Abstract:
In a mud-lubricated bearing assembly for a downhole motor, a mechanical seal is provided between the mandrel and the lower end of the bearing housing to prevent discharge of drilling fluid (mud) from the bearing assembly into the wellbore annulus. The mechanical seal is effected by mating wear-resistant annular contact surfaces provided on the mandrel and the bearing housing, with biasing means preferably being provided to keep the contact surfaces in sealing engagement during both on-bottom and off-bottom operational modes. The diverted drilling fluid passing through the bearings is redirected into the bore of the mandrel via ports through the mandrel wall to rejoin the main flow to the drill bit, such that substantially all of the drilling fluid flows through the bit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. provisional patent application Ser. No. 61/679,292 filed Aug. 3, 2012, and entitled “Mud-Lubricated Bearing Assembly,” which is hereby incorporated herein by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       FIELD OF THE DISCLOSURE 
       [0003]    The present disclosure relates in general to bearing assemblies for downhole motors used in drilling oil, gas, and water wells, and in particular to mud-lubricated bearing sections in downhole motors. 
       BACKGROUND 
       [0004]    In drilling a 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 a drill string (comprising drill pipe sections connected end-to-end) and then to rotate the drill string (by means of either a “rotary table” or a “top drive” associated with a drilling rig) so that the drill bit progresses downward into the earth to create the desired wellbore. 
         [0005]    During the drilling process, a drilling fluid (commonly referred to as “drilling mud,” or simply “mud”) is pumped under pressure downward through the drill string, out the drill bit into the wellbore, and then upward back to the surface through the wellbore annulus 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 wellbore 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 wellbore wall (to stabilize and seal the wellbore wall). To optimize these functions, it is desirable for as much of the drilling fluid as possible to reach the drill bit. 
         [0006]    Particularly since the mid-1980s, it has become increasingly common and desirable in the oil and gas industry to use “directional drilling” techniques to drill horizontal and other non-vertical wellbores, to facilitate more efficient access to, and production from, larger regions of hydrocarbon-bearing formations than would be possible using only vertical wellbores. 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 wellbore of desired non-vertical configuration. 
         [0007]    Directional drilling is typically carried out using a “downhole motor” (also referred to as a “mud motor”) incorporated into the drill string immediately above the drill bit. A typical mud motor includes the following primary components (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 (commonly comprising a positive displacement motor of well-known type, with a helically-vaned rotor eccentrically rotatable within a stator section, and with a fixed or adjustable straight or bent housing for inducing a wellbore deviation);   a drive shaft enclosed within a drive shaft housing having a central bore for conveying drilling fluid to the drill bit, 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 bearing housing, with an upper end coupled to the lower end of the drive shaft, and a lower end connectable to a drill bit.       
 
         [0012]    In drilling processes using a mud 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 is diverted through the power section of the mud 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 (i.e., when the bit is off the bottom of the borehole and is hanging from the drill string, such as when the drill string is being “tripped” out of the wellbore, or when the wellbore is being reamed in the uphole direction). Tension loads across the bearing section housing and mandrel are also induced when drilling fluid is being circulated while the drill bit is off bottom, due to the pressure drop across the drill bit and bearing assembly 
         [0014]    Accordingly, the bearing section of a mud motor must be capable of withstanding thrust loads in both axial directions, with the mandrel rotating inside the bearing housing. Suitable radial bearings are used to maintain coaxial alignment between the mandrel and the bearing housing. 
         [0015]    Thrust bearings contained within the bearing section of a mud motor may be either oil-lubricated or mud-lubricated. In an oil-lubricated bearing assembly, the thrust bearings are disposed within a sealed, oil-filled reservoir to provide a clean operating environment. The oil reservoir is located within an annular region between the mandrel and the bearing housing, with the reservoir being defined by the inner surface of the housing and the outer surface of the mandrel, and by sealing elements at the upper and lower ends of the reservoir. 
         [0016]    Mud-lubricated bearing assemblies comprise bearings (thrust bearings and/or radial bearings) that are designed for operation in drilling fluid. In conventional mud-lubricated bearings, a portion of the drilling fluid flowing to the drill bit is diverted through the bearings to provide lubrication and cooling, and then is discharged into the wellbore annulus, thus bypassing the bit. This reduces the volume of drilling fluid flowing through the bit, thus reducing the hydraulic energy available for hole cleaning and bit performance. 
       BRIEF SUMMARY 
       [0017]    The present disclosure teaches a mud-lubricated bearing assembly providing a mechanical seal between the mandrel and the lower end of the bearing housing to prevent discharge of drilling fluid from the bearing assembly into the wellbore annulus. The mechanical seal is effected by mating wear-resistant annular contact surfaces provided on the mandrel and the bearing housing, with biasing means preferably being provided to keep the contact surfaces in substantially sealing engagement during both on-bottom and off-bottom operational modes. The diverted drilling fluid passing through the bearings is redirected into the bore of the mandrel (via ports through the mandrel wall) so as to rejoin the main flow through the bit, such that substantially all of the drilling fluid flows through the bit. Preferred embodiments use a combination of hard-faced radial and thrust bearings in a configuration that results in the bearing assembly being substantially shorter than conventional bearing assemblies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Embodiments in accordance with the present disclosure will now be described with reference to the accompanying figures, in which numerical references denote like parts, and in which: 
           [0019]      FIG. 1  is a longitudinal section through a first embodiment of a bearing assembly in accordance with the present disclosure. 
           [0020]      FIG. 2  is an enlarged sectional detail of the upper and lower seal rings of the bearing assembly in  FIG. 1 . 
           [0021]      FIG. 3  is a longitudinal section as in  FIG. 1 , illustrating the fluid flow path through the bearing assembly. 
           [0022]      FIG. 4  is an enlarged sectional detail as in  FIG. 2 , illustrating the fluid flow path from the annulus between the mandrel and the bearing housing into the mandrel bore. 
           [0023]      FIG. 5  is a longitudinal section through a second embodiment of a bearing assembly in accordance with the present disclosure, incorporating a flow-restricting nozzle disposed in a lower region of the mandrel bore. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The following description is exemplary of embodiments of the disclosure. These embodiments are not to be interpreted or otherwise used as limiting the scope of the disclosure, including the claims. It will be readily appreciated by those skilled in the art that various modifications to embodiments in accordance with the present disclosure may be devised without departing from the scope of the present teachings, including modifications which may use equivalent structures or materials hereafter conceived or developed. 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 is not intended to suggest in any way that the scope of the disclosure, including the claims, is limited to that embodiment. It is to be especially understood that the scope of the claims appended hereto should not be limited by any particular embodiments described and illustrated herein, but should be given the broadest interpretation consistent with the description as a whole. It is also to be understood that the substitution of a variant of a claimed or illustrated element or feature, without any substantial resultant change in functionality, will not constitute a departure from the scope of the claims. 
         [0025]    The drawing figures are not necessarily to scale. Certain features and components disclosed herein may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. In some of the figures, one or more components or aspects of a component may be not displayed or may not have reference numerals identifying the features or components that are identified elsewhere in order to improve clarity and conciseness of the figure. 
         [0026]    The terms “including” and “comprising” are used herein, including in the claims, 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 preclude the presence or inclusion of more than one such element, unless the context clearly requires that there be one and only one such element. Also, any form of the terms “couple,” “connect,” “engage,” “attach,” “secure,” or any other term describing an interaction between elements is intended to mean either an indirect or direct connection. Thus, if a first component couples or is coupled to a second component, the connection between the components may be through a direct engagement of the two components, or through an indirect connection that is accomplished via other intermediate components, devices and/or connections. In addition, if the connection transfers electrical power or signals, whether analog or digital, the coupling may comprise wires or a mode of wireless electromagnetic transmission, for example, radio frequency, microwave, optical, or another mode. So too, the coupling may comprise a magnetic coupling or any other mode of transfer known in the art, or the coupling may comprise a combination of any of these modes. 
         [0027]    In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a given axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis. Any reference to up or down in the description and the claims will be made for purpose of clarification, with “up”, “upper”, “upwardly”, or “upstream” meaning toward the surface of the well and with “down”, “lower”, “downwardly”, or “downstream” meaning toward the terminal end of the well, regardless of the well bore orientation. In some applications of the technology, the orientations of the components with respect to the surroundings may be different. For example, components described as facing “up”, in another application, may face to the left, may face down, or may face in another direction. Still further, as used herein the terms “sealed” and “gas-tight” may be used to describe components, devices, and equipment that allow fluids to flow therethrough but prevent gases from escaping into the surrounding environment during normal operating conditions. 
         [0028]    As used herein, relational terms such as but not limited to “coaxial” and “perpendicular” 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 coaxial”) unless the context clearly requires otherwise. Wherever used in this document, the terms “typical” and “typically” are to be interpreted in the sense of representative of common usage or practice, and are not to be understood as implying essentiality or invariability. 
         [0029]      FIG. 1  illustrates one embodiment of a mud motor bearing assembly  100  in accordance with the present disclosure. Bearing assembly  100  comprises a generally cylindrical mandrel  10  which is rotatable within a generally cylindrical housing  20 . The lower end  10 L of mandrel  10  has a threaded connection  12  for connection to the drill bit or other BHA components below the motor, and the upper end  10 U of mandrel  10  comprises a threaded connection  14  for connection to the driveshaft assembly and rotor of the power section (not shown). Mandrel  10  has a longitudinal channel or bore  15  for conveying drilling fluid to the drill bit. The upper end  20 U of housing  20  comprises a threaded connection  22  for connection to the fixed or adjustable straight or bent housing and stator of the power section. 
         [0030]    Bearing assembly  100  comprises multiple bearings for transferring the various axial and radial loads between mandrel  10  and housing  20  that occur during the drilling process. An upper thrust bearing  32  and a lower thrust bearing  34  transfer off-bottom and on-bottom operating loads, respectively, while an upper radial bearing  42  and a lower radial bearing  44  transfer radial loads between mandrel  10  and housing  20 . 
         [0031]    As shown in enlarged detail in  FIG. 2 , bearing assembly  100  further comprises an annular lower seal ring  50  axially and non-rotatably secured to mandrel  10  in a region adjacent to lower end  20 L of housing  20 , plus a “floating” annular upper seal ring  60  mounted to a lower region of housing  20  such that upper seal ring  60  is non-rotatable relative to housing  20  but is axially movable relative to housing  20  within a defined range of travel. 
         [0032]    For optimal operational effectiveness, bearing assembly  100  preferably includes seal assembly  65  for sealing between upper seal ring  60  and the adjacent inner cylindrical surface of housing  20 . In this embodiment, the seal assembly  65  includes an annular seal groove  61  in the outer surface of upper seal ring  60  as shown in  FIG. 2 , and an annular sealing member  63  disposed therein. In general, the sealing member  63  can be of any suitable type, such as (by way of non-limiting example) an elastomeric O-ring disposed within the annular seal groove  61 . 
         [0033]    Lower seal ring  50  has a wear-resistant annular upper seal surface  54  in a plane perpendicularly transverse to the longitudinal axis of the mandrel, and upper seal ring  60  has a wear-resistant annular lower seal surface  64  matingly engageable with upper seal surface  54  on lower seal ring  50  so as to prevent leakage of drilling fluid across the interface  55  between seal surfaces  54  and  64  except in miniscule amounts if any. Persons skilled in the art will be aware of various materials that can be used for fabrication or hard-facing of seal rings  50  and  60  to provide seal surfaces  54  and  64  with wear resistance to suit specific requirements, and embodiments in accordance with the present disclosure are not limited or restricted to the use of any particular means or materials for providing wear resistance on seal surfaces  54  and  64 . 
         [0034]    Mandrel  10  is provided with one or more fluid ports  16  extending between bore  15  of mandrel  10  and the outer surface of mandrel  10  adjacent to upper seal ring  60 . Because flow across seal interface  55  is substantially prevented, drilling fluid diverted through the bearings will be directed through fluid ports  16  into mandrel bore  15  to join the main flow of fluid to the drill bit. For this purpose, fluid must be able to flow downward through or past upper seal ring  60  in order to reach fluid ports  16 . In the illustrated embodiment, and as best seen in  FIG. 2 , this can be facilitated by sizing upper seal ring  60  to provide an annular space  66  between the inner surface of upper seal ring  60  and the outer surface of mandrel  10 . However, bearing assemblies in accordance with the present disclosure are not limited to this particular arrangement, and persons skilled in the art will understand that fluid flow to ports  16  can be effected or facilitated in a variety of other ways. By way of non-limiting alternative example, upper seal ring  60  could be made to fit fairly closely around mandrel  10  while including one or more longitudinal grooves or channels allowing flow through seal ring  60 . 
         [0035]    Lower seal ring  50  may be non-rotatably secured to mandrel  10  by any suitable means, such as (to provide one non-limiting example) by way of an interference fit at a cylindrical interface  52  with mandrel  10  as shown in  FIG. 2 . 
         [0036]    Similarly, bearing assemblies in accordance with the present disclosure are not limited or restricted to any particular means for non-rotatably securing floating upper seal ring  60  to housing  20  or for permitting longitudinal movement of upper seal ring  60  relative to housing  20 . However,  FIG. 2  illustrates one non-limiting example of means for providing these features. In the illustrated embodiment, upper seal ring  60  is formed with one or more axially-oriented splines  62  slidable within mating grooves  25  formed in housing  20 . 
         [0037]    During operation of a mud motor incorporating bearing assembly  100 , mandrel  10  will rotate relative to housing  20 , so lower seal ring  50  will rotate relative to floating upper seal ring  60 . In the typical case, there will be limited axial travel between mandrel  10  and housing  20  as the configuration of bearing assembly  100  changes from on-bottom to off-bottom loading conditions or vice versa.  FIG. 2  illustrates the operational case in which bearing assembly  100  is under on-bottom loading, with a gap G 1  being formed between lower end  20 L of housing  20  and the adjacent portion of mandrel  10 . When bearing assembly  100  is under off-bottom loading, a slightly larger gap G 2  will be formed between lower end  20 L of housing  20  and mandrel  10  as splines  62  on upper seal ring  60  slide downward within grooves  25  in housing  20 . 
         [0038]    Preferably, upper and lower seal surfaces  54  and  64  will at all times remain matingly engaged to prevent fluid leakage across interface  55 , by virtue of biasing means provided for biasing floating upper seal ring  60  toward fixed lower seal ring  50 . Such biasing means may be provided in the form of springs  70  as shown in the Figures. Springs  70  are illustrated in the Figures in the form of a “stack” of Belleville washers. However, this is by way of non-limiting example only, and any suitable alternative biasing means (such as one or more helical springs) may be used without departing from the scope of the disclosure. In addition, differential pressure across the seal assembly  65 , and in particular seal member  63 , will also bias upper seal ring  60  toward lower seal ring  50 . 
         [0039]    During operation of the mud motor, a portion of the circulating drilling fluid is diverted through the bearings to lubricate and cool bearings  32 ,  34 ,  42 , and  44  (in the illustrated embodiment). This diverted fluid continues to flow past the bearings until reaching interface  55  between seal faces  54  and  64  of seal rings  50  and  60 , respectively. Preferably, seal faces  54  and  64  will be highly polished to minimize leakage of drilling fluid across interface  55  between seal rings  50  and  60 , such that all or substantially all of the fluid exiting the bearings is redirected through ports  16  in mandrel  10  to join the main flow of fluid in mandrel bore  15  and to proceed onward toward the bit. This fluid flow path is illustrated by flow arrows F in  FIGS. 3 and 4 . 
         [0040]      FIG. 5  illustrates a variant bearing assembly  110  generally similar to bearing assembly  100  but in which a nozzle  120  is provided near lower end  10 L of mandrel  10  above fluid ports  16 , to create a pressure drop across the bearing assembly to force the flow of drilling fluid through the bearings. In embodiments not incorporating nozzle  120 , other means may be provided to help ensure adequate fluid flow through the bearings. To provide one non-limiting example of such means, in embodiments in which upper radial bearing  42  is provided in the form of a bushing-type bearing, upper radial bearing  42  could be provided with longitudinally-oriented grooves or channels to facilitate adequate fluid flow. Another alternative would be to provide radial ports through the wall of mandrel  10  into mandrel bore  15  at a point between upper thrust bearing  32  and upper radial bearing  42 . 
         [0041]    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. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.