Patent Publication Number: US-6338388-B1

Title: Load bearing pump rotor tag bar

Description:
FIELD OF THE INVENTION 
     The present invention relates in one aspect to a method for positioning the rotor in a downhole progressive cavity pump prior to its use. In another aspect, apparatus connected to the pump stator is capable of resisting downward thrust of the rotor, thereby relieving tension in the supporting rod string. 
     BACKGROUND OF THE INVENTION 
     A progressive cavity pump is located within an oil well, positioned at the bottom end of a production tubing string which extends down the bore of the well. The pump forces fluids up the bore of the tubing string to the surface. The pump comprises a pump stator hung at the end of the tubing string, and a rotor which is both suspended and rotationally driven by a sucker rod string extending downwardly through the tubing string&#39;s bore. 
     The rotor is a helical element which rotates within a corresponding helical passage in the stator. 
     The rotor rotates in the stator and drives fluids upwardly. A downward reaction force is created, driving the rotor downwardly. Further, the rotor hangs from the rod string. The combined force of the rod string weight and the pumping reaction causes the rod string to be under significant tension. As a result, the long length of elastic sucker rod stretches and the bottom end of the rotor moves slightly downhole to a lower elevation, perhaps even extending out of the bottom of the pump. 
     Conventionally, a stop is located at the bottom of the stator and is used to set the initial elevation of the rotor. This stop is known as a tag bar, which extends across the pump&#39;s inlet, minimally impeding the fluid inlet and prevents the rotor from exiting the stator. Once the rotor contacts the tag bar (indicated by a lessening of the suspended weight), then the rod string and rotor are lifted slightly (usually about 1 foot-1½ feet) in anticipation of providing enough clearance that, even when elongated during operation, the rotor will not contact the tag bar again. 
     Often however, the estimate of dynamic stretch is often inaccurate and the rotor ends up contacting the tag bar during operation anyway. The result is rotating to stationary, metal-to-metal damage; both to the tag bar and possibly to the rotor. 
     Even if the tag bar and rotor do not contact one another, another result of high tension is the natural tendency of the rod string to assume the straightest possible path, regardless of the profile of the tubing string. The result is a large amount of wear on the tubing string wherever the rod contacts a deviation in the tubing, such as through curves. Rotation stabilizers and centralizers are some of the prior art devices which passively deal with rod tension, deviation and wear. 
     In U.S. Pat. Nos. 5,209,294 and 5,725,053, both to J. Weber, rotor placer apparatus is disclosed which is located about 30 feet above the pump and which both suppresses vibration from the eccentric rotor and absorbs the downward thrust from the rotor. One perceived difficulty these devices is the quantity of hardware provided and its impact on the flow passages, restricting flow of fluid up the tubing string. 
     So, there are several demonstrated disadvantages associated with the use of progressive cavity pumps, in particular, due to the movement of the rotor associated with the pumping reaction and the known prior art remedies provided to date. 
     SUMMARY OF THE INVENTION 
     A supportive platform is positioned beneath the bottom of the rotor of a downhole progressive cavity pump. The platform can rotate, supported by a thrust bearing which is itself supported by a tensile housing secured to the bottom of the stator. Thereby, rotor-imposed loads on the platform translate into tensile loads directed into the stator and tubing string, lessening the load on the rod string and thereby both reducing the need for extreme accuracy in rotor positioning and reducing rod string and tubing wear. Preferably, a no-turn tool transmits load from the stator into the casing. 
     In one broad aspect of the invention, a rotating tag bar is provided comprising a housing connected to the bottom end of the stator which contains a platform which supports the bottom of the rotor and transferring at least a portion of the weight of the rotor and rod string into a bearing assembly and thus into the housing, stator and tubing string. 
     Preferably, the platform is a sleeve having a bore which is sized to accept a concentric prong fitted to the bottom of the eccentric rotor. The prong extends part way into the sleeve&#39;s bore and then bears against a stop. The preferred sleeve is supported in the housing using at least one thrust bearing and stabilized using radial bearings. The prong is formed with flow passages to pass well fluid through the sleeve&#39;s bore. 
     The above apparatus results in a novel method of landing a screw pump rotor suspended from the end of a rod string, comprising: fitting the bottom of a stator with a rotating tag bar apparatus as described above; running in the rotor until its bottom end tags the sleeve stop; and lifting to position the rotor&#39;s bottom end off of the sleeve&#39;s stop a distance which is less than the anticipated stretch, so that when operated and the rotor and rod string, the bottom end of the rotor is supported by the rotating tag bar, lessening the load in the rod string. The positioning of the rotor before operation is no longer critical as contact is encouraged. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial side, cross-sectional schematic view of one embodiment of the invention illustrating an installation below a progressive cavity pump for engaging and supporting the bottom end of its rotor; 
     FIG. 2 a side, cross-sectional schematic view illustrating rod and tubing strings implementing another embodiment of the invention having a cross-over apparatus between the rotor and the tag bar platform; 
     FIG. 3 is a cross-sectional view of a preferred embodiment of a rotating tag bar, shown in isolation from the stator from which it is suspended, and the rotor&#39;s prong prior to engagement; 
     FIG. 4 a  is a cross-sectional view of the prong; 
     FIG. 4 b  is a bottom view of the prong along lines IVb—IVb of FIG. 4 a;    
     FIG. 4 c  is a partial cross-sectional view of the prong of FIG. 4 a  sectioned along lines IVc—IVc; 
     FIGS. 5 a  and  5   b  are simplified one half cross-sectional assemblies of the rotating tag bar of FIG.  3 . FIG. 5 a  is an assembled view and FIG. 5 b  is an axially exploded view with the sleeve exploded laterally for clarity; and 
     FIGS. 6 a - 6   d  are schematic representations of the rotating tag bar during landing and operation; the rotating tag bar being represented only by the sleeve for simplicity. More specifically: 
     FIG. 6 a  illustrates landing of the rotor and prong wherein the prong tags the sleeve&#39;s stop; 
     FIG. 6 b  illustrates upward positioning the rotor prior to operation, both the tubing string and rod string supporting 100% of their own weights; 
     FIG. 6 c  illustrates the bottom of the rotor having lowered to the rotating tag bar use to rod string stretch on pumping; the tubing string bearing more than 100% of its own weight by partially supporting the rod string; and for comparison 
     FIG. 6 d  illustrates a hypothetical case to demonstrate the equivalent greater extent of stretch if a rotating tag bar were not present. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Having reference to FIGS. 1 and 2, the stator  1  of a progressive cavity pump is located concentrically within the well casing  2  and is suspended from a production tubing string  3 . The pump stator  1  is secured against reactive torque rotation, relative to the well casing, using a no-turn tool  4  such as that described in U.S. Pat. No. 4,901,793, issued Feb. 20, 1990 to J. Weber. 
     A rotor  5  is suspended from a string of sucker rod  6 . A universal joint  7  or similar device may be placed between the rotor  5  and the rod string  6 . The length of the string of sucker rod  6  is such that the rotor  5  seats into the downhole pump stator  1 . The rotor  5  has a bottom end  8  which is inserted into the stator  1  and ends adjacent to the pump bottom  9 . 
     As shown in FIG. 1, in a schematic sense, the bottom end  9  of the stator  1  is threaded into an improved tag bar assembly  10 . A tensile tubular member  11  connects to the stator&#39;s bottom end  9  and extends downwardly to support the outer race  12  of a thrust bearing  13 . The inner race  14  of the thrust bearing  13  supports a rotating platform  15 . The platform  15  replaces the prior art, non-rotating tag bar with a rotating tag bar platform for supporting the axial load imposed by the rotating rotor  5  and transmitting the resulting loads into the stator  1 , and tubing string  3 , or through the no-turn tool  4  into the casing  2 . 
     As shown in FIG. 2, in an alternate embodiment, due to the eccentric path of the end of the rotor  5 , it is preferable to place an intermediate cross-over device  16 , such as a universal joint, between the platform  15  and the bottom end  8  of the rotor  5 . The cross-over device  16  has a connection  17  to the bottom  9  of the rotor  5  for supporting the compressive load imposed on it between the rotor&#39;s end  8  and the rotating platform  15 . 
     In either embodiment, the rotating platform  15  does not impede flow of the fluid into the bottom  9  of the stator  1 . 
     The platform  15  is substantially open, only having sufficient cross-sectional area to support the rotor  5  and to connect to the thrust bearing  13 . Spacing the rotating platform  15  from the ends  8 , 9  of the rotor or stator further avoid interfering with fluid inflow into the stator  1 . Additionally, the tubular member  11  need not be a continuous tubular structure, having openings  18  formed therein, thereby permitting fluids in the annulus  19  formed between the tubular member  11  and casing  2  to flow to the bottom of the stator  1 . Further, flow ports  20  are formed around the periphery of the bottom  8  of the rotor  5 . 
     More particularly, a specific embodiment of the invention is provided which embodies the objectives set forth above and also addresses the practical aspects of landing a rotor  5  in the stator  1  and dictating what tension to accept in the rod string  6 . 
     Having reference to FIG. 3, a rotating tag bar  110  is provided wherein the tensile tubular member  11  comprises a tubular housing  31 . The housing&#39;s upper end  32  connects to the stator  9  (not shown). The lower end  33   18  is adapted to connect to a no-turn tool  4  as necessary (also not shown). 
     The housing  31  forms a bore  34 . Within the bore  34  is a sleeve  35  which is rotatable and supported on bearings  13  within the housing  31 . The sleeve  35  forms the rotating platform  15 . As shown in FIGS. 4 a  and  4   c,  the eccentric pump rotor bottom is fitted with a complementary concentric prong  36 . The prong is affixed at the rotor&#39;s center of rotation, thereby minimizing eccentric loading into the sleeve  35  and housing  31 . The prong  36  engages the sleeve  35  so that at least a portion of the rotor&#39;s load is transferred into the sleeve  35 , through the bearing  13 , and into the housing  31 . The prong  36  has a leading tapered nose  37  for ease of engaging the sleeve&#39;s bore  34 . In FIGS. 4 b,    4   c,  the cross-section of the prong  36  is seen to be non-circular, having one or more fluid bypass ports  38  formed therein. Accordingly, when the prong engages the sleeve&#39;s bore  34 , fluid can flow thereby and to the stator  1 . 
     More specifically, sleeve  35  is forms a radially outward thrust shoulder  39  intermediate its upper inlet end  41  and bottom end  42 . The sleeve  35  is cylindrically tubular, forming a bore  43 , contiguous with bore  34 . At the upper inlet end  41 , the bore  43  forms an engagement portion  44  for insertably accepting the prong  36 . Within the bore  43 , and at the bottom of the engagement portion  44 , a radially inward upset forms a shoulder or stop  45 . The stop  45  engages the prong  36  and blocks further downward movement, thereby supporting the prong  36 , rod  5  and rod string  6  thereabove. 
     The engagement portion  44  aids in a smooth axial transition between the landing of the rotor  5  versus the rotor  5  when operating. The engagement portion bore  43  can be contoured or helically fluted (not shown) to aid in engaging the prong  36 . 
     The sleeve  35  is rotatably and axially supported with a system of seals and bearings  13 . Bearings  50   a  and  50   b  are located either side of the thrust shoulder  39 . These bearings  50   a,    50   b  are capable of accepting and supporting some or all of the rotor thrust. Radial bearings  51   a,    51   b  are positioned at the upper and lower ends  41 , 42  of the sleeve for stabilizing the sleeve during rotor operation. 
     Seals and a pressure compensated lubrication system ensure the bearing have the best opportunity for survival in adverse fluid conditions. To aid in assembly and installation of the sleeve, bearings and seals, the housing  41  is assembled from a plurality of tubular sub-housing components. 
     Having reference to FIGS. 5 a  and  5   b,  most preferably, the housing  41  is formed of a top sub  60  and a bottom sub  65  having threaded ends for connection to the stator&#39;s bottom  9  and a non-turn tool  4  (if used) respectively. The bottom of the top sub  60  is coupled to an intermediate upper sub  61 , sandwiching the top radial bearing  51   a  axially therebetween. A center sub  62  couples with the intermediate upper sub  61 , sandwiching the thrust bearings  50   a,   50   b,  thrust shoulder  39  and sleeve  35  to the intermediate upper sub  61 . A barrel  63  couples the center sub  62  and the bottom sub  65 . An intermediate lower sub  64  is coupled to the bottom sub and extends upwardly into the barrel  63 , sandwiching the lower radial bearing to the center sub  62 . 
     The barrel  63  and intermediate lower sub  64  forming an annular lubrication chamber  66  therebetween. An annular piston  67  is fitted to the chamber  66 . At the lower end of the lubrication chamber, a passage  68  extends to the bore  34  for communicating with the well fluid. The upper end of the lubrication chamber has passageways  69  extending to each of the bearings  51   b,   50   b,   50   a,   51   a  for providing lubricant. The annular piston  67  is fitted with seals  70  to ensure that well fluid is not able to access the bearings. The piston  67  moves to equalize and compensate for pressure changes in the bore  37  and thereby avoid driving contaminants past the bearing protection seals  51   b,   51   a.    
     Having reference to FIGS. 6 a  - 6   c,  in operation, a pump stator is suspended from a tubing string. The stator is fitted with a rotating tag bar  110 . For illustrative purposes, only the sleeve  35  and a representation of the housing  31  are shown. A fanciful tubing load indicator  81  signals whether more or less than 100% of the assembled tubing  3 , stator  1  and housing  31  weight is being suspended. A fanciful rod string load indicator  82  signals whether more or less than 100% of the assembled rod string  6  and rotor  5  weight is being suspended. 
     Before the rotor  5  is landed, the tubing load indicator  81  reads 100%. 
     Having reference to FIG. 6 a,  the rotor  5  is fitted with a concentric prong  36  and lowered downhole on the rod string  5 . The rotor  5  is lowered through the stator  1  until the prong  36  is landed in the rotating tag bar by contacting the sleeve&#39;s stop  45 , indicated by the rotor load indicator  82  at less than 100%. Some of the rod string weight is transferred to the tubing string whose load indication is shown to increase beyond 100%. 
     Referring to FIG. 6 b,  the rod string is then lifted a predetermined amount, based on a calculation about how much the rod string will stretch in operation. The amount lifted will be a value less than the anticipated stretch amount, else the rotor prong  36  would not bear on the stop  45 . At this point, the prong is suspended above the shoulder a distance less than that it is expected to stretch in operation (as suggested in FIG. 6 d ). The tubing string  3  again supports only 100% of its own weight. The prong  36  is preferably still located in the engagement section  44  so that on rotor rotation, the prong  36  is laterally supported in the sleeve  35 . 
     Referring to FIG. 6 c,  as the rotor  5  and rod string  6  become axially loaded during pumping operation, the prong  36  lowers in the sleeve, contacting the stop  45 . The rod string weight is then borne partially by the rotating tag bar which transfers the load to the tubing string. Accordingly, the fanciful load indicators  81 , 82  show that the tubing string  3  now carries greater than 100% of its weight and the rod string carries less than 100% of its weight. 
     For example, a ⅞″ diameter at full pumping load is anticipated to experience an 18″ stretch or extension in length in operation. Accordingly, one can lift the rotor about 6″ off of the shoulder prior to starting the pump. As the pump begins to pump, the rotor and prong lower into the sleeve, contacting the shoulder when the rotor stretches 6″ of the anticipated 18″. The sleeves stop then begins and continues to absorb more and more of the weight of the rotor and rod string. Without the rotating tag bar, the prong would continue to drop a further 12″ (FIG. 6 d ). That previous stretch amount of 12″ is now supported by the tubing string and could be equivalent to transferring about ⅔ of the weight of the rod string and rotor to the tubing string. 
     The release of tension from the rod string has several beneficial effects. One effect is that contact wear between the rod string and tubing is diminished. Another effect is to increase tubing string tension. This places load on the threaded connections, including the stator to tubing string connection. The increased load significantly reduces the possibility of accidental unthreading during operation. Accordingly, the stator could now be operated without the need for a no-turn tool. 
     Advantages of the present invention include: 
     providing a tag bar which can accept rotor load in instances where the operating rotor position is below the bottom of the stator, normally destructive to conventional non-rotating tag bars; 
     the flow of fluid above the pump is not impeded and the present apparatus has a large fluid inlet capacity to provide fluid to the stator inlet; 
     the ability to accept rotor loads and relieve some of the tensile forces on the rod string and thereby reduce the load-related failures and wear on the string and production tubing; and 
     the possibility of eliminating the need for a no-turn tool or torque anchor.