Abstract:
A progressing cavity rod-driven well pump utilizes a tag shoulder above a helical passage of the stator. The pump stator is located at the lower end of a string of tubing. The tag shoulder is more restrictive than a passage through the tubing. A pump rotor is secured to a string of rods and has a stop located above the rotor. The rotor is lowered on the rods until the stop lands on the tag shoulder. Then the operator lifts the rods and the rotor to accommodate for expected stretch during operation. By removing the rods and rotor, monitoring tools can be lowered through the tag shoulder and stator.

Description:
FIELD OF THE INVENTION 
   This invention relates in general to progressing cavity rod driven well pumps that are driven by a motor at the surface, and particularly to a method and apparatus for axially spacing the rotor within the stator. 
   BACKGROUND OF THE INVENTION 
   A progressing cavity pump has a stator and a rotor. The stator typically comprises an elastomeric liner within a housing. The stator is open at both ends and has a double helical passage extending through it. The rotor is normally of metal and has a single helical exterior formed on it. Rotating the rotor causes fluid to pump through the stator. Progressing cavity pumps are used for a variety of purposes. 
   As a well pump, progressing cavity pumps may be driven by a downhole electrical motor or by a string of rods extending to a motor located at the surface. With a rod driven pump, normally the stator is suspended on a string of tubing, and the drive rods are located within the tubing. When installing a rod driven progressing cavity pump, the operator first secures the stator to the string of tubing and runs the tubing into the well to a desired depth. The operator then lowers the rotor through the tubing on the string of rods and into the stator. 
   To operate the pump at desired capacity, the rotor must be at the desired axial spacing within the stator and the rods must be in tension. If the lower end of the rotor is spaced above a lower end of the stator during operation, then a lower portion of the stator will not be in engagement with the rotor and the pumping capacity will suffer. The operator thus needs to know when the rotor has fully entered the stator during installation. The operator can calculate how much the rods will stretch due to the hydrostatic weight of the column of well fluid in the tubing. With the anticipated stretch distance known and with the rotor at a known initial position in the stator, the operator can pull the rods and rotor upward a distance slightly greater than the anticipated stretch, so that during operation, the rotor will move back downward to the desired axial position relative to the stator. 
   In the prior art, prior to running the tubing, the operator secures or welds a tag bar across the bottom of the stator. During installation, downward movement of the rods will stop when the lower end of the rotor contacts the tag bar at the bottom of the stator. Upon tagging the bar, the operator pulls the rod string back toward the surface by the calculated amount of rod stretch. During operation, as well fluid fills the tubing, the rod stretches, allowing the rotor to move back downward until in full engagement with the stator. If installed properly, once the rods have stretched fully, the lower end of the rotor will be spaced above the tag bar and the rods will be in tension. 
   While this method works well enough, tag bar creates an obstruction at the bottom of the pump. The obstruction prevents the operator from lowering tooling or instruments through and below the pump for logging, tagging fill, and other monitoring related purposes. 
   SUMMARY OF THE INVENTION 
   In this invention, a tag shoulder is positioned above the stator. The tag shoulder defines a restrictive passage to the stator that is more restrictive than the passage through the tubing to the shoulder. The operator installs a stop above the rotor. The stop will freely pass through the tubing, but will not pass through the tag shoulder. 
   The operator lowers the rotor on the string of rods until the stop lands on the tag shoulder. At this point, the lower end of the rotor will be spaced below the lower end of the stator. The operator then lifts the string of rods and the rotor a selected distance that places the stop above the shoulder. This distance is calculated to be slightly more than the expected stretch of the rods due to the weight of a full column of liquid in the tubing. At this distance, the lower end of the rotor will be above the lower end of the stator. 
   Once the rods start rotating and the pump begins to lift liquid to the surface, the rods will stretch. When the tubing is completely full, the rotor will have moved downward to fully engage the stator. The lower end of the rotor will be substantially flush with the lower end of the stator, however, the stop will still be located above the shoulder. The rotor orbits within the stator during operation. The stop is dimensioned so that it will orbit also without contact with the tag shoulder. 
   The operator can retrieve the rods and the rotor, then run tools or instruments in on wireline for monitoring purposes. The tools are dimensioned to pass through the tag shoulder and inner diameter of the stator. Because there is no tag bar at the lower end of the stator, the tools can pass completely through the stator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a vertical cross-sectional view of a portion of a pump assembly constructed in accordance with this invention, and shown with the stop landed on the tag shoulder. 
       FIG. 2  is a view of the pump assembly of  FIG. 1 , showing the operator lifting the string of rods and rotor a selected amount after tagging the shoulder and before beginning operation of the pump. 
       FIG. 3  is a view of the pump assembly of  FIG. 1 , with the rotor and rods removed and a wireline tool lowered through the stator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , progressing cavity pump  11  has a stator  15  that is fixed within a housing  13 . Housing  13 , which may be considered a part of stator  15 , is normally of metal while stator  15  is normally of a deformable elastomeric material. A helical passage  17  configured in a double helix extends through stator  15  in a manner that is conventional to progressing cavity pumps. Pump  11  is suspended on the end of a string of production tubing  25 . 
   A sub  19  is mounted within tubing string  25  above stator housing  13 . Sub  19  has a passage  23  containing a tag shoulder  21 . In this embodiment, tag shoulder  21  is annular and faces upward. The inner diameter of passage  23  at tag shoulder  21  is equal to or slightly greater than the minimum inner diameter of passage  17  of stator  15 . Tag shoulder  21  is shown as a flat surface that is perpendicular to the longitudinal axis of stator  15 , but it could be conical, if desired. Passage  23  optionally may have an outward flared portion below tag shoulder  21 . 
   Sub  19  is secured by threads into the string of tubing  25 , and may be considered a part of the string of tubing  25 . Tubing  25  is conventional and may be either a plurality of individual sections of pipe screwed together or continuous coiled tubing. The inner diameter of tubing string  25  is greater than the inner diameter of passage  23  at shoulder  21 . By way of example, the inner diameter of tubing  25  might be  2⅞″ while the inner diameter of passage 23 at shoulder 21 is  2½″. The minimum inner diameter of passage  17  in a typical stator  15  for this use might be 1½″. 
   A conventional rotor  27  is shown located within stator passage  17 . Rotor  27  has a single helical configuration and is normally made of steel. A string of rods  31  extends downward from a drive motor (not shown) at the surface and connect to rotor  27  for rotating rotor  27 . Rods  31  normally comprise individual solid steel members that have threaded ends for coupling to each other. The combination of rotor  27  and rods  31  define a drive string for pump  11 . 
   A stop  29  is mounted to rods  31  above rotor  27  for movement therewith. Stop  29  may be two clamp halves, as shown, that are clamped around one of the rods  31  and secured by fasteners  30 . Alternately, stop  29  could be secured in other manners, such as by threads, retainer rings, or welding. The distance from stop  29  to the lower end of rotor  27  is greater than the distance from the lower end of stator  15  to tag shoulder  21 . When the lower end of rotor  27  is at the proper operational position in stator  15 , which is with the lower ends of stator  15  and rotor  27  substantially flush, stop  29  will be located slightly above tag shoulder  21 . 
   Stop  29  is preferably an annular enlargement having a greater outer diameter than rods  31 , the upper end of rotor  27 , and the inner diameter of passage  23  at tag shoulder  21 . The outer diameter of stop  29  is less than the inner diameter of tubing  25 . During operation, the upper end of rotor  27  orbits about the axis of stator passage  17 , thus stop  29  will also orbit, and its outer diameter is sized accordingly. 
   In operation, the operator first secures stator housing  13  to a string of tubing  25  containing sub  21 . The operator lowers the assembly into the well to a desired depth. Then, the operator assembles rotor  27  and stop  29  to a string of rods  31 , making up a drive string. The operator lowers the drive string until stop  29  contacts tag shoulder  21 , as shown in  FIG. 1 . The operator will know when this occurs because the weight indicator on the workover rig at the surface will display a weight drop off. At this point, a lower portion of rotor  27  will be protruding below the lower end of stator  15 . 
   The operator will normally have previously calculated an expected amount of stretch that will occur in the string of rods  31  during pumping operation, or he may do so at this time. The stretch is due to the weight of the fluid in the tubing  25  acting downward on pump rotor  27 . The operator will pull the string of rods  31  upward an amount that is slightly greater than the expected amount of stretch to be assured that stop  29  does not contact tag shoulder  21  during operation.  FIG. 2  illustrates rods  31  being pulled upward to accommodate stretch. At this point, the lower end of rotor  27  will be within passage  17  of stator  15  above the lower end of stator  15 . 
   Once the desired elevation of rotor  27  has been reached, the operator couples the upper end of the string of rods  31  to the motor and drive assembly (not shown) at the surface of the well. The operator begins rotating rods  31  by the motor and drive assembly. Rotor  27  rotates within stator  15 , pumping liquid to the surface. As tubing  25  fills with well fluid, rods  31  will stretch, causing rotor  27  to move downward relative to stator  15 . Preferably, when rods  31  are fully stretched, the lower end of rotor  27  will be substantially flush with the open lower end of stator  15 . This full engagement assures that pump  11  is able to pump at the desired capacity. When fully stretched, stop  29  will still be located a safe distance above tag shoulder  21 . 
   By way of example, in a typical well, the operator might lift rods  31  an amount in the range from 12″ to 24″ after stop  29  lands on tag shoulder  21 . The stretch during operation of a pump  11  in a well of typical depth would cause stop  29  to be normally above shoulder  21 . The thrust on rods  31  due to the weight of column of well fluid is accommodated by thrust bearings at the motor and drive assembly at the surface. 
   If the operator wishes to perform wireline or small diameter coiled tubing operations below stator  15 , he may do so by pulling rods  31  and rotor  27  to the surface. As shown in FIG.  3 , the operator then lowers a tool or instrument  33  through tubing  25 , preferably on wireline  35 . The outer diameter of tool  33  is less than the minimum inner diameter of passage  17  in stator  15  and also less than the inner diameter of passage  23  at tag shoulder  21 . Tool  33  thus will pass completely through stator  15  and out the open lower end. Tool  33  can be used for performing a wireline survey or logging operation, for determining the depth of fill that has occurred, or for other purposes. 
   The invention has significant advantages. The placement of a tag shoulder above the helical passage of the stator, rather than a bar below the stator, allows the operator to lower wireline tools below the stator. The tag shoulder allows a conventional tagging operation to occur much in the same manner as has been done with tag bars in the prior art. 
   While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but susceptible to various changes without departing from the scope of the invention.