Vibrationless moineau system

Abstract

A method of operating a moineau system to substantially eliminate vibrations, the method includes rotating a first rotational member, and rotating a second rotational member. Each of the first rotational member and the second rotational member includes a plurality of lobes. A first rotational speed of one of the first rotational member and the second rotational member is selected based on 1) a second rotational speed of the other of the first rotational member and the second rotational member and 2) the number of lobes of one of the first rotational member and the second rotational member to maintain eccentric force of one of the first and second rotational members below a predetermined threshold.

Description

BACKGROUND

Downhole operations often include a downhole string, also referred to as a drill string that extends from an uphole system into a formation. The uphole system may include a platform, pumps, and other systems that support resource exploration, development, and extraction. During resource exploration operations, a drill bit is guided through the formation to form a well bore. The drill bit may be driven directly from the platform or both directly and indirectly through a flow of downhole fluid, which may take the form of drilling mud passing through a motor. A downhole motor includes a stator having a plurality of lobes and a rotor having another plurality of lobes. The stator is rotated by the downhole string and the rotor by the flow of fluid. The number of lobes on the stator is one fewer than the number of lobes on the stator. In this manner, the flow of fluid drives the rotor eccentrically while the motor drives the drill bit concentrically.

The eccentric rotation of the rotor often leads to vibrations, especially when operating the drill bit at high speeds. The vibrations produced by the downhole motor are not only detrimental to the motor itself, but may also interfere with drilling operations. The vibrations may lead to a reduced overall service life of the downhole motor. Components of the downhole motor, over time, may delaminate due to prolonged exposure to vibrations. Further, the vibrations may exist at a frequency that could lead to interferences with signals passing from the drill string to uphole operators. According, resource exploration companies would be receptive to improvements in downhole motor design and operation.

SUMMARY

A method of operating a Moineau system to substantially eliminate vibrations, the method includes rotating a first rotational member, and rotating a second rotational member. Each of the first rotational member and the second rotational member includes a plurality of lobes. A first rotational speed of one of the first rotational member and the second rotational member is selected based on 1) a second rotational speed of the other of the first rotational member and the second rotational member and 2) the number of lobes of one of the first rotational member and the second rotational member to maintain eccentric force of one of the first and second rotational members below a predetermined threshold.

A vibrationless Moineau system includes a first downhole motor including a first stator having a first number of stator lobes, and a first rotor having a first number of rotor lobes, and a second downhole motor including a second stator operatively connected to the first rotor. The second stator has a second number of stator lobes. A second rotor has a second number of rotor lobes. The first and second downhole motors are selectively operated to substantially maintain eccentric forces on at least one of the first rotor and the second rotor below a predetermined threshold.

A resource exploration system includes a surface system, and a downhole system including a downhole string operatively connected to the surface system. The downhole string includes a vibrationless Moineau system including a first downhole motor having a first stator including a first number of stator lobes, and a first rotor including a first number of rotor lobes. A second downhole motor includes a second stator operatively connected to the first rotor. The second stator has a second number of stator lobes. A second rotor has a second number of rotor lobes. The first and second downhole motors are selectively operated to substantially maintain eccentric forces on at least one of the first rotor and the second rotor below a predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alike in the several Figures:

FIG. 1 depicts a resource exploration system having an uphole system operatively connected to a downhole string including a downhole motor system, in accordance with an exemplary embodiment;

FIG. 2 depicts a cross-sectional view of a downhole motor system, in accordance with an aspect of an exemplary embodiment,

FIG. 3 depicts a schematic representation of the downhole motor system, in accordance with an aspect of an exemplary embodiment;

FIG. 4 depicts a cross-sectional side view of a first downhole motor of the downhole motor system, in accordance with an aspect of an exemplary embodiment;

FIG. 5 depicts a cross-sectional side view of a second downhole motor of the downhole motor system, in accordance with an aspect of an exemplary embodiment; and

FIG. 6 depicts a schematic representation of the downhole motor system, in accordance with another aspect of an exemplary embodiment.

DETAILED DESCRIPTION

A resource exploration system, in accordance with an exemplary embodiment, is indicated generally at 2, in FIG. 1. Resource exploration system 2 should be understood to include well drilling operations, resource extraction and recovery, CO₂ sequestration, and the like. Resource exploration system 2 may include a surface system 4 operatively connected to a downhole system 6. Surface system 4 may include pumps 8 that aid in completion and/or extraction processes as well as fluid storage 10. Fluid storage 10 may contain a gravel pack fluid or slurry (not shown) that is introduced into downhole system 6.

Downhole system 6 may include a downhole string 20 that is extended into a wellbore 21 formed in formation 22. Downhole string 20 may include a number of connected downhole tools or tubulars 24 that may define a drill pipe 25. One of tubulars 24 may be connected with a vibrationless Moineau or downhole motor system 28 operatively connected to a drill bit 32. Vibrationless downhole motor system 28 cooperates with uphole devices (not shown) to rotate drill bit 32 creating well bore 21.

As shown in FIG. 2, downhole motor system 28 may take the form of a downhole motor and includes a power section 36 and a bearing assembly (not shown) that operatively connects with drill bit 32 (FIG. 1). Power section 36 includes a first rotational member defined by a stator 40 and a second rotational member defined by a rotor 42. Rotor 42 is arranged within stator 40. Stator 40 includes a stator housing 44 having a number of stator lobes 48 that define an inner lobed contour or profile 52. Stator housing 44 may be pre-formed with inner lobed profile 52. Inner lobed profile 52 is lined with an elastomeric liner 56 that includes an inner lobed profile 58. Elastomeric liner 56 may be secured within stator housing 44 with any suitable process such as molding, vulcanization, and the like.

Rotor 42 includes a number of rotor lobes 64. It is to be understood that the number of stator lobes 48 is one more than the number of rotor lobes 64. Rotor 42 is rotatably disposed inside of stator 40 and may include a rotor bore 68 that terminates at a location 70 below an upper end 74. Rotor bore 68 remains in fluid communication with drilling fluid 78 which may exit through a port 80. It is to be understood that drilling fluid 78 may take on a number of forms including drilling mud or other types of fluids, foams, gases or the like introduced into downhole motor system 28 through downhole string 20 (FIG. 1).

Stator lobes 48 and rotor lobes 64 possess a helical angle (not separately labeled) causing a seal, such as indicated at 82 at multiple discrete locations between stator 40 and rotor 42. Seals 82 create multiple axial fluid chambers or cavities, one of which is indicated at 84. Drilling fluid 78 supplied under pressure from surface system 4 flows through axial fluid cavities 84 causing rotor 42 to rotate inside stator 40 in a planetary fashion. The number and design of stator lobes 48 and rotor lobes 64 define output characteristics of downhole motor 28. More specifically, a ratio between rotations of stator housing 44 controlled by drill string 20 and rotor 42 as controlled by drilling fluid pressure defines an output torque of downhole motor 28 that is passed to drill bit 32 through a flex shaft 90.

In accordance with an exemplary aspect, rotor 42 and stator 40 may be formed of metal or alloys thereof or any other material that is suitable for downhole motor 28 in the form of a Moineau system. It is to be understood that a Moineau system has at least two rotating parts, also referred to as a first rotational member and a second rotational member. The two parts are an outer tubular part with lobes on its inner surface and an inner threaded rod part, either massive or hollow, with lobes on its outer surface.

It is also to be understood that the term “stator” as used herein refers to a slower rotating part of downhole motor 28, and the term “rotor” refers to a faster rotating part of downhole motor 28. That is, in accordance with an exemplary aspect, rotor 42 rotates faster than stator 40 in a laboratory system, while the wellbore is stationary in the laboratory system. Depending on the configuration of downhole motor 28, rotor 42 may be an inner threaded rod part arranged within an outer tubular part such as, for example, stator 40. Alternatively, stator 40 may be defined by the inner threaded rod part arranged within the outer tubular part such as, for example, rotor 42. In either example, rotor 42 is at least partially driven by fluid flow through downhole motor system 28.

In accordance with an exemplary aspect, rotation of stator 40 and rotor 42 is controlled to substantially eliminate vibrations produced by downhole motor 28. Specifically, downhole motor 28 is operated in a manner that maintains lateral accelerations of rotor 42 below about 15 g. In accordance with another aspect of an exemplary embodiment, downhole motor 28 is operated to maintain lateral accelerations of rotor 42 below about 2 g. In accordance with still another aspect of an exemplary embodiment, downhole motor 28 is operated to maintain lateral accelerations below about 0.5 g. In accordance with yet still another aspect of an exemplary embodiment, downhole motor system 28 is operated to substantially eliminate lateral accelerations of rotor 42.

Substantially eliminating lateral accelerations of rotor 42 results in substantially eliminating vibrations of downhole motor system 28. Lateral accelerations of rotor 42 may be described by formula 1. Controlling motor input through drill string 20 and establishing a selected drilling fluid pressure can establish a desired lateral acceleration of rotor 42 and thereby substantially reduce vibrations of downhole motor 28. For example, rotating downhole string 20 at about 120 RPM and delivering drilling fluid to at a flow rate causing the rotor to rotate at about 60 RM in a downhole motor having a rotor-stator lobe ratio of 2:3 will maintain lateral accelerations below about 0.5 g.
F_ecc=m_rotor*r_ecc*ω²_ecc  (1)

• • F_ecc=eccentric force on the motor;
  • m_rotor is the mass of the motor; r_ecc defines rotor eccentricity; and ω_ecc defines angular velocity defined by equation 2
    ω_ecc=ω_stator−(ω_rotor*n_{rotor lobes})=0  (2)
    ω_stator=ω_rotor*n_{rotor lobes} or ω_rotor=ω_stator/n_{rotor lobes}  (3)
  • ω_stator defines the angular velocity of the stator;
  • ω_rotor defines the angular velocity of the rotor; and
  • n_{rotor lobes} defines the number of lobes on the rotor.
    It is to be understood that motor power provided at drill bit 32 (P_motor) is a function of angular velocity of downhole motor 28. (ω_motor) and downhole motor torque (M_motor) are established by drill string 20 as shown in equations 3 and 4 below.
    ω_motor=ω_stator+ω_rotor  (4)
    P_motor=Mω_motor=M(ω_stator+ω_rotor)  (5)

It is also to be understood that controlling motor inputs to reduce vibration may impose limitations on motor power. For example, downhole string 20 may be rotated only at a limited RPM due to operational limitations. In accordance with an exemplary embodiment the rotation of the downhole string determines the RPM of the stator. Table 1 presents assumed stator RPM and the rotor RPM for typical ‘rotor lobe’/‘stator lobe’ configurations to achieve a vibrationless downhole motor. Entries including a leading “x” are not applicable to a drilling operation, because of a resulting low motor power caused by a resulting low angular velocity of the motor.

In accordance with an aspect of an exemplary embodiment, motor power and angular velocity that is too low for drilling operations may be overcome by including another downhole motor in downhole string 20 with the purpose of creating another source of rotation which is different from the rotation of downhole string 20. The addition of a second downhole motor, as detailed below, would increase motor power and angular velocity to a level that is more applicable to downhole drilling operations.

	TABLE 1
	Stator RPM
	Lobe conf.	60	90	120	150	180
	½	60	90	120	150	180
	⅔	×30	45	60	75	90
	¾	×20	×30	40	50	60
	⅚	×12	×18	×24	×30	×36
	⅞	×8.6	×13	×17	×21.4	×25.8
	9/10	×6.7	×10	×13.3	×16.7	×20

A downhole motor system, in accordance with another aspect of an exemplary embodiment is illustrated generally at 150 in FIG. 3, vibrationless downhole motor system 28 includes a first downhole motor 180 operatively connected to a second downhole motor 190. First downhole motor 180 includes a first rotational member defined by a first stator 200 having a first stator housing 204 a first number of stator lobes, one of which is indicated at 210 that define a first stator lobe profile 212. A second rotational member defined by a first rotor 220 is rotatably supported within first stator 200. First rotor 220 includes a first number of rotor lobes, one of which is indicated at 224 that interact with the first number of stator lobes 210. The first number of rotor lobes 224 number one fewer than the first number of stator lobes 210. A first elastomeric liner 230, see FIG. 4, is mounted to first stator lobe profile 212 and is selectively engaged by the number or rotor lobes to form multiple discrete axial passages 232 in a manner similar to that described above.

It is to be understood that the terms “first” and “second” are not meant to define a particular order relative to surface system 4. That is, while second downhole motor 190 is shown positioned downhole relative to first downhole motor 180, the particular order may vary. Also, it is to be understood that first and second downhole motors 180 and 190 need not be directly adjacent. It is to be understood that first and second downhole motors may be separated by one or more intervening tubulars.

First rotor 220 includes an output member 244 that operatively connects with second downhole motor 190. Output member 244 may take the form of a dampening member that restricts transmission of vibrations from first downhole motor 180 to second downhole motor 190. Output member 244 may include a seal (not separately labeled) and is rotationally isolated from first stator housing 204. Output member 244 includes a passage 250 that receives drilling fluid passing from an area (not separately labeled) between first stator 200 and first rotor 220. In the exemplary embodiment shown, first downhole motor 180 constitutes a downhole motor having a rotor-stator lobe ratio of 2:3 with the number of first rotor lobes 224 being two (2) in number and the number of first stator lobes 210 being three (3) in number.

In further accordance with an aspect of an exemplary embodiment, second downhole motor 190 includes a third rotational member defined by a second stator 250 having a second stator housing 255 and a first number of stator lobes, one of which is indicated at 258 that define a second stator lobe profile 260. A fourth rotational member defined by a second rotor 266 is rotatably supported within second stator 250. Second rotor 266 includes a second number of rotor lobes, one of which is indicated at 268 that interact with the second number of stator lobes 258. The second number of rotor lobes 268 number one fewer than the second number of stator lobes 258. A second elastomeric liner 273, see FIG. 5, is mounted to second stator lobe profile 260 and is selectively engaged by the second number or rotor lobes 268 to form multiple discrete axial passages 278 in a manner similar to that described above. In the exemplary embodiment shown, second downhole motor 190 constitutes a downhole motor having a rotor-stator ratio of 1:2 with the number of second rotor lobes 268 being one (1) in number and the number of second stator lobes 258 being two (2) in number.

Second stator housing 255 is mechanically linked to output member 244. In this manner rotational forces or torque developed in first rotor 220 are direct passed to second stator housing 255. Drilling fluid passing through passage 246 of output member 244 is directed into second stator housing 255. Second rotor 70 includes a first end portion 283 rotatably supported within second stator 250 through a support bearing (not shown) and a second end portion (also not shown) mechanically linked to drill bit 32.

In accordance with an aspect of an exemplary embodiment, rotational energy is imparted to first stator 200 through a drilling system (not shown) arranged at surface system 4 (FIG. 1). The rotational energy imparts a first rotational speed RPM1 to first stator 200. At the same time, drilling fluids at a selected pressure are pumped through downhole string 20 (FIG. 1) into first downhole motor 180. The drilling fluids at the selected pressure interact with first rotor 220 resulting in a second rotational speed RPM 2. First rotor 220 is coupled to second stator housing 255 through output member 244. In this manner, torque developed at output member 244 is directly transferred to second stator 250. As such, second stator 250 rotates at the second rotational speed RPM 2 relative to first stator 200.

Drilling fluids at the selected pressure passing into second downhole motor 190 from passage 246 enter into second stator housing 255. Those drilling fluids interact with second rotor 266 resulting in a third rotational speed RPM 3 relative to second stator 250. The drilling fluids then pass from second downhole motor 190 through an outlet portion (not shown). In this manner, first and second downhole motors 180, 190 may be operated individually at levels below which would produce vibration but at a lower than desired speeds while the operative connection produces a much higher output, e.g., RPM1+RPM2+RPM 3 to drill bit 32. Accordingly, during operating, vibrationless downhole motor system 28 produced few if any vibrations. That is, lateral acceleration of downhole motor system 150 and, more specifically first rotor 220 and lateral accelerations of second rotor 266 are maintained below about 15 g.

In accordance with another aspect of an exemplary embodiment, downhole motor system 150 is operated to maintain lateral accelerations of rotor 42 below about 2 g. In accordance with still another aspect of an exemplary embodiment, downhole motor system 150 is operated to maintain lateral accelerations below about 0.5 g. In accordance with still another aspect of an embodiment, downhole motor system 150 is operated to substantially eliminate lateral acceleration or rotor 266. In one example, RPM 1 may be about 120 RPM, RPM 2 may be 60 RPM, and RPM 3 may be 180 RPM resulting in a combined output to drill bit 32 of 360 RPM.

In another embodiment, the downhole motor system may not be operated in a manner to reduce vibration. That is RPMs first rotor 220 and first stator 200 of the downhole motor system may not be adjusted to reduce vibration. Second rotor 266 and second stator 250 of second downhole motor system 190 may be operated in a manner to reduce vibration. More specifically, the RPM of second rotor 266 and second stator 250 may be adjusted to substantially eliminate eccentric forces. This embodiment may be beneficial with respect to reduce vibration near a vibration sensitive part of the downhole string, e.g. a bottom hole assembly. In order to damp vibration of the downhole motor system, damping elements, e.g. heavy weight drill pipes (not shown), may be included in downhole string 20 to dampen vibrations that may originate from the downhole motor system and which may propagate towards the vibration sensitive part of downhole string 20.

In accordance with an aspect of an exemplary embodiment illustrated in FIG. 6, wherein like numbers represent corresponding parts in the respective views, vibrationless downhole motor system 28 may include a bypass conduit 310 that delivers fluid from upstream of first downhole motor 180 into second downhole motor 190. More specifically, bypass conduit 310 includes a first end 314 fluidically connected to downhole string 20 upstream of first downhole motor 180, a second end 315 fluidically connected to second downhole motor 190 downstream of passage 246 and an intermediate portion 318 extending therebetween.

A valve 324 may be arranged in bypass conduit 310 to selectively control fluid flow therethrough. More specifically, valve 324 may be selectively opened to adjust a flow of drilling fluids into second downhole motor 190 thereby providing additional control of torque developed in second rotor 266 to promote a desired output and/or further reduce vibrations. It is also to be understood that resource exploration system 2 may include a control system (not shown) operable to determine how much fluid may bypass valve 324 and a telemetry system (also not shown) that allows communication between downhole motor(s) and surface system 4. Telemetry may take the form of a mud pulse telemetry system, an acoustic telemetry system, and electro-magnetic telemetry system or a wired pipe telemetry system.

Embodiment 1

A method of operating a Moineau system to substantially eliminate vibrations, the method comprising: rotating a first rotational member; rotating a second rotational member, each of the first rotational member and the second rotational member including a plurality of lobes; and selecting a first rotational speed of one of the first rotational member and the second rotational member based on: 1) a second rotational speed of the other of the first rotational member and the second rotational member, and 2) the number of lobes of one of the first rotational member and the second rotational member to maintain eccentric force of one of the first and second rotational members below a predetermined threshold.

Embodiment 2

The method of embodiment 1, further comprising: inputting a fluid flow at a selected flow rate into a housing of the Moineau system, the selected flow rate establishing the first rotational speed of the one of the first rotational member and the second rotational member.

Embodiment 3

The method of embodiment 2, further comprising: establishing the second rotational speed of the other of the first rotational member and the second rotational member by rotating a drill string operatively coupled to the other of the first rotational member and the second rotational member.

Embodiment 4

The method of embodiment 1, wherein operating the Moineau system includes operating a downhole motor coupled to a drill string extending into a formation.

Embodiment 5

The method of embodiment 4, wherein operating the downhole motor includes coupling the downhole motor to another downhole motor arranged uphole of the downhole motor.

Embodiment 6

The method of embodiment 5, wherein coupling the downhole motor to the another downhole motor includes connecting the downhole motor to the another downhole motor through a dampening member.

Embodiment 7

The method of embodiment 1, wherein selecting the first rotational speed of the one of the first rotational member and the second rotational member maintains eccentric forces on the one of the first and second rotational members below about 2 g.

Embodiment 8

A vibrationless Moineau system comprising: a first downhole motor including a first stator having a first number of stator lobes, and a first rotor having a first number of rotor lobes; and a second downhole motor including a second stator operatively connected to the first rotor, the second stator having a second number of stator lobes, and a second rotor having a second number of rotor lobes, wherein the first and second downhole motors are selectively operated to substantially maintain eccentric forces on at least one of the first rotor and the second rotor below a predetermined threshold.

Embodiment 9

The vibrationless Moineau system of embodiment 8, wherein the second downhole motor is arranged downhole of the first downhole motor.

Embodiment 10

The vibrationless Moineau system of embodiment 9, wherein the first rotor is operatively connected to the second stator through a dampening member.

Embodiment 11

The vibrationless Moineau system of embodiment 9, wherein a rotational speed of the second stator is selected based upon a rotational speed of the second rotor and one of the second number of stator lobes and the second number of rotor lobes.

Embodiment 12

The vibrationless Moineau system according to embodiment 8, further comprising: a bypass conduit having a first end fluidically connected to the first downhole motor and a second end fluidically connected to the second downhole motor.

Embodiment 13

The vibrationless Moineau system according to embodiment 12, wherein the first end of the bypass conduit is fluidically connected uphole of the first downhole motor.

Embodiment 14

The vibrationless Moineau system according to embodiment 12, further comprising: a valve fluidically connected with the bypass conduit, the valve being selectively controllable to allow fluid to pass from the first end to the second end.

Embodiment 15

The vibrationless Moineau system according to embodiment 8, further comprising: a drill bit operatively connected to the second rotor.

Embodiment 16

The vibrationless Moineau system according to embodiment 8, wherein the first and second downhole motors are selectively operable to maintain eccentric forces on the second rotor below about 2 g.

Embodiment 17

A resource exploration system comprising: a surface system; and a downhole system including a downhole string operatively connected to the surface system, the downhole string including a vibrationless Moineau system comprising: a first downhole motor including a first stator having a first number of stator lobes, and a first rotor having a first number of rotor lobes; and a second downhole motor including a second stator operatively connected to the first rotor, the second stator having a second number of stator lobes, and a second rotor having a second number of rotor lobes, wherein the first and second downhole motors are selectively operated to substantially maintain eccentric forces on at least one of the first rotor and the second rotor below a predetermined threshold.

Embodiment 18

The resource exploration system according to embodiment 17, wherein the first and second downhole motors are selectively operable to maintain eccentric forces on the second rotor below about 2 g.

Embodiment 19

The resource exploration system according to embodiment 17, wherein the second downhole motor is arranged downhole of the first downhole motor.

Embodiment 20

The resource exploration system according to embodiment 19, further comprising: a bypass conduit having a first end fluidically connected to the downhole string and a second end fluidically connected to the second downhole motor.

The terms “about” and “substantially” are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” can include a range of ±8% or 5%, or 2% of a given value. It is also to be understood that the term “uphole” denotes a direction along the downhole string leading to the surface and the term “downhole” denotes a direction along the downhole string leading into the formation.

While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

Claims

1. A method of operating a moineau system including a first downhole motor mechanically linked in series to a second downhole motor to substantially eliminate vibrations, the method comprising:

rotating a first rotational member of the first downhole motor;

rotating a second rotational member of the first downhole motor coupled to a third rotational member of the second downhole motor, each of the first rotational member and the second rotational member including a plurality of lobes; and

selecting a first rotational speed of one of the first rotational member and the third rotational member based on: 1) a second rotational speed of the other of the first rotational member and the third rotational member, and 2) the number of lobes of one of the first rotational member and the second rotational member to reduce eccentric force of one of the first and second rotational members.

2. The method of claim 1, wherein operating the moineau system includes operating a downhole motor coupled to a drill string extending into a formation.

3. The method of claim 1, wherein coupling the first downhole motor to the second downhole motor includes connecting the first downhole motor to the second downhole motor through a dampening member.

4. The method of claim 1, wherein selecting the first rotational speed of the one of the first rotational member and the second rotational member maintains eccentric forces on the one of the first and second rotational members below about 2 g.

5. The method of claim 1, wherein reducing eccentric force comprises maintaining eccentric force below a predetermined threshold.

6. The method of claim 1, further comprising: inputting a fluid flow at a selected flow rate into a housing of the moineau system, the selected flow rate establishing the second rotational speed of the third rotational member of the second downhole motor coupled to the second rotational member of the first downhole motor.

7. The method of claim 1, wherein operating the moineau system comprises inputting a fluid flow at a selected flow rate into the first downhole motor and the second downhole motor.

8. A vibrationless moineau system comprising:

a first downhole motor including a first stator having a first number of stator lobes, and a first rotor having a first number of rotor lobes; and

a second downhole motor connected in series with the first downhole motor, the second downhole motor including a second stator mechanically linked to the first rotor, the second stator having a second number of stator lobes, and a second rotor having a second number of rotor lobes, wherein the first and second downhole motors are selectively operated to reduce eccentric forces on at least one of the first rotor and the second rotor.

9. The vibrationless moineau system of claim 8, wherein the first rotor is operatively connected to the second stator through a dampening member.

10. The vibrationless moineau system of claim 8, wherein a rotational speed of the second stator is selected based upon a rotational speed of the second rotor and one of the second number of stator lobes and the second number of rotor lobes.

11. The vibrationless moineau system according to claim 8, further comprising: a bypass conduit having a first end fluidically connected to the first downhole motor and a second end fluidically connected to the second downhole motor.

12. The vibrationless moineau system according to claim 11, wherein the first end of the bypass conduit is fluidically connected uphole of the first downhole motor.

13. The vibrationless moineau system according to claim 11, further comprising: a valve fluidically connected with the bypass conduit, the valve being selectively controllable to allow fluid to pass from the first end to the second end.

14. The vibrationless moineau system according to claim 8, further comprising: a drill bit operatively connected to the second rotor.

15. The vibrationless moineau system according to claim 8, wherein the first and second downhole motors are selectively operable to maintain eccentric forces on the second rotor below about 2 g.

16. The vibrationless moineau system of claim 8, wherein the second downhole motor is arranged downhole of the first downhole motor.

17. A resource exploration system comprising:

a surface system; and

a downhole system including a downhole string operatively connected to the surface system, the downhole string including a vibrationless moineau system comprising:

a first downhole motor including a first stator having a first number of stator lobes, and a first rotor having a first number of rotor lobes; and

a second downhole motor including a second stator mechanically linked in series to the first rotor, the second stator having a second number of stator lobes, and a second rotor having a second number of rotor lobes, wherein the first and second downhole motors are selectively operated to reduce eccentric forces on at least one of the first rotor and the second rotor.

18. The resource exploration system according to claim 17, wherein the first and second downhole motors are selectively operable to maintain eccentric forces on the second rotor below about 2 g.

19. The resource exploration system according to claim 17, further comprising: a bypass conduit having a first end fluidically connected to the downhole string and a second end fluidically connected to the second downhole motor.

20. The resource exploration system of claim 17, wherein the second downhole motor is arranged downhole of the first downhole motor.