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This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/275,853, filed on Mar. 14, 2001. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention pertains to tool strings, and particularly to retrievable tool strings used for underbalanced well completions. 
     2. Related Art 
     It is often desirable to isolate a portion of a well. For example, a portion of the well may be isolated during insertion or retrieval of a work string. It may also be desirable to isolate a portion of a well during perforation operations, particularly during underbalanced completion operations. 
     SUMMARY OF THE INVENTION 
     The present invention enables the retrieval of a completion string while maintaining control of a well without having to kill the well (i.e., without having to exceed formation pressure) each time the string is retrieved. 
    
    
     DESCRIPTION OF FIGURES 
     FIGS. 1A-1H are schematic views of a tool string constructed in accordance with the present invention, each figure showing contiguous portions (with slight overlap) of the tool string. 
     FIG. 2 is a sectional view of the tool string of FIGS. 1A-1H taken along section line A—A shown in FIG.  1 F. 
     FIG. 3 is a sectional view of the tool string of FIGS. 1A-1H taken along section line B—B shown in FIG.  1 G. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1A-1H, tool string  10  includes an upper string  12  and a lower string  14 . In one embodiment, upper string  12  and lower string  14  are deployed into the wellbore as a unit. In another embodiment, lower string  14  is deployed and located in the wellbore first. Subsequently, upper string  12  is deployed and stabbed into lower string  14 . 
     Lower string  14  includes a valve  20  that prohibits flow through a tool string passageway  18  when valve  20  is in the closed position, but permits flow through passageway  18  when valve  20  is in the open position. In the embodiment of FIG. 1H, valve  20  comprises a ball valve that is operated by a ball operator mandrel  22 . Sliding movement of ball operator mandrel  22  induces the opening or closing of ball valve  20 , as is known in the art. Ball operator mandrel  22  includes a ball collet  32  (FIG. 1G) that releasably locks ball operator mandrel  22  (and ball valve  20 ) in the open and closed positions. Fingers  34  of ball collet  32  are disposed within a lower ball groove  36  defined on an interior surface of a lower housing  38  when ball valve  20  is in the open position (as shown in FIG.  1 G). Ball valve  20  is, in the configuration shown, releasably locked in the open position. Sliding movement of ball operator mandrel  22  in the upward direction causes fingers  34  to snap out of lower ball groove  36  and slide on the interior surface of lower housing  38  until fingers  34  snap into an upper ball groove  40  defined on the interior surface of lower housing  38 . Ball valve  20  is, in that configuration, releasably locked in the closed position. Ball valve  20  may be moved between the closed and open positions any number of times by sliding ball operator mandrel  22  in the upward and downward directions. 
     Upper string  12  includes a stinger assembly  28  and a valve actuator mechanism  30  (FIG.  1 F). Upper string  12  may also include a safety valve  24  (FIG. 1D) that may be hydraulically actuated, and/or a crossover  26  (FIG.  1 A). 
     At its lower end, stinger assembly  28  includes a bearing  42  and a stinger collet  44 . When upper string  12  is properly positioned into lower string  14 , as shown in FIG. 1G, bearing  42  abuts ball operator mandrel  22 , and fingers  46  of stinger collet  44  are located within grooves  48  defined on the exterior surface  50  of ball operator mandrel  22 . Fingers  46  are disposed between lower housing  38  and ball operator mandrel  22 . Stinger collet  44  and bearing  42  are attached to the lower end of an actuating piston  52  which is movably disposed within an upper housing  54 . 
     A seal stack  56  is disposed around the exterior of upper housing  54 . When upper string  12  is properly positioned into lower string  14 , a portion of upper housing  54  stabs into a portion of lower housing  38 , and seal stack  56  forms a seal between upper and lower housings  38  and  54 , respectively. 
     Near the top of lower housing  38 , the interior surface of lower housing  38  includes threads  58 . In one embodiment, threads  58  are left-handed threads. A threaded collet  60 , which includes threads  62  on fingers  64  that match lower housing threads  58 , is disposed on the exterior of upper housing  54 . When upper string  12  is properly positioned into lower string  14 , finger threads  62  are engaged to lower housing threads  58 . 
     Upper housing  54  also includes a locking mechanism  66  to lock the engagement between finger threads  62  and lower housing threads  58 . Locking mechanism  66  comprises a locking piston  68  that includes an extension section  70  that slides between threaded collet  60  and upper housing  54 , ensuring that finger threads  62  are securely engaged to lower housing threads  58 . Threaded collet  60  and locking piston  68  include slots  72 ,  82  that are aligned. At least one peg  74  is attached to upper housing  54  and located within the aligned slots  72 ,  82  to prevent relative rotation between threaded collet  60  and locking piston  68 . When locking piston  68  is positioned so that extension section  70  is between threaded collet  60  and upper housing  54 , a snap ring  76  disposed within upper housing  54  snaps into a groove  78  defined on the exterior surface of locking piston  68 , thereby fixing locking piston  68  in the appropriate position. 
     The upper surface of locking piston  68  is in fluid communication with a lower chamber  84  that is in fluid communication with a lower control line  80 . Initially, locking piston  68  is located within lower chamber  84  so that extension section  70  is not between threaded collet  60  and upper housing  54 . When desired, control line  80  is pressurized to force locking piston  68  downward until extension section  70  is between threaded collet  60  and upper housing  54 , and snap ring  76  is locked within groove  78 . 
     A plurality of dogs  86 , each attached to upper housing  54  such as by screws  88 , are preferably disposed circumferentially between threaded collet fingers  64 . Extension section  70  preferably also slides underneath dogs  86 . Dogs  86  are preferably located within dog grooves  90  defined on the exterior surface of upper housing  54 . Dogs  86  serve to transfer torque to threaded collet  60 , as will be described below. 
     In operation, an operator initially stabs upper string  12  into lower string  14  so that: (1) bearing  42  abuts ball operator mandrel  22 ; (2) fingers  46  of stinger collet  44  are located within grooves  48  and disposed between lower housing  38  and ball operator mandrel  22 ; and (3) finger threads  62  of threaded collet  60  are engaged to lower housing threads  58 . Next, when an operator is prepared to lock upper string  12  to lower string  14 , control line  80  is pressurized to move locking piston  68  so that extension section  70  is between threaded collet  60  (dogs  86 ) and upper housing  54 , and snap ring  76  is locked within groove  78 . At this point, upper string  12  is mechanically locked to lower string  14 . 
     In one embodiment, ball valve  20  is in the closed position when first deployed in the well and ball collet fingers  34  are snapped into upper ball grooves  40 . As upper string  12  is positioned into lower string  14 , bearing  42  will abut the top of ball operator mandrel  22  and force ball operator mandrel  22  downward. This movement will cause ball collet fingers  34  to snap out of upper ball grooves  40  and slide downward until they snap into lower ball grooves  38 , thereby opening ball valve  20 . Thus, the stabbing of upper string  12  into lower string  14  forces ball valve  20  to move from its closed position to its open position. 
     Once upper string  12  is locked to lower string  14 , ball valve  20  may be operated (closed/opened) hydraulically. Actuating piston  52  has an annular extension  100  whose upper surface is in fluid communication with an upper chamber  102  that is in fluid communication with an upper control line  104 . In one embodiment, a rupture disk  106  is disposed between the lower surface of annular extension  100  and lower chamber  84 . Once upper string  12  is locked to lower string  14 , lower control line  80  is pressurized above the rating of rupture disk  106  to cause disk  106  to burst, providing fluid communication between lower chamber  84  and the lower surface of annular extension  100 . This pressurization forces annular extension  100 , and correspondingly actuating piston  52 , to move upward. In turn, as actuating piston  52  moves upward, fingers  46  of stinger collet  44  abut the top ends  108  of grooves  48 , thereby also forcing the upward movement of ball operator mandrel  22 . This upward movement causes ball collet fingers  34  to snap out of lower ball grooves  38  and slide upward until they snap into upper ball grooves  40 , thereby closing ball valve  20 . 
     The operator may thereafter open ball valve  20  again by pressuring upper chamber  102  through upper control line  104 , thereby causing annular extension  100  and actuating piston  52  to move downward so that bearing  42  forces ball operator mandrel  22  downward. Concurrently, stinger collet fingers  46  slide between lower housing  38  and ball operator mandrel  22 . The downward movement will cause ball collet fingers  34  to snap out of upper ball grooves  40  and slide downward until they snap into lower ball grooves  38 , thereby opening ball valve  20 . Ball valve  20  may thereafter be repeatedly closed and opened, as discussed above, by alternately pressuring lower and upper control lines  80  and  104 , respectively. 
     When the operator is ready to retrieve upper string  12 , he may do so without having to also retrieve lower string  14 . First, the operator rotates upper string  12  in the appropriate direction to unscrew the threaded connection between collet finger threads  62  and lower housing threads  58 . For instance, if lower housing threads  58  are left-handed threads, upper string  12  would be rotated to the right to disengage such threaded connection. It is noted that the fixed connection between the plurality of dogs  86  and upper housing  54  ensures that the torque applied to upper housing  54  is transferred to threaded collet  60 . This rotational motion causes the upward movement of upper housing  54 , including actuating piston  52 . As previously discussed, upward movement of actuating piston  52  in turn causes fingers  46  of stinger collet  44  to abut top ends  108  of grooves  48 , thereby also forcing the upward movement of ball operator mandrel  22 . This upward movement causes ball collet fingers  34  to snap out of lower ball grooves  38  and slide upward until they snap into upper ball grooves  40 , thereby closing ball valve  20 . 
     Continued upward movement of upper housing  54  (including after the disengagement of the threaded connection) results in the disengagement of stinger collet  44  from ball operator mandrel  22 . Once the threaded connection is disengaged and stinger collet  44  is disengaged from ball operator mandrel  22 , upper string  12  can be retrieved to the surface. It is noted that this mechanism/procedure ensures that ball valve  20  will be closed each time upper string  12  is disengaged from lower string  14 , thereby enabling retrieval of upper string  12  (including any additional components such as safety valve  24  and crossover  26 ) without having to kill the well. 
     During operation (and when ball valve  20  is open), hydrocarbons will be produced into tool string  10  below ball valve  20  and will flow upward through passageway  18 . In the embodiment including crossover  26 , flow of hydrocarbons can be diverted to an annulus  302  of the wellbore through crossover ports  300 , in which case the hydrocarbons flow to the surface within annulus  302 . In one embodiment, all of the flow is diverted to annulus  302  by including a plug (not shown) on completion string  304  above crossover  26 . In another embodiment, flow may be partially diverted to annulus  302  so that hydrocarbons flow to the surface through both passageway  18  and annulus  302 . Flowing hydrocarbons through annulus  302  is advantageous since the area of annulus  302  in some wells is smaller than the area of passageway  18 . 
     Although only a few example embodiments of the present invention are described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.

Summary:
A tool string for deployment in a wellbore includes an upper string and a lower string. The upper string includes a valve actuator, and the lower string includes a valve. The lower string is adapted to receive the upper string, and the valve actuator controls the operation of the valve once the upper string is received by the lower string.