Abstract:
A bi-directional explosive transfer subassembly ( 56 ) for coupling two explosive tools ( 52, 54 ) comprises first ( 74, 78 ) and second ( 76, 80 ) explosive carrying members that respectively define first ( 82, 86 ) and second ( 84 ) explosive cavities. A ball end ( 102 ) of the first explosive carrying member ( 74, 78 ) is slidingly received in a socket ( 114 ) of the second explosive carrying member ( 76, 80 ) such that the first ( 74, 78 ) and second ( 76, 80 ) explosive carrying members are rotatable and angularly displaceable relative to one another. A first explosive device ( 130 ) is disposed in the first explosive cavity ( 82, 86 ) and a second explosive device ( 132 ) is disposed in the second explosive cavity ( 84 ). The first ( 130 ) and second ( 132 ) explosive devices are spaced apart such that when one of the explosive devices ( 130, 132 ) is initiated, the other of the explosive devices ( 130, 132 ) will in turn be initiated.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates, in general, to perforating a subterranean wellbore using shaped charges and, in particular, to a bi-directional explosive transfer subassembly that is installed within a work string between loaded perforating guns for use in deviated wellbores. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the present invention, its background will be described with reference to perforating a subterranean formation using shaped charge perforating guns, as an example. 
     After drilling the section of a subterranean wellbore that traverses a formation, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within the wellbore. This casing string increases the integrity of the wellbore and provides a path for producing fluids from the producing intervals to the surface. Conventionally, the casing string is cemented within the wellbore. To produce fluids into the casing string, hydraulic opening or perforation must be made through the casing string, the cement and a short distance into the formation. 
     Typically, these perforations are created by detonating a series of shaped charges located within the casing string that are positioned adjacent to the formation. Specifically, numerous charge carriers are loaded with shaped charges that are connected with a detonating device, such as detonating cord, forming perforating guns. The perforating guns are then connected within a tool string that is lowered into the cased wellbore. Once the perforating guns are properly positioned in the wellbore such that the shaped charges are adjacent to the formation to be perforated, the shaped charges are detonated. Upon detonation, each shaped charge creates a jet that blasts through a scallop or recess in the charge carrier, creates a hydraulic opening through the casing and cement and then penetrates the formation forming a perforation therein. Typically, the shaped charges are fired from the near end to the far end of the formation. In the event of a misfire of the shaped charges, however, it may be necessary to reverse the firing sequence to fire the shaped charges from the far end to the near end of the formation. 
     It has been found that it is sometimes difficult to deploy the desired length of perforating guns into highly deviated or horizontal wells and wells with restrictions. Specifically, in such well configurations, large bending moments act on the string of perforating guns in the plane parallel to the centerline of the perforating guns. These large bending moments can cause failures at the connections between perforating guns, which may result in misfiring. In addition, these large bending moments can prevent relative rotation of the perforating guns about the centerline of the perforating guns such that it is difficult or impossible to orient the perforating guns to fire in the desired direction. 
     A need has therefore arisen for an apparatus that allows a string of perforating guns to be run into highly deviated or horizontal wells and wells with restrictions. A need has also arisen for such an apparatus that allows for the proper orientation of the perforating guns so that they fire in the desired direction. Further, a need has arisen for such an apparatus that allows for bi-directional firing of the perforating guns. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a bi-directional explosive transfer subassembly that can be installed within a tool string between two live perforating guns that allows a string of perforating guns to be deployed into a highly deviated well, a horizontal well or a well with restrictions. In addition, the bi-directional explosive transfer subassembly of the present invention allows for the proper orientation of the perforating guns so that they fire in the desired direction. 
     The bi-directional explosive transfer subassembly of the present invention comprises a first explosive carrying member having a ball end and a first explosive cavity and a second explosive carrying member having a socket and a second explosive cavity. The ball end of the first explosive carrying member is slidingly received in the socket of the second explosive carrying member such that the first and second explosive carrying members are rotatable and angularly displaceable relative to one another. A first explosive device including, for example, a first shaped charge is disposed in the first explosive cavity. A second explosive device including, for example, a second shaped charge is disposed in the second explosive cavity. The first and second explosive devices are spaced apart such that the first and second shaped charges face one another and are each adapted for sending an explosive jet toward the other shaped charge, thereby providing an explosive transfer therebetween. Accordingly, when one of the first and second explosive devices is initiated, the other of the first and second explosive devices will in turn be initiated. 
     The first explosive carrying member of the bi-directional explosive transfer subassembly may include a cylindrical portion extending integrally from the ball end. The second explosive carrying member may include a flange portion extending from the socket that has a conically shaped inner surface having an angle that defines the maximum allowable angular displacement between the first and second explosive carrying members. Specifically, the maximum allowable angular displacement occurs when the cylindrical portion of the first explosive carrying member contacts the flange portion of the second explosive carrying member. The maximum angular displacement between the first and second explosive carrying members may be between about 1 and about 10 degrees and is preferably about 5 degrees. 
     The first and second explosive cavities of the bi-directional explosive transfer subassembly are separated by portions of the first and second explosive carrying members. For example, the first and second explosive carrying members may respectively include first and second wall portions that are adjacent to one another, thereby separating the first and second explosive cavities. Both the first and second explosive devices of the bi-directional explosive transfer subassembly may include a booster, a length of detonating cord connected to the booster and a detonating cord initiator connected to the detonating cord. 
     In one embodiment, the bi-directional explosive transfer subassembly is positioned between first and second perforating guns in a well perforating apparatus. In this embodiment, the sliding engagement between the ball end of the first explosive carrying member in the socket of the second explosive carrying member provides for rotation and angular displacement of the first and second perforating guns relative to one another. Also in this embodiment, when one of the first and second explosive devices is initiated, the other of the first and second explosive devices will in turn be initiated thereby transferring explosive between the first and second perforating guns. 
     The bi-directional explosive transfer subassembly is also used in a method of perforating a well. Specifically, the method comprises deploying a string of perforating guns in a wellbore, the string having first and second perforating guns with a bi-directional explosive device disposed therebetween providing relative rotation and angularly displace therebetween. The method also comprises firing one of the first and second perforating guns, igniting one of the first and second explosive devices, igniting the other of the first and second explosive devices and firing the other of the first and second perforating guns, thereby transferring the explosive and sequentially firing the string of perforating guns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is a schematic illustration of an offshore oil and gas platform operating a plurality of bi-directional explosive transfer subassemblies of the present invention that are disposed between perforating guns in a work string; 
     FIG. 2 is a half sectional view of a bi-directional explosive transfer subassembly of the present invention prior to transferring the explosive; 
     FIG. 3 is a half sectional view of a bi-directional explosive transfer subassembly of the present invention after transferring the explosive; 
     FIG. 4 is a half sectional view of a bi-directional explosive transfer subassembly of the present invention prior to transferring the explosive and with first and second sections of the bi-directional explosive transfer subassembly angularly displaced relative to one another; and 
     FIG. 5 is a half sectional view of a bi-directional explosive transfer subassembly of the present invention after transferring the explosive and with first and second sections of the bi-directional explosive transfer subassembly angularly displaced relative to one another. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
     Referring initially to FIG. 1, a plurality of bi-directional explosive transfer subassemblies of the present invention operating from an offshore oil and gas platform are schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to wellhead installation  22  including subsea blow-out preventers  24 . Platform  12  has a hoisting apparatus  26  and a derrick  28  for raising and lowering pipe strings such as work sting  30 . 
     A wellbore  32  extends through the various earth strata including formation  14 . A casing  34  is cemented within wellbore  32  by cement  36 . Work string  30  includes various tools including a plurality of shaped charge perforating guns and a plurality of bi-directional explosive transfer subassemblies. When it is desired to perforate formation  14 , work string  30  is lowered through casing  34  until the shaped charge perforating guns are properly positioned relative to formation  14 . Thereafter, the shaped charge perforating guns are sequentially fired such that the shaped charges are detonated. Upon detonation, the liners of the shaped charges form jets that create a spaced series of perforations extending outwardly through casing  34 , cement  36  and into formation  14 . 
     In the illustrated embodiment, wellbore  32  has an initial, generally vertical portion  38  and a lower, generally deviated portion  40  which is illustrated as being horizontal. It should be noted, however, by those skilled in the art that the shaped charge perforating guns and the bi-directional explosive transfer subassemblies of the present invention are equally well-suited for use in other well configurations including, but not limited to, inclined wells, wells with restrictions, non-deviated wells and the like. 
     Work string  30  includes a retrievable packer  42  which may be sealingly engaged with casing  34  in vertical portion  38  of wellbore  32 . At the lower end of work string  30  is a gun string, generally designated  44 . In the illustrated embodiment, gun string  44  has at its upper or near end a ported nipple  46  below which is a time domain firer  48 . Time domain firer  48  is disposed at the upper end of a tandem gun set  50  including first and second guns  52  and  54 . In the illustrated embodiment, a plurality of such gun sets  50 , each including a first gun  52  and a second gun  54  are utilized. Each gun set  50  may have at least one orienting fin (not pictured) extending therefrom to insure that the gun set is disposed off-center with regard to casing  34  as described in U.S. Pat. No. 5,603,379 issued to Halliburton Company on Feb. 18, 1997, which is hereby incorporated by reference. While tandem gun sets  50  have been described, it should be understood by those skilled in the art that any arrangement of guns may be utilized in conjunction with the bi-directional explosive transfer subassemblies  56  of the present invention. 
     Specifically, between each gun set  50  is a bi-directional explosive transfer subassembly  56  which serves as a connector for connecting adjacent gun sets  50  together. As will be discussed in detail below, each bi-directional explosive transfer subassembly  56  has a ball and socket joint that allows adjacent tandem gun sets  50  to not only rotate relative to one another, but also, be angularly displaced relative to one another, which allows gun string  44  to be connected, deployed, oriented and fired in deviated wells. At the far end of gun string  44  is another time domain firer  58  that is attached to a second gun  54 . The other end of time domain firer  58  is attached to a ported closure  60 . 
     Referring now to FIG. 2, each bi-directional explosive transfer subassembly  56  has a housing  70  defining a housing cavity  72  therein. Housing  70  includes an upper housing portion  74 , a lower housing portion  76  and a pair of intermediate housing portions  78 ,  80 . Upper housing portion  74  defines an upper housing cavity portion  82  which is a part of housing cavity  72 . Lower housing portion  76  defines a lower housing cavity portion  84 , which is also a part of housing cavity  72 . Intermediate housing portion  78  defines an intermediate housing cavity portion  86 , which is also part of housing cavity  72 . 
     It should be apparent to those skilled in the art that the use of directional terms such as top, bottom, above, below, upper, lower, upward, downward, etc. are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. As such, it is to be understood that the downhole components described herein may be operated in vertical, horizontal, inverted or inclined orientations without deviating from the principles of the present invention. 
     Upper housing portion  74  is attached to a second gun  54  of one of the gun sets  50  of FIG. 1 at threaded connection  88 . A plurality of O-rings  90 , provides sealing engagement between upper housing portion  74  and the corresponding second gun  54 . Upper housing portion  74  is attached to intermediate housing portion  78  at threaded connection  92 . A plurality of O-rings  94  provides sealing engagement between upper housing portion  74  and intermediate housing portion  78 . 
     Lower housing portion  76  is attached to a first gun  52  of another gun set  50  of FIG. 1 at threaded connection  96 . A plurality of O-rings (not pictured) provides sealing engagement between lower housing portion  76  and the corresponding first gun  52 . Lower housing portion  76  is attached to intermediate housing portion  80  at threaded connection  98 . 
     The lower end of intermediate housing portion  78  fits within intermediate housing portion  80  and against the top of lower housing portion  76  to form a ball and socket joint  100 . Specifically, intermediate housing portion  78  has ball end  102  configured as a portion of a sphere having an external bearing surface  104  which is configured as a portion of a spherical surface centered on a center point  106 . The center point  106  is disposed on a pair of axes  108 ,  110 . Ball end  102  is integral with the cylindrical portion  112  of intermediate housing portion  78  such that ball end  102  and cylindrical portion  112  are fixed for movement together. 
     Intermediate housing portion  80  and the top of lower housing portion  76  form socket  114  of ball and socket joint  100 . Socket  114  includes socket wall  116  and socket wall  118  forming a portion of a spherical bearing surface  120  having substantially the same diameter as the spherical external bearing surface  104  of ball end  102 . Bearing surface  120  is centered on center point  106 . Accordingly, spherical external bearing surface  104  on ball end  102  is in sliding engagement with spherical internal bearing surfaces  120  of socket  114  which allows upper housing portion  74  and intermediate housing portion  78  to not only rotate relative to lower housing portion  76  and intermediate housing portion  80 , but also allows relative angular displacement therebetween. The extent of the angular displacement is limited by flange portion  122  that has a conically shaped inner surface having an angle α relative to axis  108 . 
     A first explosive device  130  is disposed in upper housing cavity  82  and intermediate housing cavity  86 , which is adapted to provide an explosive transfer between a second gun  54  and lower housing portion  76 . Similarly, a second explosive device  132  is disposed in lower housing cavity  84  and is adapted for providing an explosive transfer between a first gun  52  and upper housing portion  74  via intermediate housing portion  78 . Second explosive device  132  is substantially identical to first explosive device  130  but is positioned in an opposite direction. As will be further described, first and second explosive devices provide a bi-directional explosive path through housing  70 . 
     First explosive device  130  includes an insert  134  that is held in upper housing cavity  82  and an insert  136  that is held in intermediate housing cavity  86 . A booster  138  is disposed in the upper end of insert  134 . Booster  138  has a metallic portion that is crimped around one end of a length of detonating cord  140 . A detonating cord initiator  142  has a metallic portion that is crimped around the other end of detonating cord  140 . Detonating cord initiator  142  is positioned adjacent to shaped charge  144  which has a conical cavity  146  therein. Second explosive device  132  is made of substantially identical components as is first explosive device  130  with the exception that second explosive device  132  only has one insert  148  that houses booster  138 , detonating cord  140 , detonating cord initiator  142  and shaped charge  144 . 
     Intermediate housing portion  78  has a wall portion  150  that closes the lower end of intermediate housing cavity  86 . Similarly, lower housing portion  76  has a wall portion  152  that closes the upper end of lower housing cavity  84 . Thus, wall portions  150  and  152  are adjacent to one another. It will be seen that wall portions  150  and  152  separate intermediate and lower housing cavities  86  and  84  of housing cavity  72 . In one embodiment, but not by way of limitation, intermediate and lower housing portions  78  and  76  are made of steel, and thus, wall portions  150  and  152  provide a steel barrier between first and second explosive devices  130  and  132 . 
     In operation, work string  30  with gun string  44  forming a lower end thereof is run into in casing  34  of wellbore  32 . In the case of a deviated wellbore or a wellbore with restrictions, use of bi-directional explosive transfer subassemblies  56  improves the deployability of gun string  44  by allowing gun string  44  to bend during such deployment. Specifically, as best illustrated in FIG. 4, as gun string  44  is run into wellbore  32 , bi-directional explosive transfer subassemblies  56  provide for angular displacement between upper housing portion  74  and lower housing portion  76  via ball and socket joint  100 , thereby reducing bending moments in gun string  44  during deployment which could damage gun string  44 . In addition, use of bi-directional explosive transfer subassemblies  56  allows gun string  44  to be deployed in certain deviated wellbores into which gun string  44  could otherwise not be deployed. As illustrated, the maximum angular displacement is defined by angle α, which may be between about 1 and about 10 degrees and which is preferable about 5 degrees. It should be noted that angle α could also be greater than 10 degrees but through the use of multiple bi-directional explosive transfer subassemblies  56 , such large angular displacements are not typically required and may in fact cause deployment problems in certain wellbore configurations. 
     As illustrated in FIG. 1, first and second guns  52  and  54  of gun sets  50  have a plurality of perforating charges which are equally angularly disposed around a longitudinal axis of the guns. In this way, a plurality of substantially evenly distributed perforations may be made through casing  34 , in cement  36  and into formation  14 . On many occasions, however, it is desirable to have the perforations be more specifically directed. For example, but not by way of limitation, it may be desirable to have the perforations directed mostly downwardly and located in the lower half of casing  34 . Orienting fins (not pictured) can be used in conjunction with bi-directional explosive transfer subassemblies  56  to help orient gun sets  50  so that the perforation charges are mostly downwardly directed. Specifically, as upper housing portion  74  and lower housing portion  76  of bi-direction explosive transfer subassemblies  56  may rotate relative to one another at ball and socket joint  100 , gun sets  50  are substantially self-orienting when used in conjunction with orienting fins. 
     Once gun string  44  has been fully deployed, as seen in FIG. 1, the perforation process may begin. In a perforating operation, a firing head, such as time domain firer  48 , is actuated to initiate the uppermost first gun  52  of the uppermost gun set  50 . First gun  52  will then trigger its corresponding second gun  54  which will in turn detonate booster  138  in the uppermost bi-directional explosive transfer subassembly  56 . The explosive powder in booster  138  initiates detonating cord  140  which in turn initiates detonating cord initiator  142 . This subsequently detonates shaped charge  144  which is shaped to send a jet toward wall portion  150 . This explosive jet is sufficient to penetrate through the barrier formed by wall portions  150  and  152  and initiate the facing shaped charge  144  in second explosive device  132 . The explosive transfer occurs through second explosive device  132  in reverse order from that just described for first explosive device  130  resulting in the configuration seen in FIG.  3 . Eventually, a firing device in the first gun  52  attached to lower housing portion  76  is initiated. This sequence is repeated through the other gun sets  50  and bi-directional explosive transfer subassemblies  56 , eventually firing the lowermost second gun  54 , assuming that there is no break in the firing sequence. 
     There may be occasions when it will be desirable to initiate gun string  44  from the far end. In this event, a firing head, such as time domain firer  58 , is fired which initiates the firing of the lowermost second gun  54  which in turn triggers the lowermost first gun  52  to fire. The lowermost first gun  52  initiates second explosive device  132  in the lowermost bi-directional explosive transfer subassembly  56 . The explosive transfer in this case follows an upward path through bi-directional explosive transfer subassembly  56  to detonate the next gun set  56 . This sequence is repeated upwardly until the uppermost gun set  50  is fired. Since bi-directional explosive transfer subassembly  56  carries essentially identical first and second explosive devices  130  and  132  disposed therein and facing one another, it will be seen that bi-directional explosive transfer subassembly  56  is bi-directional, allowing firing from the top down or from the bottom up. 
     As described, this bi-directional firing capability allows the operator to select between firing gun string  44  from the top or the bottom. Also, if there is a misfire in one direction, gun string  44  may be then triggered from the other direction to fire the remaining guns, assuming there is not an additional misfire. Thus, the gun string  44  allows for one misfire situation without the necessity of removing the entire work string  30  from casing  34 . In addition, as best seen in FIG. 5, even if a bi-directional explosive transfer subassembly  56  is in an angularly displaced configuration, the explosive transfer function is nonetheless achieved as the jet formed from the first shaped charge  144  that is fired penetrates through wall portions  150  and  152  to initiate the facing shaped charge  144  even at the maximum angular displacement of angle α. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.