WO2015006325A1

WO2015006325A1 - Pressure equalizers for shaft seals of a downhole gearbox

Info

Publication number: WO2015006325A1
Application number: PCT/US2014/045749
Authority: WO
Inventors: Arthur Watson
Original assignee: Schlumberger Canada Limited; Services Petroliers Schlumberger; Schlumberger Holdings Limited; Schlumberger Technology B.V.; Prad Research And Development Limited; Schlumberger Technology Corporation
Priority date: 2013-07-08
Filing date: 2014-07-08
Publication date: 2015-01-15

Abstract

Pressure equalizers for the shaft seals of a downhole gearbox are provided. Various implementations prevent well fluid from entering a downhole gearbox through an output shaft seal, and prevent lubricating oil from leaking out of the gearbox through an input shaft seal. Small backpressure pumps may be connected to rotating shafts inside the gearbox to equalize local pressures on each side of the shaft seals. A volume compensator may communicate at a selected point along a pressure gradient in a flow tube to equalize pressures between shaft seals. Pressure tubes may be angled and transitioned in relation to a pumped fluid flow to modify fluid pressures in favor of shaft seal integrity. Sets of input and output seals may be connected by a fluid line to equilibrate pressures across all seals, and the fluid line may be favorably biased by communication with a pressure gradient in a flow tube.

Description

PRESSURE EQUALIZERS FOR SHAFT SEALS

OF A DOWNHOLE GEARBOX

RELATED APPLICATIONS

[0001] This patent application claims the benefit of priority to U.S. Provisional Patent No. 61/843,460 to Watson, filed July 8, 2013 and incorporated herein by reference in its entirety.

BACKGROUND

[0002] Some downhole centrifugal pumps are driven by a surface motor that provides torque via a long string of mechanical rods. Because the rods cannot be driven at the same speed as the centrifugal pump, a downhole gearbox is used to step up the rotational speed provided by the rods. The input and output shafts of the downhole gearbox have shaft seals that contain and secure the interior lubricating gearbox oil, while keeping well fluid out of the gearbox. However, these shaft seals are subject to pressure differentials on their two sides. The input and output shafts of the gearbox rotate at different speeds, and may have different-size gears rotating at different rates, thereby creating fluid pressure effects. Moreover, a downhole gearbox usually has multiple flow tubes running alongside its interior, for cooling, or just to give the production fluid an avenue to the surface. Although the well fluid in the flow tubes is separately compartmentalized from the gearbox oil, the openings and exits of the flow tubes are located very near the exterior of the shaft seals, and create an exterior pressure differential near the shaft seals. The well fluid outside of the shaft seal of the output shaft of the gearbox is at a higher pressure than the well fluid outside the shaft seal of the input shaft of the gearbox, because the well fluid is being pumped from bottom to top, and because the bottom of the gearbox is lower down in the well, hence higher pressure at the bottom on both counts. Thus, well fluid tries to force its way into the gearbox through the bottom output shaft seal, while gearbox oil tries to leak out of the gearbox through the higher input shaft seal. If unchecked, the gearbox soon ends up filled with well fluid, instead of lubricating oil.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The detailed description is set forth with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components.

[0004] For this discussion, the devices and systems illustrated in the figures are shown as having a multiplicity of components. Various implementations of devices and systems, as described herein, may include fewer components and remain within the scope of the disclosure. Alternately, other implementations of devices and systems may include additional components, or various combinations of the described components, and remain within the scope of the disclosure.

[0005] Fig. 1 is a diagram of an example gearbox with an example pressure equalizer for the shaft seals of the gearbox.

[0006] Fig. 2 is a diagram of an example well containing a rod-driven pump with the example gearbox of Fig. 1 .

[0007] Fig. 3 is a diagram of the example gearbox of Fig. 1 , for downhole use.

[0008] Fig. 4 is a diagram of example backpressure pumps as pressure equalizers for the shaft seals of a downhole gearbox.

[0009] Fig. 5 is a diagram of an example volume compensator with strategic intermediate communication with a pressure gradient in a flow tube for equilibrating pressures of shaft seals in a downhole gearbox.

[0010] Fig. 6 is a diagram of example instances of angled and transitioned pressure tubes for relieving pressure differentials on shaft seals of a downhole gearbox.

[0011] Fig. 7 is a diagram of a fluid communication line between sets of input and output shaft seals, with the fluid communication line in strategic fluid communication with a particular pressure gradient in a flow tube.

[0012] Fig. 8 is a flow diagram of an example method of preventing fluid from crossing shaft seals of a gearbox for a downhole pump. DETAILED DESCRIPTION

[0013] Pressure equalizers for gearbox shaft seals are described herein. The pressure equalizers described herein preserve the integrity and lifespan of oil seals used in the gearboxes (transmissions) associated with a type of pump used in severe downhole environments. For example, geared centrifugal pumps (GCPs) are downhole centrifugal pumps similar to electric submersible pumps (ESPs), but instead of being driven by a submergible motor, the GCPs are driven by a surface power source, such as a surface motor, with torque transmitted downhole via a shaft that usually consists of rods. Because the rods cannot be driven to rotate in a 1 :1 ratio at the same speed as the optimum pump speeds, the downhole "gearbox" or "transmission" is used to step-up the speed of rotation.

[0014] Fig. 1 shows an example gearbox 100 including an example gear train 102 as used to drive a downhole pump. The downhole gearbox 100 is filled with lubricating oil 104. Thermal expansion of the lubricating oil 104 itself may be conventionally compensated for by a conventional diaphragm or piston that equilibrates pressure to a comparable pressure as the fluid in the production tubing above the gearbox 100. The gearbox 100 has a set of multiple mechanical shaft seals 106 & 108, respectively at the input shaft 110, where rotational power is applied to the gearbox 100 at a given rotational speed from a drive head on the surface, and at the output shaft 112, where rotational power is provided from the gearbox 100 to the downhole pump, at a rotational speed that is some multiple (i.e., higher multiple) of the rotational rod speed input to the gearbox 100 from the surface drive head. Conventionally, overall thermal expansion of the interior volume for the lubricating oil 104 between adjacent shaft seals 106 & 108 in the gearbox 100 may be provided in the aforementioned manner of using a diaphragm or piston to allow a variable fluid volume for the gearbox oil 104.

[0015] In various implementations, one or more pressure equalizer devices 114 or related techniques are integrated into the gearbox 100 to render aberrant or unequal pressures on each side of each vulnerable shaft seal 106 & 108 less harmful to the affected seal 106 & 108 and less harmful to the interior of the gearbox 100.

[0016] Fig. 2 shows an example pump deployment 200, in which a geared centrifugal pump (GCP) 202 is lower down in the well then the transmission or gearbox 100, which is positioned above the pump 202 and directly drives the pump 202. The gearbox 100, in turn, is lower than a rod string 204 that provides power drive to the gearbox 100, extending between the gearbox 100 and a rotational power source 206 on the surface. The gearbox 100 has upper seals 106 on its upper end and may have a compensator on this upper end, and the gearbox also has lower seals 108 on its lower end, and may have a thrust bearing on this lower end.

[0017] Fig. 3 shows the exterior of an example gearbox 100 for a geared centrifugal pump (GCP) 202. The fluid discharge of the pump 202 passes by the inner workings of the gearbox 100, for example in flow tubes 302 that are integrated into the gearbox 100 as part of the gearbox design. An example gearbox 100 may have multiple of such flow tubes 302, for example "D-tubes," which are flow tubes 302 that each have a cross-sectional open channel that is D-shaped because the flow tubes 302 run along the curved exterior of the gearbox 100, as shown.

[0018] In the flow tubes 302, such as D-tubes, there is a pressure differential across the axial length of the flow tubes 302 that is dependent on flow rate and can reach approximately 10 pounds per square inch (psi) across the length of the flow tube 302. In this description, an example 10 psi differential will be used as an example to illustrate the subject matter. This pressure differential can cause two problems with the shaft seals 106 & 108. First, the pressure differential increases the leakage rate at the shaft seals 106 & 108 compared to a pressure-balanced shaft seal. Second, the pressure differential causes well fluid to leak into the output shaft seals 108 from outside the gearbox 100, and causes gear oil 104 within the gearbox 100 to leak out of the input shaft seals 106. Well fluid therefore gradually replaces gear oil 104, eventually causing gear failure in the gearbox 100.

[0019] In an implementation, example pressure equalizers 114 for such a gearbox 100 reduce the pressure differential across the shaft seals 106 & 108 and thereby reduce the leakage of well fluid into the gearbox 100. The example pressure equalizers 114 for the shaft seals 106 & 108 may be used in conventional oil wells, high temperature oil wells, and steam assisted gravity drainage (SAGD) oil wells. The example pressure equalizers 114 for shaft seals 106 & 108 can also be applied to a different variety of downhole or process pumps in which the input and output shaft seals 106 & 108 operate at different pressures.

[0020] Several techniques for reducing the pressure differential across GCP shaft seals 106 & 108 and for reducing leakage of well fluid into output shaft seals 108 and leakage of gear oil 104 out of input shaft seals 106 are described. The example techniques can be used individually, or combined together.

[0021] In an implementation, as shown in Fig. 4, one or more backpressure pumps 402 & 404 within the gearbox fluid 104 (transmission fluid) are employed. A small centrifugal or axial pump 402 or 404 on the shaft 408 inboard of each seal set 106 & 108 produces a pressure approximately equal to the pressure of the well fluid 410 on the opposite side of the seal 106 or 108. Thus, the example pump 404 located inboard of the output shaft seals 108 increases the internal pressure on the seal 108 by, for example, 10 psi. The example pump 402 located inboard the input shaft seals 106 decreases the internal pressure on the seal by, for example, 10 psi. These example pumps 402 & 404 reduce leakage into the output shaft seal 108 and out of the input shaft seal 106.

[0022] In an implementation, as shown in Fig. 5, an example intermediate equalizer 114 can be used to decrease or eliminate a pressure differential between the output shaft seals 108 and the input shaft seals 106. For example, the intermediate equalizer 114 may include components of a volume compensator (e.g., diaphragm, bellows) for the expansion and contraction of the lubricating oil 104 in the gearbox 100, but placed in fluid communication with one or more of the flow tubes 302 (e.g., D-tubes) at a strategic location intermediate along the length (at a mid-region) of the flow tubes 302, instead of fluidly connecting, as conventionally, with the flow tubes 302 at a location above the gearbox 100. This intermediate equalization device 114 or technique produces a pressure in the gearbox 100 that is intermediate between the pressure of the well fluid 410 on the output shaft seals 108 and gearbox oil 104 on the input shaft seals 106. For example, the example intermediate equalizer 114 may reduce the differential pressure on the seals 106 & 108 from 10 psi to 5 psi, reducing the maximum pressure differential and the leakage across any shaft seal 106 & 108.

[0023] As shown in Fig. 6, in an implementation, a pressure tube equalizer 602 may also be employed to equalize pressure on the output shaft seals 108 and the input shaft seals 106. Fig. 6 shows various implementations of a single configuration, wherein the equalization of pressure depends on the placement of the pressure tube 602, its diameter, its angle with respect to fluid flow 604, and so forth. In an example configuration, the outside of a volume compensator 606 communicates with the produced fluid 604 through an orifice that may be angled and transitioned in relation to the flow stream 604 to produce a certain pressure in the gearbox 100 that is intermediate between the pressure of the well fluid 410 on output shaft seals 108 and input shaft seals 106, reducing the maximum pressure differential and leakage across any shaft seal 106 & 108. [0024] In an implementation, an example gearbox 100 uses interconnected shaft seals 106 & 108, as shown in Fig. 7. For example, the oil volume between adjacent output shaft seals 108 & 108' can be communicated via a line 702 with the oil volume between adjacent input shaft seals 106 & 106'. Both sets of seals 106 & 108 can communicate with a common compensator 704, so that the sets are paired. The outside of the compensator 704 for each set of paired seals can communicate with the well fluid 410 in a flow tube 302 at a specified point in the pressure gradient along the flow tube 302 that is selected to optimize overall sealing. For example, by locating the compensator 704 for the paired sets of shaft seals 106 & 108 nearer the input shaft, any leakage of the well fluid 410 into the first output shaft seal 108 may be shunted to produce a positive internal pressure on the first output shaft seal 108, so that the well fluid 410 tends to leak out of that seal 108 instead of into the gearbox 100.

Example Methods

[0025] Fig. 8 is an example method 800 of preventing fluid from crossing shaft seals of a gearbox for a downhole pump. In the flow diagram, operations are represented by individual blocks.

[0026] At block 802, a gearbox for a downhole pump is outfitted with one or more input shaft seals and one or more output shaft seals.

[0027] At block 804, pressure is equalized across the input shaft seals and the output shaft seals to prevent well fluid from crossing the one or more output shaft seals into the gearbox, and to prevent gearbox lubricating oil from crossing the one or more input shaft seals to leak out of the gearbox.

Conclusion

[0028] Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from a deployment and retrieval system. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112 (f) for any limitations of any of the claims herein, except for those in which the claim expressly uses the words 'means for' together with an associated function.

Claims

CLAI MS WHAT IS CLAIMED IS:

1 . A system, comprising:

a gearbox for a downhole pump;

at least a shaft in the gearbox;

at least one input shaft seal in the gearbox;

at least one output shaft seal in the gearbox;

a pressure equalizer in the gearbox to equalize pressures on each side of the at least one input shaft seal and on each side of the at least one output shaft seal.

2. The system of claim 1 , wherein the pressure equalizer prevents a well fluid from crossing one or more output shaft seals or prevents a gearbox oil from crossing one or more input shaft seals.

3. The system of claim 1 , wherein the pressure equalizer comprises one or more pumps inside the gearbox.

4. The system of claim 3, wherein the one or more pumps comprise at least a backpressure pump, each backpressure pump further comprising a centrifugal or axial pump on the shaft and located inboard of an associated shaft seal to produce a pressure approximately equal to a pressure on an other side of the shaft seal.

5. The system of claim 1 , wherein the pressure equalizer comprises an intermediate equalizer to decrease or eliminate a pressure differential between the at least one output shaft seal and the at least one input shaft seal, the intermediate equalizer comprising a volume compensator in communication with an intermediate part or a mid-region of a flow channel through the gearbox.

6. The system of claim 1 , wherein the pressure equalizer comprises a pressure tube or an orifice between a volume compensator for the gearbox oil and a flow channel through the gearbox.

7. The system of claim 1 , wherein the pressure equalizer comprises a line communicating a fluid or a pressure between an output shaft seal and an input shaft seal.

8. The system of claim 7, wherein the line is in fluid communication or in pressure communication with a selected pressure location along a pressure gradient of a flow channel of the gearbox.

9. A method, comprising:

including one or more input shaft seals and one or more output shaft seals in a gearbox for a downhole pump; and

equalizing pressures on each side of each shaft seal to prevent well fluid from crossing the seal and to prevent a gearbox lubricating oil from crossing the seal.

10. The method of claim 9, further comprising pumping a lubricating fluid near the seal to equalize the pressure on each side of the seal.

1 1 . The method of claim 9, further comprising equilibrating pressures between an input shaft seal and an output shaft seal via a volume compensator in communication with an intermediate part of a flow channel through the gearbox.

12. The method of claim 9, further comprising modifying a fluid pressure by angling a pressure tube with respect to a fluid flow in a flow channel of the gearbox.

13. The method of claim 9, further comprising equilibrating pressures between a set of input shaft seals and a set of output shaft seals via a fluid communication line between the sets of shaft seals.

14. The method of claim 13, further comprising establishing a fluid or pressure communication between the fluid communication line and a pressure location along a pressure gradient of a flow channel of the gearbox.

15. An apparatus, comprising:

a transmission for a geared centrifugal pump;

an input shaft seal of the transmission;

an output shaft seal of the transmission; and

a pressure equalizer for each of the input shaft seal and the output shaft seal to prevent a fluid from crossing the input shaft seal or the output shaft seal.

16. The apparatus of claim 15, further comprising an instance of a backpressure pump for each of the input shaft seal and the output shaft seal, each backpressure pump powered by an internal mechanical motion of the transmission.

17. The apparatus of claim 15, wherein the pressure equalizer further comprises a volume compensator for a lubricating fluid of the transmission, the volume compensator in pressure communication with a fluid pressure along a pressure gradient near a midpoint of a flow channel in the transmission.

18. The apparatus of claim 15, wherein the pressure equalizer further comprises a pressure tube, the pressure tube having an angle with respect to a flowing fluid, and the pressure tube having an opening angled with respect to the flowing fluid for modifying a local fluid pressure around one side of the input shaft seal or one side of the output shaft seal.

19. The apparatus of claim 15, wherein the pressure equalizer further comprises a fluid pipe between the input shaft seal and the output shaft seal to allow fluid and pressure to equilibrate between the input shaft seal and the output shaft seal.

20. The apparatus of claim 19, wherein the fluid pipe is in pressure communication with a selected location along a flow channel of the transmission, to equilibrate a first fluid pressure in the fluid pipe with a second pressure along a pressure gradient of the flow channel.