You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Description of the Prior Art 
     Perforating guns have long been employed to achieve the perforation of a well casing and an adjacent production formation. Originally, such perforating guns were lowered into the casing to the desired location on a wire line and then electrically activated to effect their discharge. More recently, perforating guns have been conveyed on the bottom of a tool string having an uninterrupted bore through which a detonating bar could be dropped to effect the firing of the gun, or in which a fluid pressure could be developed to effect a fluid pressure actuated detonation of a perforating gun. The tubing conveyed perforating gun has the advantage that other operations, such as chemical treatment, washing and/or gravel packing of the perforations and the production screen, can be accomplished with a single trip of the tool string into the well. See for example U.S. Pat. No. 3,987,854 (Callihan, et al). 
     It has been previously suggested in the prior art that production tubing be utilized as the tool string on which the perforating gun is inserted in the well. In such case, it often becomes desirable to disconnect the perforating gun after the perforating operation has been performed so that the subsequent movements of the tool string to effect the positioning of the screen adjacent to the perforations and to effect the chemical treatment, washing and/or gravel packing of the perforations may be conveniently accomplished without requiring the movement of the additional weight of the discharged perforating gun. It is therefore desirable to provide an economical, yet reliable apparatus for effecting the release of the perforating gun from the tool string. 
     The release of a tubing conveyed perforating gun has heretofore been disclosed in the prior art. See for example U.S. Pat. No. 3,706,344 (Vann) and U.S. Pat. No. 4,040,482 (Vann) and earlier references. Such prior art references, however, contemplate the severance of a perforating gun from the tool string by a tubing cutter or a wire line operated latch releasing mechanism. Obviously, the employment of a tubing cutter or any wire line disconnecting device necessitates the introduction of substantial delay due solely to the operation of separating the perforating gun from the tool string. 
     SUMMARY OF THE INVENTION 
     This invention provides a releasable coupling device for a perforating gun which is run into the well on a tubular work string or production string which may include one or more tools, such as packers, screens, washers, perforation treatment apparatus, gravel packers or the like. The releasable coupling comprises a first tubular assemblage which is threadably secured to the top of the housing containing the perforating gun. A second tubular assembly is threadably connected to the bottom of the tubular tool string. The two tubular assemblies have nestable sleeve portions which are normally held in secured relationship by a plurality of peripherally spaced, radially shiftable locking elements. 
     The locking elements are held in their securing position with respect to the two nested sleeves by a retaining sleeve which in turn forms part of an annular piston. The upper portion of the annular piston projects into an annular fluid pressure chamber maintained at surface ambient pressure. The lower portion of the annular piston projects into an annular fluid pressure chamber which is connected by fluid conduits to the interior of the perforating gun housing so that the high pressure blast of gas which is normally created within the perforating gun housing upon discharge of the perforating gun is directed into engagement with the lower end of the annular piston. This produces an upward movement of the piston sufficient to bring an annular recess into alignment with the radially shiftable locking elements which are then cammed into such recess by the inherent weight of the perforating gun suspended therefrom and moved radially inwardly to disconnect the nested sleeves, thus permitting the second tubular assembly and the tubular tool string to be moved upwardly relative to the perforating gun, or conversely, to permit the first tubular assembly and the discharged perforating gun to slide downwardly out of engagement with the tubular tool string to lodge in the bottom of the well. 
     It is thus assured that when the perforating gun is discharged, the releasable coupling will be automatically actuated to effect the disconnection of the two tubular assemblies due to the gas pressure force developed by the discharge. Such gas pressure force may be conveniently directed to the actuating piston by positioning the hammer and the explosive primer within the lower portion of the bore of the second tubular assembly and providing an annular communicating chamber between the explosive primer and the lower annular fluid pressure chamber. If, for any reason, the gas pressure developed by the discharge of the perforating gun is inadequate to effect the shifting of the actuating piston to effect the release of the coupling mechanism, the fluid pressure existing in the well at the location of the perforating gun will flow through the newly created openings in the gun housing and into the lower annular fluid pressure chamber to operate against the lower portion of the annular actuating piston and thus assure the release of the radially shiftable locking elements. 
     Accordingly, the disconnection of the tubing string conveyed perforating gun from the tool string occurs automatically immediately following the discharge of the perforating gun, thus eliminating the necessity for any wire line or other operations for effecting the disconnection of the perforating gun. 
     Further objects and advantages of the invention will be readily apparent to those skilled in the art from the following detailed description, taken in conjunction with the annexed sheets of drawings, on which is shown a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B collectively constitute a vertical quarter-sectional view of a perforating gun coupling mechanism embodying this invention, with components thereof shown in the coupled or connected position. 
     FIG. 2 is a view similar to FIG. 1A but showing the shifting of the actuating piston to a position permitting the latching elements to shift radially inwardly to release the two components of the coupling mechanism. 
     FIG. 3 is a vertical quarter-sectional view showing the axial separation of the two disconnected components of the coupling mechanism. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to FIGS. 1A and 1B, there is shown a releasable coupling mechanism 10 embodying this invention. Such mechanism comprises an upper tubular assembly 11 having internal threads 12a for sealably connecting to the bottom end of a tubular tool string. Such tool string may include a packer and one or more other well treatment or well completion devices, such as a screen, perforation washer, perforation treatment apparatus, and/or a gravel packing apparatus. 
     Releasable coupling 10 further comprises a lower tubular assembly 20 which terminates in external threads 21a to which may be sealably secured the housing 1 of a conventional perforating gun. The two tubular assemblies 11 and 20 are normally interconnected by a plurality of peripherally spaced, radially shiftable latching lugs 30. In the connected position shown in FIG. 1A, the latching lugs 30 are shown in their radially outermost position wherein downwardly facing, inclined shoulders 30a on the lugs 30 are in abutting engagement with an upwardly facing inclined shoulder 13a provided on the bottom portion of a latching sleeve 13 which is threadably secured at its upper end by threads 13b to the bottom end of a sub 12 which forms the other component of the upper tubular assembly 11. 
     Starting from the bottom and proceeding upwardly, the lower tubular assembly 20 comprises a connecting nipple 21 which is externally threaded at 21a for sealably mounting thereto the housing 1 of a conventional perforating gun. O-rings 1a seal the threads 21a. A typical gun which may be employed is that shown in co-pending application, Ser. No. 366,267, filed May 7, 1982. Nipple 21 has a constricted axial bore 21b which is normally traversed by a length of Primacord P which transmits energy from a detonatable primer 25 contained in the upper portion of the lower tubular assembly 20 down to the plurality of horizontally and vertically spaced shaped charges (not shown) that are conventionally provided in the perforating gun housing 1. 
     An inner body sleeve 22 is secured by internal threads 22a to the top portion of nipple 21 and the threaded joint is sealed by O-rings 22b. Inner body sleeve 22 is contoured to surround the upper end of nipple 21 and is provided with an axial bore 22c extending therethrough. The bore 22c is counterbored at 22d to provide an open bottom chamber for mounting of primer 25. A second counterbore 22e is provided above counterbore 22d to mount a conventional firing head 26 by threads 22f. Firing head 26 includes a conventional hammer 27 which is mounted for axial movement upon receipt of a downwardly directed impact blow. Any such blow imparted to hammer 27 detonates the primer 25 and in turn ignites the Primacord P. 
     Since there is an unobstructed axial passage from the location of the firing head 26 up through the bore of the upper tubular assembly 12, those skilled in the art will recognize that any type of firing mechanism, such as an electrically actuated mechanism, could be employed merely by connecting an appropriate electric wire line to the firing head 26, with the electric wire line passing through the unobstructed bore of the coupling 10 and through the tubing string to the surface of the well. Preferably, however, a conventional detonating bar (not shown) is dropped through the bore of the connected tool string to impact upon hammer 27 to effect the ignition of primer 26 and the subsequent ignition of the charges in the perforating gun. To aid in the direction of the detonating bar, an axially upwardly extending guide sleeve 28 is provided which is threadably secured at its lower end by threads 28a to a nipple portion 22g of the inner body sleeve 22. O-rings 28b seal the threaded connection 28a. Guide sleeve 28 has an internal bore 28b contoured to direct the detonating bar to centrally impact on hammer 27. 
     An outer body sleeve 32 is threadably secured by internal threads 32a to the lower portions of the inner body sleeve 22. Threads 32a are of larger diameter than threads 28a. The outer body sleeve 32 extends upwardly beyond the upper end of the guide sleeve 28 where it projects inwardly and is sealingly engaged with such upper end by O-rings 32c. The annular space 33 between the inner body sleeve 22 and the outer body sleeve 32 is divided into an upper portion 33a constituting a closed fluid pressure chamber, and a lower portion 33b constituting an open bottom fluid pressure chamber, by an annular piston 35 which is sealably engaged with the inner wall 32d of the outer body sleeve 32 by O-rings 35a and 35b, and with the upper end of the exterior cylindrical wall 28c of the guide sleeve 28 and is sealingly engaged with such upper end by O-ring 32c. It will therefore be apparent that the upper annular fluid pressure chamber 33a is maintained at the surface ambient pressure as the tool string is inserted into the well. Fluid pressure chamber 33a is preferably filled with air or an inert gas so that upward movement of the piston 35 can occur, provided a suitable pressure differential exists across the upper and lower surfaces of piston 35. 
     The lower annular fluid pressure chamber 33b is in fluid communication with a plurality of peripherally spaced vertical fluid passages 22h which are formed in the inner body sleeve 22. These passages in turn communicate with an annular chamber 36 defined between a radial wall 22j formed on the inner body member 22 and the extreme upper end face 21c of the nipple 21. The lower annular pressure chamber 33b is therefore in fluid communication with the restricted bore 21b containing the Primacord P and hence is in fluid communication with the interior of the perforating gun housing 1 and thus will receive the high pressure gases generated within such housing as a result of the discharge of the shaped charges contained in the perforating gun housing. 
     As shown in FIG. 1A, the outer surface 35d of the annular piston 35 is in abutting relationship with the interior surfaces of the latching lug elements 30 and hence maintains a coupling or connection between the upper tubular assembly 11 and the lower tubular assembly 20 so long as the piston 35 remains in the illustrated position. Whenever a fluid pressure differential is built up across the top and bottom ends of the annular piston 35, and in particular, a higher fluid pressure is exerted on the bottom end of the annular piston 35 than on the top end, the piston 35 will be shifted upwardly, compressing the ambient pressure compressible gas contained in the upper enclosed annular fluid pressure chamber 33a. Such upward movement of piston 35 will bring an annular recess 35e on its periphery into alignment with the peripherally spaced latching elements 30, thus permitting such latching elements to be cammed inwardly by cooperating inclined surfaces 30a and 13a to release the latching engagement between the cooperating latching surfaces 30a and 13a, as illustrated in FIG. 2. Since the latching lugs 30 constitute the only physical connection between the upper tubular assembly 11 and the lower tubular assembly 20, the coupling 10 is effectively disconnected and the perforating gun may be axially separated from the tubular work string as illustrated in FIG. 3. 
     Latching lugs 30 are mounted in peripherally spaced slots 32f provided in outer body sleeve 32. In order to prevent the latching lugs 30 from falling into the well bore when released from the latching sleeve 13, a locking lug holding sleeve 38 is provided which is secured to the upper external portion of the internal body sleeve 22 by a plurality of radially disposed bolts 38a. Retaining sleeve 38 extends downwardly and overlies a notch 30b provided in each of the locking elements 30, thus preventing such locking elements from falling out of the assemblage when the tubular assemblages are separated, as illustrated in FIG. 3. Additionally, in order to prevent the premature movement of the piston 35, a plurality of shear pins 35d are mounted between the piston 35 and the upper body sleeve 32 to maintain the piston in the coupled position illustrated in FIG. 1 until sufficient fluid pressure force is exerted on the piston to shear the shear pins 35d and permit piston 35 to move upwardly. 
     In the event that the perforating gun does not generate sufficient fluid pressure force to cause the piston 35 to shear the shear pins 35d and move upwardly to release the radially shiftable locking lugs 30, it will be apparent that the lower annular fluid pressure chamber 33b will be promptly filled with well fluids resulting from flow through the perforations and into the interior of the perforating gun housing. Such well fluid pressures are normally substantially in excess of the surface ambient pressure existing in the upper annular fluid pressure chamber 33a, and hence will exert an upward force on the piston 35 to effect the shearing of the shear pins 35d and the movement of piston 35 to the latching lug releasing position illustrated in FIG. 2. 
     Accordingly, if firing of any portion of the charges contained in the perforating gun is accomplished, it will be readily apparent to those skilled in the art that the annular piston 35 will be actuated to release the radially shiftable locking lugs and effect the disconnection of the upper tubular assembly 11 and the lower tubular assembly 20, thus disconnecting the perforating gun from the tubular tool string as illustrated in FIG. 3. Such action is automatic and requires no time or effort on the part of the operator, thus permitting the completion of the well to be accomplished more rapidly. 
     Although the 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, since alternative embodiments and operating techniques will become apparent to those skilled in the art in view of the disclosure. Accordingly, modifications are contemplated which can be made without departing from the spirit of the described invention.

Summary:
A method and apparatus for releasing a subterranean well perforating gun from a tubular string after firing of the subterranean well perforating gun. The apparatus comprises two telescopically inter-related tubular assemblies which are interconnected for axial co-movement solely by a plurality of radially shiftable latching elements. Such latching elements are normally maintained in a locked position by an annular piston. One end of the annular piston is exposed to gas at surface ambient pressure and the other end is exposed to the gas pressure generated in the perforating gun by its discharge and subsequently to the well fluid pressures produced by flow of well fluids through the perforations. The movement of the annular piston under such fluid pressure forces permits the locking elements to be cammed radially inwardly and release the connection between the tubular tool string and the perforating gun without any action on the part of the operator.