Abstract:
There is disclosed a rotary-pad gas lift plunger. In an embodiment, the pads rotate around the mandrel. This rotation movement results from two features. The first one is the assembly of the mandrel and the pad retaining rings. The second one is the shape of the wearing pad. The rotation movement results from the plunger travel up and down within the tubing. Rotation with uniform wearing helps to maintain the sealing effect between the wearing pads and the inside of the tubing. This uniform wearing on the surface of pads improves efficiency to displace oil and water from oil and gas wells. The uniform wearing also causes a significant increase in the service life of the plunger. Because of the rotation movement, this gas lift plunger can operate in deviated wells where a typical gas lift plunger is restricted.

Description:
BACKGROUND 
       [0001]    Within the oil and gas industry, a plunger is an artificial lift device used to increase hydrocarbon production on wells whose gas flow is reduced by a weight of accumulated fluids sitting on top of the gas flow. By closing and opening the sales valve, and by other means, a plunger is made to travel toward the bottom of the tubing string and then ascend under internal gas pressure, carrying a slug of liquid above it that will be removed from the well, thereby increasing production. Inefficiency in a plunger system is related to the amount of fluid that slips past its seal during ascent (fluid slippage). A wide range of designs is necessary to accommodate a wide range of well conditions such as production rate, gas pressure, gas-to-liquid ratio, profit margin, irregularities in tubing diameter, doglegs, debris, contaminants, and corrosives. Unfortunately, since the plunger is a tight-fitting device, wear is a significant issue, including one-sided wear, eventually necessitating a replacement or repair cost. 
         [0002]    One solution to keeping costs low may be to use a bar stock plunger having few or no moving parts, the surface being arrayed with flutes or grooves stacked axially to generate a sealing turbulence. However, significant fluid slippage may occur in this simple design, especially at low travel speeds. And, only one degree of freedom exists in responding to changing tubing conditions, which is to adjust the speed of the plunger through the plunger control system. 
         [0003]    Alternatively, a higher efficiency may be obtained by use of a pad plunger in which an array of circular pads are deployed around the mandrel and spring biased against the inner surface of the tubing string, allowing individual pads to move radially in an out, and to toggle circumferentially. Thereby, two degrees of freedom exist, an improvement allowing a greater responsiveness to changes in tubing diameter, angular bends in the string, and debris, and may result in greater efficiency. But, one-sided wear may occur, especially in a non-vertical string and when encountering debris. Wearing stripes may also occur in a straight well where one side of the plunger abrades against the tubing string, causing a deterioration of sealing performance and requiring premature repair or replacement. 
         [0004]    A solution to one-sided wear is to induce pad rotation by arraying spirals or grooves onto the wear surfaces of the sealing pads. However, the grooves depend on a strong fluid flow to generate rotation and do not work as well at low travel speeds. Also, the grooves, which often double as wear indicators in the range of approximately 0.015-0.030″, eventually wear flat so that they are no longer effective. Additionally, the rotational inertia of the plunger may be high, corresponding to high weight of the plunger (typically 6 to 18 pounds), and therefore the mandrel cannot rotatably respond quickly to a change in tubing conditions, leading to wear and fluid slippage. 
         [0005]    Another solution, aiming to improve the seal, uses multiple sets of pads, such as a dual-pad plunger with articulating pads, that can better corner doglegs in the tubing and also responds with two degrees of freedom (toggling radially and circumferentially) to changing string conditions. However, the complexity of the dual-pad plunger makes it heavy, thereby causing a greater potential for damage to the tubing string that it&#39;s abrading against. 
       SUMMARY 
       [0006]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key aspects or essential aspects of the claimed subject matter. Moreover, this Summary is not intended for use as an aid in determining the scope of the claimed subject matter. 
         [0007]    In an embodiment, there is provided a plunger for lifting fluids within the tubing string of a hydrocarbon well and which may comprise an elongated mandrel having a mandrel axis and a circumferential surface. At least 2 arcuate pads may be disposed circumferentially around the mandrel, and may form a pad set. The arcuate pads may each have a pad upper end and a pad lower end. Each arcuate pad may have a wear surface cooperating to form a substantially contiguous cylindrical seal against an inner surface of the tubing string. At least one spring may be disposed between the mandrel and the arcuate pad for biasing the arcuate pad outwardly radially such that the wear surfaces are moveably in contact with the inner surface of the tubing string. At least one circular guide may be disposed on the mandrel and may each engage a complementary receptacle of the arcuate pad for substantially restricting the motion of the arcuate pad to be circumferential around the mandrel axis. An annular retainer may be disposed around the mandrel for enclosing one of the pad upper end, the pad lower end, the pad upper end of one pad set and the pad lower end of another pad set. The annular retainer may limit outward radial movement of the arcuate pad. The at least one circular guide, complementary receptacles, springs, and annular retainers may cooperate to allow rotation of the arcuate pads about the mandrel axis while radially biasing the arcuate pads against the inner surface of the tubing string, thereby lifting fluids within the tubing string. 
         [0008]    In another embodiment, there is provided a plunger for lifting fluids within the tubing string of a hydrocarbon well and which may comprise an elongated mandrel having a mandrel axis and a circumferential surface. At least 2 arcuate pads may be disposed circumferentially around the mandrel to form a pad set. Each arcuate pad may have a pad upper end and a pad lower end. Each arcuate pad may have a wear surface arcing between two pad side edges and which may cooperate to form a substantially contiguous cylindrical seal against an inner surface of the tubing string. At least one spring may be disposed between the mandrel and the arcuate pad for biasing the arcuate pad outwardly radially such that the wear surfaces are moveably in contact with the inner surface of the tubing string. An annular retainer may be disposed around the mandrel and may enclose either the pad upper end, the pad lower end, or the pad upper end of a first pad set and the pad lower end of a second pad set. The annular retainer may limit outward radial movement of the arcuate pad. A pad gap may separate the pad side edges of two adjacent arcuate pads and may provide a channel for fluids to flow between the pad upper end and the pad lower end. The arcuate pad may be shaped as a rhombus for slanting the pad gap oblique to the mandrel axis by a pad slant angle of less than approximately 35 degrees for encouraging rotation of the arcuate pads. 
         [0009]    Additional objects, advantages and novel features of the technology will be set forth in part in the description which follows, and in part will become more apparent to those skilled in the art upon examination of the following, or may be learned from practice of the technology. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Non-limiting and non-exhaustive embodiments of the present invention, including the preferred embodiment, are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. Illustrative embodiments of the invention are illustrated in the drawings, in which: 
           [0011]      FIG. 1  illustrates a side sectional view of a dual-pad plunger having a circular guide, in accordance with an embodiment of the present disclosure. 
           [0012]      FIG. 2  illustrates a side view of a dual-pad plunger having a circular guide, in accordance with an embodiment of the present disclosure. 
           [0013]      FIG. 3  illustrates a close-up side sectional of a collar lip enclosing a pad end, in accordance with an embodiment of the present disclosure. 
           [0014]      FIG. 4  illustrates an axial sectional view of a guide rail and guide slot, in accordance with an embodiment of the present disclosure. 
           [0015]      FIG. 5 a    illustrates an axial sectional view of edge spacers with pads biased outward, in accordance with an embodiment of the present disclosure. 
           [0016]      FIG. 5 b    illustrates an axial sectional view of edge spacers with pads in a deflected position, in accordance with an embodiment of the present disclosure. 
           [0017]      FIG. 6  illustrates an axial sectional view of a spring bearing supporting biasing springs, in accordance with an embodiment of the present disclosure. 
           [0018]      FIG. 7  illustrates a side view of a plunger with edge spacers, in accordance with an embodiment of the present disclosure. 
           [0019]      FIG. 8  illustrates a side view of a dual-pad plunger having a slanted pad gap, in accordance with an embodiment of the present disclosure. 
           [0020]      FIG. 9  illustrates a perspective view of a dual-pad plunger having a slanted pad gap, in accordance with an embodiment of the present disclosure. 
           [0021]      FIG. 10  illustrates a side sectional view of a dual-pad plunger having a guide wheel, in accordance with an embodiment of the present disclosure. 
           [0022]      FIG. 11  illustrates an axial sectional view of a plunger having a guide wheel, in accordance with an embodiment of the present disclosure. 
           [0023]      FIG. 12  illustrates a side sectional view of a shortened plunger having one set of double pads, in accordance with an embodiment of the present disclosure. 
           [0024]      FIG. 13  illustrates a side view of a shortened plunger having one set of double pads, in accordance with an embodiment of the present disclosure. 
           [0025]      FIG. 14 a    illustrates a side-sectional view of a bypass plunger having a circular guide, in accordance with an embodiment of the present disclosure. 
           [0026]      FIG. 14 b    illustrates a side view of a bypass plunger having a circular guide, in accordance with an embodiment of the present disclosure. 
           [0027]      FIG. 15 a    illustrates an axial sectional view of a standoff ring for a plunger, in accordance with an embodiment of the present disclosure. 
           [0028]      FIG. 15 b    illustrates a side sectional close-up view of a standoff for a plunger, in accordance with an embodiment of the present disclosure. 
           [0029]      FIG. 16  illustrates a perspective view of a plunger having two pad sets rotatable on finger rings and clocked at an angular offset, in accordance with an embodiment of the present disclosure. 
           [0030]      FIG. 17  illustrates a side view of a plunger having rhombus-shaped pads rotatable on finger rings and pad sets clocked at an angular offset, in accordance with an embodiment of the present disclosure. 
           [0031]      FIG. 18 a    illustrates a side sectional view of a dual-pad plunger having pad sets rotatable on finger rings and clocked at an angular offset, in accordance with an embodiment of the present disclosure. 
           [0032]      FIG. 18 b    illustrates a side sectional view of a finger ring mating with a clocking ring and enclosed by a central collar, in accordance with an embodiment of the present disclosure. 
           [0033]      FIG. 19 a    illustrates a perspective view of a clocking ring with axial fingers for retaining the pads, in accordance with an embodiment of the present disclosure. 
           [0034]      FIG. 19 b    illustrates a side sectional view of a clocking ring with axial fingers, in accordance with an embodiment of the present disclosure. 
           [0035]      FIG. 20 a    illustrates a perspective view of a finger ring with axial fingers for retaining the pads, in accordance with an embodiment of the present disclosure. 
           [0036]      FIG. 20 b    illustrates a side sectional view of a finger ring with axial fingers, in accordance with an embodiment of the present disclosure. 
           [0037]      FIG. 21 a    illustrates a perspective view of a central collar for retaining the pads, in accordance with an embodiment of the present disclosure. 
           [0038]      FIG. 21 b    illustrates a side sectional view of a central collar for retaining the pads, in accordance with an embodiment of the present disclosure. 
           [0039]      FIG. 22 a    illustrates a perspective view of an end collar for retaining the pads, in accordance with an embodiment of the present disclosure. 
           [0040]      FIG. 22 b    illustrates a side sectional view of an end collar for retaining the pads, in accordance with an embodiment of the present disclosure. 
           [0041]      FIG. 23  illustrates an exploded view of a plunger having two pad sets rotatable on finger rings and clocked at an angular offset, in accordance with an embodiment of the present disclosure. 
           [0042]      FIG. 24 a    illustrates a perspective view of a mandrel with threads at each end, in accordance with an embodiment of the present disclosure. 
           [0043]      FIG. 24 b    illustrates a side sectional view of a mandrel with threads at each end, in accordance with an embodiment of the present disclosure. 
           [0044]      FIG. 25 a    illustrates a perspective view of an arcuate pad having a rhombus shape. 
           [0045]      FIG. 25 b    illustrates a perspective view of the arcuate pad of  FIG. 25 a    in which there is a view of a pair of spring cavities. 
           [0046]      FIG. 25 c    illustrates a bottom view of the arcuate pad of  FIGS. 25 a  and 25 b    in which there is a view of the pair of spring cavities. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]    Embodiments are described more fully below in sufficient detail to enable those skilled in the art to practice the system and method. However, embodiments may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein. The following detailed description is, therefore, not to be taken in a limiting sense. 
         [0048]    As may be appreciated, based on the disclosure, there exists a need in the art for a lift plunger with more even wear around the sealing surface, and reduced wear in general. Further, there exists a need in the art for a plunger with improved lift efficiency, especially at slow speeds. Additionally, there exists a need in the art for a greater degree of freedom in the movement of the wear surfaces in order to better respond to dimensional irregularities and debris in the tubing string. Also, there exists a need in the art for the plunger seal to respond more quickly to changing conditions. And finally, there exists a need in the art for a plunger system that poses a lower risk of damage to the tubing string. 
         [0049]    In an embodiment, referring to  FIGS. 1-4  and  FIG. 6 , a lift plunger  10  may comprise two pad sets  11  of arcuate pads  12  stacked axially in a dual pad plunger arrangement for use in lifting fluids to the surface in a hydrocarbon well, such as a well for oil and gas. Each pad set  11  of arcuate pads  12  may be disposed circumferentially around the mandrel  30  such that wear surfaces  20  cooperate to form a substantially contiguous cylindrical seal against the inner surface of the tubing string  16  ( FIG. 6 ). A top end  34  (also known as a catching neck) may terminate mandrel  30  and may provide an annular lip  56  for enclosing and retaining a pad upper end  24  for limiting outward radial movement of the arcuate pad  12 . A bottom end  32  may terminate mandrel  30  and may provide an annular lip  56  for enclosing and retaining a pad lower end  24  for limiting outward radial movement of the arcuate pad  12 . Bottom end cavity  38  may be formed in bottom end  32  in order to provide greater gas lift. A collar  50  may wrap around circumferential surface  36  of mandrel  30  to provide a collar lip  52  on both ends for enclosing pad upper end  24  and pad lower end  22 , thereby limiting outward radial movement of arcuate pad  12 . Alternatively, an annular lip  56  may be disposed directly on the circumferential surface  36  of mandrel  30  ( FIG. 3 ). 
         [0050]    Referring to  FIGS. 1 and 6 , in an embodiment, springs  60  may be disposed between spring bearing  64  and a spring cavity  62  recessed into the underside of arcuate pad  12  for biasing arcuate pads  12  outwardly radially. As shown in this embodiment, there may be two springs  60  for each of four arcuate pads per spring bearing, and two spring bearings per arcuate pad, for a total of sixteen springs per pad set  11 . Spring bearing  64  may slide circumferentially on spring bearing shaft  66  joining an upper and lower portion of mandrel  30 . Spring bearing shaft  66  may be formed as a recess in mandrel  30 , or may be a specialized bearing joining an upper and lower portion of mandrel  30 . Spring bearing  64  axially located by the recess in mandrel  30  may comprise a circular guide for substantially restricting motion to be circumferential about mandrel axis  36  (not shown). Springs  60  and spring cavity  62  may comprise a complementary receptacle in arcuate pad  12  for enabling rotation. Arcuate pads  12  may be adjustably biased by springs  60  into contact with inner surface of the tubing string  16  in order to adapt to changes in tubing diameter or respond to the presence of debris, thereby maintaining a seal that avoids fluid slippage during ascent of plunger  10 . Since most of the pressure directed by the sealing action may be directed through biasing springs  60 , and not onto other components disposed on the mandrel  30 , a bearing surface may be most needed at the interface between spring  60  and mandrel  30 . 
         [0051]    Continuing, in an embodiment,  FIGS. 1 and 4  show, respectively, a side sectional and axial sectional view of guide rail  40  disposed on mandrel  30  and complementary guide slot  42  disposed on the underside of arcuate pad  20  and which may together form a circular guide and complementary receptacle for guiding arcuate pads  20  in rotation around the mandrel&#39;s elongated axis (not shown). The sides of guide rail  40  and guide slot  42  may fit closely to minimize axial movement of arcuate pads  12 . A rail radial gap  41  between guide rail  40  and guide slot  42  may allow the arcuate pads to flex radially between bottoming ledge  54  and the enclosing limits provided by annular lips  56  and collar lips  52 , thereby responding to diameter or pressure changes within the tubing string  14  ( FIG. 6 ). The function of bottoming ledge  54  may also be obtained from guide rail  40  by allowing rail radial gap  41  to close. Alternatively, a hooked surface (not shown) may be added to the circular guide to radially limit the arcuate pads  12  to a maximum outward movement and thereby may reduce the need for an annular lip  56  or collar lip  52 . 
         [0052]    Continuing with  FIGS. 1-4  and  FIG. 6 , in a embodiment, pad side edges  26  of adjacent arcuate pads  12  may bracket pad gap  90 , creating a channel through which fluids may flow around lift plunger  10 . Pad gap  90  may also be filled with a gasket or seal to reduce fluid slippage and improve lift efficiency. In an embodiment, a set of four springs per arcuate pad that are positioned off center may provide a toggling degree of freedom in both an axial plane and a circumferential plane. The top of guide rail  40  and mating surface of guide slot  42  may be rounded (not shown) to allow for axial toggling while still preventing a shift in axial position. Alternately, a small gap in the mating sides of guide rail  40  and guide slot  42  may also allow for axial toggling of arcuate pad  20  while sufficiently limiting axial positioning. 
         [0053]    Continuing, in an embodiment, by providing a circular guide and complementary receptacle for pad rotation, an additional degree of freedom (swiveling or rotating) may prevent undue or one-sided wear due to dimensional irregularities and debris in the tubing string. For example, a slant well (not shown) may have a non-vertical tubing string ( 14 ) which causes the plunger to lie on one side, leading to one-sided wear. And, sharp bends in the slant well may compress one or more pads and cause undue wear in a stationary or fixed-pad plunger. However, a lift plunger  10  with rotatable pads, such as the design disclosed herein, may deflect one or more arcuate pads rotationally in response to a dimensional change, or in response to a deposit of paraffin, thereby spreading wear more evenly among all arcuate pads and increasing lift efficiency, even at slow travel speeds. Additionally, unlike a fixed-pad plunger having high rotational inertia, the swivel pads may respond quickly to irregularities in the tubing string  14  because the pads have much lower inertia than the plunger as a whole. The arcuate pads may therefore rotate while the mandrel  30  stays still. 
         [0054]    Referring to  FIGS. 2, 15   a , and  15   b , in an embodiment, an axial standoff  44  may protrude radially into pad cutout  28  in the center-end of arcuate pad  12 , stabilizing arcuate pad  12  in its circumferential position and establishing a consistent pad gap  90  between pad side edges  26 . In a fixed-pad plunger (without rotation of the pads), axial standoffs  44  may be disposed directly on mandrel  30  for establishing a circumferential and axial position for each arcuate pad while allowing toggling in both planes. In an embodiment, axial standoffs  44  along all pad ends in a pad set  11  may be disposed on a standoff ring  46  rotating as a bearing on circumferential surface  36  between standoff ledges  49  of mandrel  30 , and may comprise a circular guide for substantially restricting motion to be circumferential. Axial standoff  44  and pad cutout  28  may be dimensioned to allow a full play between maximum and minimum radial biasing by springs  60 . Collar lip  52  may contact pad upper end  24  at a maximum biasing while bottoming ledge  54  may contact circumferential surface  36  at a minimum biasing ( FIG. 10 ). 
         [0055]    In an alternative embodiment not shown, one long standoff bearing sliding on circumferential surface  36  and extending between standoff ledges  49  may support an array of axial standoffs  44  functioning as a circular guide and mating with pad cutouts  28  functioning as complementary receptacles. Additionally, one or more springs  60  may be disposed between the long standoff bearing and the arcuate pads for biasing the pads outwardly and allowing axial and circumferential toggling. 
         [0056]      FIGS. 5 a , 5 b    and  FIG. 7 , in an embodiment, may suggest an alternative to using axial standoffs  44 , or similar means, for maintaining an even spacing between adjacent arcuate pads  12 . Flexible edge spacers  58  may be placed between pad side edges  26  in a way that allows independent deflection of wear surface  20  by debris  94  ( FIG. 5 b   ) or other irregularities inside the tubing string  14  while maintaining an even spacing between arcuate pads  12 . Edge spacer  58  may have concave ends for receiving pad side edge  26  while allowing pivoting of edge spacer  58  and a toggling articulation of arcuate pads  12 . Or, edge spacer  58  may clip onto an indentation or mating axle on the pad side edge  26  (not shown). Additionally, a flexible material such as Teflon may allow deflection while reducing play in the attachment of the edge spacer  58  to pad side edge  26 . 
         [0057]    In an embodiment, referring to  FIG. 7 , arcuate pads  12  having wear surfaces  20  may be disposed on a mandrel  30  (not shown) having top end  34  and bottom end  32 . A circular guide design may be combined with edge spacers  58  by including a guide rail  40  and guide slot  42  (not shown here), as mentioned above, or an alternate method of guiding circumferential rotation of arcuate pads  12 . Springs  60  may be integrated into an edge spacer with a rail and slot design, or may be provided for separately, such as with a spring bearing. Annular lip  56  may enclose pad upper end  24  and may enclose pad lower end  22  for limiting outward radial movement of arcuate pad  12 . Collar lip  52  of collar  50  may enclose pad upper end  24  and may enclose pad lower end  22  for limiting outward radial movement of arcuate pad  12 . 
         [0058]    Alternate methods of implementing a circular guide will be shown in subsequent descriptions which may provide for rotation, or swiveling, of arcuate pads  20  about the mandrel axis. Additionally, alternate methods of disposing springs  60  on mandrel  30  may include using an arched or leaf spring sliding directly on circumferential surface  36 . Or, spring  60  may slide on circumferential surface  36  using an independent ball bearing element, and may thereby reduce to one the number of bearing rings needed for each pad set  11  in order to accomplish a swivel pad design. For instance, a rail and guide bearing (not shown) may alone guide rotation with low levels of friction while edge spacers  58  control pad gap  90  and annular or collar lips restrain outward radial biasing, and may eliminate the need for standoff ring  46 . In another embodiment, the amount of rotation may be limited to a subset of 360 degrees in order to employ simpler or less expensive manufacture by methods known to those skilled in the art, and yet may still accomplish an improved seal efficiency and reduced wear of lift plunger  10 . 
         [0059]    Referring now to  FIGS. 10 and 11 , in an embodiment, the circular guide and spring biasing functions may be integrated into one structure which may comprise a guide wheel  80  having a wheel spoke  82  slideably mating with pad shaft  84  disposed on arcuate pad  12 . Spring  60  in spring cavity  62  may be interposed between pad shaft  84  and wheel spoke  82  to provide outward radial biasing. A wheel bearing  81  portion of guide wheel  80 , from which wheel spokes  82  may extend radially, may rotate on a wheel shaft  86  interposed between and coaxial with a mandrel upper portion  106  and a mandrel middle portion  104 , in case of the upper assembly. And, wheel bearing  81  may rotate on a wheel shaft  86  interposed between and coaxial with a mandrel lower portion  102  and a mandrel middle portion  104 , in case of the lower assembly. Mandrel upper, middle, and lower portions may comprise mandrel  30 . 
         [0060]    Referring to  FIGS. 10 and 11 , in an alternate embodiment not shown, pad shaft  84  may have at least one flat side mating with a complementary surface of wheel spoke  82  for preventing twisting of arcuate pad  12  out of the circumferential plane of rotation. For example, pad shaft  84  may be rectangular and slideably mate with wheel spoke  82 . By shaping wheel spokes  82  to prevent rotation, it may not be necessary to employ axial standoffs  44 , edge spacers  58 , or guide rail  40  in fabricating a plunger having outwardly radially biased rotatable pads. In another embodiment shown in  FIG. 12 , a pad set  11  may be guided for circumferential rotation by two guide wheels  80  distributed along the mandrel axis (not shown), thereby preventing the pads from twisting out of their plane of rotation, where the two springs in each pad may enable axial toggling of the pads. In yet another embodiment (not shown), referring to  FIGS. 11 and 6 , two wheel spokes supporting one pad may extend from a wheel bearing, also preventing the pads from twisting out of their plane of rotation, where the two springs in each pad may enable circumferential toggling of the pads. 
         [0061]    Continuing with  FIGS. 10 and 11 , in an embodiment, collar lips  52  of collar  50  may enclose pad upper end  24  and pad lower end  22 , and annular lips  56  may be disposed on circumferential surface  36  for enclosing and retaining pad upper end  24  and pad lower end  22 , both annular and collar lips limiting outward radial movement of the arcuate pads  12  having wear surfaces  20  biased against the inner surface of tubing string  16 . Bottoming ledge  54  may be disposed on the underside of arcuate pad  12  ( FIG. 10 ) and positioned to contact circumferential surface  36  for establishing a minimum radius of outward biasing of arcuate pads  12 . Alternately, bottoming ledge  54  may be disposed on circumferential surface  36  ( FIG. 11 ). Mandrel top end  34  may terminate mandrel upper portion  106  and bottom end  32  having bottom end cavity  38  may terminate mandrel lower portion  102 . 
         [0062]    Advantageously, in embodiments described above, including  FIGS. 10  and  11 , there may be several methods for providing the circular guide and spring biasing functions, or integrating the two functions into one structure, and where the method selected may depend upon the degrees of freedom most effective in minimizing wear and maximizing lift efficiency, upon the amount of irregularity and debris in the tubing string, and cost factors. Several design features may contribute to an improved lift plunger, and may include adding circumferential rotation not dependent on grooves or spirals embossed on the surface of the plunger, reducing rotational inertia by separating pad rotation from the mandrel, providing several means for maintaining correct pad edge spacing, and integrating spring biasing and circular guiding. Additional embodiments will be described which may depict additional methods for achieving the above advantages. 
         [0063]    Now referring to  FIGS. 8 and 9 , in an embodiment, a dual-pad plunger  10  may comprise two pad sets  11  of four arcuate pads  12  each disposed circumferentially on a mandrel  30  (not shown) terminated with top end  34  and bottom end  32 , both ends having beveled sides  37  for reducing the risk of damage to tubing string  14  (not shown). Pad gap  90  between pad side edges  26  may provide a channel for fluids to flow between the pad lower end and the pad upper end, and the channel may be oblique to the mandrel axis  31  by a pad slant angle  92  of less than approximately 35 degrees for encouraging rotation of the arcuate pads. The effect of a slanted pad gap may be to form the arcuate pad to be in the shape of a rhombus, or parallelogram. In an embodiment, a pad slant angle of less than approximately 15 degrees may generate rotation of the arcuate pads. Axial standoffs  44  protruding radially into pad cutouts  28  may be disposed directly on mandrel  30  in a fixed pad design (without pad rotation) for fixing pad location both axially and circumferentially. Alternatively, axial standoffs  44  may be disposed on a standoff ring or other structure linking multiple axial standoffs together for permitting rotation of a pad set  11  with respect to the mandrel. By providing a pad gap  90  oblique to the mandrel axis  31 , fluid flow may induce wear surfaces  20  of arcuate pads  12  to rotate with respect to the tubing string either independent of or dependently with the mandrel. 
         [0064]    Continuing with  FIGS. 8 and 9 , in an embodiment, collar lips  52  of collar  50  may enclose pad upper end  24  and pad lower end  22 , and annular lips  56  may be disposed on circumferential surface  36  for enclosing pad upper end  24  and pad lower end  22 , both annular and collar lips limiting outward radial movement of the arcuate pads  12 . In an embodiment, the upper and lower pad sets  11  may be offset by an angular offset for preventing fluid slippage by creating a less direct path for fluid flow across the pads, or may be offset in order to allow pads to independently adapt to irregularities in the tubing string. For example, in an embodiment having four arcuate pads  12  spanning approximately 90 degrees each, the two pad sets may be offset by an angular offset of approximately 45 degrees. In other embodiments, the two pad sets may be aligned (0 degrees), or may be offset by 30 degrees or 60 degrees. In a dual-pad plunger  10  having rotatable pads, collar  50  may be configured as a clocking collar (not shown) having clocking tabs protruding into the pad gap for fixing the angular offset while the two pad sets rotate together. 
         [0065]    Referring now to  FIGS. 12 and 13 , in an embodiment, a shortened lift plunger  10  may comprise a mandrel wrapped with one pad set  11  of arcuate double pads  13  having a pad upper end  24  and pad lower end  22  enclosed by annular lips  56  for limiting the maximum outward radial extension of the arcuate double pads  13 . Double pad indentation  74  may by a recess below wear surfaces  20  and may separate the upper and lower portions of arcuate double pad  13 . A top end  34  may have a beveled side  37  for streamlining the plunger  10 . A plurality of spiral grooves  72  may be disposed on bottom end  32  for inducing rotation in the plunger as a response to fluid flow within tubing string  14  (not shown). Alternately, spiral grooves  72  may also be disposed on top end  34  for encouraging rotation. 
         [0066]    Continuing with  FIGS. 12 and 13 , in an embodiment, two guide wheels  80  may support the double pads  13 , each guide wheel  80  having a wheel spoke  82  slideably mating with pad shaft  84  disposed on arcuate pad  12 . Spring  60  in spring cavity  62  may be interposed between pad shaft  84  and wheel spoke  82  to provide outward radial biasing. A wheel bearing  81  portion of guide wheel  80 , from which wheel spokes  82  may extend radially, may rotate on a wheel shaft  86  interposed between and coaxial with a mandrel upper portion  106  and a mandrel middle portion  104 , in case of the upper assembly. And, wheel bearing  81  may rotate on a wheel shaft  86  interposed between and coaxial with a mandrel lower portion  102  and a mandrel middle portion  104 , in case of the lower assembly. Mandrel upper, middle, and lower portions may comprise mandrel  30 . Bottoming ledges  54  may be disposed on the underside of the arcuate double pads  13  for contacting circumferential surface  36 , thereby setting a minimum radial extension of pads  13 . Advantageously, the shortened plunger may be more maneuverable around bends in the tubing string, and its two guide wheels may prevent the pads  13  from twisting out of their plane of rotation while allowing for an articulating wear surface  20  that can toggle axially or circumferentially. 
         [0067]    Referring now to  FIGS. 14 a  and 14 b   , in an embodiment, a dual-pad plunger  10  may comprise two pad sets  11  of four arcuate pads  12  each disposed around circumferential surface  36  and positioned toward top end  34  terminating mandrel  30 . Bottom end  32  may terminate mandrel  30  and may house bypass actuator  110  sliding to open and close bypass inlet  112  for admitting fluid flow  78  into mandrel hollow  114  for conduction to a bypass outlet  116  in the top end  34 . Pad gap  90  between pad side edges  26  may provide a channel for fluids to flow between pad lower end  22  and pad upper end  24 , and the channel may be oblique to the mandrel axis  31  by a pad slant angle  92  of less than approximately 35 degrees for encouraging rotation of the arcuate pads. The arcuate pad may have the shape of a rhombus, or parallelogram. Preferably, pad slant angle  92  may be less than approximately 15 degrees. 
         [0068]    Continuing with  FIGS. 14 a  and 14 b   , in an embodiment, collar lips  52  of collar  50  may enclose pad upper end  24  and pad lower end  22 , and annular lips  56  may be disposed on circumferential surface  36  for enclosing pad upper end  24  and pad lower end  22 , both annular and collar lips limiting outward radial movement of the arcuate pads  12 . In an embodiment, the upper and lower pad sets  11  may be offset by an angular offset for preventing fluid slippage by creating a less direct path for fluid flow across the pads, or may be offset in order to allow pads to independently adapt to irregularities in the tubing string. For example, in an embodiment having four arcuate pads  12  spanning approximately 90 degrees each, the two pad sets may be offset by an angular offset of approximately 45 degrees. In a dual-pad plunger  10  having rotatable pads, collar  50  may be configured as a clocking collar (not shown) having clocking tabs protruding into the pad gap for fixing the angular offset while the two pad sets rotate together. 
         [0069]    Continuing with  FIGS. 14 a  and 14 b   , in an embodiment, guide rail  40  disposed on mandrel  30  and complementary guide slot  42  disposed on the underside of arcuate pad  12  may together form a circular guide and complementary receptacle for guiding arcuate pads  12  with wear surface  20  in rotation around mandrel axis  31 . The sides of guide rail  40  and guide slot  42  may fit closely to minimize axial movement of arcuate pads  12 . A rail radial gap  41  between guide rail  40  and guide slot  42  may allow the arcuate pads to flex radially. Springs  60  may be disposed between spring bearing  64  and a spring cavity  62  recessed into the underside of arcuate pad  12  for biasing arcuate pads  12  outwardly radially. As shown in this embodiment, there may be two springs  60  for each of four arcuate pads per spring bearing. Spring bearing  64  may slide circumferentially on spring bearing shaft  66  joining an upper and lower portion of mandrel  30 . Spring bearing shaft  66  may be formed as a recess in mandrel  30 , or may be a specialized bearing joining an upper and lower portion of mandrel  30 . 
         [0070]    Alternately, referring still to  FIGS. 14 a  and 14 b   , in an embodiment, a bypass feature may be incorporated into a fixed pad plunger having a pad gap  90  oblique to mandrel axis  31  and having axial standoffs to position arcuate pads  12  both axially and circumferentially. In another embodiment, a bypass feature may be incorporated into a plunger having a guide wheel  80  integrating spring biasing with circularly guiding the arcuate pads in rotation. Advantageously, several design features may contribute to an improved lift plunger, and may include adding circumferential rotation not dependent on grooves or spirals embossed on the surface of the plunger, reducing rotational inertia by separating pad rotation from the mandrel, providing several means for maintaining correct pad edge spacing, and integrating spring biasing and circular guiding. 
         [0071]    In an embodiment, referring to  FIGS. 16 and 17 , a lift plunger  10  may comprise two sets of arcuate pads  12  stacked axially in a dual pad plunger arrangement for use in lifting fluids to the surface in a hydrocarbon well, such as a well for oil and gas. Each set of arcuate pads  12  may be disposed circumferentially around the mandrel  30  ( FIG. 18 ) such that wear surfaces  20  cooperate to form a substantially contiguous cylindrical seal against the inner surface of the tubing string  16  (not shown). Pad gap  90  between pad side edges  26  may provide a channel for fluids to flow between the pad lower end and the pad upper end. The channel formed by pad gap  90  may be oblique to the mandrel axis  31  (not shown) by a pad slant angle  92  (not shown) of less than approximately 35 degrees, thereby forming arcuate pad  12  in the shape of a rhombus and encouraging rotation of the arcuate pads. Alternatively, in an embodiment, the channel formed by pad gap  90  may be parallel to the mandrel axis, resulting in a rectangular shape for arcuate pad  12  and not encouraging rotation. For example, referring to  FIG. 16 , pad gap  90  may be disposed in a slight clockwise slant, going from top to bottom, or, referring to  FIG. 17 , disposed in a counterclockwise slant, both resulting in a rhombus shape and encouraging a rotation of arcuate pads  12 . 
         [0072]    Continuing with  FIGS. 16 and 17 , in an embodiment, top end  34  and a bottom end  32  may cap mandrel  30 . Top end  34  may also be known as a catching neck for retrieving the plunger from a well. The distal end of end collars  51  may be adjacent to top end  34  and bottom end  32 , and may be shaped as a cylindrical sleeve. The central end of end collars  51  may provide a collar lip  52  for enclosing a pad upper end  24  (not shown) or a pad lower end  22  (not shown) for limiting outward radial movement of the arcuate pad  12 . A central collar  53  may be a cylindrical sleeve providing a collar lip  52  on both ends for enclosing pad upper end  24  of one pad set and pad lower end  22  of the adjacent pad set, thereby limiting outward radial movement of arcuate pad  12 . Alternatively, an annular lip may be disposed directly on the circumferential surface  36  (not shown) of mandrel  30  (not shown) to enclose pad upper end  24  and pad lower end  22 . Bottom end cavity  38  may be formed in bottom end  32  in order to provide greater gas lift. 
         [0073]    Continuing with  FIGS. 16 and 17 , in an embodiment, finger ring  47  (partially shown) may bear rotatably on circumferential surface  36  near the distal ends of mandrel  30  and may project fingers axially into pad cutout  28  for retaining arcuate pad  12  while allowing radial movement under the outward biasing of one or more springs  60  (not shown) disposed between the underside of arcuate pad  12  and mandrel  30 . Finger ring  47  may lie beneath end collar  51  and may be restrained axially outwardly by top end  34  or bottom end  32 . Together, finger ring  47  and one of mandrel ends  34  or  32  may provide a circular guide substantially restricting the motion of the arcuate pad to be circumferential around the mandrel axis, thereby enabling a lift plunger  10  with rotatable pads. In an embodiment not shown, there may be only one pad set, and finger rings  47  and end collars  51  may be sufficient to enable rotation of the pad set. Alternately, an element  47  may project into pad cutout  28  and be fixed on mandrel  30  for retaining arcuate pad  28  without providing rotation. 
         [0074]    Continuing with  FIGS. 16 and 17 , in an embodiment, a plunger comprising multiple pad sets may comprise a central finger ring  47  (partially shown) bearing rotatably on circumferential surface  36  between two adjacent pad sets and may project fingers axially into pad cutout  28  for retaining arcuate pad  12  of one pad set while allowing radial movement. A plunger having multiple pad sets may further comprise a clocking ring  43  (partially shown) mating with the central finger ring  47  such that they rotate together under central collar  53 , the clocking ring  43  bearing rotatably on circumferential surface  36  and projecting fingers into the pad cutouts of the adjacent pad set. The two pad sets may be located axially by the sandwiching of finger ring  47  mated adjacently to clocking ring  43 . Central finger ring  47  and clocking ring  43  may each, in combination with pad cutout  28 , provide a circular guide for arcuate pads  12 , thereby substantially restricting the motion of the arcuate pads to be circumferential around the mandrel axis, thereby enabling a lift plunger  10  with rotatable pads. For the dual pad plunger shown in  FIGS. 16 and 17 , there may be four circular guides comprising three finger rings  47  and one clocking ring  43 , each circular guide engaging pad cutouts  28  functioning as complementary receptacles, the circular guides and complementary receptacles retaining two pads sets while allowing rotation of arcuate pads  12  around mandrel  30 . 
         [0075]    Continuing, in an embodiment, central finger ring  47  and clocking ring  43  may mate through one or more tongue and groove joints and may thereby rotate together, clocking adjacent pad sets at a fixed angular offset for restricting fluid slippage axially though pad gap  90 . For example, as shown in  FIGS. 16 and 17 , two adjacent pad sets may be offset circumferentially by an angular offset of approximately 45 degrees. Alternately, other joints may be used, or the central finger ring and clocking ring may be formed as one piece. In another embodiment not shown, central finger ring  47  and clocking ring  43  may slide against each other and not maintain any particular angular offset. For a plunger having 3 pad sets, there may be two clocking pairs of a central finger ring  47  and a clocking ring  43 , where all three pad sets rotate together around the mandrel, and where each pair of adjacent pad sets may have a particular angular offset. For example, each pad set in a triple-pad plunger may be clocked at 0, 30, and 60 degrees, or may be clocked at 0, 45, and 90 degrees. 
         [0076]    Alternately, in an embodiment not shown, elements  47  and  43  may project fingers into pad cutouts  28  and be fixed on mandrel  30  for retaining arcuate pad  28  without providing pad rotation. Elements  47  and  43  may be fixed to mandrel  30  and have fingers projected axially into pad cutouts  28  such that there is an angular offset of 0 degrees, 45 degrees, or another angular offset amount. Although, in an embodiment not shown, arcuate pads  12  may not rotate around mandrel  30 , the entire lift plunger  10  may rotate within the tubing string of the well by incorporation of rhombus-shaped arcuate pads, by the incorporation of spiral grooves in at least one of a top end and a bottom end, or by incorporating both rhombus pads and spiral grooves. 
         [0077]    Central finger ring  47  may be identical to the finger rings  47  used near the distal ends of mandrel  30  in order to reduce the unique parts count, or may be uniquely optimized for mating or spacing requirements in the central region. Central collar  53  may preferably be longer than end collar  51  in order to sleeve both finger ring  47  and clocking ring  43 . At least two arcuate pads  12  may comprise a pad set. Preferably, four arcuate pads  12  may comprise a pad set. However, other combinations, such as three, five, or six or more pads may be used to accommodate different manufacturing and application requirements, such as using a larger number of pads to accommodate a highly deviated well. Advantageously, providing a lift plunger  10  with rotation of the arcuate pads has the benefit of overcoming one-sided wear, avoiding the developing of wearing stripes on wear surfaces  20 , maintaining superior sealing against the inner surface of the tubing string, especially in a highly deviated well, and prolonging the life of the plunger. 
         [0078]    In an embodiment, referring now to  FIG. 18  through  FIG. 24 , a lift plunger  10  may comprise two sets of four arcuate pads  12  stacked axially and disposed circumferentially around the mandrel  30  such that wear surfaces  20  cooperate to form a substantially contiguous cylindrical seal against the inner surface of the tubing string  16  (not shown). Top end  34  and bottom end  32  may cap mandrel  30  and attach via mandrel threads  30   a . Finger ring  47  may bear rotatably on circumferential surface  36  near the distal ends of mandrel  30  and may project axial finger  47   a  axially into pad cutout  28  for retaining arcuate pad  12  while allowing radial movement under the outward biasing of one or more springs  60 . Finger ring  47  may be restrained axially outwardly by top end  34  or bottom end  32 , and the combination of finger ring  47 , mandrel end  32  or  34 , and pad cutout  28  may form a circular guide for enabling rotatable pads by substantially restricting the motion of the arcuate pad to be circumferential around the mandrel axis. Spring  60  may be disposed between spring cavity  62  in the underside of arcuate pad  12  and mandrel  30 , and there may be two springs per arcuate pad  12 . Alternately, two or more arcuate pads  12  may comprise a pad set. Continuing with  FIGS. 18-24 , in an embodiment, central finger ring  47  may bear rotatably on circumferential surface  36  between two adjacent pad sets and may project axial finger  47   a  axially into pad cutout  28  for retaining arcuate pad  12  of one pad set while allowing radial movement of arcuate pad  12 . Clocking ring  43  may mate with central finger ring  47  such that they rotate together under central collar  53 , the clocking ring  43  bearing directly on circumferential surface  36  and projecting axial fingers  47   a  axially into pad cutouts  28  of the adjacent pad set. The two pad sets may be located axially by the sandwiching of finger ring  47  mated adjacently to clocking ring  43 . Central finger ring  47  and clocking ring  43  may each, in combination with pad cutout  28 , provide a circular guide for arcuate pads  12 , and may thereby enable rotatable pads by substantially restricting the motion of the arcuate pad to be circumferential around the mandrel axis. Bottom end cavity  38  may be formed in bottom end  32  in order to provide greater gas lift. 
         [0079]    Referring still to  FIGS. 18-24 , in an embodiment, the distal end of end collars  51  may be adjacent to top end  34  and bottom end  32 , and may be shaped as a cylindrical sleeve enclosing finger ring  47 . The central end of end collars  51  may provide a collar lip  52  for enclosing a pad upper end  24  or a pad lower end  22  for limiting outward radial movement of the arcuate pad  12 . A central collar  53  may be a cylindrical sleeve enclosing clocking ring  43  and central finger ring  47  and may provide a collar lip  52  on both ends for enclosing pad upper end  24  of one pad set and pad lower end  22  of the adjacent pad set, thereby limiting outward radial movement of arcuate pad  12 . Alternatively, an annular lip (not shown) may be disposed directly on the circumferential surface  36  of mandrel  30  to enclose pad upper end  24  or pad lower end  22 . 
         [0080]    Continuing with  FIGS. 18-24 , in an embodiment, finger ring  47  and clocking ring  43  may contain ring bearing surface  47   b  rotatably bearing on circumferential surface  36  of mandrel  30 . Finger ring  47  may include dado groove  43   b  mating with dado-tongue  43   a  of clocking ring  43 , and may thereby cause finger ring  47  and clocking ring  43  to rotate together, clocking adjacent pad sets at a fixed angular offset for restricting fluid slippage axially though pad gap  90  (not shown). For example, as shown in  FIG. 18 , two adjacent pad sets may be offset circumferentially by an angular offset of approximately 45 degrees. Alternately, offsets of 0, 30, or 60 degrees may be chosen. Additionally, joints other than tongue-and-groove may be used, and the central finger ring  47  and clocking ring  43  may be formed as one piece. 
         [0081]    With reference to  FIGS. 18-25 , in an embodiment, a fluid bypass (not shown) for conducting fluid axially through a central portion of the mandrel may be included, as shown in  FIGS. 14 a  and 14 b   . Alternately, a plurality of spiral grooves (not shown) in at least one of a top end and a bottom end of the mandrel may be included for encouraging rotation of the plunger traveling in a fluid, as shown in  FIG. 13 . Further, the channel formed by pad gap  90  (not shown) may be oblique to the mandrel axis  31  by a pad slant angle  92  of less than approximately 35 degrees, thereby forming arcuate pad  12  in the shape of a rhombus and encouraging rotation of the arcuate pads, as shown in  FIGS. 16, 17, and 25 .  FIG. 25 a    illustrates a perspective view of an arcuate pad having a rhombus shape.  FIG. 25 b    illustrates a perspective view of the arcuate pad of  FIG. 25 a    in which there is a view of a pair of spring cavities.  FIG. 25 c    illustrates a bottom view of the arcuate pad of  FIGS. 25 a  and 25 b    in which there is a view of the pair of spring cavities. 
         [0082]    Referring to  FIG. 23 , in an embodiment, lift plunger  10  may be assembled in a slide-on manner, starting with attaching a mandrel end  32  or  34 , adding finger ring  47  and end collar  51 , and may continue with sliding on a first set of arcuate pads  12  and biasing springs  60 , followed by adding clocking ring  43  and central finger ring  47  plus central collar  53 , and may proceed with positioning a second set of arcuate pads and biasing springs  60 , adding finger ring  47  and end collar  51 , and may end with attaching the other mandrel end  34  or  32 , respectively. 
         [0083]    Advantageously, providing a lift plunger  10  with rotatable arcuate pads may have the benefit of overcoming one-sided wear, avoiding the developing of wearing stripes on wear surfaces  20 , maintaining superior sealing against the inner surface of the tubing string, especially in a highly deviated well, and prolonging the life of the plunger. 
         [0084]    Although the above embodiments have been described in language that is specific to certain structures, elements, compositions, and methodological steps, it is to be understood that the technology defined in the appended claims is not necessarily limited to the specific structures, elements, compositions and/or steps described. Rather, the specific aspects and steps are described as forms of implementing the claimed technology. Since many embodiments of the technology can be practiced without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.