Composite motor stator

Abstract

A hollow stator for a Moineau-style motor is constructed from a composite material such as fiberglass or graphite. A series of lobes arranged along its inner surface corresponds to the external lobes of a rotor inserted in the stator, and enables use of a constant width elastomer layer between the stator and the rotor. The composite stator includes a pair of box socket interface members at its ends to enable the stator to be attached to other components such as pipeline sections.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, in drilling a borehole 100 through a rock formation, such as for the recovery of oil, it is conventional practice to connect a drill bit 110 on the lower end of an assembly of drill pipe sections that are connected end-to-end so as to form a drill string 120. The drill string 120 is rotated and advanced downward, causing the drill bit to cut through the underground rock formation. A pump 130 on the surface 140 typically suctions drilling fluid (also known as drilling mud), represented by arrows 135, from a mud pit 132 and forces it down through a passage in the center of the drill string 120. The drilling fluid then exits the face of the drill bit 110, in the process cooling the face of the drill bit. The drilling mud returns to the surface 150 by an annulus located between the borehole wall and the drill string, carrying with it shavings and rock fragments from downhole.

A conventional motor (not shown) is typically located on the surface to rotate the drill string 120 and the drill bit. Often, a drill motor that also rotates the drill bit may be placed as part of the drill string a short distance 160 above the drill bit. As known to those of ordinary skill in the art, this enables directional drilling downhole, and can simplify deep drilling. One such motor is a positive displacement motor called a Moineau motor. This type of motor uses the pressure exerted on the drilling fluid 135 by the surface pump 130 as a source of energy to rotate the drill bit 110. In an effort to reduce costs, such a motor may be used in multiple drilling programs prior to being replaced.

FIG. 2 is a cut-away view of a known Moineau motor design. Moineau motor 200 includes a hollow outer body 210. Motor body 210 contains an elastomeric rubber stator 220 with multiple helical lobes 225. A stator for a Moineau motor may have as few as two lobes. A typical stator lobe makes a complete spiral in 36 inches. This distance is known as the lead length. The distance of the lead length also comprises a single stage of the motor. A typical motor may be three to four stages in length.

Inside the stator 220 is a rotor 240, the rotor 240 by definition having one lobe fewer than does the stator. The rotor has the same lead length as the stator. The rotor 240 and stator 220 engage at the helical lobes to form a plurality of sealing surfaces 260. Sealed chambers 250 between the rotor and stator are also formed. The rubber of the stator degenerates at areas 231-237 and at areas 271-277.

In operation, drilling fluid is pumped in the chambers 250 formed between the rotor and the stator, and causes the rotor to nutate or precess within the stator. The gearing action of the stator lobes causes the rotor to rotate as it nutates, resulting in the centerline of the rotor traveling in a circular path around the centerline of the stator. In the case of a six-lobed rotor, the centerline of the rotor travels in a complete circle six times for each full rotor rotation. The nutation frequency is defined as the multiple of the number of rotor lobes times the rotor revolution speed.

One drawback in such motors is the stress and heat generated by the movement of the rotor within the stator. There are several mechanisms by which this heat is generated. A first is the compression of the stator rubber by the rotor, known as interference. Interference is necessary to create the sealed chambers 250 that prevent leakage. Under typical conditions, the compression may be on the order of 0.005″ to 0.030″. A second source of stress and heat is known as hysteresis. With each cycle of compression and release of the rubber, heat is generated from internal viscous friction among the rubber molecules. A third source is the sliding or rubbing movement of the rotor combined with the forces of interference that generate friction. In addition, heat may also be introduced from high temperatures downhole.

Because elastomers are poor conductors of heat, the heat from these various sources builds up in the thick sections 231-237 of the stator lobes. In these areas the temperature rises higher than the temperature of the circulating fluid or the formation. This increased temperature causes rapid degradation of the elastomer. Also, the elevated temperature changes the mechanical properties of the rubber, weakening the stator lobe as a structural member and leading to cracking and tearing of sections 231-237, as well as portions 271-277 of the rubber at the lobe crests. The cracking and tearing of the elastomer that comprises the stator results in fluid leakage, leading to impaired sealing and reduced motor power. If severe enough, the cracking and tearing results in “chunking” of the rubber, potentially clogging of downstream parts such as fluid exit nozzles located on the face of the drill bit.

Another undesirable phenomenon that occurs to the motor downhole is uneven bending wear to the rubber. Although the stator and rotor may be constructed from the same material, the stiffness of the stator differs from that of the rotor due to their different radii. Complex directional drilling programs may require severe drilling turns. A series of severe turns also may be made in an effort to stay in a rich pay zone or strike a target. Alternately, an inexperienced or unfortunate driller may inadvertently make a series of severe turns. In any event, because the stiffness of the rotor differs from that of the stator, uneven pressure is applied to the elastomer at different locations along the rotor/stator rubber interface as a result of bending. This uneven pressure can lead to further cracking and tearing of the elastomer.

One approach to increase the durability and longevity of a Moineau motor, as disclosed in U.S. Pat. No. 6,405,762, is to create a constant width elastomer layer by means of an undulating inner profile for the stator. This approach promises a longer life motor, in that rubber degeneration in the motor can cause reduced performance and failure of the motor. A longer life motor is especially desirable because motors are frequently used until the motor fails or its performance degrades excessively.

The problems of motor failure are most severe when the motor must be replaced during the drilling program. Replacement of the motor while drilling entails “tripping” the drill string (i.e. drawing the entire drillstring from the borehole), section by section, replacing the motor, and then re-assembling the pipe sections while inserting the drillstring into the borehole. This is difficult enough for a shallow well but for a well that extends for miles, the time and effort wasted can be substantial. Because the operator of a drilling operation may be paying daily rental fees for his equipment, this lost time can be very expensive, especially after the cost of an additional motor. Even where the operator is not paying these rental fees, the oil and gas industry is highly competitive and costs are constantly being squeezed from operations. As the saying goes, “time is money” and the extra time spent on tripping the drill string could be better used elsewhere.

One novel idea would be to build the motor entirely of a composite material. However, although composite materials are finding increasing application in the replacement of metal, composites tend to be used only for special applications. Composite materials have not been used for Moineau-style downhole motors, perhaps because composite materials are generally more expensive than their steel counterparts. Relative characteristics of various composites and metals are shown below (cost is approximate):

	TABLE 1
	Graphite Composite	Graphite Composite	Fiberglass	Aluminum
	(aerospace grade)	(commercial grade)	Composite	6061 T-6	Steel, Mild
Cost $/LB	$20-$250+	$5-$20	$1.50-$3.00	$3	$.30
Strength (psi)	90,000-200,000	50,000-90,000	20,000-35,000	35,000	60,000
Stiffness (psi)	10 × 106-50 × 106	8 × 106-10 × 106	1 × 106-1.5 × 106	10 × 106	30 × 106
Density (lb/in3)	.050	.050	.055	.10	.30
Specific Strength	1.8 × 106-4 × 106	1 × 106-1.8 × 106	363,640-636,360	350,000	200,000
Specific Stiffness	200 × 106-1,000 × 106	160 × 106-200 × 106	18 × 106-27 × 106	100 × 106	100 × 106

While the cost of these materials may change somewhat over time, steel is (and is expected to remain) substantially less expensive than composites. Another drawback to use of composite materials in a downhole motor is that physical properties and characteristics of composites, such as expansion coefficient, are different from metal. This makes a direct metal-to-composite interface difficult in the highly challenging downhole environment.

Manufacture of a downhole motor from a composite material thus presents a dilemma. Not only does the composite construction not seem to make sense from an economic perspective, but switching to a composite motor would be a challenge because it would require a composite-to-metal interface that operates well enough to withstand the demanding downhole environment.

A novel motor is needed that has long life and high performance. Ideally, such a motor would be competitive from a cost perspective to known downhole motors.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the preferred embodiment of the present invention, reference will now be made to the accompanying drawings, wherein:

FIG. 1 is a schematic of a prior art drilling system;

FIG. 2 is a cut away end view of a Moineau-style motor including a stator with points of rubber degeneration;

FIG. 3 is a cut away end view of a stator built in accord with an embodiment of the invention;

FIG. 4 is a side view of a stator body built in accord with an embodiment of the invention;

FIG. 5 is an angled view of a series of fiber tensile members;

FIG. 6 is a view of a clamshell mold around a stator body;

FIG. 7 is a metal box socket according to one embodiment of the invention;

FIG. 8 is a metal box socket engaged with a stator body according to one embodiment of the invention; and

FIG. 9 is flow chart of a method according to the invention of attaching a box socket to a stator body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A Moineau motor stator according to the invention is formed from a composite material and includes lobes formed along the inner diameter of the stator. An elastomeric lining of constant thickness may be applied to the inner diameter of the composite stator. A metal-to-composite box connection is applied to one or both ends of the stator to provide an interface to steel pipelines uphole of the motor and other components. A novel method allowing the economic manufacture of such a composite stator is also disclosed.

FIG. 3 is a cut-away top-view of a Moineau-style motor 300 manufactured in accordance with an embodiment of the invention. A composite stator 320 has an inner profile 350 and an outer profile 355. Attached to stator 320 is an elastomeric layer 330. As shown, a rotor 310 inserted in the stator 320 has multiple helical lobes. Rotor 310 may be solid or hollow, and may be made from any suitable material including steel. The rotor and elastomeric layer 330 engage at the helical lobes to form sealing surfaces 340. The inner profile 350 of stator 320 follows the curvature of elastomeric layer 330 and thus the thickness of elastomeric layer 330 is constant or near-constant. Alternately, the elastomeric layer may be placed on the rotor, but the construction of the composite stator 320 will be unaffected. The outer profile 355 of stator 320 may be of any desired profile but is expected to be cylindrical as shown in FIG. 3.

Referring back to FIG. 3, the constant thickness of elastomeric layer 330 eliminates a substantial amount of rubber as compared to conventional Moineau motors. Less heat is generated because heat generation (hysteresis) in rubber is a function of strain, and under a constant load, a thinner rubber results in lower heat generation. A thinner rubber also results in less swelling of the rubber in aggressive drilling fluids and at elevated temperatures, which also helps reduce interference and its consequent heat generation. Additionally, cracking of the rubber at the crests of the stator lobes due to pressure bending of a thick elastomer profile is reduced, further reducing repetitive stress induced fatigue.

Stator 320 is also close to sealing surfaces 340. The stator 320 mechanically reinforces and supports the rubber at each sealing surface 340, which reduces tearing when high loads are applied. In addition, composites are much better at heat conduction than is rubber. The composite material of stator 320 proximate to the sealing surface 340 permits the stator 320 to dissipate a substantial amount of heat that could cause degeneration and failure of a thick rubber.

The use of lobes on the interior surface of the stator provides mechanical support to the elastomer layer, enabling higher forces to be used per stage of the motor. For example, rather than the 150 lbs/stage used for conventional motors, 600 lbs/stage or more may be used. This allows a shorter, high power density (HPD) motor to be used if so desired. The length on a high-power density motor may be, for example, 48 inches or less although benefits from the invention accrue even if the lead length is not shortened.

FIG. 4 is a partial cut-away side view of a composite stator body 400 according to one embodiment of the invention. Stator body 400 generally includes end regions 402, 404 and a hollow interior. End 402 may be considered to include terminus end 410. Interior lobes 406, 408 extend along the length of stator 320 and correspond to the stator lobes as shown and explained with reference to FIG. 3. The ends 402, 404 of the stator 320 connect to pipeline sections and/or other threaded components. As such, during initial manufacture, the ends of the stator 320 should be a geometry that facilitates connection, such as a cylindrical shape. Lobes 406, 408 therefore begin at the interior surface of stator 320 away from end 402 and ending away from end 404. This simplifies additional machining of the stator 320 to include the preferred metal-to-composite interface.

Referring to FIG. 5, commercially-available composites include a series of fiber tensile members 501-505, held together along their length by epoxy. Referring to FIG. 6, to construct the composite stator, the series of carbon or fiberglass fibers are soaked in an epoxy resin and are wrapped around an internal core 602. As the fiber members are wrapped around the outside of internal core 602, the external surface of the internal core 602 provides the structure to which the fiber members adhere. Thus, the external shape of the internal core 602 defines the shape of the inner surface for the stator 320. Consequently, the outside of internal core should be formed with a proper geometry of lobes to result in a stator having the desired number of lobes, lead length, etc. The specifics of wrapping the carbon fibers around an internal core are within the skill of one of ordinary skill in the art.

After the layers of composite fiber for the stator 320 are applied around the core 602, a clamshell mold may be applied. The clamshell mold comprises matching portions 702, 704 that are placed together. Flanges 706 are aligned and bolted 708. If the clamshell mold has a cylindrical inner diameter 710, a composite stator 320 with a cylindrical exterior results. Epoxy resin should be injected into the structure to provide additional support, and the stator heat cured to a high temperature for a suitable amount of time. This may be about 365 degrees Fahrenheit for 10 hours, resulting in a much strengthened epoxy. The clamshell mold is then taken off.

The internal core 602 is then removed. The manner of its removal depends on its composition although the core may be composed of any suitable material. If a rubber elastomer is to be molded onto the interior surface of the stator 320 according to techniques well known in the art, the interior surface of the stator should have a rough surface for the elastomer to adhere to. This rough surface may be formed on the inner stator surface initially by providing by a rough surface on the outside surface of the core 602. Such a core may be made from plastic or aluminum, it being dissolved chemically after the composite material that forms the stator has hardened. Alternately, the core may be steel with a smooth outer surface coated by a low friction coating such as Teflon. The core 602 is then slid out from the stator and the inner surface of the stator roughened, e.g., chemically or abrasively.

Stator 320 should be constructed of a composite material, such as graphite (carbon) or fiberglass. Composite materials are made by combining reinforcement (fiber) with matrix (resin), and this combination of the fiber and matrix provide characteristics superior to either of the materials alone. Composite products are designed with fiber angle and layer thickness tailored to resist the actual load configuration. The fibers carry the majority of the loads, and are the major factor in the composite properties. The greater the ratio of fiber to resin (measured by fiber-volume fraction), the greater the strength and stiffness of the composite material.

The composite engineer designs the directions and relative distribution of fiber in the manufactured part to optimally resist the design loads. The strength and stiffness of composite materials are generated by the fibers and, in a given fiber layer, are highest in the fiber direction. A stator must withstand tensile loads to transfer weight-on-bit downhole, torsional loads to withstand the twisting action of drilling, and pressure loads to withstand the radial forces from within the motor body. In the case of the stator, the tension loads are carried by fibers that are axially oriented or helically wound at low angles to the stator axis, and the pressure loads are resisted by circumferentially placed, or hoop-wound, fibers. Hoop-wound layers and low-angle, helically wound axial layers with high fiber-volume fraction can be fabricated by filament winding as known to those of ordinary skill in the art.

The matrix, or epoxy resin, serves to transfer load between fiber layers, prevents the fibers from buckling, maintains fiber orientation, and binds the materials together. The epoxy resin should be a high temperature epoxy able to withstand the temperatures to be encountered downhole.

Composite materials have high specific strength (strength divided by density) and specific stiffness (stiffness divided by density) as well as excellent corrosion resistance. Because a composite can be selected whose modulus is about 10 to 15 times lower than the modulus of a steel rotor, the composite stator may be manufactured to tailor the stiffness of the stator to that of the rotor. The stiffness of the steel rotor may also be lowered by making it hollow, but this is not preferred as it is desirable to use existing rotor stock. Ideally, the stiffness of the stator and rotor will be the same, at least to within 10% of one another. This matching of the rotor and stator should minimize the uneven bending wear that can be seen when using motors with steel stators and rotors.

Graphite/epoxy composite materials have strengths in the fiber direction on the order of high-strength steels and a density 20% of steel. Graphite-based composites have a stiffness in the fiber direction ranging from 19 million lb/sq in. (60% of steel) using standard modulus fibers, all the way to 45 million lb/sq in. (150% of steel) using ultra-high-modulus fibers. Composites based on standard modulus fibers are the most cost-efficient from a strength standpoint.

Referring to FIG. 7, a hollow metal box socket 802 includes a first stator reception end 804, having an inner diameter 806, and a second threaded end 808, with inner diameter 810. The interior of box socket 802 includes threaded surface 812 having inner diameter 814. Inner diameter 814 should be large enough to accommodate insertion of a rotor into the stator body 400 with ring 902 (FIG. 9) in place. Next to threaded area 812 of the box socket is a relieved area 816, such as a recessed groove.

FIG. 8 shows the box socket 802 with inserted stator body and wedge ring 902. The exterior of wedge ring 902 is sized and threaded to matingly engage the threads on the interior 812 of box socket 802. The end of wedge ring 902 on the side of the stator body includes a sloped surface 904 that abuts the terminus end 410 of the stator body, and forces it into relieved area 816. It is not necessary to the invention that a wedge ring exactly like that shown in FIG. 8 be used. Wedge ring 902 should be sized to clear the rotor eccentrically and to force the stator into the adjacent relieved areas. Threaded end 808 of the box socket 802 is suitable for attachment to a threaded pipeline section. An advantage of the box socket 802 is that it is formed and heat treated prior to being attached to the composite stator body, eliminating any forging, heat treating and subsequent operations that would be required if working on the end of a steel stator body.

FIG. 9 is a flow chart according to one embodiment of the invention of a method of attaching the box socket to the stator body. Where appropriate, the steps may be performed in an order other than that shown in FIG. 9. At step 1002, groove 816 is filled with an epoxy of sufficient strength to semi-permanently affix the box socket 802 to stator body 400. Enough epoxy must be provided, and groove 816 must be large enough, to create a reliable adhesion. At step 1004, terminus end 410 of the stator body 400 is inserted into the box socket 802. The terminus end 410 stator body is inserted a distance into box socket 802 adequate to place the terminus end 410 adjacent to (including in) groove 816. At step 1006, wedge ring 902 is rotatably inserted along threaded surface 812 of the box socket. Sloped surface 904 of the wedge ring forces the terminus end 410 of the stator body 400 into epoxy-filled groove 816 at step 1008. The epoxy is then allowed to harden at step 1010, forming a semi-permanent bond between the stator body 400 and box socket 802. By virtue of box socket 802, stator 320 may be connected to a metal section of pipeline or other component. A rotor is inserted in the stator 320 to form the motor.

One advantage to a composite stator is the cost of manufacture of the stator. Although inner lobes on a stator are desirable, the presence of inner lobes on a steel stator complicates manufacturing. Where the stator has a set of inner lobes, the manufacture of the composite stator is expected to be much less involved than for the steel stator. This simplification to the manufacturing process is expected to provide substantial cost savings to formation of the stator, savings that are expected to more than compensate for any extra cost of the composite material itself.

An additional benefit of a composite stator is that composite materials do not suffer from the deforming forces applied during the manufacture of an item from steel. When formed to a core, the composite stator has smaller manufacturing tolerances than would a steel stator. The lead of the lobes in the composite stator body is tightly controlled. The closer match between stator and rotor should allow more even wear over the stator rubber length. In addition, the more precise composite stator allows matching and utilization of existing rotors and rotor designs. This further minimizes the cost of the motor. The electrical characteristics of composite materials also mean that the composite motor is electrically insulated.

While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. For example, alternate composites could be used, and novel composites may be developed in the future. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims which follow, the scope of which shall include all equivalents of the subject matter of the claims.

Claims

1. A stator configured for use in a motor comprising: a hollow pipe, said pipe having a length, an inner profile and an outer profile, said pipe being made from a plurality of fibers adhered together by resin; wherein said inner profile of said pipe has a plurality of lobes, said lobes of inner profile being disposed in a helical arrangement along said length of said pipe.

2. The stator of claim 1, further comprising: a first end for said pipe, a metal box connector attached to said first end of said pipe.

3. The stator of claim 1, said plurality of fibers adhered together by resin being a fiberglass composite.

4. The stator of claim 1, said plurality of fibers adhered together by resin being a graphite composite.

5. The stator of claim 1, further comprising: a layer of elastomer applied to said inner profile.

6. The stator of claim 5, said layer of elastomer being of a constant thickness and extending the length of said plurality of lobes.

7. The stator of claim 1, further comprising: first and second ends for said pipe, a first metal box connector attached to said first end; and a second metal box connector attached to said second end.

8. The stator of claim 1, said outer profile being circular.

9. A motor, comprising: A stator having a length, an inner profile and an outer profile, said stator being made from a plurality of fibers adhered together by resin, said inner profile of said stator has a plurality of lobes, said lobes of inner profile being disposed in a helical arrangement along said length of said stator; A rotor residing inside said stator, said rotor having an outer surface defining a plurality of lobes but one fewer than defined by said stator, said lobes of said rotor being disposed in a helical arrangement along said length of said rotor.

10. The motor of claim 9, said rotor having a first stiffness and said stator having a second stiffness, said first stiffness and said second stiffness differing by at most ten percent.

11. The motor of claim 9, said rotor having a first stiffness and said stator having a second stiffness, said first stiffness and said second stiffness being the same.

12. The motor of claim 9, further comprising: a first end for said stator, a metal box connector attached to said first end of said stator.

13. The motor of claim 9, said plurality of fibers adhered together by resin being a fiberglass composite.

14. The motor of claim 9, said plurality of fibers adhered together by resin being a graphite composite.

15. The motor of claim 9, further comprising: a layer of elastomer applied to said inner profile.

16. The motor of claim 15, said layer of elastomer being of a constant thickness and extending the length of said plurality of lobes.

17. The motor of claim 9, further comprising: first and second ends for said stator, a first metal box connector attached to said first end; and a second metal box connector attached to said second end.

18. The motor of claim 9, said stator having a circular outer profile.

19. A method for joining a hollow, metal box housing to a composite stator tube, comprising: filling a relieved area on an interior surface of said metal box housing with epoxy; inserting an end of said composite stator tube into said metal box housing until said end of said stator tube is adjacent said relieved area; inserting a wedge ring into said metal box housing, said wedge ring abutting said end of said stator tube upon the insertion of both the stator and the wedge ring into said metal box housing such that said end of said stator is forced toward said relieved area; and affixing said stator tube to said metal box housing by hardening of said epoxy.

20. The method of claim 19, said composite stator tube being a graphite composite.

21. The method of claim 19, said composite stator tube being a fiberglass composite.

22. The method of claim 19, said wedge ring being rotatably inserted into said metal box housing via a threaded engagement between threads on the exterior of said wedge ring and threads on the interior of said metal box housing.

23. The method of claim 19, an interior end of said box housing being threaded.