Abstract:
In a downhole motor bearing section having a cylindrical mandrel rotatably disposed within a cylindrical housing, a bi-directional thrust bearing is disposed within a four-shouldered annular containment chamber formed partially into the inner cylindrical wall of the housing and partially into the outer cylindrical wall of the mandrel. Under on-bottom loading conditions, thrust loads are transferred through the thrust bearing by the upper shoulder of the housing and the lower shoulder of the mandrel. The radial widths of these two shoulders are both symmetrical about the centerline of the thrust bearing such that on-bottom thrust loads are transferred non-eccentrically to the thrust bearing, thereby avoiding twisting loads on the thrust bearing races, increasing the effective on-bottom load capacity of the thrust bearing and extending its service life.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is the U.S. National Stage under 35 U.S.C. §371 of International Patent Application No. PCT/US2010/026771 filed Mar. 10, 2010, which claims the benefit of U.S. Provisional Application No. 61/159,696 filed Mar. 12, 2009, entitled “Bearing Assembly For A Downhole Motor.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    The invention relates generally to bearing assemblies, and in particular to bearing assemblies for downhole motors for use in drilling of oil, gas, and water wells. 
         [0004]    In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of an assembly of drill pipe sections connected end-to-end (commonly referred to as a “drill string”), and then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. In conventional vertical borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or, in offshore drilling operations, on a seabed-supported drilling platform or a suitably adapted floating vessel). 
         [0005]    During the drilling process, a drilling fluid (also commonly referred to in the industry as “drilling mud”, or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the borehole, and then upward back to the surface through the annular space between the drill string and the wellbore. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry borehole cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the borehole wall (to stabilize and seal the borehole wall). 
         [0006]    Particularly since the mid-1980s, it has become increasingly common and desirable in the oil and gas industry to drill horizontal and other non-vertical boreholes (i.e., “directional drilling”), to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical boreholes. In directional drilling, specialized drill string components and “bottomhole assemblies” (BHAs) are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a borehole of desired non-vertical configuration. 
         [0007]    Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “drilling motor” or “mud motor”) incorporated into the drill string immediately above the drill bit. A typical downhole motor includes several primary components, as follows (in order, starting from the top of the motor assembly):
       a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component);   a power section comprising a positive displacement motor, with a helically-vaned rotor eccentrically rotatable within a stator section;   a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and   a bearing section comprising a cylindrical mandrel coaxially and rotatably disposed within a cylindrical housing, with an upper end coupled to the lower end of the drive shaft, and a lower end adapted for connection to a drill bit.       
 
         [0012]    In drilling processes using a downhole motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. However, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the power section of the downhole motor to generate power to rotate the drill bit. 
         [0013]    The bearing section must permit relative rotation between the mandrel and the housing, while also transferring axial thrust loads between the mandrel and the housing. Axial thrust loads arise in two drilling operational modes: “on-bottom” loading, and “off-bottom” loading. On-bottom loading corresponds to the operational mode during which the drill bit is boring into a subsurface formation under vertical load from the weight of the drill string, which in turn is in compression; in other words, the drill bit is on the bottom of the borehole. Off-bottom loading corresponds to operational modes during which the drill bit is raised off the bottom of the borehole and the drill string is in tension. This condition occurs, for instance, when the drill string is being pulled out of the borehole, putting the drill string into tension due to the weight of drill string components. Tension loads across the bearing section housing and mandrel are also induced when circulating drilling fluid with the drill bit off bottom, due to the pressure drop across the drill bit and bearing assembly. 
         [0014]    Accordingly, the bearing section of a downhole motor must be capable of withstanding thrust loads in both axial directions, with the mandrel rotating inside the housing. This has been accomplished in some prior art bearing assemblies by using a first thrust bearing assembly configured to resist on-bottom thrust loads only, and a second thrust bearing assembly configured to resist off-bottom thrust loads only. Suitable radial bearings are used to maintain coaxial alignment between the mandrel and the bearing housing. 
         [0015]    Each thrust bearing assembly typically comprises bearings (usually but not necessarily roller bearings) contained within a bearing cage disposed within an annular bearing containment chamber, the outer portion of which extends radially outward into the housing, and the inner portion of which extends radially inward into the mandrel. Other prior art bearing sections for downhole motors use one or more bi-directional thrust bearings rather than separate thrust bearings for on-bottom and off-bottom loads. Using this arrangement, the bearing section can be significantly shorter than a bearing section incorporating separate on-bottom and off-bottom thrust bearing assemblies. 
         [0016]    U.S. Pat. No. 5,150,972 (Wenzel) provides an example of a downhole motor bearing assembly that uses a bi-directional thrust bearing disposed within a bearing containment chamber generally as described above. The inner and outer portions of the containment chamber each define an upper shoulder and a lower shoulder. These four shoulders lie in parallel planes which are perpendicularly transverse to the coincident longitudinal axes of the housing and mandrel. The inner upper shoulder is closely adjacent to the outer upper shoulder, and the inner lower shoulder is closely adjacent to the outer lower shoulder. The relative axial positions of the inner and outer shoulders will change slightly due to relative axial movement between the housing and the mandrel as the assembly shifts between on-bottom and off-bottom loading conditions. The thrust bearing, with associated upper and lower bearing races, is disposed between these four shoulders. When the bearing section is subjected to on-bottom loading, the axial thrust loads react against the thrust bearing through the outer upper shoulder and the inner lower shoulder of the containment chamber. Under off-bottom loading, the axial thrust loads react against the thrust bearing through inner upper shoulder and the outer lower shoulder. 
         [0017]    One problem with the Wenzel bearing assembly is that under both on-bottom and off-bottom loading conditions, the thrust bearing is always being loaded eccentrically, due to the radial offset between the resultant load paths acting through the operative shoulders of the bearing containment chamber. Such eccentric loading tends to twist the bearing races, which can result in damage to the bearing assemblies and reduced bearing service life. Eccentric loading thus reduces the total thrust load that a given thrust bearing can withstand, and necessitates the use of more robust thrust bearing components than would be required if the thrust loads were transferred through the thrust bearing without eccentricity. 
         [0018]    In practical terms, this is primarily a concern with respect to on-bottom thrust loads, the highest of which are typically much greater than the highest off-bottom loads for a given drilling operation. Therefore, a bi-directional thrust bearing will typically be designed for the maximum on-bottom thrust load. Since the bi-directional thrust bearing will typically have the same maximum thrust load capacity in both directions, and since the maximum off-bottom thrust loads will be substantially less than the maximum on-bottom thrust loads, the expected off-bottom thrust loads will rarely if ever be a limiting design factor. 
         [0019]    For at least the foregoing reason, there is a need for a bearing assembly for a downhole motor in which on-bottom thrust loads are transferred through bi-directional thrust bearings without eccentricity, thus maximizing the on-bottom thrust load capacity of the bearing assembly. The embodiments described herein are directed to this need. 
       SUMMARY 
       [0020]    In general terms, embodiments described herein provide a bearing assembly having a bi-directional thrust bearing disposed within a four-shouldered containment chamber, in which the annular shoulders transferring on-bottom axial thrust loads to the thrust bearing (i.e., the outer upper and inner lower shoulders) are symmetrical about the center of the thrust bearing races. Therefore, the load paths (i.e., resultant force vectors) of the axial thrust loads reacting against the outer upper and inner lower shoulders under on-bottom loading conditions are substantially in alignment, and pass through the center of the bearing race, thus eliminating eccentric axial loading on the thrust bearing. 
         [0021]    In one embodiment, a bearing assembly comprises an outer cylindrical member that has a longitudinal axis, a generally cylindrical bore, and a cylindrical inner surface. A first annular recess formed into the cylindrical inner surface defines axially-spaced outer upper and outer lower shoulders. The assembly further includes an inner cylindrical member rotatably disposed within the bore of the outer tubular member. The inner tubular member has an outer cylindrical surface into which is formed a second annular recess defining axially-spaced inner upper and inner lower shoulders. The inner tubular member is longitudinally movable within the outer tubular member between a first position in which the inner upper shoulder and the inner lower shoulder are displaced below the outer upper shoulder and the outer lower shoulder, respectively; and a second position in which the inner upper shoulder and the inner lower shoulder are displaced above the outer upper shoulder and the outer lower shoulder, respectively. 
         [0022]    The first and second annular recesses, in juxtaposition, define an annular containment chamber for receiving a thrust bearing assembly of generally annular configuration. The outer upper shoulder is planar and has a first radial width. The inner lower shoulder is planar and has a second radial width. The outer upper shoulder and the inner lower shoulder lie in parallel planes transverse to the longitudinal axis, and the first and second radial widths are symmetrical about a common centerline parallel to the longitudinal axis. 
         [0023]    In a another embodiment, a bearing assembly particularly adapted for use in association with a downhole motor comprises a generally cylindrical housing having a longitudinal axis, a generally cylindrical bore, and a cylindrical inner surface. A first annular recess formed into the cylindrical inner surface defines axially-shaped outer upper and an outer lower shoulder shoulders. The bearing assembly further includes a generally cylindrical mandrel rotatably disposed within the bore of the housing. The mandrel has an outer cylindrical surface into which is formed a second annular recess defining axially-spaced inner upper and inner lower shoulders. The mandrel is longitudinally movable within the housing between a first position in which the inner upper shoulder and the inner lower shoulder are displaced below the outer upper shoulder and the outer lower shoulder, respectively; and a second position in which the inner upper shoulder and the inner lower shoulder are displaced above the outer upper shoulder and the outer lower shoulder, respectively. 
         [0024]    The first and second annular recesses, in juxtaposition, define an annular containment chamber which houses a thrust bearing assembly of generally annular configuration. The outer upper shoulder is planar and has a first radial width. The inner lower shoulder is planar and has a second radial width. The outer upper shoulder and the inner lower shoulder lie in parallel planes transverse to the longitudinal axis, and the first and second radial widths are symmetrical about a common centerline parallel to the longitudinal axis. 
         [0025]    In preferred embodiments, the thrust bearing assembly comprises a plurality of roller bearings retained within a circular bearing cage, with the bearing cage being disposed between an upper bearing race and a lower bearing race. The bearing case and bearing races have a common circular bearing centerline which is coincident with the common circular centerline of the first radial width of the outer upper shoulder and the second radial width of the inner lower shoulder. Accordingly, when axial thrust loads urge the mandrel toward the above-mentioned second position, thereby bringing the upper bearing race into contact with the outer upper shoulder and bringing the lower bearing race in contact with the inner lower shoulder, and displacing the inner upper shoulder and the outer lower shoulder clear of the thrust bearing assembly, the resultant axial force vectors acting on the outer upper shoulder and the inner lower shoulder will pass though the thrust bearing assembly along a path substantially coincident with the bearing centerline. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Embodiments of the invention will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which: 
           [0027]      FIG. 1  is a longitudinal cross-sectional view of an embodiment of a downhole motor bearing assembly in accordance with the principles described herein, shown under off-bottom axial thrust loading; and 
           [0028]      FIG. 2  is a longitudinal cross-sectional view of the downhole motor bearing assembly of  FIG. 1 , shown under on-bottom axial thrust loading. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
         [0030]    Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
         [0031]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one such element. Any use of any form of the terms “connect”, “engage”, “couple”, “attach”, or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the subject elements, and may also include indirect interaction between the elements such as through secondary or intermediary structure. Relational terms such as “parallel”, “perpendicular”, “planar”, “coaxial”, “coincident”, “concentric”, “intersecting”, “symmetrical”, “centered”, “equal”, and “equidistant” are not intended to denote or require absolute mathematical or geometrical precision. Accordingly, such terms are to be understood as denoting or requiring substantial precision only (e.g., “substantially parallel”) unless the context clearly requires otherwise. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a longitudinal or central axis (e.g., central axis of a structure), while the terms “radial” and “radially” generally mean perpendicular to the longitudinal or central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. 
         [0032]      FIGS. 1 and 2  illustrate a downhole motor bearing assembly  10  in accordance with one embodiment, shown under off-bottom and on-bottom axial thrust loading conditions respectively. In  FIG. 1 , force arrows T 1  signify off-bottom thrust loads putting the drill string and bearing assembly  10  in tension, while in  FIG. 2 , force arrows T 2  signify on-bottom thrust loads putting the drill string and bearing assembly  10  in compression. 
         [0033]    Bearing assembly  10  comprises a cylindrical housing  20  and a cylindrical mandrel  30  rotatably disposed within housing  20 , with housing  20  and mandrel  30  having a common longitudinal axis X. An upper portion  20 U of housing  20  has an inner upper cylindrical surface  21 U, and a lower portion  20 L of housing  20  has an inner cylindrical surface  21 L, with the diameter of upper cylindrical surface  21 U being greater than the diameter of lower cylindrical surface  21 L. In the illustrated embodiment, housing  20  is provided with threaded joint J, separating upper and lower portions  20 U and  20 L, to facilitate assembly of bearing assembly  10 . The location and configuration of threaded joint J, as shown in the Figures, is by way of example only. 
         [0034]    Formed into lower cylindrical surface  21 L of housing  20  is an outer annular recess  22  defining a cylindrical outer recess wall  23  extending between an outer upper annular shoulder  24  and an outer lower annular shoulder  26 . Outer upper shoulder  24  and outer lower shoulder  26  of housing  20  are substantially parallel to each other, and lie in planes perpendicularly transverse to longitudinal axis X. Outer recess wall  23  is contiguous with an annular surface  25  outwardly adjacent to outer upper shoulder  24 , with outer upper shoulder  24  being proud of (i.e., projecting from) annular surface  25  as shown. Outer upper shoulder  24  has a radial width W 1 . 
         [0035]    As conceptually illustrated in  FIGS. 1 and 2 , upper and lower radial bushing-type bearings  60  and  62  will typically be provided above and below annular recess  22 , in corresponding annular bushing recesses  27  and  29  formed into cylindrical surfaces  21 U and  21 L, respectively, of housing  20 , to maintain coaxial alignment of housing  20  and mandrel  30 . 
         [0036]    Mandrel  30  has a central bore  38  for circulation of drilling fluid. An upper portion of mandrel  30  has an outer upper cylindrical surface  31 U, and a lower portion of mandrel  30  has a outer lower cylindrical surface  31 L, with the diameter of upper cylindrical surface  31 U being less than the diameter of lower cylindrical surface  31 L. Formed into upper cylindrical surface  31 U is an inner annular recess  32  defining a cylindrical inner recess wall  33  extending between an inner lower annular shoulder  34  and an inner upper annular shoulder  42 . Inner upper shoulder  42  and inner lower shoulder  34  of mandrel  30  are substantially parallel to each other, and lie in planes perpendicularly transverse to longitudinal axis X. Inner recess wall  33  is contiguous with an annular surface  35  inwardly adjacent to inner lower shoulder  34 , with inner lower shoulder  34  being proud of annular surface  35  as shown Inner lower shoulder  34  has a radial width W 2 , which preferably (but not necessarily) will equal to radial width W 1  of inner upper shoulder  24 . 
         [0037]    In the illustrated embodiment, inner upper shoulder  42  is defined by a radially-projecting lower surface of a split ring  40  disposed partially within an annular groove  36  formed into upper cylindrical surface  31 U of mandrel  30 . Split ring  40  is retained within annular groove  36  by a retainer ring  44 , with retainer ring  44  being maintained in position by a snap ring  46  disposed within a corresponding annular groove in split ring  40 . An upper annular recess  28  is formed into upper cylindrical surface  21 U of upper portion  20 U of housing  20  to receive (with appropriate clearances) the sub-assembly comprising split ring  40 , retainer ring  44 , and snap ring  46 . 
         [0038]    When housing  20  and mandrel  30  are assembled as shown in the Figures, outer annular recess  22  of housing  20  and inner annular recess  32  of mandrel  30  define an annular bearing chamber for receiving a thrust bearing assembly  50 , with outer upper shoulder  24  of housing  20  being closely adjacent to inner upper shoulder  42  of mandrel  30 , and with outer lower shoulder  26  of housing  20  being closely adjacent to inner lower shoulder  34  of mandrel  30 . The axial distance between outer upper and outer lower shoulders  24  and  26 , and the axial distance between inner upper and inner lower shoulders  42  and  34 , are both slightly greater than the axial length of thrust bearing assembly  50 . This provides thrust bearing assembly  50  with an axial clearance within the bearing containment chamber, sufficient to permit relative axial movement of housing  20  and mandrel  30  between the two positions illustrated in  FIGS. 1 and 2 . 
         [0039]    In the illustrated embodiment, thrust bearing assembly  50  includes a plurality of bearings  51  (typically but not necessarily roller bearings) contained within a bearing cage  52 ; an upper bearing race  54  disposed above bearings  51 ; and a lower bearing race  56  disposed below bearings  51 . Upper bearing race  54  has an upper face  54 A, and lower bearing race  56  has a lower face  56 A. In alternative embodiments, spacers (not shown) may be provided in association with bearing races  54  and  56 . Typically, an upper bearing race bushing  55 , a lower bearing race bushing  55 , and a bearing cage bushing  58  will be provided between inner recess wall  33  of mandrel  30  and upper bearing race  54 , lower bearing race  56 , and bearing cage  52 , respectively, to keep all components of thrust bearing assembly  50  concentric about longitudinal axis X. As shown, a suitable outer clearance gap G is also provided between bearing races  54  and  56  and cylindrical wall  23  of outer annular recess  22  of housing  20 . 
         [0040]    In the Figures, bearings  51 , bearing cage  52 , bearing races  54  and  56 , and bearing cage bushing  58  are illustrated as conceptual blocks only. Persons skilled in the art will appreciate that the specific details of these components can take various forms, and that the embodiments described herein are not in any way limited or restricted to use with thrust bearing components of any particular design or configuration. 
         [0041]    Thrust bearing assembly  50  will have a circular bearing centerline C representing an optimal load path for thrust bearing assembly  50  such that axial compressive loads acting on upper face  54 A of upper bearing race  54  and lower face  56 A of lower bearing race  56  will be distributed with optimal uniformity across the radial width of bearings  51 . Preferably, as in the illustrated embodiment, radial width W 1  of outer upper shoulder  24  and radial width W 2  of inner lower shoulder  34  will be both symmetrical about bearing centerline C. 
         [0042]    During operation of bearing assembly  10  under off-bottom thrust loading as in  FIG. 1 , mandrel  30  rotates within housing  20 , upper bearing race  54  rotates with mandrel  30  (with an inner portion of upper face  54 A of upper bearing race  54  in contact with inner upper shoulder  42 ), and lower bearing race  56  rotates with housing  20  (with an outer portion of lower face  56 A of lower bearing race  56  in contact with outer lower shoulder  26 ). Because inner upper shoulder  42  and outer lower shoulder  26  are radially offset, off-bottom thrust loading conditions will result in eccentric loading of thrust bearing assembly  50 . However, this will rarely if ever be a practical concern due to the fact that thrust bearing assembly  50  will most commonly be designed for much higher on-bottom thrust loads. 
         [0043]    Upon transfer from off-bottom to on-bottom thrust loading conditions as in  FIG. 2 , upper bearing race  54  abruptly changes from rotating with mandrel  30  to rotating with housing  20  (with a central portion of upper face  54 A of upper bearing race  54  in contact with outer upper shoulder  24 ), and lower bearing race  56  abruptly changes from rotating with housing  20  to rotating with mandrel  30  (with a central portion of lower face  56 A of lower bearing race  56  in contact with inner lower shoulder  34 ). Because radial width W 1  of outer upper shoulder  24  and radial width W 2  of inner upper shoulder  42  are both symmetrical about bearing centerline C, the contact pressures between outer upper shoulder  24  and upper bearing race  54  and between inner lower shoulder  34  and lower bearing race  56  are substantially uniform, such that the corresponding resultant axial force vectors are substantially coincident with bearing centerline C, thereby effectively eliminating eccentric loading on thrust bearing assembly  50 . 
         [0044]    While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Although embodiments described herein have been specifically described in the context of a bearing section for a downhole motor, the principles may also be adapted for other practical applications for bi-directional thrust bearings, without departing from the scope of the invention.