Patent Application
20120177308
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 pressure compensation systems for oil-sealed bearing assemblies.
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).
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). This condition occurs, for instance, when the drill string is being pulled out of the wellbore, 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
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, 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 (usually but not necessarily roller bearings contained within a bearing cage) disposed within an annular bearing containment chamber.
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 bearings are located within an oil-filled reservoir in an annular region between the mandrel and the housing, with the reservoir being defined by the inner surfaces of the housing and the outer surface of the mandrel, and by sealing elements at each end of the reservoir. Because of the relative rotation between the mandrel and the housing, these sealing elements must include rotary seals.
Mud motor bearing sections also include multiple radial bearings to maintain coaxial alignment between the mandrel and the bearing housing. In an oil-sealed assembly, the radial bearings can be provided in the form of bushings disposed in an annular space between the inner surface of the housing and the outer surface of the mandrel. It is desirable to maximize radial support for the mandrel in order to maximize the mandrel's resistance to flexural stresses induced when drilling non-straight wellbores.
An oil-sealed bearing assembly must incorporate pressure compensation means, whereby the volume of the annular oil reservoir is automatically adjusted to compensate for changes in oil volume due to temperature changes. In addition, certain types of elastomeric rotary seals (such as KALSI SEALS®) are designed to slowly pump oil underneath the seal interface, and this causes a gradual reduction in oil volume which also must be compensated for. For optimum performance of the rotary seal, it is ideal for the sealing surface of the mandrel to be as wear-resistant as possible, while having a very fine surface finish.
A common method of providing pressure compensation in an oil-sealed bearing assembly uses an annularly-configured piston disposed within an annular region (or “piston chamber”) between the housing and mandrel. The outer diameter (O.D.) of the piston is sealed against the inner bore of the housing (by means of one or more sliding seals, such as O-rings), and also may incorporate anti-rotation seals to ensure that the piston does not rotate relative to the housing. The inner diameter (I.D.) of the piston is sealed against the mandrel by means of a rotary seal, which rotates relative to the mandrel during operation, and also slides axially along the mandrel as the piston moves. The rotary seal and sliding seals associated with the piston thus define the upper end of the oil reservoir within the bearing assembly.
A sufficient length of the mandrel below the piston's initial position must remain uninterrupted to accommodate the piston travel that will occur as oil volume varies over time (whether due to temperature change or oil loss). The housing bore must be similarly uninterrupted along this length, forming a cylindrical oil reservoir. The uppermost radial support is thus located at a point below the oil reservoir. Therefore, a significant length of the mandrel in a conventional oil-sealed mud motor bearing section is not radially supported.
Alternatively, radial support for the mandrel may be provided to some extent by the pressure-compensating piston itself. However, the length of radial support is limited to the length of the piston (which desirably should be minimized), and the mandrel will still be unsupported along the length of the oil reservoir (said length of which will be greatest when the oil reservoir is full and the piston is at its uppermost position).
For optimum performance of the rotary seal, it is ideal for the sealing surface of the mandrel to be as wear-resistant as possible, with a very fine surface finish. This is typically provided through the use of a surface treatment such as an abrasion-resistant, diamond-ground coating. To accommodate axial translation of the piston within the piston chamber, the surface treatment of the mandrel needs to be provided over a length corresponding to at least the range of travel of the piston's rotary seal, and preferably the full length of the piston chamber.
Accordingly, there remains a need in the art for a pressure compensation system for oil-sealed mud motor bearing assemblies that provides radial support for the portion of the mandrel corresponding to the stroke of the pressure-compensating piston. Embodiments disclosed herein are directed to these needs.
In accordance with at least one embodiment disclosed herein, a cylindrical sleeve is mounted, internally and coaxially, within the cylindrical housing of an oil-sealed bearing assembly in a mud motor, such that the sleeve is non-rotatable relative to the housing, and such that a cylindrical chamber is formed between the O.D. of the sleeve and the I.D. of the housing. The mandrel of the bearing assembly rotates coaxially within the sleeve, with suitable bearing means (such as a bushing) disposed between the I.D. of the sleeve and the O.D. of the mandrel. The sleeve effectively provides radial support to the corresponding length of the mandrel by virtue of the sleeve's flexural stiffness, such that flexural stresses induced in the mandrel during well-drilling operations will be less than they would be in a bearing assembly not having the radial support sleeve.
The above-noted cylindrical chamber between the O.D. of the radial support sleeve and the I.D. of the housing forms part of a generally annular oil reservoir in which one or more oil-lubricated thrust bearings are disposed. An annularly-configured pressure-balancing piston is disposed within the cylindrical chamber, and is axially movable within the chamber in response to variations in the volume of oil in oil reservoir. Because the radial support sleeve is non-rotating relative to the housing, the piston simply slides within the cylindrical chamber, and therefore can use simple sliding seals rather than rotary seals, which are generally more costly and susceptible to wear than non-rotary seals. As well, there is no need to provide the piston with anti-rotation seals, thus considerably reducing the seal friction that must be overcome as the piston translates during compensation. Accordingly, in addition to providing radial support for the mandrel along the length of the cylindrical chamber (unlike in conventional oil-sealed bearing assemblies), the radial support sleeve provides the significant further benefit of eliminating the need for rotary seals in the pressure-balancing piston. Instead, the upper rotary seal for the oil reservoir is housed in a fixed location within the housing rather than being associated with the piston, such that it does not translate during operation. Therefore, the length of the mandrel requiring wear-resistant surface treatment for the rotary seal can be kept to a minimum, resulting in significant cost savings.
Accordingly, at least one embodiment disclosed herein teaches an oil pressure compensation system for a mud motor bearing section, where the pressure compensation system comprises:
In another aspect, at least one embodiment disclosed herein teaches a bearing section for a mud motor, where the bearing section comprises:
In a further aspect, at least one embodiment disclosed herein teaches a method of providing increased radial support for a mandrel rotatable within the cylindrical housing of a mud motor bearing section having a bearing chamber, where the method comprises the steps of:
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 numerical references denote like parts, and 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.
Any use of any form of the terms “connect”, “mount”, “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”, “coincident”, “intersecting”, 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 the illustrated prior art bearing section 10, a bearing assembly 50 is disposed within an annular bearing chamber between mandrel 20 and housing 30, at roughly mid-length of bearing section 10. For illustration purposes, bearing assembly 50 is shown as comprising a lower bearing 52 (with associated bearing races) for resisting off-bottom thrust loads; an upper bearing 54 (with associated bearing races) for resisting on-bottom thrust loads; and a split ring 56 mounted to mandrel 20 to provide load-transferring shoulders for transferring thrust loads to bearings 52 and 54. However, the structural and operational details of bearing assembly 50 are not directly relevant to embodiments of the present invention, and therefore are not described in further detail in this patent specification. Between bearing assembly 50 and lower end 30L of housing 30, a lower radial bearing (shown in the form of a lower bushing 24) is provided in an annular space between mandrel 20 and housing 30, to provide radial support to mandrel 20 as it rotates within housing 30.
Referring now to
Piston chamber 70 has an upper end 70U and a lower end 70L, defining a piston travel length LPT through which piston 80 can travel. An upper radial bearing (shown in the form of an upper bushing 26) is provided in an annular space between mandrel 20 and housing 30 in a region between bearing assembly 50 and lower end 70L of piston chamber 70. However, a portion of mandrel 20 having a length corresponding to piston travel length LPT has no radial support (except to the variable extent of any radial support provided by piston 80).
At a point above (and preferably directly above) bearing assembly 50, a cylindrical sleeve 90 is mounted inside, and coaxial with housing 30, such that sleeve 90 is non-rotatable relative to the housing, and such that an annular piston chamber 170 (with upper end 170U and lower end 170L) is formed between the outer cylindrical surface 91 of sleeve 90 and the inner cylindrical surface 31 of housing 30. In general, sleeve 90 may be non-rotatably mounted to housing 30 in any suitable way known in the art. By way of non-limiting example, this is achieved in the embodiment shown in
The upper end 96 of sleeve 90 is anchored to housing 30 by any suitably secure means (such as but not limited to friction due to makeup torque applied to threaded connection 94A). An upper bushing 126 is provided in an annular space between mandrel 20 and the inner cylindrical surface 92 of sleeve 90, to facilitate rotation of mandrel 20 within sleeve 90 (optionally with lubrication channels 28 provided in the inner cylindrical surface 92 of sleeve 90 to allow passage of oil to lubricate bushing 126 and upper rotary seal 182).
An annular pressure-balancing piston 180 is disposed within piston chamber 170, and is axially and bi-directionally movable therein. Piston 180 has an outer face 180A for sliding engagement with inner surface 31 of housing 30 in conjunction with an outer seal 93A, and an inner face 180B for sliding engagement with outer surface 91 of sleeve 90 in conjunction with an inner seal 93B. Since sleeve 90 is non-rotatable relative to housing 30, piston 180 does not rotate relative to both housing 30 and sleeve 90. Accordingly, outer seal 93A and inner seal 93B can be sliding seals (such as O-rings or lip seals) rather than rotary seals.
A generally annular oil reservoir is thus formed between lower rotary seal 15, upper rotary seal 182, piston 90 (with sliding seals 93A and 93B), outer surface 91 of sleeve 90, outer surface 21 of mandrel 20, and inner surface 31 of housing 30, and includes piston chamber 170 and the bearing chamber associated with bearing assembly 50. Piston 180 is also shown with an optional bushing 184 engaging outer surface 91 of sleeve 90.
Sleeve 90 effectively provides radial support to the corresponding length of mandrel 20 by virtue of the flexural stiffness of sleeve 90. Furthermore, since piston 180 does not rotate relative to either housing 30 or sleeve 90, rotary seals and anti-rotation seals within piston 180 are unnecessary. Whereas the upper rotary seal 82 in the prior art assembly of
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.
