Patent Publication Number: US-10309381-B2

Title: Downhole motor driven reciprocating well pump

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional application 61/920,292, filed Dec. 23, 2013 and to provisional application 61/985,614, filed Apr. 29, 2014. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates in general to reciprocating well pumps and in particular to a reciprocating well pump operated by a downhole electrical motor. 
     BACKGROUND 
     Many oil wells require pumping in order to produce the well fluid. One common type employs a reciprocating downhole pump. A sucker rod extends down the well to the plunger of the pump. A lifting mechanism at the surface strokes the sucker rod to lift the well fluid. Extending a sucker rod string down to a pump is problematic for deep wells and wells where the pump is located in an inclined lower portion. 
     Rotary pumps driven by a downhole electrical motor are also utilized to a large extent. The pump may be a centrifugal pump having many stages of impellers and diffusers. Rotary oil well pumps also include progressing cavity pumps, in which a rotor rotates within an elastomeric stator. The rotor and the stator have helical contours. 
     Also, various proposals have been made to drive a reciprocating pump with a downhole electrical motor. One type employs a motor that rotates a drive shaft. A helical screw mechanism converts the rotation to linear to stroke the pump. In another type proposed, a linear motor is employed to stroke the pump. The linear motor has electromagnet coils and a mover with permanent magnets located within a bore of the coil assembly. When energized with one type of pulse, the mover strokes linearly in one direction. Another type of pulse causes the mover to stroke in an opposite direction. 
     For various reasons, reciprocating pumps with downhole electrical motors are not in commercial use to any extent. 
     SUMMARY 
     The submersible well pump assembly disclosed herein has a pump housing with a pump discharge on upper end. A pump barrel is located within the pump housing, defining an annular passage between the barrel and the pump housing. A plunger is reciprocally carried in the barrel. A motor is mounted below the pump housing and operatively coupled to the plunger for causing the plunger to reciprocate between an upstroke and a down-stroke. A valve means within the pump housing directs well fluid in the barrel below the plunger into the annular passage and out the discharge during a down stroke of the plunger. The valve means admits well fluid into the barrel below the plunger during the up stroke of the plunger. 
     The valve means comprises a barrel outlet port below the plunger that places well fluid in the barrel in fluid communication with well fluid in the annular passage. A connecting rod extends between the motor and the plunger. The connecting rod is in tension during the down-stroke. The valve means may also comprises a well fluid inlet at the upper end of the pump housing that admits well fluid from an exterior of the assembly into the pump housing. 
     In some of the embodiments, the well fluid inlet is in fluid communication with an interior of the barrel above the plunger. The well fluid inlet may direct well fluid from the inlet into the barrel above the plunger during the upstroke of the plunger as well as the down-stroke. 
     In some of the embodiments, a plunger passage extends axially through the plunger. A traveling valve is mounted to the plunger for movement therewith. The traveling valve opens the plunger passage to allow well thud in the interior of the barrel to flow downward through the plunger passage during the upstroke. The traveling valve closes during the down-stroke, preventing well fluid below the plunger from flowing upward through the plunger passage. 
     In some of the embodiments, the motor has a motor outer housing and a motor inner housing mounted concentrically in the motor outer housing. The motor inner housing has a smaller outer diameter than an inner diameter of the motor outer housing, defining a windings chamber. A coil winding is located within the windings chamber and immersed within a dielectric fluid contained in the windings chamber. A mover is located within the motor inner housing, the mover comprising a shaft with a plurality of magnets extending along a length of the shaft. Electrical power supplied to the coil winding causes the mover to move linearly along the axis. The mover is operatively coupled to the plunger for causing the upstroke and down-stroke movement of the plunger. 
     An expansion chamber may be coupled to the motor outer housing. The expansion chamber has a movable element that contains a dielectric fluid. The movable element is movable in response to a difference between well fluid pressure exterior of the expansion chamber and the dielectric fluid pressure. A dielectric fluid communication passage leads from the expansion chamber into the windings chamber in fluid communication with the dielectric fluid in the windings chamber. The motor may have a motor well fluid passage extending into the interior of the motor inner housing, immersing the mover in well fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       So that the manner in which the features, advantages and objects of the disclosure, as well as others which will become apparent, are attained and can be understood in more detail more particular description of the disclosure briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the disclosure and is therefore not to be considered limiting of its scope as the disclosure may admit to oilier equally effective embodiments. 
         FIG. 1  is a side view of a first embodiment of an electrical submersible pump assembly in accordance with this disclosure and installed in a well. 
         FIGS. 2A and 2B  comprise a sectional view of the pump of the pump assembly of  FIG. 1 . 
         FIG. 3  is a transverse sectional view of the pump of  FIG. 2 , taken along the line  3 - 3  of  FIGS. 2A and 2B . 
         FIG. 4  is a transverse sectional view of the pump of  FIG. 2 , taken along the line  4 - 4  for  FIGS. 2A and 2B . 
         FIGS. 5A and 5B  comprise a sectional view of the linear motor of the pump assembly of  FIG. 1 . 
         FIG. 6  is a schematic view of a second embodiment of an electrical submersible pump assembly in accordance with this disclosure and installed in a well. 
         FIGS. 7A and 7B  comprise a sectional view of the pump of the assembly of  FIG. 6 . 
         FIG. 8  is a sectional view of the pump of  FIGS. 7A and 7B , showing the plunger in a different position. 
         FIG. 9  is a perspective view of a third embodiment of a pump in accordance with this disclosure. 
         FIGS. 10A and 10B  comprise a sectional view of a fourth embodiment of a pump in accordance with this disclosure. 
         FIGS. 11A, 11B and 11C  comprise a sectional view of a linear electrical motor coupled to the pump of  FIGS. 10A and 10B . 
         FIG. 12  is a sectional view of a portion of a an expansion chamber unit for use with the motor of  FIGS. 11A-11C . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The methods and systems of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The methods and systems of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. 
     It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. 
     Referring to  FIG. 1 , a well  11  has casing  13  that is perforated to admit well fluid. A pump assembly  15  is illustrated as being supported on production tubing  17  extending into well  11 . Alternately, pump assembly  15  could be supported by other structure, such as coiled tubing. Although shown installed vertically, pump assembly  15  could be located within an inclined or horizontal section of well  11 . Pump assembly  15  could be employed to feed well fluid to the intake of an upper pump assembly (not shown) located above. 
     Pump assembly  15  includes a linear motor  19  connected to a lower end of a reciprocating pump  21 . The terms “upper” and “lower” are used herein for convenience only since pump assembly  15  could be oriented horizontally. A power cable  23  extends downward from a wellhead to motor  19  to supply power. In this example, pump assembly  15  is double acting, having an upper intake  25  and a lower intake  27 , both of which are located above linear motor  19 . Alternately, pump assembly  15  could be single acting, and if so, preferably the power stroke that lifts the well fluid up tubing  17  occurs during the down-stroke, and the fill stroke to admit fluid to the pump  21  during the upstroke. 
     Referring to  FIG. 5B , motor  19  has a lower end or base  29 . A cylindrical outer housing  31  has a lower end that secures to base  29 . A cylindrical inner housing  33  is concentrically mounted to base  29  within outer housing  31  along a longitudinal axis  32  of pump assembly  15 . A set of electromagnetic coils or windings  35  are located in an annular space  34  between inner and outer housings  33 ,  31 . Coils  35  may be in a slotted or slot-less arrangement. The lower end of coils  35  is spaced above base  29  a selected distance. 
     A mover  37  within inner housing  33  comprises a shaft having permanent magnets  39  along its length. Mover  37  moves linearly in inner housing  33  along axis  32  in response to an electromagnetic field generated by coils  35  affecting magnets  39 . A control circuit (not shown) located adjacent to a wellhead cycles power supplied to coils  35  to cause mover  37  to stroke upward and downward. The distance from the uppermost magnet  39  to the lowermost magnet  39  is about twice the axial length of coils  35 . Alternately, the axial distance between the uppermost and lowermost magnets  39  could be one-half the axial length of coils  35 . Magnets  39  are illustrated as having outer diameters greater than mover  37 . Magnets  39  may slidingly engage the inner surface of inner housing  33 , however, they do not form seals with inner housing  33 . Magnets  33  may be magnetized radially, axially, or in a Halbach arrangement. 
     A dielectric lubricant optionally may be located in inner housing  33  that is sealed from well fluid on the exterior of motor  19 . If so, the stroking of mover  37  does not cause pumping action of any lubricant in inner housing  33 . Similarly, a sealed dielectric fluid may be located in annular space  34  between inner housing  33  and outer housing  31 , and optionally sealed from any lubricant within inner housing  33 . Optionally, a pressure equalizer or expansion chamber (not shown) will communicate hydrostatic well fluid pressure to any lubricant and/or dielectric fluid contained in outer housing  31  and inner housing  33 . 
     A connecting rod  41  located on axis  32  couples to mover  37  with a connector  43 . Referring to  FIG. 5A , inner housing  33  sealingly secures to the lower end of a motor head  45 . Connecting rod  41  extends through the upper end of inner housing  33  and through motor head  45 . Connecting rod  41  extends sealingly through an axial passage in motor head  45 . Power cable  23  connects to motor  19  with a power cable connector  47 . Motor leads (not shown) extend from power cable connector  47  through annular space  34  to coils  35 . Motor head  45  has a connector  49  on its upper end that may comprise threads, either internal, as shown, or external. Alternately, a bolted flange connection may be employed. 
     Referring to  FIG. 2B , pump  21  has a pump base  53  on its lower end that couples to motor connector  49 , such as by threads or by a bolted flange connection. Pump  21  has a pump housing  55  that is cylindrical and concentric relative to axis  32 . Pump  21  has an upper valve assembly  57 , which contains upper intake  25 , and a lower valve assembly  59 , which contains lower intake  27 . A barrel  61  extends concentrically between upper valve assembly  57  and lower valve assembly  59  within pump housing  55 . Upper valve assembly  57  connects to production tubing  17  ( FIG. 1 ) and has a pump discharge passage  63  on its upper end that is in communication with the interior of tubing  17 . Pump housing  55  and barrel  61  define a pump annulus  65  between them. A pump piston or plunger  67  slidingly engages the inner diameter of barrel  61 . Connecting rod  41  connects to the lower end of plunger  67  to cause plunger  67  to stroke in unison with motor mover  37  ( FIG. 5B ). 
     Upper valve assembly  57  includes an upper valve body  68  having an upper intake valve  69  that is in a passage parallel to and offset from axis  32 . Upper intake valve  69  is a cheek valve that may be of a variety of types. In this example, upper intake valve  69  has a ball  71  that moves between a seat  73  above it and a cage  75  below. Upper intake  25  is above and leads downward to seat  73  of upper intake valve  69 . 
     An upper crossover member  70  secures between upper valve body  68  and the upper end of pump housing  55  with threaded connections or by a flanged connection. Upper crossover member  70  could be integrally formed with upper valve body  68 . Upper crossover member  70  has an upper crossover intake passage  77  that extends upward and outward from the lower to the upper end of upper crossover member  70 . Upper crossover intake passage  77  has a lower end in fluid communication with the interior of barrel  61  and an upper end in fluid communication with the lower side of upper intake valve  69 . When plunger  67  strokes downward, ball  71  rests on cage  75  and well fluid flows through upper intake  25 , upper intake valve  69  and into barrel  61  above plunger  67 . 
     Upper valve assembly  57  has an upper discharge valve  79  that is offset from axis  32  in a direction opposite from upper intake valve  69 . Upper discharge valve  79  may be identical to upper intake valve  69  but inverted. An upper crossover discharge passage  81  in upper crossover member  70  extends upward and outward from the interior of barrel  61  to an upper discharge valve bore  83  in upper valve body  68 . Upper discharge vale bore  83  extends to pump discharge passage  63 . Upper discharge valve  79  is mounted in upper discharge valve bore  83 , and when plunger  67  strokes upward, well fluid in barrel  61  above plunger  67  flows through upper crossover discharge passage  81 , upper discharge valve  79  and into pump discharge passage  63 . 
     As shown by the dotted lines in  FIG. 2A  and in the transverse cross-sectional view of  FIG. 3 , au annulus upper passage  85  extends from annulus  65  through upper crossover member  70  and valve body  68  to pump discharge passage  63 . Annulus upper passage  85  is parallel to and offset from axis  32  and upper discharge valve bore  83 . The upper end of annulus upper passage  85  is in fluid communication with the upper end of upper discharge valve bore  83  above upper discharge valve  79 . In this example, annulus upper passage  85  extends to pump discharge passage  63 , as shown in  FIG. 3 . 
     Referring to  FIG. 2B , lower valve assembly  59  has a lower valve body  89  that secures, such as a by a threaded connection, to pump base  53 . A lower crossover member  91  secures to the upper end of lower valve body  89  and the lower end of pump housing  55 . A lower intake valve  93  that may be identical to but inverted relative to upper intake valve  69  ( FIG. 5A ) is mounted in lower valve body  89  offset from axis  32 . Lower intake valve  93  is above and in fluid communication with lower intake  27 . A lower crossover intake passage  95  in lower crossover member  91  has an upper end in fluid communication with the interior of barrel  61  below plunger  67 . Lower crossover intake passage  95  extends downward and outward from barrel  61  to lower intake valve  93 . When plunger  67  moves in the upstroke, well fluid flows through lower intake  27 , lower intake valve  93  and lower crossover intake passage  95  to barrel  61  below plunger  67 . 
     A lower discharge valve  97  is mounted in a lower discharge valve bore  99  in lower valve body  89  parallel to and 180 degrees offset from lower intake valve  93 . Lower discharge valve  97  may be identical to upper discharge valve  79  but inverted. A lower crossover discharge passage  101  is in fluid communication with the discharge side of lower discharge valve  97 . Lower crossover discharge passage  101  extends upward and inward through lower crossover member  91  offset from lower crossover intake passage  95 . The upper end of lower crossover discharge passage  101  extends from the interior of barrel  61  below plunger  67  to lower discharge valve bore  99  above lower discharge valve  97 . 
     The discharge or lower side of lower discharge valve  97  is in fluid communication with an annul us lower passage  103  (shown by dotted lines) that extends through lower crossover member  91 . Annulus lower passage  103  communicates the lower side of discharge valve  97  with annulus  65 . Annulus lower passage  103  is parallel to and offset from lower discharge valve bore  99 , as shown in  FIG. 4 . A linking passage  105  in lower valve body  89  extends partially in a circumferential direction to connect the lower ends of lower discharge valve bore  99  and annulus lower passage  103  with each other. 
     In operation of the embodiment of  FIGS. 1-5 , a control circuit supplies alternating current power in a phased manner to coils  35  to interact with magnets  39  to produce linear movement along axis  32  of motor mover  37  ( FIG. 5B ). Motor mover  37  causes linear movement of pump plunger  67  ( FIGS. 2A and 2B ). Assuming that the movement is in an upstroke direction, well fluid flows in lower intake  27 , through lower intake valve  93  and lower crossover intake passage  95  to the interior of barrel  61  below plunger  67 . Well fluid in barrel  61  above plunger  67  from a previous down-stroke is pushed upward during the upstroke through upper crossover discharge passage  81  and upper discharge valve  79  to pump discharge passage  63 . The well fluid in pump discharge passage  63  flows upward into production tubing  17  ( FIG. 1 ). 
     The upward movement of well fluid in barrel  61  during the upstroke of plunger  67  does not flow out upper intake  25  because upper intake valve  69  will close, with ball  71  seating against seat  73 . Similarly, during the upstroke, well fluid being discharged into pump discharge  63  does not flow down annulus upper passage  85  into annulus  65  because the discharge pressure in pump discharge  63  will communicate with lower discharge valve  97  to close. This discharge pressure in pump discharge  63  communicates with annulus  65 , which communicates with the lower side of lower discharge valve  97  via annulus lower passage  103 , linking passage  105  and lower discharge valve bore  99 . Thus during the upstroke of plunger  67 , lower intake valve  93  and upper discharge valve  79  are open and lower discharge valve  97  and upper intake valve  69  are closed. 
     Sensors (not shown) will signal the control circuit when motor mover  37  is reaching the upper end of the upstroke. The controller (not shown) coordinates the power to coils  35  to cause mover  37  to begin a down-stroke. During the down-stroke, well fluid flows in upper intake  25  through upper intake valve  69  and upper crossover intake passage  77  to the interior of barrel  61 . At the same time, well fluid in barrel  61  below plunger  67  will be pushed out during the down-stroke. The well fluid being pushed out flows through lower crossover discharge passage  101  and through lower discharge valve  97  and linking passage  105  to annulus lower passage  103 . The well fluid flows up annulus lower passage  103  into annulus  65 . The well fluid flows from annulus  65  through annulus upper passage  85  to pump discharge passage  63  and up production tubing  17  ( FIG. 1 ). 
     The discharge pressure during the down-stroke does not cause well fluid to flow out lower intake  27  because it will cause lower intake valve  93  to close. The discharge pressure in pump discharge  63  during the down-stroke does not cause well fluid to flow through upper discharge valve  79  because it will close upper discharge valve  79 . Thus, during the down-stroke, lower discharge valve  97  and upper intake valve  69  are open and upper discharge valve  79  and lower intake valve  93  are closed. During the down-stroke, connecting rod  41  is in tension even though the down-stroke is a power stroke causing well fluid to be lifted in production tubing  17 . 
     A second embodiment of a pump is shown in  FIGS. 6-8 . Referring to  FIG. 6 , a reverse acting piston pump assembly  111  is disposed in a wellbore  113  along a generally vertical axis “A”. Although pump assembly  111  is illustrated as being installed in a generally vertical section of wellborn  113 , pump assembly  111  could alternatively be located within an inclined or horizontal section (not shown) of wellbore  113 . Wellbore  113  is lined with casing  115  that is perforated or has openings  117  to exchange well fluid with the surrounding geologic formation “F”. 
     Pump assembly  111  is illustrated as being supported on production tubing  119  extending into the wellbore  113  in an up-hole direction from the pump assembly  111 . Alternately, pump assembly  111  could be supported by coiled tubing or another structure operable to carry well fluids to and from a surface location (not shown). Pump assembly  111  is coupled to a motor or actuator  121  disposed below, or down-hole with respect to the pump assembly  111 . As described in greater detail below, actuator  121  is operable to axially move a connecting rod  123  of the pump assembly  111  in a reciprocating manner. Actuator  121  can include a submersible, rotary electric motor having a rotary to linear motion converter, and can be powered by an electric cable (not shown) extending to the surface location. In other embodiments, actuator  121  can include a hydraulic actuator, electrical linear motor, or other actuators operable to induce linear reciprocating motion of connecting rod  123 . 
     In operation of the embodiment of  FIGS. 6-8 , actuator  121  is activated to move connecting rod  123  alternatingly on a down-stroke (in a down-hole direction) and on an upstroke (in an up-hole direction). As described in greater detail below, the down-stroke draws well fluid into the interior of pump assembly through inlet ports  125 . The well fluid moving toward the inlet ports  125  between casing  115  and pump assembly  111  along arrows “L” defines a relatively low pressure flow. The wellbore fluid reverses direction upon entering the inlet ports  125 . This reversal can induce gas to separate from liquid in the wellbore fluid, similar to the operation of a reverse flow gas separator, to minimize gas entering the pump assembly  111 . The down-stroke also provides the pressure to discharge the well fluid from the pump assembly  111  into the production tubing  119 . The well fluid moving into production tubing  119  along arrows “H” defines a relatively high pressure flow. During the upstroke, well fluids are exchanged within the pump assembly  111 . A flow path through the pump assembly is described below. 
     As appreciated by those skilled in the art, the power or down-stroke places the connecting rod  123  in tension, while the fill or upstroke places the connecting rod in compression. The compression of the upstroke is not as great as the tension of the down-stroke since the upstroke serves primarily to exchange fluids within the pump assembly  111  in a “refilling cycle”, whereas the down-stroke is the “power cycle” that energizes the well fluid to move up-hole through the production tubing  119 . This arrangement mitigates the probability that the connecting rod  123  will buckle during operation, and thereby offers a reliable operation of the pump assembly  111 . 
     Referring to  FIGS. 7A and 7B , pump assembly  111  includes an annular pump housing  131  having an upper end  131   a  and a lower end  131   b . Relative terms such as “upper”, “lower” and the like are used herein only for convenience, since pump assembly  111  is also operable in horizontal or obliquely inclined orientations as described above. A pump head  133  is coupled to the upper end  131   a  of the pump housing  131 . The pump head  133  includes a central interior chamber  135 , which is fluidly coupled to inlet ports  125 . Discharge ports  137  are defined through pump head  133  and radially spaced about interior chamber  135 . The discharge ports  137  are fluidly coupled to a connector  139  defined in the head  133 , which is provided for mechanically and fluidly coupling pump assembly  111  to production tubing  119  ( FIG. 1 ). 
     A standing valve  143  is coupled to the pump head  133  and supported therefrom in a fixed or stationary manner within the pump housing  131 . The standing valve  143  includes a closure member  145 , which is operable to selectively permit or restrict flow of wellbore fluids from passing through the standing valve  143 . As illustrated, closure member  145  is a ball that is passively operable to open and permit flow of wellbore fluid through standing valve  143  when a pressure below the standing valve  143  is less than a pressure above the standing valve  143 , as will occur during the down-stroke. Conversely, closure member  145  passively closes against a seat when the pressure below the standing valve  143  is greater than the pressure above the standing valve  143  as occurs during the upstroke. Alternately, the closure member  145  could be a flapper, or other mechanism passively or actively controlled to open during the down-stroke, and close during the upstroke. 
     A pump barrel  149  extends below the standing valve  143  within the pump housing  131 . The pump barrel  149  is a constructed of a tubular body having threads defined at upper and lower ends thereof. The threads at the upper end of the pump barrel  149  are engaged with a first adapter  151 , which is coupled to the standing valve  143  by a second adapter  153 . An internal cavity  155  is defined on an interior of pump barrel  149 , and an annular passageway  157  is defined between the pump barrel  149  and the pump housing  131 . The internal cavity  155  is fluidly coupled to the standing valve  143 , and the annular passageway  157  is fluidly coupled to the discharge ports  137  defined in the pump head  133 . Redirection ports  159  are defined through the tubular body of pump barrel  149 , and are fluidly coupled to annular passageway  157 . The threads at the lower end of the pump barrel  149  are engaged with a collar member  161 . 
     Collar member  161  is also coupled to the lower end  131   b  of pump housing  131  by threads. The collar member  161  thus maintains a radial separation between the pump barrel  149  and the pump housing  131 . Connecting rod  123  is radially surrounded by collar member  161 , which, in some embodiments, can include guide flanges  163  such that collar member  161  serves as a bearing to support the reciprocating axial movement of the connecting rod  123 . Collar member  161  supports a base member  165  at a lower end thereof. The base member  165  houses a seal  167  that engages the connecting rod  123  and operates to isolate wellbore fluids on tire exterior of the pump assembly  111  from relatively higher pressure wellbore fluids on an interior of the pump assembly  111 . Seal  167  also operates to prohibit wellbore fluid from entering actuator  121  ( FIG. 1 ). Seal  167  can include elastomeric o-rings, bellows members or other dynamic seal mechanisms known in the art for sealing about a reciprocating member. 
     Coupled to an upper end of connecting rod  123 , is a perforated cylinder  171 , a plunger  173  and a traveling valve  175 . Each of the perforated cylinder  171 , plunger  173  and traveling valve  175  reciprocate along with connecting rod  123  within pump barrel  149 , and are closely fit within the pump barrel  149 . Perforated cylinder  171  includes radial openings  177  defined therein through which wellbore fluid can pass. Plunger  173  includes an axial opening  179  extending therethrough which fluidly couples the perforated cylinder  171  and traveling valve  175 . Traveling valve  175  includes a closure member  181 , which is operable to open during the upstroke and close during the down-stroke. As illustrated, closure member  181  is a ball arranged below a seat such that a higher pressure below the ball, e.g., within axial opening  179 , than above the ball, e.g., within internal cavity  155 , induces the ball to seal against the seat. As described below, closure member  181  passively opens and closes in response to the differential pressure induced by the reciprocation of the connecting rod  123 . 
     In operation, during the down-stroke, the connecting rod  123 , perforated cylinder  171 , plunger  173 , and toweling valve  175  are all drawn downward by the actuator  121  ( FIG. 1 ) from the configuration illustrated in  FIGS. 7A and 7B  toward the configuration illustrated in  FIG. 8 . At least since the plunger  173  is closely fit within the pump barrel  149 , this downward motion pressurizes wellborn fluids below the closure member  181 , and thereby maintains traveling valve  175  in a closed configuration during the down-stroke. The wellbore fluid below the closure member  181  is pushed downward and through redirection ports  159 , where the wellbore fluid reverses direction into the annular passageway  157 . At the top of the annular passageway  157 , the wellbore fluid enters the discharge ports  137  defined in the pump head  133  and exits pump assembly  111 . The discharged wellbore fluid flows into the production tubing  119  ( FIG. 1 ), and up-hole toward the surface location. 
     Also during the down-stroke, a pressure vacuum or a reduced pressure is generated above the closure member  181  of the traveling valve  175 . This creates a lower pressure in the internal cavity  155  above the traveling valve  175  than a pressure in the interior chamber  135  of the pump head  133 . This differential pressure causes the closure member  145  to disengage its seat and permits wellbore fluid to flow through the standing valve  143 . Wellbore fluid thus flows into the pump assembly  111  through inlet ports  125  and through the interior chamber  135  of the pump head  133 , and then through the stationary valve  143 . This flow of fluid fills the internal cavity  155  with wellbore fluid. 
     When the down-stroke is complete, the upstroke begins us the actuator  121  ( FIG. 6 ) reverses the direction of the connecting rod  123 , perforated cylinder  171 , plunger  173  and traveling valve  175 . This upward movement increases the pressure above the plunger  173 , and thereby induces the closure member  181  to disengage its seat and open the traveling valve  175 . This increase in pressure above the plunger  173  thereby induces the standing valve  143  to close, and causes wellbore fluid that entered the internal cavity  155  during the previous down-stroke to flow through the traveling valve  175  and through the axial opening  179  of the plunger  173 . The upstroke thus causes the volume below the plunger to be refilled with wellborn fluid, and this fluid is produced on the subsequent down-stroke. The down-stroke and upstroke cycle is repealed to produce wellbore fluids up-hole. 
       FIG. 9  illustrates a third embodiment of a pump. Although constructed differently, pump assembly  201  is similar in operation to pump assembly  111  described above. Pump assembly  201  includes a pump head  203  and a base member  205  supporting an outer annular pump housing  207  and inner annular pump barrel  209  therebetween. A standing valve  211  is coupled to the pump head  203 , and a toweling valve  213  is coupled to a plunger  215  and a connecting rod  217 . The connecting rod  217  can be coupled to an actuator  121  ( FIG. 6 ) disposed below the pump assembly  201  as described above. 
     The connecting rod  217  includes radial openings  219  defined therein to permit the exchange of wellbore fluid from interior portions of the connecting rod  217  to an exterior of the connecting rod  217 . Redirection ports  221  are defined in the base member  205 , instead of in barrel  149  as redirection ports  159  ( FIG. 7B ) of the second embodiment. Redirection ports  221  are operable to redirect a downward flow of wellbore fluids from within pump barrel  209  to an upward flow in an annular passageway  223  defined between the bump barrel  209  and the pump housing  207 . 
     In operation of the embodiment of  FIG. 9 , a first down-stroke allows a relatively low pressure fluid on an exterior of the pump housing  207  (arrows “A”) to enter the pump assembly  201  through inlet ports  225  (arrow “B”). The low pressure fluid flow is redirected to a downward flow (arrow “C”) where the fluid passes through an open standing valve  211  (arrow “D”) to a closed traveling valve  213 . A subsequent upstroke induces the traveling valve  113  to open to permit the low pressure fluid flow to pass the traveling valve  213  to a space defined below the plunger  213  (arrow “E”). A second down-stroke then pressurizes the fluid below the traveling valve  213  and induces a relatively high pressure fluid How downward and through redirection ports  221  (arrows “F”). The high pressure fluid flow continues through the annular passageway  223  and through discharge ports  227  (arrows “G”). The high pressure fluid flow can then continue up-hole through a conduit such as production tubing  119  ( FIG. 6 ). 
       FIGS. 10-12  illustrate a fourth embodiment of a pump assembly. Referring to  FIG. 10A , pump  301  has a discharge adapter  303  on an upper end that typically connects to a string of production tubing  304  leading upward to a wellhead assembly. A pump head  305  secures with threads to discharge adapter  303 . A cylindrical pump housing  307  secures with threads to pump head  305 . Well fluid discharge ports  309  extend through pump head  305  from a lower end to an upper end. A well fluid intake or inlet port  311  extends from the exterior of pump head  305  to a central cavity  313  in pump head  305 , central cavity  313  having a closed upper end within pump head  305 . 
     A standing valve  315  secures to an upper end of pump head  305  within discharge adapter  303 . Standing valve  315  has a lower seat  317  with a hall  319  below. When the pressure on hall  319  from above is higher than below, ball  319  closes, blocking downward flow from production tubing  304  into discharge ports  309 . When the pressure on ball  319  from above is less than below, ball  319  opens to allow upward flow of well fluid from discharge ports  309  out the upper end of discharge adapter  303  into production tubing  304 . Standing valve  315  has no effect on well fluid inlet  311 , which may remain open at all times. 
     A cylinder or barrel  321  concentrically locates within pump housing  307 . A collar  323  on the upper end of barrel  321  sealingly couples barrel  321  to a depending isolation tube  325  extending downward from pump head cavity  313 . Barrel  321 , which does not move within pump housing  307 , defines an annular passageway  327  between barrel  321  and pump housing  307 . Barrel  321  has an open bore  329  that is coaxial with a longitudinal axis  331  of pump  301 . Collar  323  places well fluid from pump head cavity  313  in fluid communication with barrel bore  329 . 
     Referring to  FIG. 10B , a lower end of barrel  321  connects to a barrel adapter  333 , which may be considered to be a part of barrel  321 . Barrel adapter  333  has a lower end that secures to a pump base  335 , which secures to the lower end of pump housing  307 . Redirect or outlet parts  337  extend through barrel adapter  333 , creating a flow path for well fluid in barrel bore  329  to flow outward into a lower portion of annular passageway  327 . 
     A plunger  339  slides sealingly within barrel bore  329  along axis  331 . Plunger  339  has an axial plunger passage  340  extending therethrough. Plunger  339  is movable from the lower end of barrel bore  329  to the upper end. A connecting rod  341  has an upper end that secures to plunger  339  for moving plunger  339  in unison between an upstroke and a down-stroke. Seals  343  seal between connecting rod  341  and pump base  335 . The upper end of connecting rod  341  has the same outer diameter as plunger  339 . A downward facing shoulder  342  on connecting rod  341  separates the larger diameter portion of connecting rod  341  from a lower smaller outer diameter portion of competing rod  341 . Shoulder  342  may be considered to be a lower end of plunger  339  in that any fluid in barrel  321  below shoulder  342  will be pushed downward during the down-stroke. 
     In this example, connecting rod  341  has plunger ports  345  located within a connecting rod cavity  347  at the upper end of connecting rod  341 . Plunger ports  345  communicate well fluid in plunger passage  340  with well fluid in barrel bore  329 . Alternately, plunger ports  345  could be located directly in the side wall of plunger  339 . 
     A traveling valve  349  mounts to an upper end of plunger  339  for axial movement therewith. Traveling valve  349  has an upper seat  351  that is engaged by a ball  353  while plunger  339  is in down-stroke movement. The engagement closes traveling valve  349 , causing downward movement of plunger  339  to push well fluid located in barrel bore  329  below plunger  339  outward. The outward flowing well fluid will flow through redirect ports  337  into annular passageway  327  until the lower end of plunger  339  passes below redirect ports  337 . During the upstroke, traveling valve  349  opens, allow well fluid that has entered barrel bore  329  above plunger  339  to flow through traveling valve  349  and out plunger ports  345  info the portion of barrel bore  329  below plunger  339 . 
     During the down-stroke of plunger  339 , well fluid is pumped upward in annular passageway  327  out discharge adapter  303  to lift the column of well fluid in production tubing  304 . The down-stroke may be considered to be a power stroke, and during the down-stroke, plunger  339  moves in an opposite direction to the flow of well fluid into production tubing  304 . During the down-stroke, traveling valve  349  closes. Plunger  339  pushes well fluid that previously entered barrel bore  329  below plunger  339  out redirect ports  337  until shoulder  342  passes below redirect ports  337  near the end of the down-stroke. The well fluid flowing into annular passageway  327  will be pushed upward through discharge ports  309  and through standing valve  315 , which is open during the down-stroke. 
     During the upstroke, traveling valve  349  will open, allowing fluid that enters intake port  311  to flow into bore barrel  329  above plunger  339 . This incoming well fluid flows downward through traveling valve  333  into plunger passage  340 . The incoming well fluid flows downward in plunger passage  340  out plunger ports  345  into barrel bore  329  below plunger  339 . The well fluid entering barrel bore  329  will be in fluid communication with the well fluid in annular passageway  327 . The upstroke thus replenishes well fluid in barrel bore  329  below plunger  339 . Standing valve  315  will be closed during the upstroke, blocking downward flow of well fluid in production tubing  304 . When plunger  339  reaches the top of the upstroke, connecting rod  341  reverses, starting another down-stroke. 
     During the down-stroke, connecting rod  341  will be moving downward and will be in tension. During the upstroke, connecting rod  341  will be in compression, but the level of compression is far less than the tension because pump  301  is not lifting a column of well fluid during the upstroke. 
       FIGS. 11A-C  illustrate an example of a linear motor  355  for stroking connecting rod  341 . Linear motor  355  has a head  357  that connects to the lower end of pump  301  ( FIGS. 10A  and  10 B) in this embodiment. A cylindrical outer housing  359  secures with threads to a lower end of motor head  357 . A seal  301  seals around the reciprocating connecting rod  341 , the seal being retained by a retaining nut  363 . Motor head  357  has a mover stop  365  that limits upward movement of connecting rod  341  beyond the top of the upstroke. Motor well fluid ports  367  extend through motor head  357  below seal  361  to admit well fluid to a central portion of the interior of motor outer housing  339 . An electrical connector  369  in motor head  357  connects to a motor lead of a power cable (not shown) to supply electrical power to linear motor  355 . 
     A cylindrical inner housing  371  has an upper end that secures to motor head  357 . Inner housing  371  is concentrically located in outer housing  359  and has a smaller outer diameter than the inner diameter of outer housing  3591  defining an annular chamber  373 . Inner housing  371  is formed of a nonmagnetic material, which may be a metal or a composite material. Well fluid is admitted to the interior of inner housing  371  via well fluid ports  367  in motor head  357 . 
     A lower end of connecting rod  341  secures to a mover head  374 , as shown in  FIG. 11B . Mover head  374  is part of a mover  376 , which includes a shaft or inner tube  375  carried concentrically with inner housing  371  and of a smaller outer diameter than mover head  374 . Permanent magnets  377  are mounted around and extend along a length of inner tube  375 . Mover head  374  is only slightly smaller in outer diameter than the inner diameter of inner housing  371 . Mover inner tube  375  may receive well fluid in its interior. 
     Outer housing  359  is illustrated as being in sections. A sensor and bearing connector  379  connects an upper section of outer housing  359  to a next lower section of outer housing  359 . Sensor and bearing connector  379  has an inner diameter that fits closely around inner housing  371  to provide radial support. In this example, inner housing  371  extends continuously throughout the length of linear motor  355 , but it also could be formed in sections. A sensor  381  mounts to sensor and bearing connector  379  within a lower end of annular chamber  373 . Sensor  381  detects the proximity of a portion of mover  376  to determine the top of the upstroke and tire bottom of the down-stroke. Sensor  381  may be a Hall effect magnetic sensor that transmits a magnetic field inward across inner housing  371  to detect the approach of mover head  374  as mover  376  nears the bottom of the down-stroke. Additional connectors (not shown) similar to sensor and bearing connector  379  may connect additional segments of outer housing  359 . Those connectors would not need to have sensors  381 . 
     A number of electromagnetic coil windings  383  are located in an annular windings chamber  385 . Windings chamber  385  comprises an annular space between inner housing  371  and a segment of outer housing  359  extending downward from sensor and bearing connector  379 . The upper end of windings chamber  385  is defined by sensor and bearing connector  379 . Windings chamber  385  is filled with a dielectric liquid or fluid that immerses coil windings  383 . A passage (not shown) in sensor and bearing connector  379  communicates dielectric thud in chamber  385  with dielectric fluid in annular chamber  373 . When supplied with electrical pulses, coil windings  383  emit electromagnetic fields across inner housing  371  to interact with mover magnets  377  and cause mover  376  to stroke. Coil windings  383  do not extend a full length of windings chamber  385 . The axial length of mover magnets  377  is greater than the axial length of coil windings  383 . The electrical wires (not shown) for coil windings  383  extend from electrical connector  369  ( FIG. 11A ), through a wire passage  386  to annular chamber  373 . The electrical wires extend through passages in sensor and bearing connector  379  ( FIG. 11B ) to coil windings  383 . 
     Referring to  FIG. 11C , a coil spring  387  encircles a lower end of mover inner tube  375  and supports the lower end of the array of mover magnets  377 . A mover base  389  of larger diameter than mover inner tube  375  supports coil spring  387 . Coil spring  387  is under compression between mover base  389  and mover magnets  377 , urging the array of mover magnets  377  against mover head  374  ( FIG. 11B ). Mover magnets  377  are axially slidable on mover inner tube  375 , and the bias of coil spring  387  accommodates thermal expansion that occurs between the different materials of mover inner tube  375  and mover magnets  377 . 
     A thin magnet sleeve  390  encloses mover magnets  377  and extends from mover base  389  ( FIG. 11C ) to mover head  374  ( FIG. 11B ). Magnet sleeve  390  moves in unison with mover  376  and slides within motor inner housing  371 . Magnet sleeve  390  protects protection to magnets  377  against wear and is not sealed from well fluid located within motor inner housing  371 . 
     A motor base  391  secures with threads to a lower end of the lowest segment of outer housing  359 . The lower end of inner housing  371  seals to a counterbore in motor base  391 . A well fluid port  393  extends into motor base  391  to admit well fluid into the interior of inner housing  371  as well as the inferior of mover tube  375 . Motor base  391  has a transverse barrier wall  395  below well fluid port  393  that closes the interior of inner housing  371  from a central passage  396  in motor base  391  located below. Motor base  391  has dielectric fluid passages  397  that extend from central passage  396  into windings chamber  385 . 
     Referring to  FIG. 12 , an expansion chamber unit  399  secures to motor base  391  in this embodiment. Expansion chamber unit  399  has a head  401  that secures to motor base  391 , either with a threaded rotatable collar or bolts. Expansion chamber head  401  has an axially extending dielectric fluid passage  403  that communicates with motor base cavity  396 . A guide tube  405  extends coaxially downward from expansion chamber head  401  in registry with dielectric fluid passage  403 . A movable member, such as a flexible elastomeric bag  407  encircles guide tube  405 . Alternately, the movable member could be a bellows or a piston. An expansion chamber housing  409  surrounds bag  407  and connects to expansion chamber head  401 . Bag  407  is filled with a dielectric fluid, and guide tube ports  411  communicate that fluid between bag  407  and windings chamber  385  via passages  403 ,  396  and  397 . A well fluid inlet  413  in expansion chamber housing  409  admits well fluid to the exterior side of bag  407 . Bag  407  seals dielectric fluid in its interior from the well fluid in housing  409  and equalizes the pressure of the dielectric fluid in windings chamber  385  with the hydrostatic pressure of well fluid. 
     A lower connector  415  secures to the lower end of housing  409 . Additional segments of housing  409  and additional bags  407  may be mounted below lower connector  415  in tandem. Alternately, lower connector  415  could be configured to comprise the lower end of expansion chamber unit  399 . The upper end of bag  407  seals around guide tube  405  above guide tube ports  411 . The lower end of bag  407  seals around lower connector  415 . 
     When pump assembly  301  is deployed in the well, the temperature of the well fluid often increases with depth. The increasing temperature causes thermal expansion of the dielectric fluid contained within windings chamber  385 . Also, when linear motor  355  operates, more heat is generated, causing thermal expansion of the dielectric fluid in windings chamber  385 , annular chamber  373  and electrical wire passage  386  ( FIG. 11A-C ). The thermal expansion is accommodated by allowing hag  407  to expand. When linear motor  355  is shut off, it will cool, causing the dielectric fluid to contract thermally. Bag  407  contracts to accommodate the contraction. 
     A controller (not shown) at the surface adjacent the wellhead will supply a first pulse, preferably DC, to coil windings  383 , causing mover  376  to stroke connecting rod  341  in a first direction. Assuming the first direction to be an upstroke, when near the top of the upstroke stroke, sensor  381  will detect the proximity of mover base  389 , and provide a signal to the controller. The controller reverses the polarity to coil windings  383 , causing mover  376  to begin the down-stroke of connecting rod  341 . When nearing the bottom of the down-stroke, sensor  381  will detect the proximity of mover head  374 , and provide a signal to the controller to again reverse the direction. 
     The pump assembly may also have various additional sensors (or detecting well fluid conditions. For example, a significant reduction in amperage being sent to linear motor  355  may indicate that a large gas bubble in the well fluid is flowing into pump  301 . The controller may in response take various remedial actions, such as providing much more rapid pulses to cause vibration of the pump assembly to break up the gas bubble. Another remedial action may be to stop powering linear motor  355  for a selected time to allow the gas bubble to dissipate. 
     While the invention has been shown only in a few of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes.