Patent Application
20120195542
Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to bearing assemblies for mud motors used in drilling of oil, gas, and water wells. More particularly, the invention relates to mud motor bearings for resisting on-bottom and off-bottom thrust loads.
2. Background of the Technology
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 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 wellbore. In conventional vertical wellbore 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 wellbore (or, in offshore drilling operations, on a seabed-supported drilling platform or a suitably adapted floating vessel).
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 wellbore, 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 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.
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 subsurface 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.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “mud motor”) incorporated into the drill string immediately above the drill bit. A typical mud motor includes several primary components, as follows (in order, starting from the top of the motor assembly):
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 exiting the lower end of the drill pipe is diverted through the power section of the mud motor to generate power to rotate the drill bit.
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 wellbore. Off-bottom loading corresponds to operational modes during which the drill bit is raised off the bottom of the wellbore and the drill string is in tension (i.e., when the bit is off the bottom of the wellbore 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 circulating drilling fluid with the drill bit off bottom, due to the pressure drop across the drill bit and bearing assembly.
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 housing. A mud motor bearing section may be configured with one or more bearings that resist on-bottom thrust loads only, and with another one or more bearings that resist off-bottom thrust loads only. Alternatively, one or more bi-directional thrust bearings may be used to resist both on-bottom and off-bottom loads. A typical thrust bearing assembly comprises bearings (commonly but not necessarily roller bearings contained within a bearing cage) disposed within an annular bearing containment chamber. Suitable radial bearings (e.g., journal bearings or bushings) are used to maintain coaxial alignment between the mandrel and the bearing housing.
Thrust bearings contained in the bearing section of a mud motor may be either oil-lubricated or mud-lubricated. In an oil-sealed bearing assembly, the thrust bearings are disposed within an oil-filled reservoir to provide a clean operating environment. The oil reservoir is located within an annular region between the mandrel and the 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.
Mud-lubricated bearing assemblies comprise bearings that are designed for operation in drilling fluid (“mud”). A small portion of the drilling fluid flowing to the drill bit is diverted to flow through the bearings to provide lubrication and cooling.
Oil-sealed bearing assemblies offer several advantages over mud-lubricated bearing assemblies. Because of the clean operating environment, oil-sealed components tend to have a much longer service life. Since conventional mud-lubricated bearing assemblies require a portion of the drilling fluid to be diverted through the bearings and to the wellbore annulus, the total flow of fluid through the drill bit is reduced, thereby reducing the effectiveness of the drilling fluid hydraulics through the bit. Oil-sealed assemblies do not require drilling fluid to be diverted and can be configured such that all the drilling fluid is directed through the bit, thus optimizing drilling fluid hydraulics through the bit. This can be particularly advantageous when running additional drilling tools between the mud motor and the drill bit, such as a rotary steerable system, where full flow of drilling fluid to the tool is required for optimum operation.
However, mud-lubricated bearings have their own advantages. In particular, mud-lubricated bearings with planar bearing contact surfaces can provide static thrust load capacities considerably greater than is achievable with conventional rolling-element bearings. In addition, mud-lubricated bearings can operate reliably in harsh environments, without need for a sealed bearing chamber.
As previously noted, separate thrust bearings may be used for on-bottom and off-bottom thrust loads, or bi-directional thrust bearings may be used to resist both on-bottom and off-bottom thrust loads. In either case, the mandrel must incorporate a load-transferring shoulder situated above the off-bottom bearing, for transferring off-bottom loads from the mandrel to the housing. This is commonly accomplished in prior art bearing assemblies through the use of a ring machined with an array of high-tolerance annular grooves and ribs sized to mate with corresponding high-tolerance annular ribs and grooves on the mandrel. The ring is necessarily provided in the form of a split ring to allow assembly onto the mandrel. When assembled on the mandrel, the split ring provides the necessary shoulder for off-bottom loads, which are transferred from the off-bottom thrust bearing (or, alternatively, a bi-directional thrust bearing) to the mandrel through the mating annular grooves and ribs of the mandrel and split ring. The spacing of the grooves and ribs in the mandrel and the split ring must be very precise so that axial load is shared equally between each adjacent set of mating groove/rib faces.
A rolling-element bearing (i.e., a bearing incorporating any type of rolling element, such as balls, cylindrical rollers, tapered rollers, and spherical rollers) will have static and dynamic load ratings that define allowable load limits during operation. An off-bottom thrust bearing can experience high static loads if the drill bit becomes stuck in the wellbore and the drill string needs to be put in tension in an attempt to pull the bit free. If the static load limit of the off-bottom bearing is exceeded, the motor will not be operable once the bit is pulled free, and the motor will need to be removed from the wellbore and replaced before drilling can continue.
For at least the reasons discussed above, there remains a need in the art for an oil-sealed mud motor bearing section in which the mandrel is provided with a load-transferring shoulder for reacting off-bottom thrust loads, but without the need for high-tolerance machining of the mandrel and associated shoulder components. Further, there remains a need in the art for a mud motor bearing section incorporating an off-bottom thrust bearing having a static load limit much greater than provided by rolling-element bearings. Still further, there remains a need in the art for a mud motor bearing section incorporating a mud-lubricated off-bottom bearing assembly in which the mud flow through the off-bottom bearing assembly is returned to the main mud flow through the bearing section, rather than exiting into the wellbore annulus and thereby reducing the total mud flow reaching the drill bit. Embodiments disclosed herein are directed to such needs.
Embodiments described herein generally teach mud motor bearing assemblies having an oil-sealed bearing chamber which houses at least one oil-sealed thrust bearing for resisting on-bottom thrust loads, with off-bottom thrust loads being resisted by a mud-lubricated thrust bearing disposed within an off-bottom thrust bearing chamber located above the oil-sealed bearing chamber. Radial loads acting on the bearing assemblies are resisted by radial bearings located within the oil-sealed chamber. Being oil-sealed, the radial bearings and the on-bottom thrust bearing are in an optimum operating environment, and there is no need to divert any drilling mud through the on-bottom thrust bearing chamber. Drilling mud used to lubricate and cool the off-bottom thrust bearing rejoins the flow of mud to the drill bit rather than being discharged into the wellbore annulus.
In accordance with embodiments described herein, the lower end of a drive shaft adapter connected to the mandrel effectively serves, either directly or through intermediary structure, as the load-transferring shoulder required in association with the mandrel for transfer of off-bottom thrust loads. This eliminates the need for an intermediate support shoulder along the mandrel such as the split ring shoulder used in prior art assemblies, thus eliminating the need for the high-tolerance machining entailed by such split ring shoulders. In addition, utilization of the drive shaft adapter for transfer of off-bottom thrust loads shortens the overall length of the bearing assembly. Furthermore, by eliminating the use of a rolling-element bearing for off-bottom thrust loads, the static load limit of the off-bottom thrust bearing assembly is significantly increased, such that when the drill string is being pulled to free a stuck drill bit, there is little or no risk of overloading the off-bottom thrust bearing and thus making the mud motor inoperable after the bit has been pulled free.
Accordingly, embodiments described herein teach a bearing section for a mud motor, comprising: an elongate mandrel rotatably and coaxially disposed within an elongate cylindrical housing; a first (or upper) annular bearing chamber laterally bounded by the outer surface of the mandrel, the inner surface of the housing, an annular abutment associated with the housing, and the lower end of a cylindrical drive shaft adapter mounted to the upper end of the mandrel; and a mud-lubricated thrust bearing assembly disposed within the first bearing chamber such that the mud-lubricated thrust bearing assembly will be in compression between the annular abutment and the drive shaft adapter when the bearing section is in tension, thereby resisting off-bottom thrust loads. The mandrel is generally cylindrical, with a central bore for passage of drilling mud, and a generally cylindrical wall. One or more mud ports are formed through the mandrel wall such that drilling mud flowing through the first bearing chamber to lubricate and cool the thrust bearing assembly will exit the bearing chamber through the one or more mud ports, joining the main flow of drilling mud through the central bore of the mandrel and downward to the drill bit.
In alternative embodiments, the bearing section also incorporates an annular oil reservoir bounded by the outer surface of the mandrel, the inner surface of the housing, and upper and lower rotary seals between the mandrel and the housing. A portion of the oil reservoir defines a second (or lower) annular bearing chamber, bounded at its lower end by an annular lower shoulder associated with the mandrel, and at its upper end by an annular upper shoulder associated with the housing. A thrust bearing is disposed within the second bearing chamber such that it will be in compression between the upper and lower shoulders when the bearing section is in compression, thereby resisting on-bottom thrust loads.
In some embodiments, the bearing section further incorporates a mud-lubricated radial bearing assembly disposed within the first (or upper) bearing chamber.
Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.
For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:
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.
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.
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 . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the 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.
As best shown in
An oil-lubricated lower thrust bearing 50, with lower bearing race 51L and upper bearing race 51U, is disposed within bearing chamber 25 below and immediately adjacent to lower shoulder 41L of split ring 40. Shims 55 may be provided as shown in association to facilitate positioning of bearing 50 within bearing chamber 25. Off-bottom (tensile) thrust loads are transferred from mandrel 20 to split ring 40 (via annular grooves 28 and 48 and annular ribs 29 and 49); thence via lower shoulder 41L of split ring 40 to upper bearing race 51U, lower thrust bearing 50, and lower bearing race 51L; and thence to a lower shoulder 32 formed in housing 30.
An oil-lubricated upper thrust bearing 60, with lower bearing race 61L and upper bearing race 61U, is disposed within bearing chamber 25 above and immediately adjacent to upper shoulder 41U of split ring 40. As best shown in
Accordingly, bearing chamber 25 of conventional bearing section 10 is defined by outer surface 21 of mandrel 20, inner surface 31 of housing 30, lower shoulder 32 in housing 30, and upper shoulder 34 in housing 30. Between bearing chamber 25 and lower end 30L of housing 30, a lower radial bearing (shown in the form of a lower bushing 24) is provided below bearing chamber 25 in an annular space between mandrel 20 and housing 30, to provide radial support to mandrel 20 as it rotates within housing 30. Similarly, an upper radial bearing (shown in the form of an upper bushing 26) is provided above bearing chamber 25 in an annular space between mandrel 20 and housing 30.
Bearing section 10 in
Referring now to
As shown in
A lower radial bushing 24 is provided below lower bearing chamber 125 in an annular space between mandrel 20 and housing 30 to provide radial support to mandrel 20 as it rotates within housing 30. Similarly, an upper radial bushing 26 is provided above lower bearing chamber 125. Lower bearing chamber 125 and cylindrical chamber 70 are contained within an annular oil reservoir sealed at its lower end by a lower rotary seal 115 between mandrel 20 and housing 30, and at its upper end by an upper rotary seal 135 between mandrel 20 and housing 30.
As shown in
Radial bearing races 142 and 144 may be formed from, or may have their respective contact surfaces 142A and 144A hard-faced with, a highly-polished and wear-resistant material such as tungsten carbide or cemented carbide. Optionally, either or both of contact surfaces 142A and 144A may be provided with flow channels (not shown) to facilitate the flow of lubricating mud over the interface between contact surfaces 142A and 144A. Although optional and not essential to the broadest embodiments described herein, radial bearing assembly 140 is advantageous to provide additional radial support to upper end 20U of mandrel 20 as it rotates within housing 30.
The operation of bearing section 100 may be readily understood with reference to the Figures and to the foregoing description. In addition to being rotatable relative to housing 30, mandrel 20 can also move axially relative to housing 30 over a short range of travel determined by the dimensions and positions of various components of the on-bottom and off-bottom thrust bearing assemblies. More specifically, when bearing section 100 is under on-bottom loading, such as when the drill bit is under load on the bottom of a wellbore, mandrel 20 is shifted slightly upward into housing 30 such that on-bottom thrust bearing 60 and its associated races 61U and 61L are in compression between load-transfer shoulder 27 of mandrel 20 and load-transfer shoulder 34 of housing 30. The compressive on-bottom thrust loads are thus transferred from mandrel 20 to housing 30 through thrust bearing 60.
This upward shift of mandrel 20 into housing 30 has the effect of shifting threaded ring 86 slightly upward relative to housing 30, thus opening a gap between lower face 84L of bearing race 84 and upper face 82U of bearing race 82. However, when compressive load on bearing section 100 is relieved (by lifting the drill bit off the bottom of the wellbore), bearing section 100 will then be under off-bottom (tensile) thrust loading, and gravity and/or fluid pressure will cause mandrel 20 to shift axially downward relative to housing 30, thereby bringing lower face 84L of bearing race 84 into tight mating contact with upper face 82U of bearing race 82, as seen in
During operation of the mud motor, drilling mud is pumped downward through drive shaft housing annulus 97 and then is directed into central bore 22 of mandrel 20 through mud flow channels 99 in drive shaft adapter 92. A small portion of the mud flow is diverted through off-bottom thrust bearing assembly 80 to provide lubrication and cooling for thrust bearing assembly 80 (and radial bearing assembly 140 when included) before rejoining the main flow of mud in central bore 22. This is illustrated more specifically in
In an unillustrated alternative embodiment, in which bearing race 84 is not fixed axially to threaded ring 86, fluid flow through the bearing assembly will keep faces 82U and 84L together, and a gap will open up between bearing race 84 and threaded ring 86, rather than between bearing race 84 and bearing race 82. However, the operation of the assembly will be otherwise as described above.
As previously noted, the radial bearing assembly 140 illustrated in
In the mud-lubricated off-bottom bearing assembly 80 shown in
Another alternative embodiment would use mud-lubricated roller bearings and races, such as mud-lubricated roller bearings available from QA Bearing Technologies Ltd. of Edmonton, Alberta and QA Bearing Technologies (USA) Inc. of Houston, Tex. Although not providing static load capacities as high as are available with other types of mud-lubricated bearings, these alternative bearings would nonetheless provide advantages over prior art bearing arrangements by virtue of not requiring a high-tolerance split ring to provide load-transferring shoulders (as in the prior art bearing section of
In alternative embodiments, radial bearings 112 and 114 could be provided in the form of PDC insert bearings or ball bearings.
In alternative embodiments radial bearing assembly 140 could be located below mud-lubricated off-bottom thrust bearing assembly 80, rather than above it as in the embodiment shown in
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.
