GB2408776A

GB2408776A - Helical Moineau pump having small radius peaks on rotor and stator

Info

Publication number: GB2408776A
Application number: GB0328498A
Authority: GB
Inventors: Andrei Plop
Original assignee: Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 2003-12-06
Filing date: 2003-12-06
Publication date: 2005-06-08
Also published as: GB0328498D0

Abstract

A Moineau type pump has a rotor 34 with helical lobes 42 having relatively sharp, small radius peaks. The corresponding helical troughs 40 of the stator 36 also have a small radius. The rotor peaks define a sealing line between the rotor and the stator, and may have tip seals 46 possibly in the form of wiper seals, which may be energised by fluid pressure within the rotor. The rotor 34 and stator 36 may both be made of metallic materials, to improve robustness for downhole use.

Description

PROFILE FOR USE IN A DOWNHOLE MUD MOTOR

FIELD OF THE INVENTION

The invention relates generally to rotor/stator profiles for use with positive displacement drilling motors. More specifically, the invention relates to a novel profile for use in a mud motor power section.

BACKGROUND OF THE INVENTION

Positive Displacement Motors (PDMs) are known in the art and are commonly used to drill wells in earth formations. PDMs operate according to a reverse mechanical application of the Moineau principle wherein pressurized fluid is forced though a series of channels formed on a rotor and a stator. The channels are generally helical in shape and may extend the entire length of the rotor and stator. The passage of the pressurized fluid generally causes the rotor to rotate within the stator. For example, a substantially continuous seal may be formed between the rotor and the stator, and the pressurized fluid may act against the rotor proximate the sealing surfaces so as to impart rotational motion on the rotor as the pressurized fluid passes through the helical channels.

A rotor and a stator mounted together form what is usually referred to as the "power section".

Figures 1-3 illustrate a typical PDM power section known in the art.

Referring to Figure 1, an end view of a prior art rotor 10 is provided. The typical prior art rotor 10 includes at least one lobe 12 (wherein, for example, channels 14 are formed between lobes 12), a major diameter 8, and a minor diameter 6. The rotor 10 may be formed of metal or any other suitable material. The rotor 10 may also be coated to withstand harsh drilling environments experienced downhole.

A prior art stator 20 is illustrated in Figures 2 and 3. Figure 2 provides an end view of the stator 20 only, while Figure 3 provides an end view of the assembled rotor 10 and stator 20. A typical prior art stator 20 comprises at least two lobes 22, a major diameter 7, and a minor diameter 5. Note that if the rotor (10 in Figure 1) includes "n" lobes, the corresponding stator 20 used in combination with the rotor 10 generally includes "n+1" lobes.

The typical stator 20 generally includes a cylindrical external tube 24 having an inner surface 29 that is generally cylindrical in shape. An inner liner 26 comprising an elastomer, a plastic, or other synthetic or natural material is formed along the inner surface 29 of the stator 20. Typically, the liner 26 is injected into the cylindrical external tube 24 around a mold (not shown) that has been placed therein. The liner 26 is then cured for a selected time at a selected temperature (or temperatures) before the mold (not shown) is removed. The thickness 28 of the liner 26 and the shape of the lobes 22 is generally controlled by changing the dimensions of the mold (not shown). \ A rotational frequency and, for example, an amount of torque generated by the rotation of the rotor 10 within the stator 20 may be selected by determining a number of lobes on the rotor 10 and stator 20. Likewise, the major and minor diameter of the rotor 10 and stator 20 affect the rotation frequency and the amount of torque generated by the PDM power section.

A lower end of the rotor 10 may be coupled either directly or indirectly to, for example, a drill bit. In this manner, the PDM provides a drive mechanism for a drill bit independent of any rotational motion of a drillstring generated proximate the surface of the well by, for example, rotation of a rotary table on a drilling rig. Accordingly, PDMs are especially useful in drilling directional wells where a drill bit is connected to a lower end of a bottom hole assembly (BHA).

The motor may be comprised of, for example, a power section, a transmission assembly, a bent housing assembly, a bearing section. The BHA may be formed, for instance, by a motor and the drill bit. The rotor 10 may transmit torque to the drill bit via a drive shaft or a series of drive shafts that are operatively coupled to the rotor 10 and to the drill bit. Therefore, when directionally drilling a wellbore, the drilling action is typically referred to as "sliding" because the drill string slides through the wellbore rather than rotating through the wellbore (as would be the case if the drill string were rotated using a rotary table) because rotary motion of the drill bit is produced by the PDM. However, directional drilling may also be performed by rotating the drill string and using the PDM, thereby increasing the available torque and drill bit rpm.

Rotation of the rotor 10 within the stator 20 causes the rotor 10 to nutate within the stator 20. The motion of the rotor 10 within the stator 20 may be defined by a circle O which defines a trajectory of a point A disposed on a rotor axis as point A moves around a stator axis B during a series of notations.

Note that an "eccentricity" e of the assembly may be defined as a distance between the rotor axis A and the stator axis B when the rotor 10 and stator 20 are assembled to form a power section.

As illustrated in Figure 3, in a typical prior art PDM power section, the lobes 12 of the rotor 10 and the corresponding lobes 22 of the stator 20 have large radius peaks. Thus, notation of the rotor 10 results in a moving seal point between the rotor 10 and the stator 20. Because the seal line is constantly moving, the prior art PDM requires the use of compliant sealing material (such as rubber or polyurethane) along the entire rotor/stator profile, either on the rotor 10 or on the stator 20 or on both.

There are several problems that may be encountered as a result of using compliant sealing material along the entire rotor/stator profile. For instance, rotation of the rotor 10 within the stator 20 can, under certain conditions, shear off portions of the stator lobes 22. This process, which may be referred to as "chunking," deteriorates the seal formed between the rotor 10 and stator 20 and may cause failure of the PDM. Chunking may be increased by swelling of the liner 26 or thermal fatigue. Swelling and thermal fatigue may be caused by elevated temperatures and exposure to certain drilling fluids and formation fluids, among other factors. Moreover, flexibility of the liner 26 may lead to incomplete sealing between the rotor 10 and stator 20 such that available torque may be lost when the rotor compresses the stator lobe material, thereby reducing the power output of the PDM.

Accordingly, there is a need for a rotor/stator design that provides increased power output and increased longevity in harsh downhole environments.

SUMMARY OF INVENTION

In one aspect, the invention comprises a positive displacement motor power section. The positive displacement motor power section comprises a stator and a rotor. The stator comprises an outer surface and an inner surface.

The inner surface of the stator comprises at least three radially inwardly projecting lobes that extend helically along a selected length of the stator. The stator lobes define small radius peaks. The rotor of the positive displacement motor is adapted to nutate within the stator. The rotor has outwardly projecting lobes that extend helically along the length of the rotor. The rotor lobes define small radius peaks. The rotor has one fewer outwardly projecting lobe than the number of inwardly projecting lobes of the stator. The interaction between the inner surface of the stator and the outwardly projecting lobes of the rotor form the sealing points between the stator and the rotor.

In another aspect, the invention comprises a liner-less stator for a positive displacewcnt motor power section. The liner-less stator comprises an external tube having an outer surface and an inner surface. The inner surface of the stator comprises at least three radially inwardly projecting lobes extending helically along a selected length of the external tube. The inwardly projecting lobes define small radius peaks. The inner surface of the external tube is s adapted for receipt of a rotor having one fewer outwardly projecting lobes than the number of inwardly projecting lobes of the stator.

In another aspect, the invention comprises a positive displacement motor power section. The positive displacement motor power section comprises a stator and a rotor. The stator has at least three small radius lobes and the rotor has one fewer small radius lobe than the stator. The small radius lobes of the rotor define the sealing points between the stator and the rotor.

Other aspects and advantages of the invention will be apparent from the

following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

Figure I shows a prior art rotor.

Figure 2 shows a prior art stator.

Figure 3 shows an assembled view of a prior art positive displacement motor.

Figure 4 shows an embodiment of the present invention wherein the rotor has small radius cusps.

Figure 5 shows a cross-sectional view of an embodiment of the cusped rotor of the present invention.

DETAILED DESCRIPTION

Figure 4 shows an embodiment of a positive displacement motor (PDM) power section 30 comprising at least one aspect of the present invention. The PDM 30 comprises a stator 32 and a rotor 34. The stator 32 comprises an external tube 36 that may be formed from, for example, steel or another material suitable for downhole use in a drilling environment. As shown, the stator 32 of the present invention does not incorporate a liner. However, it should be understood that depending upon the application and operational conditions, a suitable liner may be incorporated and remain within the scope of the present invention.

The stator 32 comprises a shaped inner surface 38 that is defined by a profile that is rotated and translated through space so as to generate a geometry having a defined pitch length. The resulting lobes 40 of the stator 32 define small radius peaks and are helically formed along a selected length of the external tube 36 so that the lobes 40 define a helical pattern along the selected length.. The stator 32, including the inner surface 38, may be helically shaped by any means known in the art including machining, extrusion, and the like.

Although the illustrated stator 32 has four (4) peaked lobes 40, it should be understood that any number of lobes 40 may be utilized depending upon the number of lobes existing on the corresponding rotor 34. The present invention utilizes a rotor/stator profile of the "n/n+l" type. Thus, in the present invention, the number of lobes 40 formed on the stator 32 is always one (1) more than the number of lobes 42 formed on the rotor 34.

The rotor 34 illustrated in Figure 4 has a profile corresponding to the profile of the stator 32. For the rotor 34 to nutate and remain at all times with its axis parallel to the axis of the stator 32 (but not coaxial), the pitch of the rotor 34 must always be smaller than the pitch of the stator 32 by a factor equal to the ratio of the number of lobes 42 on the rotor 34 and the stator 32. Thus, for example, in the embodiment shown the rotor 34 should have a pitch that is 3/4 times smaller than that of the stator 32. By generating a rotor 34 and stator 32 in such a way, the rotor 34 will nutate within the stator 32 such that the lobes 42 of the rotor 34 are always in contact with the inner surface 38 of the stator 32. Accordingly, each of the cavities 48 formed between the rotor 34 and the stator 32 can be sealed from each adjacent cavity 48.

During notation of the rotor 34, the sealing points between the rotor 34 and the stator 32 are the peaks of the rotor lobes 42. Accordingly, it is only necessary to provide compliant sealing material at the peaks of the rotor lobes 42 to seal the cavities 48 between the rotor 34 and the stator 32. In one embodiment of the present invention, the peaks of the rotor lobes 42 are comprised of an elastomeric, polymeric, or otherwise suitable sealing material.

In another embodiment, shown in Figure 5, the peaks of the rotor lobes 42 further comprise cusps 44 adapted for the receipt of lip or wiper-type seals 46.

The wiper-type seals 46 can be energized from inside the rotor 34 using the pressure differential from the operating power section. The differential pressure between the top and the bottom of the rotor 34 is communicated to the seals 46 through a central bore within the rotor 34.

Because the sealing points between the rotor 34 and the stator 32 are the peaks of the rotor lobes 42, the stator 32 does not require a liner (liner-less stator 32) and can be made entirely of metallic materials or other structural materials that ensure no deformation under pressure and improved efficiency/reliability at extreme pressures, temperatures, and other downhole environmental conditions. The sealing materials that are more susceptible to such conditions are located only in discrete positions along the rotor 34. Thus, aside from the discrete sealing locations, the rotor 34 can be made entirely of metallic materials.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

What is claimed is: A positive displacement motor power section, comprising: a stator comprising an outer surface and an inner surface, the inner surface comprising at least three radially inwardly projecting lobes defining a pitch and extending helically along a selected length of the stator, the inwardly projecting lobes defining small radius peaks; a rotor adapted to nutate within the stator, the rotor having outwardly projecting lobes defining a pitch and extending helically along the length of the rotor, the outwardly projecting lobes defining small radius peaks corresponding with the small radius peaks of the stator, the rotor having one fewer outwardly projecting lobe than the number of inwardly projecting lobes of the stator; and wherein the interaction between the inner surface of the stator and the peaks of the outwardly projecting lobes of the rotor form the sealing points between the stator and the rotor.
2. The positive displacement motor power section of claim I, wherein the stator is entirely comprised of metallic materials.
3. The positive displacement motor power section of claim 1, wherein the rotor is substantially comprised of metallic materials.
4. The positive displacement motor power section of claim 1, wherein the peaks of the outwardly projecting lobes of the rotor are comprised of sealing materials.
5. The positive displacement motor power section of claim 1, wherein the peaks of the outwardly projecting lobes of the rotor are further adapted for the receipt of wiper-type seals.
6. The positive displacement motor power section of claim 5, wherein the wiper-type seals are energized from inside the rotor using the differential pressure from the operating power section.
7. The positive displacement motor power section of claim 6, wherein the differential pressure between the top and the bottom of the rotor is communicated to the seals from the top of the rotor through a central tunnel inside the rotor.
8. The positive displacement motor power section of claim 1, wherein the pitch of the rotor is less than the pitch of the stator.
9. A liner-less stator for a positive displacement motor power section, comprising: an external tube comprising an outer surface and an inner surface, the inner surface comprising at least three radially inwardly projecting lobes extending helically along a selected length of the external tube, the inwardly projecting lobes defining small radius peaks; and wherein the inner surface of the external tube is adapted for receipt of a rotor having one fewer outwardly projecting lobes than the number of inwardly projecting lobes of the stator.
10. The stator of claim 9, wherein the stator is entirely comprised of metallic materials.
11. The stator of claim 9, wherein the outwardly project lobes of a rotor disposed within the stator define the sealing points between the inner surface of the stator and the rotor.
12. A positive displacement motor power section, comprising: a stator having at least three small radius lobes; a rotor having one fewer small radius lobe than the stator; and wherein the small radius lobes of the rotor define the sealing points between the stator and the rotor.
13. The positive displacement motor power section of claim 12, wherein the stator is entirely comprised of metallic materials.
14. The positive displacement motor power section of claim 12, wherein the rotor is substantially comprised of metallic materials.
15. The positive displacement motor power section of claim 12, wherein the small radius lobes of the rotor are comprised of sealing materials.
16. The positive displacement motor power section of claim 12, wherein the small radius lobes of the rotor are further adapted for the receipt of wiper-type seals.
17. The positive displacement motor power section of claim 12, wherein the wiper-type seals are energized from inside the rotor.
18. The positive displacement motor power section of claim 12, wherein the pitch of the rotor is less than the pitch of the stator.