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 
   The present invention relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides equipment and methods for use in underbalanced well completions. 
   At times it is useful to be able to isolate a portion of a tubular string, such as a production tubing, drill pipe, liner or casing string, from the remainder of the tubular string. For example, while drilling underbalanced, it is useful to be able to periodically trip a drill string in and out of the well without killing the well. In that instance, a valve may be interconnected in a casing string, the valve being opened upon tripping in the drill string, and the valve being closed when the drill string is tripped out of the well. A valve suitable for such an application is described in U.S. Pat. No. 6,152,232, the entire disclosure of which is incorporated herein by this reference. 
   Other uses include running completion assemblies (including perforated or slotted liners) after drilling underbalanced, drilling overbalanced in areas of lost circulation to prevent kicks and loss of mud while tripping the drill string, and drilling in deep water where pore pressure and fracture gradient provide a narrow window for acceptable mud density and use of lower mud density is desired. 
   From the foregoing, it can be seen that it would be quite desirable to provide improvements in underbalanced well drilling and completions, in other operations, and in equipment utilized in these operations. 
   SUMMARY 
   In carrying out the principles of the present invention, in accordance with an embodiment thereof, an apparatus is provided which is an improvement over prior equipment utilized in the operations described above. 
   In one aspect of the invention, a well system is provided. The well system includes an apparatus positioned in a well and a tool conveyed through the apparatus in a container. The container engages the apparatus, actuating the apparatus and separating from the tool, as the tool is displaced through the apparatus. 
   In another aspect of the invention, an apparatus for use in a subterranean well in conjunction with a tool conveyed through the apparatus in a container is provided. The apparatus includes an engagement device which engages the container, preventing relative displacement between the container and the apparatus, as the tool is conveyed through the apparatus. 
   In yet another aspect of the invention, a valve for use in a subterranean well in conjunction with a tool conveyed through the valve in a container is provided. The valve includes a passage formed longitudinally through the valve, a closure assembly which selectively permits and prevents flow through the passage, and an engagement device which engages the container as the tool is conveyed through the passage. The closure assembly permits flow through the passage when the container is conveyed into the passage, and the closure assembly prevents flow through the passage when the container is removed from the passage. Engagement between the container and the engagement device separates the tool from the container. 
   These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of a representative embodiment of the invention hereinbelow and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic partially cross-sectional view of a well system embodying principles of the present invention; 
       FIG. 2  is a cross-sectional view of an apparatus used in the well system of  FIG. 1 , the apparatus embodying principles of the invention, and the apparatus being depicted in an initial configuration; 
       FIG. 3  is a cross-sectional view of the apparatus depicted in a configuration in which an engagement device of the apparatus has engaged a container containing a tool being conveyed through the apparatus; and 
       FIG. 4  is a cross-sectional view of the apparatus depicted in a configuration in which the tool is being used to cut through a portion of the container. 
   

   DETAILED DESCRIPTION 
   Representatively illustrated in  FIG. 1  is a well system  10  which embodies principles of the present invention. In the following description of the system  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. 
   As depicted in  FIG. 1 , the system  10  includes an apparatus  12  interconnected in a tubular string  14  positioned in a wellbore  16 . Representatively, the apparatus  12  is a valve which selectively permits and prevents flow through an interior passage  18  of the string  14 , and the string is a casing string cemented in the wellbore  16 . However, it should be clearly understood that the invention is not limited to these, or any other, specific details of the illustrated system  10 . For example, the casing string  14  could instead be a production tubing string, drill string, etc. 
   Another tubular string  20  is positioned in the casing string  14 . The tubular string  20  is used in the system  10  to convey a tool  22  through the passage  18 . Representatively, the string  20  is a drill string. However, the string  20  could be another type of conveyance, such as a production tubing string, a wireline, etc., in keeping with the principles of the invention. 
   The tool  22  could be a drill bit, a perforated or slotted liner, a mud motor, a production tool, a completion tool, a drilling tool, a packer, a multilateral tool, or any other type of well tool. Representatively, the tool  22  is a drill bit used to drill a wellbore extension  24  below the casing string  14 . In this situation, it may be desirable to close the valve  12  while the string  20  is tripped in and out of the wellbore  16 , such as when drilling overbalanced or underbalanced, but the valve would be opened when the drill bit  22  is conveyed therethrough into the wellbore extension  24  for further drilling. 
   In a unique feature of the invention, the drill bit  22  is conveyed in a container  26  attached to the drill string  20 . As the container  26  is conveyed into the valve  12 , the container engages the valve, operates the valve to open a closure assembly  28  of the valve, and then the container disengages from the tool, allowing the tool  22  to be conveyed into the wellbore extension  24  on the drill string  20 , without the container. 
   One advantage of this system lo is that the container  26  may be configured so that it can accommodate a variety of tools, and so a different container does not have to be constructed for each tool conveyed through the valve  12 . For example, the container  26  may be used to convey the drill bit  22  through the valve  12  during drilling operations, and then the same or a similar container may be used to convey an item of completion equipment (such as a packer, etc.) through the valve after drilling operations are completed. 
   Referring additionally now to  FIG. 2 , an enlarged cross-sectional view of the valve  12  is representatively illustrated. In this view it may be seen that the closure assembly  28  is depicted as including a flapper  30  pivotably supported relative to a seat  32 . 
   When closed as shown in  FIG. 2 , the flapper  30  prevents flow through the passage  18 . However, when pivoted downward about a pivot  34 , the flapper  30  no longer contacts the seat  32 , and flow is then permitted through the passage  18 . Note that other types of closure assemblies may be used in place of, or in addition to, the assembly  28 . For example, the closure assembly  28  could include a ball closure, a sleeve closure, etc. 
   Referring additionally now to  FIG. 3 , the valve  12  is depicted with the drill string  20  conveyed through the casing string  14 . The drill bit  22  is contained within the container  26 , which is shown engaged with the valve  12 . This engagement includes sealing engagement between a sleeve  36  of the container  26  and seals  38  axially straddling the closure assembly  28 , and contact between the sleeve and an internal shoulder  40  formed in the valve  12  which prevents further downward displacement of the sleeve through the passage  18 . 
   The drill bit  22  is contained in the sleeve  36  between a shoulder  42  formed internally on the sleeve and a plug or abutment  44  closing off a lower end of the sleeve. If desired, the drill bit  22  may additionally be secured relative to the sleeve  36 , for example, using shear screws  46  or another type of securing device. However, preferably the drill bit  22  is permitted to rotate and/or reciprocate within the container  26 . 
   The abutment  44  may be secured relative to the sleeve  36  using shear screws  48 , or another type of securing device. Preferably, the abutment  44  is made of a tough but relatively easily drillable material, such as a composite material, relatively soft metal, etc. The abutment  44  may be bonded to the sleeve  36 , for example, using adhesives or other bonding agents. 
   The sleeve  36  could also be made of a composite material (or another relatively easily drillable material), in which case the sleeve and abutment  44  could be molded together, or otherwise integrally formed. If the sleeve  36  is made of a composite material, then the seal surfaces  50  may also be made of a composite material, or another relatively easily drillable material. 
   As the container  26  is conveyed into the valve  12 , the abutment  44  contacts the closure assembly  28  and pivots the flapper  30  downward, thereby opening the passage  18 . Damage to the flapper  30  and seat  32  is prevented in part by the abutment  44  being made of the relatively easily drillable material. 
   The sleeve  36  then enters and maintains the flapper  30  in its opened position. Again, damage to the flapper  30  and seat  32  may be prevented by the sleeve  36  being made of the relatively easily drillable material. Sealing engagement between the seals  38  and seal surfaces  50  formed externally on the sleeve  36  isolates the closure assembly  28  from debris, etc. in the passage  18 . 
   For example, during drilling operations this sealing engagement may prevent cuttings from becoming lodged in the closure assembly  28 . The sleeve  36 , or a similar sleeve, may be positioned in the valve  12  while the casing  14  is cemented in the wellbore  16 , in which case the sleeve would prevent cement from contacting the closure assembly  28 . 
   As described above, a lower end of the sleeve  36  contacts the shoulder  40 , preventing further downward displacement of the sleeve relative to the valve  12 . If the shear screws  46  or other securing devices are used, then at this point a downwardly directed force may be applied to the drill bit  22  (such as by slacking off on the drill string  20  to apply the drill string weight to the bit) in order to shear the screws  46 . However, if the drill bit  22  is not secured to the sleeve  36  (other than being contained between the shoulder  42  and abutment  44 ), then this step is not needed. 
   Referring additionally now to  FIG. 4 , the valve  12  is depicted after the shear screws  46  have been sheared and the drill bit  22  has been displaced downward relative to the sleeve  36 . The drill bit  22  now contacts the abutment  44 . 
   As illustrated in  FIG. 4 , the drill bit  22  is being used to cut through the abutment  44  while the abutment remains attached to the sleeve  36 . This will release the drill bit  22  from within the container  26 , allowing the drill bit and the drill string  20  to displace through the open valve  12 . The alternative configuration depicted in  FIG. 4  has the abutment  44  bonded to the sleeve  36 . 
   However, if the abutment  44  is releasably attached to the sleeve  36 , such as by using the shear screws  48  as depicted in  FIG. 3 , then the downward displacement of the drill bit  22  into contact with the abutment  44  may operate to shear the screws and release the abutment from the sleeve. In that case, the drill bit  22  may not cut into the abutment  44  until after the abutment falls (or is pushed) to the bottom of the wellbore extension  24 . 
     FIG. 4  also depicts another type of engagement device  52  used to provide engagement between the sleeve  36  and the valve  12 . The engagement device  52  includes a snap ring  54  (such as a C-shaped or spiral ring) engaged with a groove  56  formed internally on the valve  12 . The snap ring  54  is preferably carried externally on the sleeve  36  and, when the sleeve is properly positioned relative to the valve  12 , the snap ring snaps into the groove  56 , thereby releasably securing the sleeve relative to the valve. Note that the engagement device  52  may be used as an alternative to, or in addition to, the engagement between the lower end of the sleeve  36  and the shoulder  40 . 
   After the drill bit  22  has cut through or otherwise released the abutment  44  from the sleeve  36 , the drill bit and drill string  20  are used to drill the wellbore extension  24 . When the time comes to trip the drill string  20  out of the wellbore, or otherwise raise the drill bit  22  back up through the valve  12 , the drill bit will eventually contact the internal shoulder  42  in the sleeve  36 . As the drill bit  22  is raised further, the sleeve  36  will also be raised therewith, and with the sleeve no longer maintaining the flapper  30  in its open position, the closure assembly  28  will close off the passage  18 . 
   Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, 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 and their equivalents.

Summary:
Equipment and methods which may be used in conjunction with an underbalanced well completion. In a described embodiment, a valve for use in a subterranean well in conjunction with a tool conveyed through the valve in a container is provided. The valve includes a passage formed longitudinally through the valve, a closure assembly which selectively permits and prevents flow through the passage, and an engagement device which engages the container as the tool is conveyed through the passage. The closure assembly permits flow through the passage when the container is conveyed into the passage, and the closure assembly prevents flow through the passage when the container is removed from the passage. Engagement between the container and the engagement device separates the tool from the container.