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FIELD OF THE INVENTION 
       [0001]    The invention disclosed herein relates to improvements in plungers used in gas/fluid lift systems in wells producing both fluids and gases, such as petroleum and natural gas, under variable pressures. More specifically, the present invention is concerned with a pad subassembly of a particular configuration for sealingly and slidingly engaging a plunger within the well tubulars. 
       BACKGROUND OF THE INVENTION 
       [0002]    Petroleum and natural gas producing wells typically employ a plunger disposed within tubing of the well. The plunger provides lift to liquids accumulated above the plunger in the wellbore, powered by gas and pressures below the plunger from formations in the earth which are in communication with the lower part of the well, below the plunger, relying on variable fluid pressures within the well-bore, above and below the plunger. The well-bore is typically lined with tubular materials of relatively uniform internal surface diameter, but operators expect the internal passageway of the tubular to be somewhat uneven or imperfect. It is optimal if the gap between the outer sides of the plunger and the inner surface of the tubular is kept small, as this will make the lift system operate more efficiently, as less pressure and fluid from beneath will bypass the plunger, and less fluid above the plunger can drop below, past the plunger. In essence, it would be ideal to have a plunger which was perfectly sealed to the tubular but moved frictionlessly along its length in either direction, powered by fluid pressure variations above and below the plunger (at least on the up-stroke lift portion of the plunger system&#39;s cycle). It is also useful to have replaceable surfaces on the outer sides of the plunger as that surface will wear from contact with the tubular&#39;s inner wall; an outer surface of different materials from the plunger&#39;s body may also be advantageous as different materials can be used to provide different structural, mass and density, permeability, chemical reactivity, formability or machineability, resilience, tooling, frictional, or wear or other characteristics as required for manufacturing, operation, assembly, repair, or function in place of different parts of the plunger. 
         [0003]    In the prior art, a variety of mechanical plungers for use in gas-lift systems for production of fluids from wells have been disclosed or are known. Each has disadvantages. Some examples follow: 
         [0004]    U.S. Pat. No. 6,725,916 to Gray et al. (“Gray”) discloses a plunger with a system of floating, spring-loaded pads between a plunger&#39;s body and the tubular within which it operates, together with a novel seal and internal passage, with the aim of facilitating rapid descent of the plunger from its upper-most part of a stroke in its lift-cycle (by opening the inner passage at the top of the stroke, and reclosing it at the bottom). Gray provides a good example of state-of-the art pad systems. Gray&#39;s “jacket” comprises a series of interlocking pads held to the plunger&#39;s body but spring-loaded to bias outwardly toward the tubular&#39;s walls. The aspect in Gray&#39;s jacket which is relevant, is the provision of “labyrinthine passages” between the jacket&#39;s elements (the spaces between the pads), which in Gray are formed by the interlocking teeth of each pad with the adjacent pads—when the jacket (pads) is expanded, the spaces between the interlocking pads increases, providing larger and larger flow-paths for fluid communication past the plunger in the tubular during use. This is undesirable, and Gray has attempted to resolve the issue by making these passages between the interlocking pads “labyrinthine” or following a toothed, notched, or circuitous pathway. Notably, the notch-finger interlocking region between pads in Gray are also stepped and matched with a step in the extended end of each finger (and a mating void in the recess or notch into which the finger fits when assembled) which is stepped up and down in a direction radial to the linear centre of the plunger (to its longitudinal axis), while the rest of the adjacent pads&#39; mating surface edges are not stepped in that way. While providing some resistance to fluid flow past the plunger in the annulus between the plunger and the tubular, there is still a void and passageway for fluid communication with a large cross-section. 
         [0005]    US Patent Application 2012/0080196 by Laing (“Laing”) discloses a plunger lift and safety valve system with a variable outside diameter plunger where the diameter is variable by the retraction and expansion of pads deployed about the outer circumference of the plunger&#39;s body between the plunger and the tubular (when expanded) or between the plunger and a smaller-diameter (than the tubular) safety valve (when the pads are retracted). The pads are spring-biased toward the inner surface of the tubular from the plunger, and are interlocking with each other to permit them to radially expand and contract but to be firmly held linearly in position with the plunger (linearly along the direction of the plunger&#39;s longitudinal axis). The relevance of Laing as an example of prior art plunger pad systems is that the pads interlock and are biased outwardly by springs, but when expanded the spaces between the pads open up, providing a large cross-section (viewed longitudinally along the plunger&#39;s axis to a cross-section of the plunger and pads), the openings between the pads are the relevant flow-paths for fluid flow past the plunger, which is undesirable both in terms of efficiency of operation, as well as contamination of the plunger&#39;s working parts with materials produced with the hydrocarbon fluids in the well (debris, sand, silt, corrosive materials, etc). 
         [0006]    It is an object of the present invention to obviate or mitigate at least one disadvantage of previous related art. 
       SUMMARY OF THE INVENTION 
       [0007]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the drawings, like elements are assigned like reference numerals. The drawings are not necessarily to scale, with the emphasis instead placed upon the principles of the present invention. Additionally, each of the embodiments depicted are but one of a number of possible arrangements utilizing the fundamental concepts of the present invention. The drawings are briefly described as follows: 
           [0009]      FIG. 1  is a longitudinal drawing blending a cross-section and surface elevation of a preferred assembled plunger assembly of the invention; 
           [0010]      FIG. 2  shows a side elevation of the external surfaces of an exemplar of adjacent pads to show an aspect of their interlocking features; 
           [0011]      FIGS. 3 ,  4 , and  5  show cross-sections perpendicular to the longitudinal axis of a preferred plunger assembly, each figure with a slight variant of the pads&#39; overlap features, highlighting their approximately axial interfaces; 
           [0012]      FIGS. 6 and 7  show elevations in perspective of the inner surface and the outer surface of a preferred pad element of the invention; and 
           [0013]      FIGS. 8 and 9  show elevations in perspective of the plunger&#39;s body and a preferred retaining means for holding the pad elements of the invention in place. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    The present invention provides an improved plunger assembly for use in downhole tubulars in wells that produce fluids and/or gases under variable pressure. In particular, the present invention provides a subassembly of expandable interlocking pad elements radially arrayed around the plunger body for sealing an annulus between the plunger and its pads and the tubular within which the plunger is deployed as part of a gas-lift system. 
         [0015]    The present invention more specifically provides for an improved configuration of overlapped joints between adjacent pad elements which, when the pad elements are expanded within the tubular of the well, slidably sealing the plunger to the tubular. The pad elements are configured to overlap both longitudinally, by mating notch and finger joints or similar geometric arrangements, and radially, along the length of the gap or slot along the edge of each mating notch and finger by inner and outer surfaces radially spaced from the plunger&#39;s axis, such that one edge of a pad element will radially overlap the mating edge of an adjacent pad element. This can have the effect of reducing the available flowpath past the plunger along the seams between pad elements by reducing the cross-sectional surface area of the voids in those seams accessible to fluids in the annulus between the tubular and the plunger&#39;s outer surfaces when the pad elements are expanded. 
         [0016]    When describing the present invention, all terms not defined herein have their common art-recognized meanings. To the extent that the following description is of a specific embodiment or a particular use of the invention, it is intended to be illustrative only, and not limiting of the claimed invention, which should be given the broadest interpretation consistent with the description as a whole. 
         [0017]    Referring now to  FIG. 1 , therein illustrated is a preferred plunger assembly of the present invention generally designated  100 . The plunger assembly  100  comprises a plunger body  200  that can be composed of any rigid material, including any metal or metal alloy, rigid plastics and polymers, ceramics, etc., or any combinations thereof. 
         [0018]    As illustrated in both  FIGS. 1 and 8 , the body of the plunger  200  includes an elongated central mandrel  1  for support. The mandrel  1  can be substantially cylindrical in shape and in the form of an elongated rigid non-flexible solid rod. 
         [0019]    The plunger body  200  may include a fishing profile  2  that has a head or cap  4  located above a neck  3 , as depicted in the drawings. Also illustrated in the drawings is the bottom face  13  of the fishing profile. 
         [0020]    Referring now to  FIGS. 1-7 , depicted therein is the preferred pad subassembly  300  of the present invention. The pad subassembly  300  generally provides a system of expandable interlocking pad elements radially arrayed around the body of the plunger  200  for sealing the annulus between the plunger and the pad assembly and the tubular within which the plunger is deployed as part of a gas-lift system. In the preferred embodiment, four pad elements  14 ,  15 ,  16  and  17  can collectively form the pad subassembly  300 . While four pad elements are depicted, the pad subassembly may comprise alternate numbers of pad elements. The pad elements can be made of any relatively rigid material such as metal or metal alloys, rigid plastics or rubber, graphite, etc, or any combinations thereof. In a preferred embodiment, the pad elements may be composed of a different material from the plunger&#39;s body  200 , that may be advantageously used to provide different structural, mass and density, permeability, chemical reactivity, formability or machineability, resilience, tooling, frictional, or wear or other characteristics as required for manufacturing, operation, assembly, repair, or function, in different parts of the plunger. 
         [0021]    The plunger&#39;s pad elements  14 ,  15 ,  16 ,  17  are generally adapted to engage or interlock with each other, slidably held to the plunger body  200 , between the body and the inner surface of a tubular. The interlocking nature of the pad elements  14 ,  15 ,  16 ,  17  is both to permit the elements to move away from each other when the diameter of the assembly is expanded, and to move radially from the plunger when the diameter of the assembly is expanded. 
         [0022]    As illustrated in the drawings, the pad elements  14 ,  15 ,  16 ,  17  can be generally rectangular in shape. However, the elements may be a variety of geometric shapes, sizes, and dimensions. Further, in a preferred embodiment, the pad elements  14 ,  15 ,  16 ,  17  may have a convex or substantially convex outer surface with a concave or substantially concave inner surface. 
         [0023]    Referring now to  FIGS. 6 and 7  depicted therein are elevations in perspective of the inner and outer surfaces of a preferred embodiment of one of the pad elements  14 . As shown, a pad element  14  may have a generally rectangular shape having a substantially convex outer surface and a generally cylindrical inner surface. 
         [0024]    A pad element of the present invention may comprise a tabbed or protruding portion on a first side and a notched or slotted portion on a second side, with the tabbed or slotted portion being mutually engageable with the corresponding tabbed or slotted portion of an adjacent element, so as to minimize or prevent leakage from between the elements. 
         [0025]    As illustrated, in the preferred embodiment, the tabbed portion of the pad element  14  comprises an outer tongue  14   a  which can be defined by an upper side face  14   b  and a lower side face  14   c , and an inner tongue  14   v  which can be defined by an upper side face  14   y  and an inner end face  14   u . The inner tongue  14   v  may be stepped inward, such that the outer tongue  14   a  can be elevated from the inner tongue  14   v , and the inner tongue  14   v  may extend out from the outer tongue  14   a . Also, depicted in  FIG. 7  is a side skirt upper face  14   w  and a side skirt lower face  14   x  of a pad element  14 . The side skirt upper face  14   w  may be continuous with the inner tongue  14   v  and situated generally above and stepped inward from the outer tongue  14   a , such that the outer tongue  14   a  may be elevated from the side skirt upper face  14   w . The side skirt lower face  14   x  may be continuous with the inner tongue  14   v  and situated generally below and stepped inward from the outer tongue  14   a , such that the outer tongue  14   a  maybe elevated from the side skirt lower face  14   x.    
         [0026]    As illustrated in  FIGS. 6 and 7 , the notched portion of a pad element  14  may be defined by an upper side face  14   d  and lower side face  14   e . Also depicted on the inner surface, is an internal face of the notch  14   r , an internal face above the notch  14   s  and an internal face below the notch  14   t.    
         [0027]    Referring now to  FIGS. 2-5 , depicted therein are the preferred engaging or interlocking capabilities of the pad elements of the present invention.  FIGS. 3 ,  4 , and  5  particularly depict aspects of the overlap features of pad elements  14 ,  15 ,  16 ,  17 . 
         [0028]      FIG. 2  is a side elevation that depicts the external surfaces of adjacent pad elements  14 ,  15  to show an aspect of their interlocking features. As illustrated, the outer tongue  15   b  of a pad element  15  engages or interlocks with the notch portion of an adjacent pad element  14 . In particular,  FIG. 2  shows the mated external surfaces between the pad elements  14 ,  15  as the outer tongue  15   a  of the tabbed portion engages the notched portion. As depicted, the notch upper side face  14   d  of a pad element  14  may directly contact the outer tongue upper side face  15   b  of an adjacent pad element  15 . The outer tongue lower side face  15   c  may also come into direct contact with the notch lower side face  14   e .  FIG. 3  provides a bottom cross-sectional view of the pad subassembly  300  with each of its constituent pad elements  14 ,  15 ,  16 ,  17  as the pad subassembly  300  is expanded. 
         [0029]    Referring now to  FIG. 4 , illustrated therein is a top cross-sectional view of the pad subassembly  300  of the present invention, depicting particular overlap features of adjacent pad elements  14 ,  15 ,  16 ,  17 . More specifically, overlapping of the tabbed portion by an inner surface of the notched portion is illustrated. In the preferred embodiment depicted, there may be an overlap of the side skirt upper face  14   w ,  15   w ,  16   w ,  17   w  of a pad element by the internal face above the notch  14   s ,  15   s ,  16   s ,  17   s  of an adjacent pad element. For example, the internal face above the notch  14   s  of pad element  14  can overlap the side skirt upper face  15   w  of adjacent pad element  15 . Also, as can be seen, such an overlap could be maintained even as the pad subassembly  300  was further expanded. 
         [0030]    Referring now to  FIG. 5 , illustrated therein is a top cross-sectional view of the pad subassembly  300  of the present invention depicting a different aspect of the overlap features of adjacent pad elements  14 ,  15 ,  16 ,  17 . In particular, depicted therein is the overlap of the inner tongue outer face  14   v ,  15   v ,  16   v ,  17   v  of a pad element by the notch internal face  14   r ,  15   r ,  16   r ,  17   r  of an adjacent pad element. For example, the notch internal face  14   r  of pad element  14  can overlap the outer face of the inner tongue  15   v  of adjacent pad element  15 . Also, as can be seen, such overlap could be maintained even as the subassembly was further expanded. 
         [0031]    While it may be known to have the gaps between pad elements form a labyrinthine route for fluid to flow past the plunger, by defining the gaps between the pad elements by shaping notches and fingers or tabs in the overall shapes of the pad elements as interlocking “T” shapes, or other mating and moveable geometries, the present invention provides a second type of overlap, radially between adjacent pad elements, such that the pad elements slide apart with restricted flow paths linearly along the direction of the plunger body&#39;s axis being restricted by tight gaps between adjacent pad elements&#39; interfaces which can be perpendicular to the plunger body&#39;s axis which may not expand when the pad elements slide apart, but also providing a second slideable interface between adjacent pad elements which can overlap along a circumferential direction along a radial surface which can be within the radial depth of the pad elements, where a radially inner surface of a pad element along the gap between two pad elements can mate with a radially outer surface of an adjacent pad element along the same gap. 
         [0032]    The pad element subassembly  300  of the present invention can be biased outwardly for slidably engaging the well tubular, while providing an external seal against the interior of the tubulars. The pad element subassembly  300  has the largest diameter of the plunger assembly when it is in its most radially expanded position and sealingly engaging the tubular. The pad elements may be biased outwardly against the tubulars by built up internal pressure and/or springs. 
         [0033]    As illustrated in  FIGS. 1 and 8 , the pad subassembly of the preferred embodiment may have biasing means comprising springs  22 , such as a helically wound spring, coil spring, leaf spring or any other element which has the ability to rebound or recoil after being compressed. The springs  22  may be disposed between the pump assembly body  200  and the inner surface of pad elements  14 ,  15 ,  16 ,  17 . In a preferred embodiment, there are two springs  22  between each pad element  14 ,  15 ,  16 ,  17  preferably disposed at an upper end and lower end of each element. Recesses on the upper end  10  and the lower end  9  of the pump assembly  200  may accommodate and hold the spring in place. As depicted in  FIG. 6 , the underside of a pad element  14  may also comprise an upper spring recess  14   n  and a lower spring recess  14   o.    
         [0034]    The radial surface of the pad subassembly  300  may be either parallel to the outer surface of the plunger body, or may be sloped with relation to a circumferential theoretical surface within the plungers&#39; body thickness, and if sloped, could provide a further biasing force to assist or perhaps replace some or all of the radially expanding forces typically provided by springs or other similar mechanisms (hydraulic or mechanical) between the plunger body and any or each pad element to bias the pad(s) to expand to meet the tubular. 
         [0035]    Referring back to  FIG. 1 , the plunger assembly  100  of the present invention comprises a pad subassembly  300  disposed about the plunger body  200  that is preferably held in place by retaining means, such as an upper and a lower retaining ring  18 .  FIG. 9  specifically depicts the preferred retaining means for holding the pad elements of the present invention in place. Retaining rings  18  may be substantially cylindrical in shape having a hollow inner surface of slightly larger diameter than the plunger body  200  with a shape that can correspond with that of the plunger body  200 . 
         [0036]    In a particularly preferred embodiment, the leading and/or trailing edge at the upper and/or lower end of the pad elements  14 ,  15 ,  16 ,  17  may be skirted to slide and seal with a retaining ring in the assembled plunger assembly  300 , which may improve their seal to the plunger body  200 . 
         [0037]    As illustrated in  FIGS. 6 and 7 , the upper and the lower end of a pad element can comprise a corresponding upper and lower end skirt. In the preferred embodiment depicted, the upper end skirt has a notch side  14   g  and a tongue side  14   h , whereas the lower end skirt can also comprise a notch side  14   j  and a tongue side  14   g . The pad elements of the preferred embodiment may also comprise an upper tab  14   f  and a lower tab  14   i  which may be stepped outward from the skirt. An upper tab  14   f  may also extend generally upwards from the skirt while a lower tab  14   i  may also extend generally downwards from the skirt. 
         [0038]    Referring back to  FIG. 9 , a retaining ring  18  may further comprise a plurality of end skirt recesses  18   b  for overlapping with the notch side  14   g ,  15   g ,  16   g ,  17   g  and the tongue side  14   h ,  15   h ,  16   h ,  17   h  of the upper end skirt of a pad element, with respect to an upper retaining ring  18 , or the notch side  14   j ,  15   j ,  16   j ,  17   j  and the tongue side  14   k ,  15   k ,  16   k ,  17   k  of the lower end skirt of a pad element, with respect to a lower retaining ring  18 . The retaining ring  18  may further comprise a plurality of pad tab recesses  18   a  that overlap a pad element upper tab  14   f ,  15   f ,  16   f ,  17   f , with respect to an upper retaining ring  18 , or a pad element lower tab  14   i ,  15   i ,  16   i ,  17   i , with respect to a lower retaining ring  18 . Also depicted is a thru bore  18   c  for a locking pin  19  for securing the retaining rings  18 . As illustrated in  FIG. 8 , the plunger body comprises a corresponding thru bore for a lower retaining ring locking pin  6   a  and a thru bore for the upper retaining ring locking pin  6   b.    
         [0039]    In a preferred embodiment, the underside or inner surface of the pad element further comprises an upper and a lower rib stepped inwardly from the skirt at the upper and lower ends of the pad element, respectively. As illustrated in  FIGS. 6 and 7 , a pad element  14  can comprise an upper internal rib  14   l  and lower internal rib  14   p  that protrude radially inwardly toward the body  200  of the plunger. The internal ribs  14   l ,  14   p  of the preferred embodiment may further comprise a tabbed portion extending vertically from the ribs  14   l ,  14   p . As illustrated in  FIGS. 6 and 7 , an upper tab internal rib  14   m  is depicted extending substantially vertically and upwards from the upper internal rib  14   l . A lower tab internal rib  14   q  is also depicted extending substantially vertically and downwards from the lower internal rib  14   p.    
         [0040]    The plunger assembly of the present invention may further comprise a bottom sub  21  on the bottom end of the plunger body  200 , as illustrated in  FIG. 1 . The bottom sub  21  may have tapered end. Referring also to  FIG. 8 , illustrated therein is a threaded connection  5  a bottom sub locking pin  23  and thru bore for the locking pin  6 . 
         [0041]    In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments of the invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the invention. 
         [0042]    The above-described embodiments of the invention are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto. 
       LEGEND FOR DRAWINGS 
       [0043]    Legend for  FIGS. 1-9 
     1 . Mandrel     2 . Fishing Profile (contains  3  and  4 )     3 . Fishing Neck     4 . Fishing Head or Cap     5 . Bottom Threaded Connection     6 . Thru Bore for Bottom Sub Locking Pin
         6   a . Thru Bore for lower Retaining Ring locking pin     6   b . Thru bore for upper Retaining Ring locking pin         7 . Lower Tab ( 14 Q) recess     8 . Lower pad internal rib ( 14   p ) recess     9 . Pad coil spring recess lower end     10 . Pad coil spring recess upper end     11 . Upper pad internal rib ( 14 L) recess     12 . Upper pad tab ( 14   m ) recess     13 . Fishing Profile bottom face     14 . Plunger Pad Element
       a. Outer tongue   b. Outer tongue upper side face   c. Outer tongue lower side face   d. Notch upper side face   e. Notch lower side face   f. Pad upper tab outer surface   g. Upper end skirt notch side   h. Upper end skirt tongue side   i. Pad lower tab outer surface   j. Lower end skirt notch side   k. Lower end skirt tongue side   l. Upper internal rib   m. Upper tab internal rib   n. Upper coil spring recess   o. Lower coil spring recess   P. Lower internal rib   q. Lower tab internal rib   r. Notch internal face   s. Internal face above notch   t. Internal face below notch   u. Inner tongue end face   v. Inner tongue outer face   w. Side skirt upper face   x. Side skirt lower face   y. Inner tongue upper side face         15 . Plunger Pad Element
       a. a thru y same as for  14           16 . Plunger Pad Element
       a. a thru y same as for  14           17 . Plunger Pad Element
       a. a thru y same as for  14           18 . Retaining Ring
       a. Pad tab recess (overlaps  14   f  or  14   i )   b. End skirt recess (overlaps  14   j  &amp;  14   k  or  14   g  &amp;  14   h )   c. Thru bore for locking pin         19 . Locking pin for retaining ring ( 18 )     20 . Lock ring for bottom sub locking pin ( 23 )
       a. Lock ring access hole for locking pin   b. Lock ring interior surface         21 . Bottom sub     22 . Pad coil spring     23 . Bottom sub locking pin     24 . Pad internal face     25 . Mandrel face     26 . Bottom sub thru bore for locking pin

Summary:
An improved plunger with expandable mating pad elements arrayed circumferentially about the plunger&#39;s body sealed to the plunger and biased to expand the plunger assembly&#39;s outer circumferential surface toward the inner surface of the tubular within which the plunger assembly is designed to operate. The gaps between the pad elements are minimized by having the adjacent pad elements slidable against each other along two sets of surfaces along essentially the length of the interface between adjacent pad elements, one surface set being approximately axial to the plunger and the other set being approximately radial to the plunger&#39;s longitudinal axis, and in this way reducing the available pathway in the gap between adjacent pad elements for fluid to bypass the plunger assembly.