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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on provisional application Ser. No. 60/060,691 filed Sep. 23, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a fluid control device for use in an oil and/or gas well and, more particularly, to such a device for selectively controlling the flow of production fluid from a producing formation adjacent the well, through the well, and to the ground surface. 
     In a typical oil and gas production well, a casing is provided to line the well and is provided with perforations adjacent the formation to receive the production fluid. A tubing string is run into the casing and has an outer diameter less than that of the inner wall of the casing to form an annulus. A packer is placed in the annulus to direct the production fluid into the lower end of the tubing string for passage upwardly through the tubing string for recovery above ground. 
     It is often advantageous, and sometimes necessary, to utilize hydraulically-actuated packers and other ancillary devices, especially when operating in deviated or horizontal well sections. To this end, the flow of production fluid into and through the tubing string is blocked, and well fluid is introduced into the tubing string from the ground surface, to create a relatively high fluid pressure which is used to actuate these devices. After this operation is completed the tubing string must be opened to permit the flow pf production fluid through the string and to the ground surface. Therefore, pump-out plugs, or the like, are often provided in the tubing string which normally block fluid flow through the string and which are ejected from the string when the flow of production fluid is desired. However, these plugs are relatively large and, when ejected, must either be removed from the wellbore by coiled tubing or the like, which is very expensive, or left in the wellbore, which may cause problems during the life of the well. 
     Also, disc subs have been used which incorporate a disc that normally blocks fluid flow through the tubing string and which breaks in response to fluid pressure acting thereon when flow is desired. However, these disc subs suffer from the fact that the pressure that has to be applied to break the disc is often excessive and unpredictable. Therefore, other techniques have been devised to break the discs to permit fluid flow. For example, steel bars have been used which are dropped into the well or run on wireline or coiled tubing. This has disadvantages since the broken disc forms debris in the wellbore and, if the well has a deviated or horizontal section, a drop bar or wireline run is very unreliable. 
     Still other techniques for selectively blocking the flow of production fluid through the tubing string involve wireline set/retrieved tubing plugs. However, these devices require a “profile” sub that has to be added to the tubing string and require the use of wireline intervention, as well as increased risk and expense. 
     Therefore, what is needed is a relatively inexpensive and reliable device for selectively controlling the flow of production fluid through a tubing string in an oil and/or gas well which minimizes the amount of debris left in the wellbore yet which can be activated with a predictable and relatively low amount of fluid pressure. Also what is needed is a device of the above type which does not require a profile sub or any actuation device to be dropped into the tubing string or run into the string on wireline or coiled tubing. 
     SUMMARY OF THE INVENTION 
     The present invention, accordingly, is directed to a device for selectively controlling the flow of production fluid through a tubing string in an oil and gas well according to which one end of a housing is connected to a tubing string for insertion into the well, and well fluid is passed from the ground surface the one end of the housing. The other end of the housing is closed to establish well fluid pressure in the housing to actuate a packer and/or other ancillary devices. The other end of the housing can be opened by increasing the pressure of the well fluid in the housing above a predetermined value, thus permitting the flow of production fluid from the formation, through the housing and the tubing string, and to the ground surface. 
     Several advantages result from the device and method of the present invention. For example, they are relatively inexpensive and reliable, yet minimize the amount of debris left in the wellbore. Also, the device can be activated with a predictable and relatively low amount of fluid pressure, and does not require a profile sub or any actuation device that must be dropped into the tubing string or run into the string on wireline or coiled tubing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial elevation-partial sectional view depicting an installation in an oil and/or gas well including the device of the present invention. 
         FIGS. 2 and 3  are vertical sectional views of the device of the present invention depicting two operational modes of the device. 
         FIGS. 4 and 5  are views identical to those of  FIGS. 2 and 3 , respectively, but depicting an alternate embodiment of the device of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The well fluid control device of the present invention is designed to be used downhole in an oil and/or gas wall depicted in FIG.  1 . The reference numeral  10  refers, in general to a well casing that lines the well bore and receives a tubing string  12  having an outer diameter that is less than the casing to define an annulus  14  between the tubing string and the casing. The tubing string  12  can be lowered into the casing  10  from the ground surface in any conventional manner such as by using a wireline, coiled tubing, or the like. A packer  16  is disposed in the annulus  14  and extends around a lower portion of the tubing string  12 , as viewed in FIG.  1 . The packer  16  is preferably hydraulically actuated and since it is conventional, it will not be described in detail. A plurality of perforations  10 a are formed through the casing  10  below the end of the tubing string  12 . The perforations  10 a permit production fluid from a formation zone F to flow into the casing  10  and through the tubing string to the ground surface, in a manner to be described. 
     The control device of the present invention is referred to, in general, by the reference numeral  20 , and is attached to the lower end portion of the tubing string  12 . The control device  20  is adapted to selectively control the flow of the production fluid through the tubing string  12  and to the ground surface, and to permit well fluid from the ground surface to be introduced into the tubing string  12  and pressurized sufficiently to actuate the packer, and any ancillary devices. 
     To this end, and with reference to  FIG. 2 , the control device  20  comprises a sub  22  which is internally threaded at its upper end portion  22 a, as viewed in  FIG. 2 , to mate with a corresponding externally threaded lower end portion of the tubing string  12  (FIG.  1 ). The control device  20  also includes a tubular housing  24  having an internally threaded upper end portion  24 a that threadedly engages a corresponding externally threaded lower end portion  22 b of the sub  22 . A plurality of set screws  26 , one of which is shown in  FIG. 2 , are angularly spaced around the circumference of the upper end portion  24 a of the housing  24  and extend through aligned opening in the latter end portion and the lower end portion  22 b of the sub  22 , to secure the sub to the housing. A seal ring  28  extends between an outer surface portion of the sub  22  and a corresponding inner surface portion of the housing  24 . 
     A lower sub  30  is also provided which has an internally threaded upper end  30 a portion that threadedly engages a corresponding externally threaded lower end portion  24 b of the housing  34 . A plurality of set screws  32 , one of which is shown in  FIG. 2 , are angularly spaced around the circumference of the upper end portion  30 a of the lower sub  30  and extend through aligned opening in the latter end portion and the lower end portion  24 b of the housing  24 , to secure the connection between the sub and the housing. A seal ring  34  extends between and outer surface portion of the housing  24  and a corresponding inner surface portion of the sub  30 . The lower end portion of the lower sub  30  is externally threaded so as to enable internally threaded subs of ancillary equipment (not shown) to be attached to the device  20  as needed. 
     A tubular piston  40  is slidably mounted in the housing  24  and its outer surface is stepped to define an upper end portion  40 a, an intermediate portion  40 b extending just below the upper end portion, and a portion  40 c that extends from the intermediate portion  40 b to the lower end of the piston. The outer diameter of the intermediate portion  40 b is greater than the diameter of the portions  40 a and  40 c, and a pair of axially spaced seal rings  42 a and  42 b extend between the outer surface portion of the intermediate portion  40 b and corresponding inner surface portions of the housing  24 . The lower end of the piston  40  tapers to a relative sharp point for reasons to be described. 
     A ring  46  is disposed in a space defined between the outer surface of the upper end portion  40 a of the piston  40  and the corresponding inner surface of the housing  24 . The ring  46  receives a plurality of angularly-spaced shear pins  48  that extend through aligned openings in the ring  44  and the upper end portion of the piston  40 . The shear pins  48  thus normally retain the piston  40  in its upper position shown in  FIG. 2 , but are adapted to shear in response to a predetermined shear force applied thereto to release the piston and permit slidable movement of the piston downwardly in the housing  24 , as will be explained. A plurality of angularly-spaced openings  40 d, one of which is shown in the drawings, extend through the upper end portion  40 a of the piston  40  just below the openings that receive the shear pins  48 , for reasons that will also be explained. 
     The inner surface of the housing  24  is stepped so that the inner diameter of its lower portion is less than that of its upper portion to define an annular chamber  50  between the inner surface of the upper portion of the housing  24  and a corresponding outer surface of the piston  40 . The relatively large-diameter intermediate portion  40 b of the piston  40  defines the upper boundary of the chamber  50 , and the reduced-diameter portion of the housing  24  defines its lower boundary. The chamber  50  accommodates movement of the intermediate portion  40 b of the piston  40  during its downward movement. A seal ring  52  extends between an outer surface portion of the piston portion  40 c and a corresponding inner surface portion of the reduced-diameter portion of the housing  24 . Thus, the chamber  50  extends between the seal rings  42 b and  52  to isolate the chamber from fluids and to maintain the pressure in the chamber at atmospheric pressure for reasons to be described. 
     The lower sub  30  has a stepped inner surface that defines a shoulder that receives a frangible disc  56 , and a seal being  58  extends between the shoulder and the disc. The disc  56  is made of frangible material, such as glass that is adapted to shatter when impacted by the pointed lower end of the piston  40  with sufficient force. The end of the housing  24  abuts the disc  56 , and a seal ring  60  is disposed between the latter end and the disc. A seal ring  62  extends between the outer surface of the disc  56  and the corresponding inner surface of the sub  30 . The disc  56  is capable of withstanding relatively large differential pressures acting on its respective upper and lower surfaces far in excess of the amount of force required to shears the pins  48  as will be described. 
     In operation, a well fluid is introduced into the casing  10  from the ground surface at a sufficient pressure to block the flow of production fluid from the formation zone F ( FIG. 1 ) through the perforations  10 a and into the casing  10 . When it is desired to recover the production fluid, the tubing string  12  is run into the casing  10  with the device  20  attached to the lower end of the string, and with the packer  16  provided in a section of the string just above the device  20 . 
     The presence of the disc  56  in the lower end portion of the device  20  permits well fluid from the ground surface to be introduced into the tubing string  12  at an increased pressure to establish a hydrostatic load to allow the packer  16 , and/or any ancillary devices to be hydraulically set in a conventional manner. During this operation, the pressure of the well fluid in the device  20  acts on the upper end of the piston  40  in a downwardly direction and on the lower end of the piston in an upwardly direction. Since the area of the annular upper end surface of the piston  40  is greater that the area of its annular lower end surface, a differential force is established which applies a shear force to the pins  48 . However, the pins  48  are designed to normally resist the force and thus maintain the piston in its upper, static position of FIG.  2 . This increased fluid pressure in the device  20  is controlled so that the resultant differential pressure across the disc  56  caused by the latter pressure acting on the upper surface of the disc  56 , and the well fluid in the annulus  14  acting on the lower surface of the disc, does not exceed the design limit of the disc. 
     When the packer  16 , and any ancillary devices, have been set in accordance with the above and it is then desired to recover production fluid from the formation zone F, the pressure of the well fluid in the tubing string  12  is increased. Since the upper end surface of the piston  40  has a larger area than its lower end, the shear force applied to the pins  48  will be increased until the pins are sheared, with the openings  40 d increasing the volume of well fluid available to act on the upper surface of the piston  40 . The piston  40  is thus forced downwardly and its pointed lower end strikes the disc  56  with enough force to shatter it. It is noted that the relatively low atmospheric pressure existing in the chamber  50  does not impede this downward movement of the piston  40  and that the above increase in hydrostatic load is selected so that the disc  56  can withstand the resulting differential pressure acting on its upper and lower surfaces. The pressure of the well fluid in the tubing string  12  is then reduced as necessary to allow the well fluid in the annulus, and then the production fluid from the formation zone F, to flow through the device  20  and the tubing string  12  to the ground surface and be recovered. 
     The device  20  thus enjoys several advantages. For example, it is relatively inexpensive and reliable, yet can withstand a great deal of differential fluid pressure and be activated with a predictable and relatively low amount of fluid pressure. Also, the amount of debris left in the wellbore is minimized since the material used in the frangible disc  56  is such that, one broken by the piston  40 , it is reduced to small slivers or particles that can be flowed or circulated from the well. Further, the device  20  does not restrict the inner diameter of the well bore and thus allows other tools to pass through it and it does not require a profile sub or any actuation device that must be dropped into the tubing string or run into the string on wireline or coiled tubing. 
     The embodiment of  FIGS. 4 and 5  is similar to the embodiment of  FIGS. 2 and 3  and identical components are given the same reference numerals. According to the embodiment of  FIGS. 4 and 5 , a device  20 ′ is provided which is identical to the device  20  of the embodiment of  FIGS. 2 and 3  with the exception that, in the former device, a plurality of angularly-spaced ports, one of which is shown by the reference numeral  24 c in  FIGS. 4 and 5 , are provided through the wall of the housing  24 . The ports  24 c are axially located relative to the housing  24  so that they register with the lower portion of the chamber  50  when the piston  40  is retained in its upper, static position by the shear pins  48  as shown in FIG.  4 . Thus, the above-mentioned well fluid that is initially in the annulus  14  to maintain the production fluid in the formation zone F, as discussed above, will enter the chamber  50  through the ports  24 c and exert an upwardly-directed pressure against the lower annular surface of the relative large diameter portion  40 b of the piston  40 . 
     As in the previous embodiment, the upper surface of the piston  40  has a greater surface area than the lower surface due to the relatively large diameter portion  40 b. Therefore, there is one downwardly-directed force caused by the well fluid in the interior of the housing  24  acting on the upper surface of the piston  40  as described above and an upwardly directed force caused by the well fluid in the interior of the housing acting on the lower surface of the piston, also as described above. In addition, there is an additional upwardly-directed force by the well fluid in the annulus  14  acting on the lower annular surface of the relatively large diameter portion  40 b of the piston. Also as in the previous embodiment, the shear pins  48  are designed to shear at a predetermined shear force applied thereto based on the difference of the above-mentioned forces acting on the piston  40 . However, in this embodiment, the shear force can be much less than that of the embodiment of  FIGS. 2 and 3  due to the presence of the last-mentioned upwardly directed force. Otherwise the operation of the device  20 ′ is identical to that of the device  20  of the embodiment of  FIGS. 2 and 3 . 
     The device  20 ′ of the embodiment of  FIGS. 2 and 5  thus enjoys all of the advantages of the device  20  of the embodiment of  FIGS. 2 and 3  and, in addition, the amount of shear force required to shear the pins  48 , and therefore actuate the piston  40  of the former device is mush less than that of the latter device. 
     It is understood that variations can be made in the foregoing without departing from the scope of the invention. For example, although the tubing string  12  and the devices  20  and  20 ′ are shown extending vertically, it is understood that this is only for the purpose of example and that, in actual use, they can extend at an angle to the vertical. Therefore, the use of the terms “upper”, “lower”, “upwardly”, “downwardly”, and the like, are only for the purpose of illustration only and do not limit the specific orientation and position of any of the components discussed above. 
     It is understood that other modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.

Summary:
A device and method for selectively controlling the flow of production fluid through a tubing string in an oil and gas well according to which a housing is connected to a tubing string for insertion into the well, and well fluid is passed from the ground surface into the housing. The housing is provided with a plug to establish well fluid pressure in the housing to actuate a packer and/or other ancillary devices. The plug can be removed from the hosing by increasing the pressure of the well fluid in the housing above a predetermined value, thus permitting the flow of production fluid from the formation zone, through the housing and the tubing string, and to the ground surface.