparatus and associated methods for effecting the downhole latching of a firing head to a previously lowered perforating gun. It is to this need that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, perforating apparatus is provided which is operatively positionable in a subterranean wellbore and includes a firing head and an associated perforating gun. Cooperatively engageable first and second latching structures are preferably of fixed geometry configurations, are respectively carried by the firing head and the perforating gun, and are operative to couple them, while in the wellbore, in a manner such that subsequent operation of the firing head responsively fires the perforating gun. According to an aspect of the present invention, a portion of one of the first and second latching structures is shearable in a manner permitting the firing head, after being coupled to the perforating gun in the wellbore, to be disengaged from the perforating gun and retrieved from the wellbore. 
     In an illustrated embodiment of the present invention, the first latching structure is representatively a tubular latch collar portion of the firing head and has a circumferentially spaced plurality of shearable lugs extending radially inwardly into its interior. The second latching structure is representatively a stinger portion of the perforating gun, is telescopingly receivable in the latch collar, and has a circumferentially spaced plurality of J-slot recesses formed on an exterior sidewall portion thereof. As the latch collar is telescoped onto the stinger, the shearable studs enter the stinger J-slots to couple the firing head to the perforating gun. 
     In accordance with a method of the invention, the perforating gun is lowered into the wellbore to a predetermined depth therein and held at such predetermined depth. The firing head is then lowered, on a suitable lowering structure such as a slickline, into the wellbore until the latching portions of the firing head and perforating gun are interengaged. The slickline is then pulled up to verify, via a sensed increase in its tension, that the lowered firing head has been properly latched to the previously lowered perforating gun. After proper firing head/perforating gun latching has been verified the firing head is appropriately actuated to responsively fire the perforating gun. Subsequent to the firing of the perforating gun, the spent firing head is pulled up with sufficient force to shear the shearable portion of the latching structure, for example the shearable latch collar studs, thereby releasing the firing head from the perforating gun and permitting the unlatched firing head to be pulled out of the wellbore. 
     According to another aspect of the present invention, cooperative auxiliary connecting structures are provided on the interengageable latching portions of the firing head and perforating gun which permit them to be fixedly secured to one another in a manner permitting the perforating gun and firing head to be simultaneously lowered into the wellbore in an operatively connected state instead of being sequentially lowered into the wellbore and operatively latched together therein. Representatively these cooperative auxiliary connecting structures include a circumferentially spaced plurality of openings formed in the sidewall of the latch collar and alignable with side surface depressions in the stinger, and connecting members extendable through the collar openings into the stinger depressions to longitudinally and rotationally lock the collar onto the stinger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1F are cross-sectional views of longitudinally successive portions of a representative firing head/perforating gun assembly having incorporated therein a specially designed firing head/perforating gun latching system embodying principles of the present invention; 
     FIG. 2 is an exploded perspective view of latching collar and stinger portions of the latching assembly; 
     FIG. 3 is an enlarged scale cross-sectional view through a lower end of the latching collar structure illustrated in FIG.  2  and showing diametrically opposed shearable stud members incorporated therein; and 
     FIG. 4 is a developed side elevational view of the stinger structure shown in FIG.  2  and illustrating an opposed pair of external J-slots formed thereon for operatively receiving the shearable stud members. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides specially designed latching apparatus  10  (see FIGS. 1E and 2) useable to releasably latch a firing head  12  (see FIGS. 1C-1E) to a perforating gun  14  (see FIG. 1F) downhole within a subterranean wellbore  16 , portions of which are illustrated in FIGS. 1A and 1F. Representatively, the wellbore  16  is lined in a conventional manner with a cemented-in tubular casing structure  18 , but the principles of the present invention are also applicable to uncased wellbores. As subsequently described herein, the latching apparatus  10  includes a tubular latching collar structure  20  (see FIGS. 1E and 2) that defines a lower end portion of the firing head  12 , and a tubular stinger structure  22  (see FIGS. 1E and 2) having a pair of external J-slots  24  formed on opposite external side portions thereof. The stinger structure  22  defines an upper end portion of the overall perforating gun assembly. 
     The schematically depicted perforating gun  14  (FIG. 1F) is of a conventional construction and has a reduced diameter threaded upper end portion  26  which is connected to a threaded tubular crossover member  28  which, in turn, is threadingly coupled to the lower end of an outer tubular structure  30  used to lower the perforating gun  14  through the casing  18  to a predetermined depth therein adjacent a subterranean formation (not shown) to be penetrated as a result of firing the perforating gun  14 . The upper end of the tubular structure  30  (see FIG. 1A) is threadingly coupled to the lower end of a tubing structure  32  extending to the surface. 
     Extending upwardly from the perforating gun  14  is an extension tube  34  (see FIGS. 1E and 1F) which is threaded at its upper end into the lower end of the stinger  22 . A detonator cord  36  extends through the interior of the extension tube  34 , and into the interior of the stinger  22 . As illustrated in FIG. 1E, the upper end of the detonator cord  36  is communicated with an initiator  38  within an upper end portion of the stinger  22 , and as illustrated in FIG. 1F the lower end of the detonator cord  36  is communicated with a booster  40  in an upper end portion of the perforating gun  14 . 
     The firing head  12  is representatively of a conventional mechanically actuated type, but could be of another known type such as, for example, a pressure-actuated firing head. As previously mentioned, the latch collar  20  (see FIGS. 1D and 1E) defines a lower end portion of the firing head  12 . The threaded upper end portion  26  of the latch collar  20 , as shown in FIG. 1D, is threaded into the lower end of a tubular crossover member  42  having an upper end  44  that is threaded into the lower end of an inner tubular structure  46  (see FIGS.  1 A-A- 1 D) coaxially received within the outer tubular structure  30  and axially movable relative thereto. For purposes later described herein, an open upper end portion  48  of the inner tubular structure  46  (see FIG. 1A) has an annular latching profile  50  formed on its inner side surface. 
     Turning now to FIG. 1D, an extension tube  52  is coaxially received in the inner tubular structure  46  and has a lower end portion threaded into the upper end portion  44  of the crossover member  42 . At its upper end, the extension tube  52  is coupled to a somewhat larger diameter tubular member  54  (see FIG.  1 C). Operatively secured to the upper end of the tubular member  54 , and defining an upper end portion of the firing head  12 , is an upper releasing pin  56  which is disposed above a firing piston  58  slidably carried within the tubular member  54 . As illustrated in FIG. 1C, the firing piston  58  is disposed in an upwardly spaced relationship with an initiator  60  carried within the tubular member  54 . Initiator  60  is operatively coupled to a detonator cord  62  (see FIGS. 1C-1E) that extends downwardly from the initiator  60 , through the extension tube  52  and the crossover member  42 , to a shape charge assembly  64  secured within an upper interior end portion of the latch collar portion  20  of the firing head  12 . 
     With reference now to FIGS. 2 and 3, the latch collar  20  has a tubular body  66  with an open lower end  68 . Four internally threaded circular holes  70 , 70   a  are equally spaced, in diametrically opposite pairs, around the circumference of a lower end portion of the collar body  66 . For purposes later described, shearable metal studs  72  are threaded into a diametrically opposite pair of holes  70  so that inner end portions of the studs  72  extend into the interior of the collar body  66  as illustrated in FIGS. 1E and 3. 
     Turning now to FIGS. 2 and 4, each of the previously mentioned opposite J-slots  24  externally formed on the outer side surface of the stinger  22  has a longitudinally extending upper entry portion  74  positioned between a pair of deflector portions  76  of the stinger having generally inverted V-shaped apex sections  78 . Each J-slot entry portion  74  is communicated with a circumferentially offset, longitudinally extending receiving portion  80  by a downwardly sloping transfer portion  82 . Each receiving portion  80  has an upper portion  80   a,  and a lower portion  80   b.  The entry portions  74  of the J-slots  74  are diametrically opposite from one another, as are the receiving portions  80  of the J-slots  74 . 
     To operatively attach the collar  20  to the stinger  22 , as later described herein, the collar  20  is simply dropped onto the upper end of the stinger  22 . The inwardly projecting end portions of the shearable studs  72  either drop directly into the J-slot entry portions  74  or are rotationally deflected by the apexed deflectors  76  into the entry portions  74  (thus causing the collar  22  to rotate relative to the stinger  22 ). The lugs  72  are then directed into the J-slot receiving portions  80  via the J-slot transfer portions  82  (thereby further rotating the collar  20  relative to the stinger  22 ) whereupon the lugs drop into the lower receiving slot portions  80   b . When the collar  20  is subsequently lifted, the lugs  72  enter the upper receiving slot portions  80   a , thereby locking the collar  20  the stinger  22 . 
     For purposes later described herein, the in-place collar  20  may be fixedly secured to the stinger  22  which it coaxially overlaps using threaded studs  84  (see FIG. 2) These studs  84  are threaded into the diametrically opposite pair of collar holes  70   a  (see FIGS. 2 and 3) until the studs  84  enter a diametrically opposite pair of circular recesses  86  formed in the outer side surface of the stinger  22 . This translationally and rotationally locks the collar  20  to the stinger  22 . 
     The use of the perforating gun  14 , utilizing the specially designed firing head/perforating gun latching apparatus  10  of the present invention, will now be described with reference to FIGS. 1A-1E. To position the perforating gun  14  for subsequent firing, the gun  14  (see FIG. 1F) is lowered to a preselected depth in the wellbore  16  on the outer tubular structure  30  secured to the lower end of the upper tubing structure  32  (see FIG.  1 A). The firing head  12  is prepared for lowering into the outer tubular structure  30  by latching a schematically depicted pulling tool  88  (see FIG. 1A) into the internal profile  50 , and interconnecting the latched-in pulling tool  88  to a lowering structure, such as the illustrated slickline  90 , via a conventional telescoped weight and jar assembly  92 , 94  which is schematically depicted in FIG.  1 A. Lowering structures other than the representatively illustrated slickline  90 , such as coiled or jointed tubing, or wireline, could be alternatively utilized if desired. 
     The slickline-supported firing head structure  12 , whose lower end is defined by the specially designed latch collar  20 , is lowered into the outer tubular structure  30  toward the upper stinger end portion  22  of the in-place perforating gun  14  until the latch collar  20  telescopes over the stinger  22  and the shearable collar studs  72  (see FIGS. 1E and 3) enter the lower end portions  80   b  of the stinger J-slot receiving portions  80  (see FIG.  4 ). As previously described, during the downward movement of the collar  20  over the stinger  22 , the shearable studs  72  sequentially pass downwardly through the J-slot portions  74 , circumferentially and downwardly through the transfer portions  82 , and then downwardly into the lower end portions  80   b  of the J-slot receiving portions. 
     To verify that the lowered collar  20  is latched to the stinger  22 , thereby operatively coupling the firing head  12  to the perforating gun  14 , the slickline  90  is pulled upwardly in a manner causing the inner end portions of the collar studs  72  to move upwardly in the J-slot receiving portions  80  until they enter the upper portions  80   a  thereof and bottom out against their upper ends. A resulting sensed substantial increase in the slickline tension verifies that the collar  20  has been operatively latched to the stinger  22 . 
     After the operative collar/stinger latching has been verified in this manner, slack is appropriately introduced into the slickline  90  in a manner causing the weight  92  to strike and “shear down” the slickline pulling tool  88  out of its associated tubing profile  50 . The slickline  90  is then pulled upwardly to remove the now unlatched pulling tool  88  from the wellbore  16  leaving the firing head  12  operatively latched to the perforating gun  14 . As will be appreciated, as alternatives to the weight and jar structure  92 , 94 , other types of jarring mechanisms or other types of unlatching mechanisms may be utilized to decouple the pulling tool  88  from the inner tubular structure  46  Subsequent to the removal of the pulling tool  88  in this manner, a suitable drop bar  96  (see FIG. 1B) is dropped through the inner tubular structure  46  and permitted to fall on the upper releasing pin portion  56  of the firing head structure  12 . In response to the impact of the drop bar  96  on the releasing pin  56 , the firing piston  58  is driven downwardly against the underlying initiator  60  to thereby cause a depending firing pin  98  on the piston  58  to penetrate the initiator  60  and ignite the explosive material therein. This ignites the detonator. cord  62  (see FIGS. 1C-1E) which, in turn, operates the booster  64  to thereby drive a shape charge  100  therein downwardly through the upper end wall of the stinger  22 . The shape charge penetration of the upper stinger end wall operates the stinger initiator  38  in a manner igniting the perforating gun detonating cord  34  (see FIGS. 1E and 1F) and, in turn, operating the perforating gun booster  40  (see FIG.  1 F). Operation of the booster  40  fires the perforating gun  14  and, in a conventional manner, drives its shape charges (not shown) outwardly through the cased wellbore  16  into the surrounding subterranean formation (also not shown). 
     After the perforating gun  14  has been fired, the spent firing head  12  may be retrieved by lowering the pulling tool  88  on the slickline  90  (see FIG. 1A) into latched receipt with the inner tubular structure profile  50 , and then pulling upwardly on the slickline  90  with sufficient force to shear the collar lugs  72 , thereby freeing the collar  20  from the stinger  22  and correspondingly freeing the firing head structure  12  from the perforating gun  14 . Once freed in this manner from the perforating gun  14 , the firing head  12  may be simply pulled out of the wellbore  16  on the slickline  90 . This also permits the drop bar  96  to be brought to the surface without the necessity of a separate trip. 
     As an alternative to first lowering the perforating gun  14  into the wellbore  16  and then separately lowering the firing head  12  into the wellbore  16  and latching the separately lowered firing head  12  to the perforating gun  14 , the same collar  20  may be used to operatively secure the firing head  12  to the perforating gun  14  in a manner permitting the firing head and perforating gun to be simultaneously lowered into the wellbore  16 . This alternate connection of the firing head  12  and the perforating gun  14  may be achieved simply by latching the collar  20  to the stinger  22 , using the studs  72  threaded into the collar holes  70   a  until inner ends of the studs  72  enter the stinger side recesses  86 . This longitudinally and circumferentially locks the collar  20  to the stinger  22 , thereby locking the firing head  12  to the perforating gun  14  for simultaneous lowering into the wellbore  16 . 
     As can be seen, in contrast to the use of resilient collet fingers to operatively couple a firing head to an associated perforating gun, the present invention representatively utilizes latching structures (i.e., the latching structures  20  and  22 ) which preferably have fixed geometry configurations. As used herein, the term “fixed geometry” with respect to these latching structures means that their configurations are not appreciably altered during the latching operation. The latching operation is thus not dependent on the resilient deflection of any portion of the structures  20  and  22 , and neither structure is appreciably susceptible to deformation or other damage while being lowered through the wellbore. Additionally, because of the rigid yet intentionally shearable nature of the firing head/perforating gun latching interconnection, both the verification of proper latching and the subsequent separation of the latched firing head and perforating gun are substantially facilitated. 
     The unique latching apparatus of the present invention thus provides for more reliable downhole latching of a firing head to a perforating gun and, via the shearable interconnection between the firing head and the perforating gun, permits the easy retrieval of the spent firing head from the perforating gun. The same firing head, however, may be alternatively attached directly to the perforating gun, as described above, to facilitate the joint lowering of the firing gun and attached perforating gun into the wellbore. Additionally, by using a drop-away attachment instead of the threaded crossover member  28  illustrated in FIG. 1F, the perforating gun  14  and attached firing head  12  may be simply dropped into the wellbore  16  after the perforating gun  14  is actuated by the firing head  12 . Moreover, if well parameters change such that a different firing head is required, the firing head in place can be retrieved and a new firing head redeployed without having to trip the perforating gun. 
     The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.