Abstract:
A method for producing modular down hole, hydraulic motor components involving the formation of replaceable stator slugs to be collectively housed within a stator housing to form a stator assembly, including, in some embodiments, replaceable lobe components for the stator slugs for altering the interference with a selected rotor for such motor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of The Invention 
         [0002]    The present invention relates to down hole tools and equipment used in oil and gas production. 
         [0003]    2. Background Information 
         [0004]    The idea of down hole motors for driving an oil well drill bit is more than one hundred years old. Modern down hole motors are powered by circulating drilling fluid (known in the industry as mud) that also acts as a lubricant and coolant for the drill bit.  FIG. 1  shows a conventional state-of-the-art down hole motor assembly. 
         [0005]    The drilling assembly  10  generally includes a rotatable drill bit  12 , a bearing/stabilizer section  14 , a transmission section  16  which may include an adjustable bent housing (for directional drilling), a motor power section  18 , and a motor dump valve  20 . The bent housing  16  and the dump valve  20  are not essential parts of the down hole motor assembly. The bent housing is only used in directional drilling. The dump valve  20  is used to allow drilling fluid to enter the motor as it is lowered into the borehole and to allow drilling fluid to exit the motor when it is pulled out of the borehole. The dump valve also shuts the motor off when the drilling fluid flow rate drops below a threshold. During operation, drilling fluid pumped through the drill string (not shown) from the drilling rig at the earth&#39;s surface enters through the dump valve  20 , passes through the motor power section  18  and exits the drilling assembly  10  through the drill bit  12 . 
         [0006]    Prior art  FIGS. 2 and 3  show details of the power section  18  of the down hole motor. The power section  18  generally includes a housing  22  that houses a motor stator  24  within which a motor rotor  26  is rotationally mounted. The power section  18  converts hydraulic energy into rotational energy by reverse application of the Moineau pump principle. The stator  24  has a plurality of helical lobes,  24   a - 24   e,  which define a corresponding number of helical cavities,  24   a ′- 24   e ′. The rotor  26  has a plurality of lobes,  26   a - 26   d,  which number one fewer than the stator lobes and which define a corresponding plurality of helical cavities  26   a ′- 26   d′.    
         [0007]    Generally, the greater the number of lobes on the rotor and stator, the greater the torque generated by the motor. Fewer lobes will generate less torque but will permit the rotor to rotate at a higher speed. The torque output by the motor is also dependent on the number of “stages” of the motor, a “stage” being one complete spiral of the stator helix. 
         [0008]    In state-of-the-art motors, the stator  24  is made of an elastomeric lining that is molded into the bore of the housing  22 . The rotor and stator are usually dimensioned to form a positive interference fit under expected operating conditions, as shown at 25 in prior art  FIG. 4 . The rotor  26  and stator  24  thereby form continuous seals along their matching contact points that define a number of progressive helical cavities. 
         [0009]    When drilling fluid (mud) is forced under pressure through these cavities, it causes the rotor  26  to rotate relative to the stator  24 . The interference fit  25  is defined by the difference between the mean diameter of the rotor  26  and the minor diameter of the stator  24  (diameter of a circle inscribed by the stator lobe peaks). Motors that have a positive interference fit of more than about 0.559 millimeters (0.022 inches) are very strong (capable of producing large pressure drops) under down hole conditions. However, a large positive interference fit will provoke an early motor failure. This failure mode is commonly referred to as “chunking”. 
         [0010]    In practice, the magnitude of the interference fit (at the time of assembly) is dictated by the expected temperature of the drilling fluid and down hole pressure. High temperatures will cause the elastomeric stator of a motor with negative or zero interference fit to expand and form a positive interference fit. For use at lower temperatures, it may be necessary to assemble the motor with a positive interference fit. As mentioned above, a motor with excessive interference fit will fail early. On the other hand, a motor with insufficient interference fit will be a weak motor that stalls at relatively low differential pressure. A motor stalls when the torque required to turn the drill bit is greater than the torque produced by the motor. When this happens, mud is pumped across the seal faces between the rotor and the stator. The lobe profile of the stator must then deform for the fluid to pass across the seal faces. This results in very high fluid velocity across the deformed stator lobes. 
         [0011]    In addition to temperature, certain types of drilling fluids may have an adverse effect on the operation of the drilling motor. For example, certain types of oil-based drilling fluid and drilling fluid additives can cause elastomeric stators to swell and become weak. Therefore, the composition of the drilling fluid must also be considered when choosing a motor with the appropriate amount of interference fit. 
         [0012]    Those skilled in the art will appreciate that the elastomeric stator of drilling motors is a vulnerable component and is responsible for many motor failures. However, it is generally accepted that either or both the rotor and stator must be made compliant in order to form a hydraulic seal. 
         [0013]    Accordingly, what is needed in the art is a drilling motor stator that does not suffer from the deficiencies of the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  illustrates a conventional state-of-the-art down hole motor assembly; 
           [0016]      FIG. 2  illustrates details of the power section  18  of the down hole motor; 
           [0017]      FIG. 3  illustrates further details of the power section  18  of the down hole motor; 
           [0018]      FIG. 4  illustrates a positive interference fit of an elastomeric lining that is molded into the bore of the housing; 
           [0019]      FIG. 5  illustrates a down hole motor stator assembly according to the present disclosure; 
           [0020]      FIG. 6A  illustrates a mold system comprising a mold housing, a mold core, and first and second end caps; 
           [0021]      FIG. 6B  illustrates a motor stator slug as formed from the mold of  FIG. 6A ; 
           [0022]      FIG. 7  illustrates a cross section of a motor stator showing a replaceable stator lip; and 
           [0023]      FIG. 8  illustrates a cross section illustrating negative interference. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    To address the above-discussed deficiencies of the prior art, the present disclosure provides a method of manufacturing a down hole motor stator and the product thereof: a down hole motor stator manufactured by that process. Refer now to  FIG. 5 . To this end, a down hole motor  500  comprising a stator tube  510 ; a plurality of stator slugs  520 ; first and second slip washers  531 ,  532 , respectively; first and second threaded compression rings  541 ,  542 , respectively; and first and second snap rings  551 ,  552 , respectively; is provided. A longitudinal cross section of the assembled down hole motor  500  is also shown in  FIG. 5 . 
         [0025]    Referring now also to  FIGS. 6A and 6B , the mold system  600  comprises: a mold housing  610 ; a core  620 ; first and second alignment disks  631 ,  632 , respectively; and first and second end caps  641 ,  642 , respectively. 
         [0026]    The mold housing  610  may have an inner diameter  611  slightly smaller than the inner diameter (ID) of the stator tube  510  (See  FIG. 5 ) for which the molded stator slugs  650  are intended, in order to facilitate final motor assembly. Alternatively, the stator slug  650  may be machined to a desired outer diameter after forming. In one embodiment, the inner diameter  611  of the mold housing  610  is slightly smaller in diameter than the intended down hole motor housing (stator tube)  510 . The mold housing inner diameter  611  relative to the inner diameter id of the intended stator tube  510  will be determined by prototyping. The required outer diameter  651  of the stator slug  650  may be dependent on the number of slugs in the finished motor  500 . The mold housing  610  may further comprise an injection port  612  and a relief port  613 . 
         [0027]    The mold core  620  has a mold functional portion  621 ; first and second end central shafts  622 ,  623 , respectively; and first and second alignment slots  624 ,  625 , respectively. It is advantageous that the mold system  600  produces one full cycle (or stage) of the stator for the intended down hole motor  500 . A “stage” is one complete spiral of the stator helix. Thus, the mold core  620  will have a functional portion length l equal to one full cycle of the intended stator. The functional portion  621  must be in the form of the void that will be left when the final motor stator slug  650  has been formed, typically having n+1, e.g., ten, lobes when the final motor rotor has n, i.e., nine (9), lobes. This is necessary to employ the reverse Moineau principle for the down hole motor. 
         [0028]    The first and second alignment disks  631 ,  632 , respectively, are substantially first and second washer bodies  633 ,  634 , respectively, having an inner diameter  635 ,  636 , respectively, to fit closely around the first and second end central shafts  622 ,  623 , respectively, and an outer diameter  637 ,  638  to fit closely inside the mold housing  610 . The first and second alignment disks  631 ,  632 , respectively, may further comprise index tabs  639 ,  640 , respectively, extending radially inwardly into the washer hole from the first and second washer bodies  633 ,  634 , respectively. The first and second alignment disks  631 ,  632 , respectively, further comprise a plurality of mold pins  660  extending longitudinally from the inner face of each of the first and second washer bodies  633 ,  634 . The plurality of mold pins  660  is spaced apart so that each pin fits between adjacent flutes of the mold functional portion  621 . This location of the pins  660  is assured by predefining the angular relationship of the pins  660  to the index tabs  639 ,  640  and the first and second alignment slots  624 ,  625 , respectively. The plurality of mold pins  660  will create spaced-apart alignment apertures  652  in each end of the final motor stator slugs  650 . 
         [0029]    First and second end caps  641 ,  642  for the ends of the mold system  600  are provided. Each of the first and second end caps  641 ,  642 , respectively, further may have first and second central apertures  643 ,  644 , respectively, therein for receiving the first and second end central shafts  622 ,  623 , respectively, of the core  620  therein. The end caps  641 ,  642  may be a slip fit over the first and second end central shafts  622 ,  623 , respectively, and inside the mold housing  610  to be held in place during molding by clamps or a fixture (not shown). The end caps  641 ,  642  may further comprise internal tabs  645 ,  646 , respectively, extending radially-inward to cooperate with the grooves  624 ,  625  of the central shafts  622 ,  623 . In an alternative embodiment, the end caps  641 ,  642  may not have through-apertures  643 ,  644 , but rather may be partial apertures and therefore have closed ends. Additional seals may be required in the mold system  600  not specifically noted herein but that are within the knowledge of one who is skilled in the art. 
         [0030]    The inner surface of the mold housing  610 , the outer surface of the core  620 , as well as inner surfaces of the first and second alignment disks  631 ,  632 , respectively, may be coated with a parting fluid (not shown) prior to injection of the forming gel (not shown). This will ease removal of the core  620  from the finished stator slug  650  and the slug  650  from the mold housing  610 . 
         [0031]    The manufacturing process comprises forming the plurality of discrete stator slugs  650  within the mold system  600  outside of the stator tube  510  and then assembling the stator slugs  650  and stator tube  510  into a finished motor  500 . Refer again to  FIG. 6A and 6B . A plurality of discrete stator slugs  650  is formed using the mold system  600 . Each stator slug  650  is formed by injecting a form-in-place polymer, e.g., a polymeric material comprising a high molecular weight, high density polyethylene (HMW-HDPE) and/or composite through injection port  612  into the mold system  600 . Excess polymer exits the mold system  600  through relief port  613  to assure complete filling of the mold  600 . While a final motor rotor  26  (See  FIGS. 2 and 3 ) may have n lobes, e.g., nine ( 9 ) lobes, the inner core  620  that shapes the motor stator inner surface  653  must have n+1 lobes, i.e., ten ( 10 ) lobes, to form the required corresponding ten (10) cavities  654  in the motor stator surface  653  of the reverse Moineau-principle motor. 
         [0032]    Because of the physical nature of the core  620  having  10  spiral lobes, the core  620  will have to be rotated with respect to the formed stator slug  650  in order to be removed after each stator slug is formed and cured. Therefore, the core  620  may have a recessed socket  628  in an end thereof so that the core  620  may be un-screwed from the slug  650  with a suitable tool before the slug  650  is removed from the mold housing  610 . A pushing ram (not shown) may be required to force the finished slug  650  from the mold housing  610 . Such a device is within the knowledge of one who is of skill in the art. The preferred order of removal of the core  620  from the slug  650  and the slug  650  from the mold housing  610  may be determined by experimentation. 
         [0033]    Multiple castings from the mold system  600  may be made to assemble the desired number of stages for a given motor. For ease of manufacturing, the number of stator slugs  650  to be used in a specific motor may equal the number of stages to be desired in the final motor, where a “stage” is one complete spiral of the stator helix. That is, where the number of stages is m, e.g., four (4), then four discrete stator slugs  650  (see also  521 - 524 ) would be used in the finished stator  520 . (See  FIG. 5 ) Thus, with the mold system  600  producing a full cycle of the stator  520  of the down hole motor  500 , it is easy to extend the stator for any number of cycles desired in the final motor stator by adding additional discrete products from the mold system  600 . The slugs  650  may further be machined to final dimensions before stator assembly. 
         [0034]    MOTOR ASSEMBLY. Refer now back to  FIG. 5 . The motor stator  520  is formed with the required number of sections (slugs  650 )  521 - 524 . A plurality of alignment pins  570  is inserted into alignment apertures  652  as shown. Of course, alternatively, the alignment apertures and pins may be replaced, with suitable provisions in the mold, with any suitable indexing method to assure the motor cycles are continuous. The slugs  521 - 524  are then assembled sequentially inside the steel stator tube  510 , i.e., a down hole motor housing. One method of assembly is: the second slip washer  532  may be assembled to the last slug  524 , then the last slug  524  is inserted into the stator tube  510 . The slugs  520  may require external lubricant during assembly. The third slug  523  is then assembled to the last slug  524  using the alignment pins  570  to align the two slugs  523 ,  524 . The third slug  523  is then slid into the stator tube  510 , etc. This procedure is then followed until the first slug  521  is inserted into the stator tube  510  and the first slip washer  531  is assembled to the first slug  521 . The first and second threaded compression rings  541 ,  542 , respectively, are inserted and the first threaded compression ring  541  is seated to the desired depth and the first snap ring  551  is placed in the stator tube  510 . The second threaded compression ring  542  is then torqued to the desired value, thereby intentionally compressing the stator slugs  520 . Lubrication may also be required on the compression-ring side of the first and second slip washers  531 ,  532  to prevent deformation of the first and fourth slugs  521 ,  524  during threaded compression ring seating. The second snap ring  552  is then set in place to prevent the second threaded compression ring  542  from backing off. One who is skilled in the art will readily understand the threaded compression ring and snap ring arrangement disclosed and possibly design other methods for retention of the motor portions. The threaded compression ring and snap ring retention finishes the forming of the down hole motor stator. Of course, one who is of skill in the art will readily design alternative sequences of assembly that remain within the scope of the present disclosure. 
         [0035]    The mold system  600  may alternatively be divided into two half-cycle systems for convenience of molding should the molding of full cycles be unwieldy. In this embodiment, each core  621  will constitute one-half of a cycle. The procedure for forming the half-cycle slugs and for assembly of the motor stator parallels the above description. 
         [0036]    Referring now to  FIG. 7 , illustrated is a cross section of a motor stator  700  showing an example of a replaceable stator lip  710 . In one embodiment, the lip  710  of the stator lobes  721   a - 721   f  may be affixed with rubber inserts  730   a - 730   f.  This embodiment may be useful when positive interference is desired with the motor rotor. Design of the mold core  620  of  FIG. 6A  may be such that the rubber insert  730   a - 730   e  is replaceable, thereby extending the useable lifetime of the motor stator  700 . One who is of skill in the art is familiar with how a replaceable lip may be designed into the mold core  620 . Furthermore, with the presently disclosed assembly, the final motor stator may be readily disassembled for replacement of the rubber insert  730   a - 730   e  in contrast to the prior art wherein the stator is formed entirely within the stator tube and is not removable without destruction. 
         [0037]    Refer now to  FIG. 8  with continuing reference to  FIG. 7 . In one embodiment, the stator lobes  721   a - 721   f  formed hereby and the rotor lobes may have a positive or a negative interference as shown by representative stator lobe  721   a  and rotor lobe  722   c.  That is, a positive interference such that there is no gap between the stator lobes  721   a - 721   f  and rotor lobes  722   a - 722   e  or a negative interference, such that there is a designed gap between the stator lobes  721   a - 721   f  and rotor lobes  722   a - 722   e.  With the interference kept to a precise limit by use of the replaceable rubber inserts, this configuration will increase the efficiency and power output of the down hole motor and reduce wear to the motor stator. It should be noted that the amount of interference of a particular motor may be varied by changing the diameter of the core  620  in the mold system  600  or by changing the dimensions of the rubber inserts  721   a - 721   f  in an embodiment employing changeable inserts. 
         [0038]    Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limited sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the inventions will become apparent to persons skilled in the art upon the reference to the description of the invention. It is, therefore, contemplated that the appended claims will cover such modifications that fall within the scope of the invention.