Well Perforating Apparatus Including an Underbalancing Valve

1. FIELD OF THE INVENTION

The invention relates to a well perforating gun incorporating a valve. Upon firing of the gun, the valve is operated to open ports in the tubing string to permit immediate flow of production fluid upwardly through the tubing string.

2. HISTORY OF THE PRIOR ART

The procedure for effecting the perforation of a production formation of a subterranean well in a so-called "underbalanced" condition is an expedient well-known in the art. Briefly, the procedure contemplates the suspension of a packer and a perforating gun on a tubing string. The packer is set above the production formation so as to position the perforating gun adjacent the formation. Steps are then taken to reduce the fluid pressure in the tubing string to a level substantially below the anticipated fluid pressure to be developed by the production formation after perforation; for example, the tubing string may be filled with a light-density fluid, or a swabbing operation conducted therein. The perforating gun is discharged and then one or more ports or fluid passages in the wall of the tubing string adjacent the production formation are opened so as to permit an immediate flow of production fluid from the perforated formation into the tubing string and up to the surface of the well. Due to the pressure differential, or the "underbalanced" condition of the well at the time of perforating, the flow from the production formation is generally quite rapid and results in a flushing of the debris normally resulting from a perforating operation from the perforations in the production formation. The removal of such debris greatly enhances the productivity of the well.

A number of valving devices have heretofore been proposed for opening ports in the tubing string immediately adjacent the production formation subsequent to the firing of the perforating gun. For example, U.S. Pat. No. 4,299,287 to VANN proposes to use a freely falling detonating bar to shift a sleeve valve mounted in series relationship between the tubing string and the perforating gun. Other valving mechanisms are disclosed in the co-pending application Ser. No. 6-551,764, filed Nov. 14, 1983, and assigned to the Assignee of the invention. All of these prior art underbalancing valves were characterized by the incorporation of relatively complex mechanisms with the attendant risk that such mechanisms would fail to operate and thus defeat the entire purpose of perforating the well in an underbalanced condition.

This invention provides an underbalancing valve in the form of a single sub that may be threadably interconnected between the top end of a tubing-carried perforating gun and the bottom end of the tubing string. Such sub comprises a hollow conduit having one or more pairs of axially aligned, radial holes formed in the sidewalls thereof. The axis of each pair of holes preferably comprises a diameter of the bore of the hollow housing. A frangible rod or plug is then inserted through each pair of holes and thus traverses the bore of the housing. The ends of the frangible rod are provided with sealing elements to effect a seal of the particular hole that it is inserted in. Each rod end is slidable relative to the hole that it is inserted in, but is prevented from passing out of the hole by a snap ring engaging the end of the rod.

Upon the dropping of a detonating bar through the tubular string and through the bore of the housing to impact upon a primer located in the upper ends of the perforating gun, the detonating bar will successively contact the various frangible rods and effect the breakage of such rods. The remaining ends of the rods, if any, are then displaced inwardly by any pressure differential existing outside of the housing over the tubing pressure. Since in a typical underbalanced perforating operation the tubing pressure is substantially less than the anticipated fluid pressure subsequent to the perforating operation, the remaining ends of the frangible rods are thus readily moved inwardly into the conduit and, hence, open each of the holes in which they were mounted for free flow of production fluids from the newly perforated formation. Thus, the full benefits of underbalanced perforating are obtainable with a high degree of assurance of success, since the perforating gun cannot be fired by the detonating bar unless all of the frangible rods disposed above the primer of the perforating gun are successively broken by the fall of the detonating bar. Upon such breakage, the remaining ends of the rods are readily displaced from their sealing position in the holes in the conduit by the fluid pressure differential and permit a free flow of pressured fluids from the newly perforated formation into the bore of the tubing string.

As an additional advantage in the use of the present invention, build up of solid contaminants on the firing head is eliminated by provision of the present device which permits debris to pass across it, thereby not interferring with the downward travel of the bar. Moreover, the present invention also may be utilized to permit pressure within the tubing string conduit thereabove to be "held" for activation of a hydraulically or pneumatically set well packer apparatus.

Further advantages of the invention will be readily apparent from the following detailed description, taken in conjunction with the annexed sheets of drawings, on which is shown a preferred embodiment of the invention.

FIG. 1 is a schematic sectional view of a subterranean well with a perforating gun and underbalancing valve embodying this invention mounted therein.

FIG. 2 is a view similar to FIG. 1 but showing the condition of the well after the dropping of detonating bar and the firing of the perforating gun.

FIG. 3 is an enlarged-scale, vertical sectional view of an underbalancing valve embodying this invention.

FIG. 4 is a sectional view taken on the plane 4--4 of FIG. 3.

Referring to FIG. 1, there is schematically illustrated the environment in which an underbalancing valve embodying this invention is employed. A packer 10 is set at a location within the subterranean well 1 so as to position a depending perforating gun 15 in a position adjacent to a production zone 2. Packer 10 may be any conventional tubing-carried type that is settable either by application of fluid pressure or by manipulation of the tubing string 5. The perforating gun 15 likewise is of conventional construction and incorporates a plurality of shaped charges 15a disposed in angular and vertically spaced relationship. An impact detonatable primer 5b is mounted above the shaped charges 15a and connected thereto by a conventional primer cord (not shown). The perforating gun 15 is suspended from the packer 10 by a hollow conduit 12 which, in effect, constitutes a continuation of the tubing string 5.

Serially connected in conduit 12 is an underbalancing sub or conduit 20. The top end of conduit 20 is threadably and sealably secured to the bottom end of conduit 12 and the bottom end of conduit 20 is sealingly secured to the top end of the perforating gun 15. Thus, conduit 20 is located in closely adjacent relationship to the production zone 2 which is to be perforated.

Referring now to FIGS. 3 and 4, the conduit 20 will be seen as comprising a tubular element having threads 20b formed at one end for connection at the bottom of the conduit 12 and threads 20a formed in the other end for connection to the top of the perforating gun 15. A plurality of axially aligned pairs of holes 21 are then formed in the sidewalls of the conduit 20. The number of such holes is determined by the cross-sectional area of such holes and the total cross-sectional area of the holes should preferably equal the flow passage area of the bore 20c of the housing 20. Thus, one or more pairs of holes 21 may be provided.

Within each pair of holes 21, a frangible rod or plug 22 is slidably and sealably mounted. The seal is provided by an 0-ring 21a mounted in the bore of each hole 21. While the ends 22a of the frangible rods 22 are freely slidable relative to the holes 21, the rods are retained in position, and particularly restrained against radially outward movement, through the provision of a C-ring 23 which is mounted adjacent the outer face of each rod end 22a and snaps into a suitable groove 21b formed in the respective hole 21. Thus, so long as the frangible rod 21 is intact, it is prevented from movement relative to the holes 21 within which it is mounted and hence such holes are effectively sealed. If desired, the central portion 22b of each frangible rod 22 may be of reduced diameter in order to facilitate the breakage of such rod by the downward impact of a detonating bar.

Referring now to FIG. 2, there is schematically illustrated the effects of dropping a detonating bar 18 through the bore of the tubing string, hence through the bore 20c of the conduit 20, thus impacting each of the frangible rods 21 and breaking such rods as the detonating bar passes downwardly to impact against the primer 15b of the perforating gun 15. The detonation of primer 15b by the detonating bar 18 discharges the shaped charges 15a and effects the perforation of the wall of well 1 and the adjoining production formation 2.

In accordance with the preferred technique for perforating wells, prior to the firing of the perforating gun, the pressure within the tubing string 5 is reduced to a level substantially below the fluid pressure anticipated to exist within the production formation 2. Hence, immediately subsequent to the firing of the perforating gun, such fluid pressure differential will cause production flow from the production formation and will exert an inward force on the remaining end portions 22a of the frangible rods 22, thus forcing such rods end portions inwardly in the holes 21 to fall through the bore 20c of the conduit 20 and downwardly around the detonating bar 18 as indicated by the debris 22c in FIG. 2. Such removal of the ends 22a of the broken frangible bars 22 effects an immediate opening of each of the holes 21 and thus provides a fluid flow passage into the bore 20c of the housing 20 adequate to transfer all of the production flow into the bore of the tubing string 5. Due to the lower fluid pressure existing in the conduit 20 and tubing string 5 at the moment of firing the perforating gun, the flow is at a fairly rapid rate and thus insures that debris inherently associated with the firing of the perforating gun will be flushed out of the production formation, and thus will facilitate the free flow of production fluids from such formation.

Those skilled in the art will recognize that the aboredescribed underbalancing valve represents not only a very economical construction, but an entirely reliable valving arrangement that will function to open the tubing string 5 to production flow from the newly perforated formation immediately upon the firing of the perforating gun by the detonating bar 18.

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.