Patent Publication Number: US-10774829-B2

Title: Hydraulic artificial lift for driving downhole pumps

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 62/417,107, filed Nov. 3, 2016. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to artificial lift systems for reciprocating a pump rod in a wellbore to drive a downhole pump in order to produce well fluids up to the surface, and more specifically to hydraulic artificial lift systems using a hydraulic linear actuator to drive such reciprocal motion of the pump rod. 
     BACKGROUND 
     Hydraulic lift systems of the forgoing type for driving downhole pumps in well applications are known in the art, and include those disclosed in US2012/0148418, US2014/0234122, US2014/0079560, US2015/0176573, US2015/0285243, U.S. Pat. Nos. 7,562,701, 8,083,499, and 8,562,308. 
     Among these references, U.S. Pat. No. 8,083,499 discloses offsetting of the piston rod from the central longitudinal axis of the cylinder in order to resist rotation of the piston relative to the cylinder, thereby preventing damage to a position sensor probe along which the piston is slidable. In this reference, the piston rod extends vertically upward from the hydraulic linear actuator and is indirectly coupled to the pump rod via a cable routed over a sheave that is carried atop the piston rod. 
     U.S. Pat. No. 7,562,701 also discloses prevention of piston rotation relative to the cylinder in a hydraulic lift apparatus by offsetting of components relative to the central longitudinal axis of the cylinder, but does so for the purpose of enabling rotational manipulation of downhole equipment. The hydraulic linear actuator is installed within an uppermost portion of the wellhead casing rather than atop the wellhead, and so hydraulic supply lines enter the upper end of the cylinder and are routed downwardly through the piston in order to pressurize the cylinder below the piston to drive the upstroke, and a hollow ram accommodates passage of the well fluid to the surface. The ram and fluid supply lines are offset from the central longitudinal axis of the cylinder to prevent rotation of the piston. 
     US20140234122, US20120148418 and US20150176573 disclose hydraulic lift systems that, like U.S. Pat. No. 8,083,499, employ a magnetorestrictive probe to monitor the position of the sliding piston, but place this probe externally of the cylinder and have the piston rod extending downwardly from the cylinder for inline connection to the pump rod. 
     Disclosures concerning piston rotation prevention and piston position detection in the general area of piston cylinder assemblies used in other applications be found in JP2005054977 and U.S. Pat. No. 7,493,995. 
     Applicant has developed a new hydraulic lift design incorporating unique features neither shown or suggested by the prior art 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a hydraulic artificial lift apparatus for operating a downhole pump of a well to produce fluids therefrom, the artificial lift apparatus comprising: 
     a housing having a top end and an opposing bottom end spaced apart in a longitudinal direction of the housing; 
     a piston slidably disposed within a hollow interior space of the housing for movement back and forth the longitudinal direction between an uppermost travel limit and an opposing lowermost travel limit, said piston being centered on a central longitudinal axis of the housing and sealed to a circumferential wall of the housing by at least one piston seal; 
     a piston shaft attached to the piston and extending downward therefrom and exiting the housing through the bottom end thereof, which features a sealed closure of the housing around said piston shaft, a lower end of the piston shaft being disposed outside the housing below the bottom end thereof and connected or connectable to the upper end of a pump rod for reciprocal driving of the downhole pump by said movement of the piston; 
     an upstroke supply port connected or connectable to a source of pressurized hydraulic fluid and entering the housing, and communicating with the interior space thereof, at a lower portion of the housing disposed between the sealed closure and a lowermost position occupied by the at least one piston seal at the lowermost travel limit of the piston, whereby the hydraulic fluid drives an upstroke of the piston; and 
     at least one anti-rotation rod running longitudinally of the hollow interior space of the housing and through the piston at a position radially offset outwardly from the central longitudinal axis, the piston being longitudinally slidable on said at least anti-rotation rod between the uppermost and lowermost travel limits. 
     According to a second aspect of the invention, there is provided a hydraulic artificial lift apparatus for operating a downhole pump of a well to produce fluids therefrom, the artificial lift apparatus comprising: 
     a housing having a top end and an opposing bottom end spaced apart in a longitudinal direction of the housing; 
     a piston slidably disposed within a hollow interior space of the housing for movement back and forth the longitudinal direction; 
     a piston shaft attached to the piston and extending downward therefrom and exiting the hollow interior space of the housing through the bottom end thereof, which features a sealed closure of the housing around said piston shaft, a lower end of the piston shaft being disposed outside the housing below the bottom end thereof and connected or connectable to the upper end of a pump rod string for reciprocal driving of the downhole pump by said movement of the piston; 
     a upstroke supply port connected or connectable to a source of pressurized hydraulic and communicating with the hollow interior space of the housing at a lower portion thereof to drive an upstroke of the piston under introduction of the pressurized hydraulic fluid through said upstroke supply port; 
     a hollow interior bore extending axially into a top end the piston shaft and communicating with the hollow interior space of the housing above the piston, and 
     a rod running longitudinally of the hollow interior space of the housing from a supported position above an upper travel limit of the piston, and extending downwardly through the piston into the hollow interior bore of the piston shaft; 
     wherein the piston is movable back and forth along said rod in the longitudinal direction. 
     According to a third aspect of the invention, there is provided a hydraulic artificial lift apparatus for operating a downhole pump of a well to produce fluids therefrom, the artificial lift apparatus comprising: 
     a housing having a top end and an opposing bottom end spaced apart in a longitudinal direction of the housing; 
     a piston slidably disposed within a hollow interior space of the housing for movement back and forth the longitudinal direction between an uppermost travel limit and an opposing lowermost travel limit, said piston being sealed to a circumferential wall of the housing by at least one piston seal; 
     a piston shaft attached to the piston and extending downward therefrom and exiting the hollow interior space of the housing through the bottom end thereof, which features a sealed closure of the housing around said piston shaft, a lower end of the piston shaft being disposed outside the housing below the bottom end thereof and connected or connectable to the upper end of a pump rod for reciprocal driving of the downhole pump by said movement of the piston; 
     an upstroke supply port connected or connectable to a source of pressurized hydraulic fluid and entering the housing, and communicating with the interior space thereof, at a lower portion of the housing disposed between the sealed closure and a lowermost position occupied by the at least one piston seal at the lowermost travel limit of the piston, whereby the hydraulic fluid drives an upstroke of the piston, the housing lacking a downstroke port at an upper portion of the housing above an uppermost position occupied by the at least one piston seal at the uppermost travel limit of the piston; and 
     a leak detection fluid passage passing through the piston, communicating with the hollow interior space of the housing at the upper portion thereof, and communicating with a leak detection port at the lower portion of the housing, whereby, in the event of leakage of the pressurized hydraulic fluid upwardly past the piston, leaked fluid above the piston is forced into the leak detection fluid passage as the piston reaches the upper travel limit during the upstroke, and detection of hydraulic fluid in or from the leak detection port confirms occurrence of said leakage. 
     According to a fourth aspect of the invention, there is provided a hydraulic artificial lift apparatus for operating a downhole pump of a well to produce fluids therefrom, the artificial lift apparatus comprising: 
     a housing enclosing a hollow interior space and having opposing top and bottom ends spaced apart along a longitudinal axis of the housing, the housing comprising a rotatable portion supported for rotation about said longitudinal axis; 
     a piston slidably disposed within the rotatable portion of the housing for movement back and forth along the longitudinal axis of the housing within the hollow interior space thereof, the piston being locked against rotation relative to the rotatable portion of the housing; 
     a piston shaft attached to the piston and extending downward therefrom and exiting the hollow interior space of the housing through the bottom end thereof, which features a sealed closure of the housing around said piston shaft, a lower end of the piston shaft being disposed outside the housing below the bottom end thereof and connected or connectable to the upper end of a pump rod string for reciprocal driving of the downhole pump by said movement of the piston; 
     a upstroke supply port connected or connectable to a source of pressurized hydraulic and communicating with the hollow interior space of the housing at a lower portion thereof to drive an upstroke of the piston under introduction of the pressurized hydraulic fluid through said upstroke supply port; and 
     a rotational actuation device operable to effect controlled rotation of the rotatable portion of the housing about the longitudinal axis thereof, said rotational actuation device comprising a motor mounted in a stationary position relative to the well and a drive train comprising an input member rotationally driven by the motor and an output member connected to the rotatable portion of the housing in a position centered on the longitudinal axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described in conjunction with the accompanying drawings in which: 
         FIG. 1  is a front elevational view of an artificial lift unit according to a first embodiment of the present invention. 
         FIG. 2  is an overhead plan view of the artificial lift unit of  FIG. 1 . 
         FIG. 3  is a bottom plan view of the artificial lift unit of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of an upper portion of the artificial lift unit of  FIG. 2  in a vertical plane denoted by line A-A thereof. 
         FIG. 5  is a cross-sectional view of a lower portion of the artificial lift unit of  FIG. 2  in the vertical plane denoted by line A-A thereof. 
         FIG. 6  is a cross-sectional view of the lower portion of the artificial lift unit of  FIG. 3  in the vertical plane denoted by line B-B thereof. 
         FIG. 7  is a cross-sectional view of the lower portion of the artificial lift unit of  FIG. 2  in the vertical plane denoted by line C-C thereof. 
         FIG. 8  is a cross-sectional view of the lower portion of the artificial lift unit of  FIG. 6  in the horizontal plane denoted by line D-D thereof. 
         FIG. 9  is a rear elevational view of an artificial lift unit according to a second embodiment of the present invention. 
         FIG. 10  is a partial rear elevational view of the artificial lift unit of  FIG. 9 , with a main cylinder housing and a cap cover thereof omitted to reveal internal components of the unit. 
         FIG. 11  schematically illustrates a hydraulic control system controlling operation of the artificial lift units of  FIG. 1  and  FIG. 10 , inclusive 
     
    
    
     In the drawings like characters of reference indicate corresponding parts in the different figures. 
     DETAILED DESCRIPTION 
       FIGS. 1 to 8  show a first embodiment of an artificial lift system for reciprocally driving a pump rod within the production tubing of a well in order to operate a downhole pump that produces well fluids to the surface through the production tubing. With reference to  FIG. 1 , The system features a hydraulic linear actuator  10  with a housing having a main hollow cylinder  12  supported in a vertically upright position and closing concentrically around a vertically oriented central longitudinal axis  14 . A cap  16  of the housing is fitted atop the hollow main cylinder  12  in a sealed relationship therewith in order to close off a top end of the hollow interior space of the housing in a fluid-tight manner. 
     A stationary base  18  of the housing resides at a distance beneath the bottom end of the main cylinder  12 . With reference to  FIGS. 5 to 7 , the stationary base  18  has an axial through bore  20  passing through it on the central longitudinal axis  14 . An outer diameter of the base  18  is stepped at two locations to divide the base into three distinct sections, namely a lower section  22  of smallest inner and outer diameter, an upper section  24  of largest inner and outer diameter, and a middle section  26  of intermediately sized inner and outer diameters relative to the upper and lower sections. 
     The lower section  22  of the base  18  passes vertically downward through a mounting opening in a horizontal drive support flange  28  that features an array of bolt holes  30  spaced circumferentially apart from one another around the mounting opening. A first downward-facing shoulder  31  defined by the step in outer diameter between the base&#39;s lower section  22  and intermediate section  26  is seated atop the drive support flange  28  around the mounting opening therein, and features a matching array of bolt holes through which the drive support flange and the base are axially bolted together to both fix the base  18  and the drive support flange  28  together axially and prevent rotation therebetween about the central longitudinal axis  14 . 
     The middle section  26  of the base  18  has a ring gear  32  disposed circumferentially therearound at the top end of the middle section just below a second downward-facing shoulder defined between the base&#39;s upper section  24  and intermediate section  26  at the change in outer diameter therebetween. The ring gear  32  is centered on and rotatable about the central longitudinal axis  14  relative to the base  18 . The drive support flange  28  extends radially away from the central longitudinal axis  14  to one side of the mounting opening therein to carry a motor mount  34  at a distance radially outward from the ring gear  32  in a position standing upward from the drive support flange  28 . A hydraulic motor  36  is mounted atop the motor mount  34  with its output shaft  37  reaching downwardly from the motor housing on an interior side of the motor mount  34 , where the output shaft  37  of the motor carries a pinion gear  38  in a position mating with the toothed periphery of the ring gear  32  at a location between the ring gear and the motor mount  34 . Accordingly, driven rotation of the pinion gear  38  by the hydraulic motor  36  will drive rotation of the ring gear  32  about the central longitudinal axis of the main cylinder  12 . As described in more detail below, driven rotation of the ring gear drives rotation of the main cylinder  12  of the housing, and so the pinion and ring gears respectively define input and output gears of a gear train for transmitting rotational power from the motor to the main cylinder  12  of the housing. 
     While the illustrated embodiments each employ a ring gear drive train in of which the input member is pinion gear and the output member is a ring gear rotatably supported on the base, other drive types may be used to similar effect. In the case of a ring gear drive chain, one or more intermediate gears may be used to indirectly couple the input and output gears of the drive chain. Alternatives include belt-driven or chain-driven drives, in which the input member is a pulley or sprocket on the motor shaft and the output motor is a pulley or sprocket rotatably supported on the base, and rotationally coupled to the input pulley/sprocket by a belt or chain. Toothed or untoothed belts and pulleys may be employed. Regardless of the particular drive train employed, the motor may be hydraulically, pneumatically or electrically powered. 
     The second downward-facing exterior shoulder of the base  18  defined by the stepped outer diameter between upper and middle sections thereof is arcuately contoured in a concave manner, as shown at  39 , and the topside of the ring gear  32  features a corresponding recess of concavely arcuate cross-section  40  encircling the inner periphery of the ring gear  32  around the base&#39;s middle section  26 . The concave recess  40  of the ring gear  32  aligns with the arcuately contoured shoulder  39  of the base  18  to define a spherical raceway between the ring gear  32  and the upper section  24  of the base  18 . Spherical roller elements  42  are received within this raceway to define a first bearing between the ring gear  32  and the base  18 . 
     A seal insert  44  is seated within the upper section  24  of the base  18  atop an interior upward facing shoulder  46  thereof where the through-bore  20  of the base  18  decreases in diameter near the transition between the upper and middle sections  24 ,  26  thereof. A second array of circumferentially spaced apart bolt holes  47   a  are provided in the first downward-facing exterior shoulder  31  of the base  18 , and are circumferentially offset from the first set of bolt holes (not shown) through which the drive support flange  28  is coupled to the base  18 . The second array of bolt holes  47   a  in the first exterior shoulder  31  of the base  18  extend upwardly through the upwardly facing interior shoulder  46  of the base&#39;s upper section  24 , and the seal insert  44  has a matching set of bolt holes  47   b  extending upwardly thereinto at an annular downward-facing surface of the seal insert  44  that overlies the interior shoulder  46  of the base&#39;s upper section. Through these aligned bolt holes  47   a ,  47   b  in the base  18  and the seal insert  44 , bolts (not shown) are used to fasten the seal insert  44  to the base  18 , which in turn is fastened to the drive support flange  28  by another set of bolts (not shown), as described above. These two sets of bolts thereby axially couple the drive support flange  28 , base  18  and seal insert  44  together and prevent relative rotation between these three components about the central longitudinal axis  14 . 
     A base cover  48  fits over the base  18  and the seal insert  44  at the annular upper end of the base&#39;s upper section  24 . The base cover  48  features a cylindrical outer rim  50  that resides over the annular upper end of the base&#39;s upper section  24 . The outer rim  50  of the base cover  48  circumferentially surrounds an upper portion of the seal insert  44  that reaches upwardly past the upper end of the base  18 . The annular upper end of the base  18  and the annular bottom end of the base cover&#39;s outer rim  50  are both concavely contoured in vertical planes emanating radially outward from the central longitudinal axis so to cooperatively define another circular raceway  51  like that defined between the topside of the ring gear  32  and the downward facing shoulder at the lower end of the base&#39;s upper section  24 . Spherical roller elements  42  are once again disposed within this second raceway  51 , thereby defining a second bearing enabling relative rotation between the base  18  and the base cover  48 . In the vertically cross-sectioned figures, only one such roller element  42  in shown in each spherical raceway to enable clear labelling of the both the raceway and the roller element contained therein, though it will be appreciated that a full set of roller elements is provided in each raceway. 
     In addition to the outer rim  50 , the base cover  48  also features an inner body  52  of externally cylindrical shape, and an upper web  54  radially interconnecting the outer rim  50  and inner body  52  at the upper end of the base cover  48 . The inner body  52  is spaced radially inwardly from the outer rim  50  and extends downwardly from the web  54  into an internal through-bore of the seal insert  44  by a distance reaching past the bottom end of the base cover&#39;s outer rim  50 . A plurality of circumferential grooves are provided in the boundary wall of the seal insert&#39;s internal through-bore and contain ring-shaped seals  56  therein to form fluid-tight seals between the seal insert  44  and the inner body  52  of the base cover  48 . 
     The web  54  at the upper end of the base cover  48  features an annular slot  58  recessed thereinto just inside the outer rim  50 . The lower end of the hollow main cylinder  12  is received within the annular slot  58 . A downward-opening containment collar  60  has a circumferential wall  62  closing around the upper section  24  of the base  18  and the base cover  48  mounted thereatop. An internal flange  64  of the containment collar  60  at the upper end thereof overlies the outer rim  50  of the base cover  48  around the hollow main cylinder  12 . An array of bolt holes  66   a  extend downwardly through the internal flange  64  of the containment collar  60  at circumferentially spaced positions therearound and align with a respective circumferential array of bolt holes  66   b  in the annular upper end of the outer rim  50  of the base cover  48 , whereby the containment collar  60  and the base cover  48  are axially coupled together and rotationally locked to one another by another set of bolts (not shown). With reference to  FIG. 6 or 7 , another array of circumferentially spaced bolt holes  67   a  open upwardly into the circumferential wall  62  of the containment collar  60  at the bottom end thereof and align with a matching circumferential array of bolt holes  67   b  passing axially through the ring gear  32 , whereby the ring gear  32  is bolted to the containment collar  60 . As a result, the containment collar  60  and the base cover  48  rotate together with the ring gear  32  under driven operation of the hydraulic motor  36 . With the lower end of the hollow main cylinder  12  fixed in the annular slot  58  of the base cover  48 , the hollow main cylinder  12  is thus rotatable about its central longitudinal axis  14  by driven operation of the hydraulic motor. 
     With reference to  FIG. 4 , a piston  70  is slidably sealed to the interior surface of the main cylinder  12  by piston seals  70   a  and is centered on the central longitudinal axis  14  for back and forth longitudinal sliding of the piston within the hollow main cylinder  12 . A piston shaft  72  is attached to the piston  70  and extends downwardly therefrom along the central longitudinal axis  14  of the cylinder  12 . The piston shaft  72  reaches downwardly through the axial bore  20  of the base  18  via an aligned axial through-hole of the base cover  48 . A set of anti-rotation rods  74 ,  76 ,  78  extend axially from the cap  16  of the hydraulic linear actuator  10  down to the base cover  48  at respective positions spaced circumferentially around the central longitudinal axis  14  at a distance radially outward from the piston shaft  72 . The base cover  48  features a set of threaded blind holes extending axially thereinto at the upper end thereof for threaded receipt the bottom ends of the anti-rotation rods, and the piston  70  contains a set of axial through bores therein via which these anti-rotation rods  74 ,  76 ,  78  pass through the piston. The piston thus slides back and forth along the anti-rotation rods during its travel back forth on the central longitudinal axis  14  within the confines of the hollow main cylinder  12 . The offset position of each anti-rotation rod from the central longitudinal axis  14  of the hydraulic linear actuator prevents relative rotation between the piston and the main cylinder  12  about the central longitudinal axis. Therefore, rotation of the main cylinder  12  under driven operation of the hydraulic motor  36  causes the piston  70  and the attached piston shaft  72  to rotate with the surrounding main cylinder  12 . With the hydraulic linear actuator  10  mounted in an upright position atop a wellhead, the piston shaft  72  passes downwardly through the wellhead into a production tubing string of the well, where the lower end of the piston shaft is connected to a pump rod that continues downward through the production tubing to a downhole pump for producing well fluids to the surface through the production tubing. As is known in the art, the pump rod may be a continuous rod, or a string of discrete rods axially coupled together by matingly threaded ends of the rods. 
     With reference to  FIG. 5 , the middle section  26  of the base  18  features an upstroke supply port  80  extending radially through its circumferential wall into the axial through-bore  20  of the base  18  at one side thereof. Referring to  FIG. 11 , a hydraulic supply line  80   a  is connected to this upstroke supply port  80  to deliver pressurized hydraulic fluid into to the base  18  of the hydraulic linear actuator from a hydraulic pump P that sources the hydraulic fluid from a fluid reservoir R. A check valve V 1  is installed in the upstroke supply port  80  or on the supply line  80   a  to prevent backflow of hydraulic fluid into the supply line  80   a  from the axial through-bore  20  of the base of the hydraulic linear actuator  10 . Turning to  FIG. 6 , at another side of the base  18 , a separate return port  82  extends radially through the circumferential wall of the middle section of the base  18  into the axial through-bore  20  of the base. Referring again to  FIG. 11 , a hydraulic return line  82   a  is connected to this return port  82  to convey hydraulic fluid from the base  18  back to the fluid reservoir during a downstroke of the hydraulic linear actuator  12 . The exterior diameter of the piston rod  72  is less than the internal diameters of the base&#39;s upper and middle sections  24 ,  26 , and also less than the internal diameters of the seal insert  44  and the inner body  52  of the base cover  48 . The interior of the lower section  22  of the base  22  carries a seal (not shown) through which the piston shaft  72  extends in a manner slidable therethrough but fluid-tight therewith, thereby providing a sealed closure of the interior space of the hydraulic linear actuator at the based-defined bottom end of its housing. 
     The axial passage through the inner body  52  of base cover  48  at the central longitudinal axis  14  to accommodate passage of the piston rod  72  therethrough has a three-lobed shape spanning radially outwardly from the piston rod at areas between the three anti-rotation rods  74 ,  76 ,  78 , as shown at  83  in  FIG. 8 . Accordingly, the piston rod  72  is surrounded by open space throughout its travel through the base cover  48 , the seal insert  44  and the upper and middle sections  24 ,  26  of the base  18 , whereby pressurized hydraulic fluid fed into the base  18  through the upstroke supply port  80  can fill this space and rise upwardly into to the main cylinder  12  in order to drive the upstroke of the piston. The lowermost travel position of the piston is limited by eventual impact against the top end of the base cover  48 , and so the positioning of the upstroke supply port  80  in the middle section  26  of the base  18  places it in a lower portion of the housing&#39;s interior space between the lowermost travel limit of the piston  70  and the sealed closure of the housing at the lower end of the base. Accordingly, introduction of pressurized fluid through the upstroke supply port  80  delivers the hydraulic fluid into the interior space of the housing at a point situated below the lowermost attainable position of the piston seals  70   a  at the bottom end of the downstroke so that this fluid will force the piston upward to initiate the upstroke. 
     To achieve such pressurization of the hydraulic linear actuator beneath the piston during the upstroke, a control valve V 2  installed at the return port  82  or on the return line  82   a  coupled thereto is held closed during the upstroke. The upstroke of the piston is caused by termination of the incoming supply of pressurized fluid to the hydraulic linear actuator, and opening of the return line&#39;s control valve V 2  so that the hydraulic fluid can drain from the base of the hydraulic linear actuator back to the reservoir R through the return line  82   a . In the illustrated embodiment, the upstroke supply port  80  is the only hydraulic fluid supply port, but there is also a leak detection passage described in later detail below that opens up to the interior space of the housing near the capped top end of the housing at which the cap  16  defines the uppermost travel limit of the piston. Therefore, the hydraulic linear actuator  12  is a two way linear actuator that lacks hydraulic pressure return on the downstroke. As a result, the downstroke of the piston  70  is effected gravitationally by the weight of the piston  70 , piston shaft  72  and attached pump rod. The combined weight of these components pulls the piston  70  downwardly, which forces the hydraulic fluid out of the hydraulic linear actuator through the return port. On downstroke the chamber above the piston is atmospherically controlled though the leak detection passage that is described in further detail below and is collectively formed by elements  74   a ,  96 ,  98 ,  102 ,  104  in  FIG. 7 . Attached to the piston, for example by a threaded connection thereto, the piston shaft  72  is driven upwardly and downwardly by the upstroke and downstroke of the piston to drive the downhole pump via the pump rod. With the main cylinder  12  being rotatable relative to the wellhead by the hydraulic motor  36 , and with the piston and piston shaft being rotationally locked to the cylinder  12  by the anti-rotation rods, the driven rotation of the cylinder  12  likewise drives rotation of the piston  70  and thus the pump rod coupled thereto by the piston shaft  72 . Accordingly, the cylinder  12  can be rotated in either direction about its longitudinal by operation of the reversible hydraulic motor in a respective direction in order to drive any downhole tools or equipment requiring rotational input. 
     With reference to  FIG. 4 , to control the timing of the start and end of the hydraulically powered upstroke, the hydraulic linear actuator incorporates a positional detection device operable to detect positional information concerning travel of the piston  70  back and forth within the housing of the hydraulic linear actuator. The positional detection device of the first illustrated embodiment is a magnetostrictive linear-position sensor with a sensing rod  84  passing axially through and downwardly from the cap  16  of the hydraulic linear actuator to the base cover  48  on the central longitudinal axis  14 , thus spanning an entirety of the piston&#39;s available travel range between the underside of the cap  16  and the upper end of the base cover  48 . The piston shaft  72  is hollow over at least a substantial majority of its length, and therefore has a hollow interior bore  72   a  extending axially thereinto from its top end that is coupled to the piston  70 . The piston features an axial through bore  70   b  having a threaded lower portion into which the top end of the piston shaft is threaded at the bottom end of the piston. The piston&#39;s axial bore  70   b  continues upwardly from the top end of the hollow piston shaft  72  to the topside of the piston. The sensing rod  84  extends downwardly through the axial bore  72   b  of the piston  70  into the hollow interior bore  72   a  of the piston shaft  72 . The combined axial bore through the piston and piston shaft from the topside of the piston to the bottom end of the piston shaft exceeds the length by which the sensing rod  84  extends downward from the cap  16  so that the sensing rod never fully reaches the bottom end of the piston shaft, even at the uppermost limit of the piston&#39;s travel. The piston features a ring-shaped magnet  86  in a position spanning circumferentially around the central opening thereof, for example sandwiched between a bolt-on cap  87  of the piston that is axially bolted to the top end of a main seal-carrying body  90  of the piston, to which the piston shaft is attached. 
     Accordingly, the magnet  86  spans circumferentially around the sensing rod  84 , whereby a signal processing head  88  of the magnetostrictive position sensor positioned outside the hydraulic linear actuator above the cap thereof can detect the current position of the piston  70  along the sensing rod  84  at any given moment based on the detected position of the magnet  86  therealong. The head  88  of the sensor is connected to an electronic controller C responsible for initiating and terminating supply of pressurized hydraulic fluid to the hydraulic linear actuator from the hydraulic pump. When the sensor detects arrival of the piston at a preselected lower-limit of the piston&#39;s desired travel range under gravitational fall of the piston during the downstroke, the controller closes the control valve V 2  and activates the pump P to initiate the supply of hydraulic fluid to hydraulic linear actuator  10  through the base  18  thereof, thereby pressurizing the lower portion of the housing&#39;s interior space below the piston, and thus initiating the upstroke. When the sensor detects arrival of the piston at a preselected upper-limit of the piston&#39;s travel range during the upstroke, the controller C deactivates the pump to terminate the supply of the hydraulic fluid and opens up the return port control valve V 2 , thereby depressurizing this lower portion of the housing to enable initiation of the gravitationally driven downstroke. The controller may be programmable to enable user-specification or adjustment of the selected lower and upper limits of the piston travel range, which may be selected to precede the hard maximum limits set by the cap and the base cover so that physical impact of the piston with the cap and base cover is prevented during normal operation. While the detailed embodiment uses a magnetostrictive position sensor, other linear displacement sensor devices could be used. For example, a hall effect sensor mounted to a bottom end of a plain rod or shaft could be used to form a detection rod to cooperate with magnetically coded areas on the piston shaft to provide contactless monitoring of the shaft position. As another option, contact switches on either the piston shaft interior or detection rod exterior could cooperate with raised areas on the other for contact-based linear position detection. However, the need for only a singular magnet for operation of a magnetostrictive sensor allows for simple placement of the magnet externally of the piston rod at or near the upper end thereof, for example within the piston itself, avoiding the need for more complicated placement of magnetic elements or switches within the hollow piston shaft. 
     As shown in  FIG. 7 , one of the anti-rotation rods  74  is hollow so as to define an axial passage  74   a  extending fully therethrough between its top and bottom ends. As shown in  FIG. 4 , a threaded nut or cap  92  is fitted on the top end of the hollow anti-rotation rod  74  outside the hydraulic linear actuator in order to close off the top end of the hollow rod&#39;s axial passage  74   a . Likewise, each other anti-rotation rod, whether hollow or not, is fitted with a threaded nut or cap  92  at the top end of the anti-rotation rod to clamp downward on the top end of the main cylinder  12 , which holds the bottom end of the cylinder down in the annular slot  58  of the base cover  48 . Just below the cap  16 , at least one radial hole  93  passes through the circumferential wall of the hollow anti-rotation rod  74  so as to fluidly communicate the axial passage  74   a  thereof with the interior space of the housing at a location above the upper travel limit of the piston seals  70   a  during the upstroke of the piston. Turning back to  FIG. 7 , the respective through hole  94  in the base cover  48  that receives the open lower end of the hollow anti-rotation rod  74  is open to the outer periphery of the of the inner body  52  of the base cover  48  at the bottom end of this blind hole  94  by way of a radial port  96  machined into the exterior of the base cover&#39;s inner body  50  to intersect with the bottom end of the through hole  94 . This radial port  96  opens into an annular space  98  that exists in an axial gap between the web  54  of the base cover  48  and an annular outer rim  100  of the seal insert  44 , which stands upward from the remainder of the seal insert  44  at the top end thereof. An axial drain channel  102  runs downwardly through the seal insert  44  from this annular space  98  to the interface between the annular downward-facing surface of the seal insert  44  and underlying interior shoulder  46  of the base  18 , from which the drain channel  102  continues into the middle section  26  of the base  18 , where the drain channel  102  intersects with a leak detection port  104  that extends radially outward to the exterior of the base&#39;s middle section  26  at a position below the ring gear  32 . The leak detection port  104  does not fully penetrate the circumferential wall of the base&#39;s middle section  26 , and instead terminates short of the interior bore  20  of the base  18  so that the leak detection port  104  is fluidly isolated therefrom. 
     Accordingly, the axial passage  74   a  of the hollow anti-rotation rod  74 , the respective blind hole  94  of the base cover  48 , the radial port  96  of the base cover, the annular space  98  between the base cover and the seal insert  44 , and the axial drain channel  102  of the base  18  and seal insert  44  all cooperate to form a leak detection passage from the uppermost area of the cylinder&#39;s interior space down to the leak detection port  104 . Seals  105   a  between the interior of the base cover&#39;s outer rim  50  and the exterior of the seal insert  44  and seals  105   b  between the exterior of a reduced-diameter lower end of the seal insert  44  and a reduced-diameter portion of the interior of the base&#39;s upper section  24  below the upward facing shoulder  46  thereof cooperate with the interior seals of the seal insert  44  to fluidly isolate the leak detection passage from the interior space of the housing below the piston  70 . In the event of a piston seal failure by which the hydraulic fluid introduced into the lower portion of the housing through the upstroke supply port can leak across the piston into the upper portion of the housing above the piston, the upstroke of the piston will force this leaked fluid upwardly toward the cap  16  of the housing and into the axial passage  74   a  of the hollow anti-rotation rod  74   a  via the radial holes  93  therein. The leaked fluid will thus drain down through the leak detection passage to the leak detection port  104 , where the presence of fluid will thus indicate the existence of a leak across the piston. A leak detection line  106  is coupled to the leak detection port  104  and leads to a leak containment tank  108  which receives the leaked fluid and isolates same from the surrounding environment. A leak detection sensor is cooperable with the leak detection passage, port, line and tank in order to trigger an alarm or notification, and/or cause shut-down of the hydraulic linear actuator, upon detecting presence or accumulation of leaked hydraulic fluid within this leak detection system. For example, the sensor may be a float sensor  110  mounted in the leak containment tank for actuation upon accumulation of a predetermined level of fluid within the containment tank. The sensor may be connected to the controller C or to a shut-down switch of the hydraulic pump P so that triggering of the sensor terminates operation of the pump to shut down operation of the linear actuator until an inspection and reset of the system can be performed. 
     The first illustrated embodiment provides a hydraulically powered artificial lift system for reciprocally driven downhole pumps that can additionally be used to operate rotationally driven downhole equipment, that places its position-detection rod internally within the housing while using a hollow piston shaft to isolate the position-detection rod from the pressurized hydraulic fluid introduced in the lower portion of the housing, that incorporates a leak detection and containment solution to prevent environmental contamination, and that provides all fluid line connections at the bottom of the housing for convenient access and leak containment. 
       FIGS. 9 and 10  show a second embodiment artificial lift unit  10 ′ that differs from first in its type of positional detection device, and in the addition of a multi-function processing module  200  mounted inside a cap cover  202  at the top end of the main cylinder  12 .  FIG. 9  shows the fully assembled lift unit  10 ′, in which cap cover  202  is engaged over the cap  16 ′ of the main cylinder  12 . In this embodiment, the cap  16 ′ features a central stand-off  204  that reaches vertically upward from the cap  16 ′ on the central longitudinal axis of the housing  12  in order to carry the processing module  200  in an elevated position over the nuts/caps  92  of the anti-rotation rods  74 ,  76 ,  78 . The stand-off  204  is hollow, and its bottom end communications with a central through-bore of the main cylinder cap  16 ′. The sensing rod of the first embodiment is replaced with a screw rod  84 ′ (e.g. a ball screw rod) that extends downwardly from the cap  16 ′ on the central longitudinal axis of the main cylinder  12 , through the central bore of the piston  70  and into the piston shaft  74 , just like the sensing rod of the first embodiment. Instead of a magnet, the piston  70  in the second embodiment carries a nut  86 ′ that is fixed on bolted cap  87  of the piston  70  and is mated with the screw rod  84 ′. As a result, linear displacement of the piston  70  in the longitudinal direction of the main cylinder  12  by hydraulic or gravitational action causes the screw shaft  84 ′ to rotate due to its mated engagement with the piston-carried nut  86 ′. 
     A smooth walled upper extension  84   a  of the screw rod  84 ′ reaches upwardly through the cap  16 ′ of the main cylinder through a fluid-tight rotation-allowing seal. The rod extension  84   a  continues upwardly through the hollow interior of the standoff  204  and into the bottom end of the module  200 . Inside the module&#39;s outer enclosure, the rod extension  84   a  passes axially through a rotary encoder  206  and then further upward to an electrical generator  208 . The rotary encoder  206  is operable to monitor the rotation of the rod extension  84   a  and attached screw rod  84   a  about the central longitudinal axis  14  of the main cylinder  12 . The same rotation of the rod extension  84   a  is operable to drive the generator and thereby provide power for electrical components of the lift unit  10 ′, which in the illustrated examples include both the rotary encoder  206  and a wireless transmitter  210 . The transmitter  210  is communicably coupled to the rotary encoder to receive electronic signals therefrom that represent current position of the piston along the screw rod based on the detected direction and angulation of the screw-rod&#39;s rotation. 
     Hydraulic lifting of the piston  70  rotates the screw rod  84 ′ in one direction, while gravitational fall of the piston  70  rotates the screw rod  84 ′ in the other direction. Accordingly, monitoring of the direction and number of rotations of the screw rod by the rotary encoder  206  serves to monitor the movement and position of the piston  70  along the longitudinal axis of the cylinder  12  relative to an initial starting position of the piston. The electrical power gained from the generator during any such rotation of the screw rod is stored in one or more capacitors, batteries or other electrical stores, and is used to power the rotary encoder  206  and the wireless transmitter  210 . The module  200  thus replaces the signal processing head  88  in the first embodiment and wirelessly communicates the data signals from the rotary encoder concerning the positional information on the piston to the separate electronic controller responsible for controlling the supply and relief of the hydraulic fluid to and from the main cylinder  10 . 
     Operation of the second embodiment artificial lift unit  10 ′ is similar to that described above for the first embodiment in relation to  FIG. 11 , except that a wired connection from the top of the artificial lift unit down to a ground level controller C is not required due to the inclusion of the wireless transmitter in the module  200 . In the first embodiment, if a wired connection is used instead of a wireless transmitter, then a slip ring is employed at the top end of the main cylinder to provide electrical connection between the wired connection and the sensor head  88  at the top of the rotatable cylinder to accommodate the rotational motion thereof. 
     Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same made within the scope of the claims without departure from such scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.