A centralizer apparatus for centralizing tubular casing in a borehole having two stop collars for securing to the casing in a spaced apart relationship and two moving collars, one slidably and rotatably coupled to each stop collar, a plurality of bow springs, each bow spring having a first end spaced apart from a second end, each bow spring biased outwardly from the center axis, the first ends connected to the first moving collar, the second ends connected to the second moving collar.

BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention is directed to casing or tubular centralizers with flexible bow springs for use in bore bole drilling operations, and particularly to centralizers that can be radially compressed to fit within a small annular space and which can later expand to center casing within the bore hole.  
           [0003]    2. Description of Related Art  
           [0004]    Casing is tubular pipe used to line a bore hole that is drilled to recover naturally occurring oil or gas deposits. A joint of casing is typically about 36 to 44 feet in length, and generally has threaded connections at each end to facilitate coupling to adjacent joints of casing to form a casing string. Casing prevents drilled bore holes from washing out or caving in during subsequent drilling and completion operations. Casing strings are cemented into place in the bore hole by circulating cement from the surface down the interior of the casing string, and then displacing the cement into the annular space between the exterior surface of the casing string and the wall of the bore hole. It is important that the casing string be positioned prior to cementing as closely as possible to the center of the bore hole in order to ensure full circumferential cement placement around the casing and to thereby effectively isolate and seal off penetrated geologic formations and to prevent unwanted fluid flow. It is desirable to install casing periodically to maintain the bore hole, and the length of a casing string that can be effectively installed and cemented into place in the bore hole is limited. Progressively smaller diameter casing strings are lowered into the bore hole through the interior of previously installed and cemented casing. Consequently, progressively smaller diameter casing is installed as the depth of the drilled bore hole increases.  
           [0005]    It is advantageous to case the bore hole adjacent to producible hydrocarbon zones with casing of a reasonably large diameter to facilitate completion and production operations. This advantage is achievable using bi-centered drill bits. Bi-centered drilling bits are drilling bits that can be used to drill a bore hole of a size larger than the inside diameter of cemented casing through which the drilling bit passes. For example, FIG. 1A illustrates a bore hole having casing  90  with 13⅜ inch outside diameter and 12¼ inch inside diameter. Using conventional drilling bits, the bore hole below the 12¼ inch I.D. casing  90  can be drilled only to 12¼ inch in diameter  71 , and the largest standard size of casing that can be effectively installed and cemented in the bore hole is 9⅝ inch O.D. casing  82 . However, using a bi-centered bit allows drilling the bore hole beneath the end of the 12¼ inch I.D. casing  90  to 14¾ inch diameter  72  as shown in FIG. 1B. This advance in drill bit technology allows 11⅞ inch O.D. casing  80  to be effectively cemented in place beneath the 12¼ inch I.D. casing  90  as an annulus  75  comparable to the annulus between the 9⅝ O.D. inch casing and the 12¼ O.D. inch bore hole in FIG. 1A is obtained around the 11⅞ inch O.D. casing in FIG. 1B.  
           [0006]    The bore hole is easier to drill and complete if larger diameter tools can be used. As wells are drilled to deeper depths, and as more high angle and horizontal wells are drilled, the number of discrete sizes of casing used in the casing system increases, and the available annular clearance between adjacent strings of casing necessarily decreases. For example, referring again to FIG. 1B, if a bi-centered bit is used to drill the bore hole below a string of 12¼ inch I.D. casing  90  to 14¾ inch, and if 11⅞ inch O.D. casing  80  is lowered through the 12¼ inch I.D. casing  90  and into the 14¾ inch diameter  72  bore hole, the thickness of the annular clearance  74  between the inside of the (fixed) 12¼ inch casing  90  and the outside of the 11⅞ inch O.D. casing  80  is {fraction (3/16)} inch. The use of bi-centered bits gives rise to the need for a centralizer that substantially radially collapses to fit within the thin {fraction (3/16)} inch annular space  74 , and that can later re-deploy to center the 11⅞ inch O.D. casing  80  in a 14¾ inch diameter  72  bore hole. However, as the thickness of the annular clearance  74  becomes very small, it becomes difficult to obtain optimal pre-cement centralization of casing  80  using conventional bow-spring centralizers because they require excessive annular clearance  74  between adjacent casing strings  90  and  80 .  
           [0007]    Bow-spring centralizers are used to center casing inside a drilled borehole in order to obtain uniform annular placement of cement in the casing/bore hole annulus  75 . Bow springs extend radially outwardly from the center bore of the centralizer to provide desirable casing stand-off from the wall  76  of the bore hole. Wide-deployment centralizers are centralizers designed to provide substantial stand-off from a nearby object such as the wall of a bore hole. For example, centering a 11⅞ inch O.D. casing within a 14¾ inch diameter bore hole requires radial stand-off of about 1{fraction (7/16)} inch. By comparison, a limited deployment centralizer may be used to center a 4½ inch O.D. diameter casing within a 5½ inch diameter bore hole, and requires only a radial stand-off of about 0.5 inch. While a limited stand-off centralizer may have bow springs that are sufficiently stiff to resist radially outward collapse upon being pushed into or through a restriction, most wide-deployment centralizers have bow springs that are flexible, and the leading ends of the bow springs must be secured against the casing in order to prevent radially outward collapse of the centralizer ribs. As shown in FIG. 2A and FIG. 2B, bow-spring centralizers are secured to the exterior of the casing and centralizer ribs project radially outwardly therefrom. In order to provide optimal centralization of casing installed at lower depths in the well, it is essential that centralizers be resiliently collapsible to fit within the annular space between the exterior surface of the casing being installed and the interior surface of the larger, installed casing. It is also desirable that the centralizer ribs  42  collapse radially inwardly to achieve a minimum annular clearance, and that the centralizer does not prevent the flow of drilling fluid through the annulus between the smaller and larger casing.  
           [0008]    Another factor to be considered in designing a low annular clearance centralizer is the prevention of centralizer rib damage. No wellbore is perfectly vertical and uniform. As casing is lowered into a well, the high strength steel resiliently twists, turns and flexes as it passes through restrictions and bends in the non-linear and non-uniform bore hole. It is important to prevent damage to centralizer ribs by securing the centralizer to the casing in a manner that allows free rotation of the centralizer relative to the casing to which it is secured.  
           [0009]    Another factor to be considered in designing low annular clearance centralizers is related to securing the centralizer in place on the exterior of casing that is to be lowered into the well. As the bow springs radially inwardly collapse, the ends of each bow spring must longitudinally separate one from the other. Longitudinal elongation of the centralizer requires that at least one of the collars to which the end of bow springs are secured must remain longitudinally movable relative to the casing on which the centralizer is secured. If the other collar is secured to the casing, then the centralizer will be unidirectional; that is, the centralizer ribs will collapse only when the centralizer passes through a restriction in one direction, and the centralizer ribs will not collapse and pass through the larger casing unless the collar that is secured to the casing is the leading collar to first enter the larger, installed casing. However, a centralizer needs to be bi-directional in order to allow casing to be reciprocated or withdrawn from the bore hole if problems arise during casing installation, and this requires that collars at each end of the bow springs be longitudinally movable relative to the casing.  
           [0010]    One attempt to provide a bi-directional centralizer involves the fixed placement of a stop collar longitudinally between sliding end collars secured to the ends of the bow springs, and securing the stop collar to the exterior of the casing to be centralized in the bore hole. This configuration provides the desirable bi-directional function of the centralizer because the centralizer will slide along the casing as the ribs resist collapse and entry into the larger casing until the leading end collar abuts against the stop collar. The ribs will then flatten as they enter the opening or restriction, and the longitudinal elongation of the centralizer slidably displaces the trailing end collar in a direction away from the leading end collar abutted against the stop collar.  
           [0011]    The problem with locating a stop collar longitudinally between sliding end collars is that the thickness of the stop collar prevents the centralizer ribs from completely radially inwardly collapsing to lie flat along the length of the casing to which the centralizer is slidably secured. This design causes the stop collar to consume valuable annular space and thereby prevents optimal sizing of subsequent casing.  
           [0012]    Attempts have been made to develop low annular clearance centralizers; that is, centralizers that collapse radially inwardly to fit within a thin annulus. U.S. Pat. No. 5,575,333 discloses a bow spring centralizer that integrates the bow springs into a specially manufactured thin-walled tubular member, or sub, that threadably couples at each end to standard joints of casing. These tubular members have very thin-walls of high strength material. A problem with the centralizer disclosed in the '333 patent is that the centralizer ribs are not freely rotatable about the joint of casing to which the centralizer is secured and are, therefore, subject to breakage and damage as the casing is lowered into the bore hole. Broken ribs may cause the casing to be cemented off-center in the bore hole, thereby greatly increasing the likelihood of fluid flow behind casing, casing failure and loss of productivity of the well. Also, broken centralizer ribs may obstruct the bore hole and prevent installation of casing on completion of the well. Another problem with the centralizer disclosed in the '333 patent is that the overall mechanical integrity of the resulting non-uniform casing string is compromised by the inclusion of the non-standard, thin-walled subs. It is desirable to use casing that conforms to standards promulgated by the American Petroleum Institute (API), and the necessity of installing thin-walled subs requires frequent interruptions in drilling and completion operations. Another problem with the centralizer disclosed in the '333 patent is that the use of centralizing subs inhibits the placement of centralizers in close proximity one to the other without specially fabricated subs made to accommodate two centralizers in close proximity or specially fabricated subs of varying lengths. What is needed is a low annular-clearance bi-directional bow-spring centralizer that can be secured to the exterior surface of a joint of standard casing, radially compressed to its annular configuration, and passed along with the casing through the interior of a slightly larger diameter casing, and later radially outwardly deployed within the lower, uncased bore hole to centralize the casing. What is needed is a centralizer that radially collapses to fit within a very thin annular space, and one that still permits sufficient flow of drilling fluid through the annular space between the smaller and the larger casing to reduce risks of swabbing or surging the well as casing is run into or withdrawn from the bore hole.  
         SUMMARY OF THE PRESENT INVENTION  
         [0013]    One embodiment of the present invention provides a bi-directional low annular clearance bow-spring centralizer having, at each end, a displacement assembly comprising a moving collar and a stop collar, and a longitudinal bore therethrough to accommodate the casing to which the centralizer is secured. Each moving collar has a collet with a radially outwardly flanged portion thereon that is movably received within a circumferential groove or bore within its mating stop collar. A plurality of flexible bow springs, each having a first end and a second end, are secured at each end to a moving collar, and the two moving collars are maintained in a variable spaced-apart relationship by the bow springs. The variance in the longitudinal distance between the two opposing moving collars is determined by the configuration of the bow springs. The longitudinal distance between the moving collars is smallest when the bow springs are in their radially outwardly deployed configuration as shown in FIG. 2A and FIG. 2B, and the longitudinal distance between the moving collars is greatest when the bow springs are radially inwardly collapsed so as to be substantially flattened along the exterior length of the casing to which the centralizer is secured. The position of the flanged portion of each moving collar within the circumferential groove or bore of its mating stop collar is determined by the configuration of the bow springs and by the mechanical interaction between the leading end of centralizer and the larger, cemented and installed casing string through which the centralizer passes. One embodiment of the present invention provides for each stop collar adapted for having three distinct internal bores of differing diameters: the securing bore, the receiving bore and the reciprocation bore, from smallest to largest. The securing bore is sized for securing the centralizer to the exterior circumferential surface of the casing to be centered in the bore hole using the centralizer. The receiving bore is sized to slidably receive the exterior generally cylindrical surface of the collet of the mating moving collar. The larger diameter reciprocation bore is disposed between the securing bore and the receiving bore, and is adapted to accommodate longitudinal reciprocation of a radially flanged portion on the collet of the mating moving collar.  
           [0014]    One embodiment of the present invention provides for each moving collar being longitudinally movable relative to its mating stop collar to accommodate longitudinal displacement of the ends of the bow springs. The range of longitudinal movement of the moving collar within its mating stop collar is limited in one direction by a stop wall in the reciprocation bore and in the other direction by the collapsed and flattened length of the bow springs. Each bow spring is radially outwardly biased towards its deployed configuration, bowed away from the center axis of the casing to which the centralizer is secured. The stop collars are secured to the casing at a distance one from the other such that, when the bow springs are in their fully deployed configuration, the radially flanged portion on the collet of the moving collar is in its extreme inboard position abutted against the stop wall of the reciprocation bore of the stop collar. The bow springs collapse toward the longitudinal center of the centralizer as the bow springs are forced radially inwardly by contact with the surface of a bore hole or with the interior wall of larger casing. In the preferred embodiment, the bow springs flexibly collapse to lie substantially flat along the exterior longitudinal length of the casing to which the centralizer is secured.  
           [0015]    In the preferred embodiment, each end of each bow spring is secured to a moving collar, and each moving collar is rotatably and slidably coupled to its mating stop collar, so that each end of each bow spring is longitudinally movable relative to the casing to which the centralizer is secured. The stop collars of the centralizer may be secured to the external surface of the casing using a variety of securing means. The stop collars may be heat shrunk onto the casing, adhered to the casing using epoxies or other adhesives, or secured in place by using welding or using connective pins or set screws. Alternately, stop collars may be made in two or more angular pieces, emplaced together to substantially encircle the casing and secured together to form a collar by welding, adhesives, epoxies, or by using connective pins or set screws.  
           [0016]    The present invention may be adapted for centering down hole tools that are lowered into the well using a wireline. Many wireline tools need to be centered in the bore hole for optimal performance, and are lowered into the bore hole through restrictions such as tubulars or bore hole deviations. The present invention provides for centralization of wireline tools while maintaining down hole access by wireline and through instructions.  
           [0017]    Optionally, the centralizer of the present invention may be used to position an elongate body within a bore hole at a place other than at the center. For example, the centralizer of the present invention may provide only one radially outwardly extending flexible rib for biasing a tool, such as a survey tool, perforating gun, resistivity through-casing tool or other wire line tool, against the interior longitudinal surface of the casing or bore hole in which the tool is disposed. In this application, the present invention may include a device for decentralizing or biasing a tool, and the minimum annular clearance aspect of the present invention is applicable to this use.  
           [0018]    The present invention provides a bi-directional, freely rotatable and a radially collapsible centralizer for centralizing a joint of casing to which the centralizer is secured when the casing is lowered into a bore hole. The present invention provides a centralizer that is freely rotatable and substantially radially collapsible to fit within a very thin annular space between two generally concentric casing strings. The present invention provides a centralizer in which each bow spring is substantially collapsible to lie in a substantially flattened configuration along the longitudinal length of the exterior surface of a joint of casing to which the centralizer is secured. The present invention provides a centralizer with two spaced apart moving collars, each moving collar rotatably and slidably coupled to a mating stop collar that is adapted for securing the centralizer to the exterior surface of a joint of casing, and having a plurality of radially outwardly biased bow springs extending between the moving collars and secured at each end to a moving collar. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0019]    A more particular description of embodiments of the invention briefly summarized above may be had by references to the embodiments which are shown in the drawings which form a part of this specification. These drawings illustrate certain preferred embodiments and are not to be used to improperly limit the scope of the invention which may have other equally effective or legally equivalent embodiments.  
         [0020]    [0020]FIG. 1A is an illustration of a bore hole having a string of 13⅜ inch O.D., 12¼ inch I.D. casing cemented in place and a portion of uncased bore hole beneath drilled to 12¼ inch in diameter using conventional drilling bits. The largest standard casing that can be effectively cemented into the 12¼ inch bore hole is 9⅝ inch O.D. casing.  
         [0021]    [0021]FIG. 1B is an illustration of a bore hole having a string of 13⅜ inch O.D., 12¼ inch I.D. casing cemented in place and a portion of uncased bore hole beneath drilled to 14¾ inch in diameter using a bi-centered drilling bit. The largest standard casing that can be effectively cemented into the 14¾ inch bore hole is 11⅞ inch O.D. casing.  
         [0022]    [0022]FIG. 2A is a side elevation view of a centralizer of the present invention.  
         [0023]    [0023]FIG. 2B is an end view of a centralizer of the present invention.  
         [0024]    [0024]FIG. 3A is a perspective view of the outer end of one embodiment of the flanged collet of the moving collar of the centralizer of the present invention.  
         [0025]    [0025]FIG. 3B is a perspective view of the outer end of one embodiment of the stop collar of the centralizer of the present invention.  
         [0026]    [0026]FIG. 3C is an illustration of the interaction of the cross-section of the wall of the moving collar of FIG. 3A with the wall of the stop collar of FIG. 3B.  
         [0027]    [0027]FIG. 4 is a side cross-sectional view of a displacement assembly of the present invention showing the interaction of a moving collar received with a stop collar.  
         [0028]    [0028]FIG. 5 is a side elevation view of a centralizer of the present invention secured to a joint of casing being drawn into the end of a larger casing.  
         [0029]    [0029]FIG. 6A is a side elevational view of a displacement assembly of the present invention in the leading position and secured to a joint of casing being moved through larger casing.  
         [0030]    [0030]FIG. 6B is a side elevational view of a displacement assembly of the present invention in the trailing position and secured to a joint of casing being moved through larger casing.  
         [0031]    [0031]FIG. 7 is an exploded side elevational view showing the relationship among components of a centralizer of the present invention.  
         [0032]    [0032]FIG. 8 is a side elevational view showing the configuration of the leading displacement assembly of a centralizer of the present invention secured to a joint of casing and being drawn into the end of a larger casing.  
         [0033]    [0033]FIG. 9 is a side elevational view showing the configuration of the trailing displacement assembly of a centralizer of the present invention secured to a joint of casing and being moved through larger casing. 
     
    
     DETAILED DESCRIPTION  
       [0034]    A centralizer  10  according to the present invention as shown in FIG. 2A has two displacement assemblies  20 , one at each end of the centralizer  10 . Each displacement assembly  20  comprises a stop collar  32  having a bore centered at an axis  50  therethrough. The stop collar  32  is adapted for securing the centralizer  10  to the external surface of a tubular body such as a joint of casing received within the bore. The stop collar  32  is secured to the casing using set screws  35 . The bow springs  42  are secured at each end to a moving collar  22  that is slidably and rotatably coupled to a mating stop collar  32 .  
         [0035]    [0035]FIG. 2B is an end view of one embodiment of the centralizer  10  of the present invention having four ribs  42 . The centralizer  10  of the present invention may have from as few as one to as many ribs as can be mechanically integrated, but the preferred number of ribs is between three and fourteen, more preferably from four to ten, inclusive. The centralizer  10  is generally centered about a center axis  50  which is in the center of the bore  14 . The centralizer ribs  42  in FIG. 2A and FIG. 2B are biased towards and shown in their radially outwardly deployed configuration.  
         [0036]    As the centralizer of the present invention is bi-directional, either displacement assembly of the centralizer may be placed in a leading position relative to the opposing displacement assembly; that is, either displacement assembly may be the first to enter a restriction, such as the end opening of casing or a deviation in the bore hole, depending on the direction of movement of the casing to which the centralizer is secured. For this reason, it is helpful to describe components that may be included in the centralizer as having an inner end and an outer end. The inner end would be the end that is longitudinally disposed towards the center of the centralizer; that is, towards the ribs. The outer end would be the end that is disposed longitudinally away from the ribs.  
         [0037]    [0037]FIG. 3A is a perspective view of one embodiment of the flange-type moving collar  22  of the present invention. Each moving collar  22  has an outer end  122  and an inner end  222 , the inner end  222  being longitudinally disposed toward the ribs  42  (see FIG. 2A) of the centralizer  10 , and the outer end  122  being longitudinally disposed toward and received into the inner end  232  of the stop collar  32  (see FIG. 3B). The moving collar  22  has a collet  23  disposed on its outer end  122  that is slidably and rotatably received in the receiving bore  37  of stop collar  32 . A plurality of bow springs  42  (see FIG. 2A), each having two ends and an outwardly bowed center portion  43 , are coupled at their ends to the inner end  222  of the moving collar  22 . The moving collar  22  shown in FIG. 3A has a radially outwardly protruding flange  26  disposed on the collet  23  of the moving collar  22 . The collet  23  also has a plurality of longitudinal channels  27  machined along its length. The inner end  222  of the moving collar  22  is adapted for coupling to a plurality of flexible ribs  42  (see FIG. 2A). The longitudinal grooves  31  in the exterior surface of the moving collar  22  are adapted for improving fluid flow within the annulus formed between the exterior surface of the moving collar  22  and the interior wall of the casing in which the centralizer collapses. The channels  27  are adapted for imparting limited radial inward collapsibility to the collet  23  of the moving collar  22  for being received into the receiving bore  37  and coupling with the stop collar  32 . The channels  27  in the collet  26  also improve fluid flow within the annulus in which the centralizer collapses.  
         [0038]    It should be noted that when the centralizer  10  of the present invention is secured to the exterior surface of a joint of casing  80  (see FIG. 2A) of the proper size, the displacement assembly becomes self-locking; that is, the casing  80 , when received within the bore of the moving collar  22 , prevents the collet  23  from radially inward collapse, thereby locking the flange  26  into the reciprocating bore  34  of the stop collar  32 . This self-locking design prevents inadvertent release of the moving collar  22  from the stop collar  32  while the centralizer  10  is secured to a joint of casing  80 . In this configuration, the moving collar  22  can only be released from the stop collar  32  by a force sufficient to exceed the yield strength of the material of the flange  26 , of the collet  26  adjacent to the flange  26  or at the reciprocation bore  34  of the stop collar  32 .  
         [0039]    [0039]FIG. 3B is a perspective view of the outer end  132  of the stop collar  32 . The stop collar  32  has three or more generally concentric bores therethrough of different diameters: a securing bore  33 , a reciprocating bore  34  having a stop wall  36 , and a receiving bore  37 . The stop wall  36  of the reciprocating bore  34  may be a shoulder, rim, node, upset or other structure defining the inwardly end of the reciprocating bore  34  by imposing an obstacle to further inward movement of the flange  26  of the moving collar  22 . The flange  26  of the moving collar  22  (see FIG. 3A) is received into and reciprocates within the reciprocating bore  34  of the stop collar  32 . The range of longitudinal reciprocation of the moving collar  22  relative to the stop collar  32  is limited in the direction toward the ribs  42  by the stop wall  36  of the reciprocating bore  34  and in the direction of the outer end  132  of the stop collar  32  by the flattened length of the ribs  42  (see FIG. 2A). When the centralizer  10  is in its relaxed, deployed configuration as shown in FIG. 2A, the flange  26  abuts against the stop wall  36  of the reciprocating bore  34 . When the centralizer  10  is in its radially inwardly collapsed configuration with the ribs  42  substantially flattened along the longitudinal length of the exterior wall of the casing on which the centralizer  10  is secured, the flange  26  is at its maximum distance from the stop wall  36  within the reciprocation bore  34 . The grooves  31  in the wall  38  of the stop collar  32 , together with the longitudinal channels  27  in the collet  23 , facilitate drilling fluid flow through the annulus to prevent inadvertent swabbing or surging of the bore hole during casing operations.  
         [0040]    [0040]FIG. 3C shows the interaction of the cross-section of the wall of the moving collar of FIG. 3A with the wall of the stop collar of FIG. 3B. The relative thicknesses of the wall of the stop collar  32  and the wall of the moving collar  22  reveal the efficient use of annular space. In a centralizer adapted for use in the bore hole and casing shown in FIG. 1B, the stop collar  32  wall thickness would be approximately {fraction (3/16)} inch for the securing bore wall  51 , {fraction (1/16)} inch for the reciprocating bore wall  52 , ⅛ inch for the receiving bore wall  53 , {fraction (1/16)} inch for the collet wall  54  and {fraction (3/16)} inch for the moving collar wall  55  adjacent to the point of coupling to the ribs  42 .  
         [0041]    It should be noted that the radially outwardly protruding flange  26 , in its preferred embodiment, forms an acute angle with the portion of the collet  23  extending towards the inner end  222  of the moving collar  22 . Preferably the angle formed by the radially outwardly protruding flange  26  and the collet  23  is less than 90 degrees and more than 45 degrees, and more preferably from 60 degrees to 80 degrees. The acute angle promotes better seating of the flange  26  in its abutting position against the stop wall  36  so that minor variations in the diameter of the casing do not compromise the seating of the flange  26 .  
         [0042]    [0042]FIG. 4 is a side cross-sectional view of the displacement assembly  20 . The outer end  122  of the moving collar  22  is received within the inner end  232  of the stop collar  32 . The securing bore  33  is adapted for securing the stop collar  32  to the external surface of a joint of casing  80 . For example, the securing bore  33  of a stop collar  32  of a centralizer  10  for use in centralizing an 11⅞ inch O.D. casing  80  would have an 11⅞ inch bore. The receiving bore  37  of the stop collar  32  has a larger diameter than the securing bore  33  and is adapted for slidably and rotatably receiving the collet  23  of the moving collar  22 . The reciprocating bore  34  of the stop collar  32  is designed to accommodate and limit reciprocation of the flange  26  and the moving collar  22  within the stop collar  32  by imposing an obstacle to longitudinal movement of the flange  26  in the direction of the ribs  42 . When the ribs  42  of the centralizer  10  are in their radially outwardly deployed configuration as shown in FIG. 2A, the flange  26  of the moving collar  22  abuts against the stop wall  36  of the reciprocating bore  34 . When the ribs  42  of the centralizer  10  are in their radially inwardly collapsed configuration, the flange  26  of the moving collar  22  may abut the inner wall  36  of the stop collar  32  or the flange  26  may be displaced away from the stop wall  36  towards the outer end  232  of the stop collar  32 , depending on the direction of movement of the joint of casing  80  to which the centralizer  10  is secured. An advantage of the design of the displacement assembly  20  of FIG. 4 is that the internal location of the flange  26  within the reciprocation bore  34  prevents fouling and accumulation of debris that might otherwise obstruct smooth operation of the displacement assembly  20 . Another advantage of the design of the displacement assembly  20  of FIG. 4 is that the ribs  42  and the moving collar  22  remain freely rotatable relative to the stop collar  32  and to the casing  80  to which the centralizer is secured. The position of the flange  26  within the reciprocating bore  34  when the centralizer  10  is in its radially inwardly collapsed configuration is explained further in connection with the FIG. 5, FIG. 6A and FIG. 6B.  
         [0043]    [0043]FIG. 5 is a cross-sectional view of displacement assembly  20  of the centralizer  10  of the present invention secured to a joint of casing  80  and being introduced into an end opening  86  of larger casing  90 . The centralizer  10  is secured to the casing  80  with the ribs  42  in their relaxed and outwardly deployed configuration and the moving collars  22  positioned longitudinally inward toward the ribs  42  with the flange  26  of the moving collar  22  abutting against the stop wall  36  of the reciprocation bore  34  of the stop collar  32 , corresponding to the configuration of the displacement assembly shown in FIG. 4. It is possible that, after the centralizer ribs  42  have been radially compressed to their inwardly collapsed position to lie substantially flat along the length of the casing  80 , the displacement assembly  20  may not fully return to the configuration shown in FIG. 4. Notwithstanding the relaxed configuration of the displacement assembly  20 , upon initial contact of the ribs  42  with the end opening  86  of the casing  90  into which the centralizer  10  will be drawn, the moving collar  22  at the bottom and leading end of the centralizer  10  is forced towards its extreme withdrawn position within the stop collar  32 , corresponding to the condition of the displacement assembly  20  shown in FIG. 4 with the flange  26  of the moving collar  22  abutting the stop wall  36  of the reciprocation bore  34 . Once the flange  26  abuts against the stop wall  36  of the reciprocation bore  34 , the adjacent ends of the bow springs  42  are longitudinally secured against further movement away from the leading stop collar shown already introduced into casing  90 . Further movement of the lowered casing  80  into the larger casing  90  results in inwardly collapsing of the ribs  42 , and the ribs  42  are progressively collapsed radially inwardly as the centralizer  10  and casing  80  move into the casing  90 . The trailing moving collar  22  of the centralizer  10  (shown at the top of FIG. 5) is progressively displaced upwardly into its mating stop collar  32  as the bow springs  42  collapse to lie substantially flat along the length of the casing  80 .  
         [0044]    A plurality of teeth  44  are disposed on the radially inwardly surface of the ribs  42  at a position immediately adjacent to the point at which the end of the ribs  42  couple to the moving collar  22 . Upon initial inward collapse of the ribs  42 , the teeth engage the exterior surface of the casing  80  to provide additional gripping of the centralizer  10  to the casing  80 . The teeth  44  may be ridges, surface roughness or a plurality of protrusions, and should be positioned to prevent interference with free rotation of the centralizer when in its deployed configuration.  
         [0045]    [0045]FIG. 6A is a slide elevational view of the leading (bottom) displacement assembly  20  of FIG. 5. A substantial portion of the collet  23  is exposed, thereby indicating that the moving collar  22  is in its extreme withdrawn position from the stop collar  32  and the flange  26  is in an abutting position against the stop wall  36  of the reciprocation bore  34  (see FIG. 4).  
         [0046]    [0046]FIG. 6B shows the position of the trailing displacement assembly  20  corresponding to the radially inwardly collapsed configuration of the ribs  42  of the centralizer  10  in which the ribs  42  are substantially flattened along the external length of the joint of casing on which the centralizer  10  is secured. FIG. 6B shows the trailing moving collar  22  at its extreme received position within the stop collar  32  and displaced towards the outer end  232  of the stop collar  32 .  
         [0047]    [0047]FIG. 7 is an exploded view of the centralizer  10  of the present invention. The moving collars  22  are shown coupled to each end of the ribs  42  of the centralizer  10 , and longitudinally aligned with the stop collars  32  into which the moving collars  22  will be received upon assembly of the centralizer  10 .  
         [0048]    [0048]FIG. 8 shown the leading displacement assembly  20  of the centralizer  10  secured to a joint of casing  80  and being drawn, in the collapsing direction  82 , into an end opening  86  of larger casing  90 . The configuration of the displacement assembly  20  in FIG. 8 corresponds to FIG. 4 where the flange  26  is forced into an abutting position against the stop wall  36  near the inner end  132  of the stop collar  32  by the force of resistance of the ribs  42  to collapse and entry into the opening  86  of the casing  90 . A plurality of rib teeth  44  disposed on the radially inward side of the ribs  42  promote securing of the centralizer  10  to the casing  80 .  
         [0049]    [0049]FIG. 9 shows trailing end of the displacement assembly  20  that is at the opposite end of the centralizer  10  from the displacement assembly  20  shown in of FIG. 8, but after the entire centralizer  10  has been drawn into the casing  90 . In FIG. 9, the displacement assembly  20  at the trailing end of the centralizer  10  accommodates the longitudinal expansion of the ribs  42  as the ribs  42  collapse radially inwardly to lie substantially flat along the casing  80 . The collet  23  of the moving collar  22  of the trailing displacement assembly  20  is slidably received to its extreme position towards the outer end  132  of the stop collar  32 . The flange  26  does not necessarily contact the end of the reciprocation bore  34  opposite the stop wall  36  because the stroke of the flange  26  is limited by the flattened length of the ribs  42 ; that is, with the flange  26  of the collet  23  of the leading moving collar  22  (see FIG. 8) abutted against the stop wall  36 , the position of the flange  26  in the trailing moving collar  22  (see FIG. 9) is determined by the flattened length of the ribs  42  as they compress radially inwardly to lay flat along the length of the casing  80 .  
         [0050]    It should be noted that is within the scope of the present invention to reverse the structures of the displacement assembly components. For example, the collet and flange may be disposed on the stop collar, and the mating receiving bore and reciprocating bore may be disposed on the moving collar to receive the collet and flange, respectively. This reversed arrangement provides the same advantages as the structures described above and depicted in the appended drawings.  
         [0051]    It should be noted that the present invention is equally useful with centralizers having spiral ribs as it is to those having ribs that are longitudinally aligned with the bore through the centralizer. Those skilled in the art will recognize that the advantages obtained using the present invention are equally attainable with centralizers having spiral ribs because the freely rotatable moving collar accommodates angular or rotational displacement of collapsing or deployment of spiral centralizer ribs.  
         [0052]    Centralizers and all parts thereof may be made of any suitable high strength material including, but not limited to, metal, plastic, fiberglass, composites, aluminum or aluminum alloys, brass, copper, zinc or zinc alloys. However, given that there is sliding movement between the external surface of the moving collar and the receiving bore of the stop collar, and between the internal bore of the moving collar and the external surface of the casing, the selection of materials should be done to prevent or minimize wear and galling. One embodiment of the present invention provides an alloy of beryllium copper for the stop collar and steel for the moving collar. This materials selection is favorable for centralizers having steel ribs and integral couplings between the ribs and the moving collar. Other embodiments may utilize other self-lubricating materials. These materials may be switched between the sliding components or other suitable materials may be used.  
         [0053]    In certain embodiments of the present invention it is preferred that the parts, grooves, and recesses are sized, configured, and disposed so that the collars and bow springs, upon collapse of the bow springs against the casing, do not project beyond the exterior surface of the stop collar  
         [0054]    In conclusion, therefore, it is seen that the present invention and the embodiments disclosed herein and those covered by the appended claims are well adapted to carry out the objectives and obtain the benefits thereof set forth. Certain changes can be made in the subject matter without departing from the spirit and the scope of this invention. It is realized that changes are possible within the scope of this invention and it is further intended that each element or step recited in any of the following claims is to be understood as referring to all equivalent elements or steps.