Patent Publication Number: US-8113289-B2

Title: Flow regulator for downhole progressing cavity motor

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for regulating the amount of flow through a progressing cavity motor within a downhole collar and other tubular. The rotational speed of the progressing cavity motor is a direct function of the volumetric flow rate passing through the stator of the motor. By modulating this flow, the rotational speed of the system may be modulated to a predetermined rate. 
     BACKGROUND OF THE INVENTION 
     A generator has been designed and constructed for creating electrical energy downhole in an oil well. The generator is driven with a progressing cavity motor as opposed to more classical methods, such as turbines. The progressing cavity motor is mechanically linked to the generator with a semi-rigid shaft. This shaft, referred to as a “flex shaft” is rigid enough to transmit a required amount of torque yet flexible enough to accommodate the eccentric neutation of the progressing cavity motor. The flex shaft in turn drives an electrical generator generally comprised of permanent magnets rotating about or within windings of an electrically conductive material such as copper wire. This results in the creation of an electrical charge capable of producing enough current to sustain electrical downhole instrumentation or other electrical devices. Further details regarding this generator are disclosed in Ser. No. 12/167,003 filed Jul. 2, 2008. 
     For a plurality of reasons, drilling fluid is pumped through the tubular string containing one or more drill collars from a pump located on the surface. A portion of this fluid is forced through the progressing cavity motor (pcm) located within a drill collar as the fluid travels downhole to pass to the drilling bit and return to the surface. The rotational speed of the pcm is directly proportional to the amount of fluid passing through the pcm. Under normal operations, this proportional amount is a minor portion of the total flow being supplied by the surface pump and passing through the drill collar. 
     In the process of drilling, the drilling mud may be pumped over a fairly wide flow range. This flow range may be 200 gallons per minute (gpm), up to and including 600 gpm. The output from the motor, however, desirably is a constant value. Flow through the pcm may be as much as 80 gpm or more. 
     While various designs exist for regulating fluid flow and pressure, this system modulates internal flow of drilling fluid within the motor. Substantially different yet related devices are taught in U.S. Pat. Nos. 3,974,876; 5,282,490; 5,301,713; 5,431,183; 6,053,196, and 6,129,112. 
     The disadvantage of the prior art is overcome by the present invention, an improved flow regulator for downhole pcm is hereinafter disclosed. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a system for regulating the fluid flow through a progressing cavity motor positioned downhole within a tubular includes an annulus restriction for restricted flow through an annulus passageway radially outward of the motor, and an annular biasing member for biasing the annulus restriction toward the closed position. Fluid flowing in the annulus exerts an opening force on the annulus restriction. The system further includes a central passageway through the motor for restricting flow, and a central biasing member for biasing the central restriction toward an open position, with fluid flow in the central passageway exerting a closing force on the central restriction. The system is particularly well-suited for providing a constant rpm for a downhole motor to power an electrical generator. 
     These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a progressing cavity pump powering an electrical generator. 
         FIG. 2  is a more detailed view of a portion of the pump shown in  FIG. 1 . 
         FIG. 3  is a cross-sectional view with substantially no flow through the pump. 
         FIG. 4  is a cross-sectional view with maximum flow through the pump. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides a system to achieve modulation of flow through a downhole motor while the flow outside the motor may vary.  FIG. 1  shows a sectional view of the system for regulating fluid flow through a progressing cavity motor  10 . The motor  10  may be suspended in the well from a work string  6 . The system uses a pressure differential across the motor for rotating the rotor  12  with respect to stator  14 . This pressure drop is also created in the annulus  20  between the O.D. of the stator  14  and the I.D. of the tubular  8  enclosing the motor  10 , with a spring loaded restriction  22 , as shown in  FIG. 2 . The tubular  8  at this depth is commonly a drill collar. This restriction is appropriately sized such that at the lower end of the flow spectra, the spring  24  will force the annular opening to its minimum, creating a greater pressure drop. As the annular flow is increased, the increased pressure drop across the restriction creates a larger force and the spring  24  is depressed, opening the restriction. Thus, the pressure drop in the annulus is largely proportional to the amount of fluid being forced through the annulus. The annular flow typically may be several magnitudes greater than the flow through the motor. 
       FIG. 2  depicts both the annular spring  24  and the inner smaller spring  26  partially depressed. Fluid flowing through the interior of the motor must pass by a central restriction  28 , which cooperates with reduced diameter neck portion  30 . Restriction  28  contains a plurality of circumferentially spaced recesses  32 , which allow minimum flow past the restriction even if the restriction  28  is fully seated in a closed position on the neck portion  30 . These recesses  32  provide continuous flow through the motor regardless of the axial position of the restriction with respect to neck portion  30 . Unlike the annular restriction  22 , as the flow is increased, the restriction  28  is forced toward it maximum resistance. The restriction  22  is positioned within the annulus  20 , and is biased toward a closed position by the spring  24 . The depression of the smaller inner spring  26  is a function of the drag force on the obstruction.  FIG. 2  shows the system in what would be a steady state position for a given flow within the operational range. Both springs  24 ,  26  are thus sized in conjunction with the geometry of both the annular and inner flow restrictors to give a dynamic balance for a desired flow condition. 
     The inner, smaller spring  26  and its flow restriction  28  are designed such that once a steady state flow is established, flow through the motor  10  is at a substantially constant rpm, thereby applying a constant rpm to electrical generator  40  powered by the motor. As shown in  FIG. 1 , generator  40  is provided above the motor, with a flex shaft  41  interconnecting the top of the rotor  12  with the generator  40 . A plurality of circumferentially spaced gaps  43  are provided in the tubular enclosing the flex shaft  41 , and allow fluids in the annulus between the work string  6  and the drill collar  8  to pass freely to and through the stator of the motor. In other embodiments, the generator  40  may be provided below the motor  10 . As the electrical load on the generator increases, the rpm is slowed and the flow through the tool will begin to be inhibited. If the inner flow is inhibited, the inner spring  26  will drive the obstruction to a more open position, allowing more fluid through the inner passage of the motor, thereby bringing the rpm back to the desired state. Likewise, should the generator&#39;s electrical load drop, the rpm will accelerate, allowing more fluid to pass through the inner portion of the tool. In turn, the inner restriction  28  will be driven to a more restrictive position, thus lowering the motor rpm. 
     The outer larger spring  24  is used to create a usable pressure drop across the motor for a spectrum of flow rates of drilling mud. The inner, smaller spring  26  is used to regulate the rpm of the pcm. Both springs are designed to act synchronously to produce a steady state flow condition. 
     As shown in  FIGS. 1 and 2 , the motor  10  may be positioned within upper collar  8 . The upper collar may have a mortise machined to accept a flanged stabilizer secured with bolts. The typical upper collar may have a 2 13/16″ bore with a 6⅝ IF machined drill collar joint. 
     Each of the annular restriction and the central restriction are biased axially in a selected direction, and preferably the axial bias is provided by a coil spring. Each of the annular restriction and the central restriction thus move axially relative to a conical shaped member to vary the flow past the restriction. More particularly, as shown in  FIG. 2 , the annular restriction  22  has a lower sleeve portion  34  for containing the spring  24 . The lower end of the motor  10  includes a sleeve-shaped member  36 , which in turn is bolted to the lower end of the drill collar, and contains a frustoconical portion  38  which has an exterior surface with a diameter increasing in an axially upward direction. As the restriction  22  moves upward relative to the conical section  38 , flow area is reduced. Sleeve-shaped member  42  in turn is positioned above the upper end of sleeve  36 , and preferably above the conical section  38  discussed above. The central restriction  28  is guided by sleeve  44  for axial movement, and moves downward to compress the spring  26  as a restriction moves toward the neck portion  30 . 
       FIG. 3  represents a maximum flow condition. Referring now to  FIG. 3 , the inner restriction  28  is seated on the neck portion  30 , although preferably limited flow through the progressing cavity motor is provided by the circumferentially-spaced flow passageways  32 . In  FIG. 3 , the spring  26  is thus fully compressed, and the spring  24  is also fully compressed since the annulus retainer  22  is forced downward by fluid pressure passing through the annulus. 
       FIG. 4  represents a minimum flow condition. In  FIG. 4 , the annulus spring  24  is fully extended to minimize flow through the annulus  20 , although gap between restriction  22  and conical portion  38  preferably still allows some fluid to pass through the annulus  20  and thus bypass the motor. In  FIG. 4 , spring  26  is fully extended, biasing the central restriction  28  away from the neck portion  30  to allow maximum flow through the motor. 
     According to the present invention, the annular biasing element provides a biasing force proportional to the opposing mildly movable due to the flow rate through the annulus passageway acting on the axially movable annulus restriction  22 , so that the biasing force increases with increased flow through the annular passageway. Similarly, the central biasing element provides a biasing force which is proportional to the force due to the flow rate through the central passageway acting on the axially movable central restriction  24 , so that the central biasing force will increase with increased flow through the central passageway. The annulus biasing member  24  biases the annular restriction or valve member  22  upward toward the conical seat  38 , although in alternate embodiments an inward protrusion could be provided on the interior of the drill collar  8 , in which case an annular biasing member may bias the valve member toward a seat which has a larger diameter than the largest restriction of the valve member. 
     The present invention is considered significantly better than concepts which utilize a more traditional governor with spinning weights to control fluid flow. In this application, such a governor concept would be difficult to achieve with the available diametrical space of a downhole progressing cavity motor, and the reliability of such a system would be questionable in view of acessibility and bearing problems associated with a spinning weight design. 
     A significant advantage of the present invention is that the system generates a substantially constant rpm for the output of progressing cavity motor which then results in a substantially constant voltage output from the electrical generator. Downhole tools which are powered by the electrical generator have a known voltage requirement, and thus a substantially constant voltage may be obtained from the generator without driving the motor at an excess speed, which may cause excessive, premature wear, as well as producing a higher than desired voltage. In the latter case, the additional voltage would have to be discarded, and may present significant problems with respect to heating down hole. 
     Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.