BACKGROUND

Field of the Invention

The invention relates generally to downhole electric equipment, and more particularly to systems and methods for preventing rotation of a motor shaft in an electric submersible pump (ESP) when the ESP is not in operation, as well as offering a solution to extend the useful life of the equipment and widen the hydraulic operating range. When energized an ESP can rotate in a clockwise (CW) or counterclockwise (CCW) rotational direction. This invention applies this bidirectional feature to solve challenges related to the erosion of pump internal components and expand the performance envelope of the multistage centrifugal pump.

Related Art

Oil is typically produced by drilling wells into oil reservoirs in geological formations and then pumping the oil out of the reservoirs through the wells. Commonly, the oil is produced using ESPs that are deployed in the wells. Electric power suitable for the respective ESPs is normally generated by electric drive systems that are positioned at the surface of each well, and is conveyed from the drive to the ESP via a power cable that extends from the drive system to the deployed ESP.

An ESP typically includes a pump section, a seal section, and a motor section. The power from the electric drive system is provided to the motor, which drives the seal section and pump section. Frequently, the motor is a rotary motor which drives a shaft that is coupled to the shaft of a centrifugal pump. The rotating motor shaft causes the pump shaft to rotate, lifting the fluid out of the well.

The motor is typically one of two types: an induction motor; or a permanent magnet motor. In the case of an induction motor, power (usually three-phase AC power) is provided to the windings of the motor's stator, causing the stator to generate rotating magnetic fields in the stator. These rotating magnetic fields induce currents and corresponding magnetic fields in a rotor, causing the rotor and the motor shaft to rotate and drive the pump. In the case of a permanent magnet motor, three-phase AC power is provided to the motor's stator windings, generating rotating magnetic fields as in the induction motor. The rotor of the permanent magnet motor, however, has a set of permanent magnets which cause the rotor to rotate in the rotating magnetic fields generated by the stator.

The pump typically contains a rotating impeller and stationary diffuser. The matching impeller and diffuser can be designed for CW or CCW rotational operation.

As explained above, in normal operation, power supplied to a conventional induction type or permanent magnet ESP motor causes the motor to rotate, which causes fluid (e.g., oil) to flow through the pump. What is less frequently considered, however, is that the reverse of this sequence may also be true. In other words, the motor can act as a generator. If fluid is caused to flow through the pump, this will cause the pump to rotate, which will in turn cause the motor to rotate and generate an AC voltage which is applied to the conductors of the power cable. The generated voltage is often unexpected since the motor normally consumes electrical energy, and it may be dangerous or even fatal to persons working on the system. It would therefore be desirable to provide means to protect these people from the electric potential that may be generated by an ESP motor acting as a generator.

The motor section, seal section, and pump section internals have rotating dynamic bearing interfaces. Wear patterns develop reducing effective system life. Furthermore, when solids or sands are entrained in the produced fluid, aggressive erosive and abrasive wear of the pump internals can occur. Erosive wear occurs when the solid particles entrained in the fluid impact surfaces of the pump internals eroding their surfaces. Abrasive wear occurs when solid particles are deposited in between rotating surfaces grinding away material from surfaces.

A pump section has a hydraulic operating range defined by a minimum and maximum rate at a given rotational speed. A fluid rate that is too low can result in excessive impeller downthrust. A fluid rate that is too great can result in excessive impeller upthrust. The performance of a producing well can change, particularly unconventional wells, resulting in the pump section operating outside its operating range. Continued operation of the pump outside the operating range can destroy the pump internals. SUMMARY

This disclosure is directed to systems and methods for manufacturing and operating pumps and motors that reduce or eliminate one or more of the problems above.

By combining a pump system that normally operates in a clockwise direction with a pump system that is designed to operate in a counterclockwise direction one or more of the problems stated previously are addressed.

One embodiment comprises installing two centrifugal pumps on the motor shaft with one designed for CW rotation and the second for CCW rotation. During CW rotational operation the CW designed pump performs most of the fluid lifting function, whereas the CCW designed pump would be inefficient at contributing to the produced fluid. However, when fluid is forced through the pump the CCW pump would effectively counter the torque generated by the CW pump, and prevent the motor shaft being rotated, hence preventing the motor from acting like a generator.

Another embodiment uses the combined CW and CCW pumps to extend the operating range of the ESP system by having dissimilar operating ranges. One example would be that while in CW operation the CW pump does most of lifting of the fluid while the CCW pump would introduce some inefficiency. Once the well performance has changed the ESP would be operated in the CCW rotation with the CW pump contributing inefficiency. The system could alternately start in the CCW direction and then the CW direction.

Another embodiment would be combing CW and CCW pumps with the same operating range performance to extend system life. The system would alternate operation in CW and CCW rotation for periods of time. Internal components in the motor section, seal section, and pump section would benefit from alternating wear patterns. This embodiment would also address erosive and abrasive wear in pump section internals.

Numerous other embodiments may also be possible. BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention may become apparent upon reading the following detailed description and upon reference to the accompanying drawings.

FIG. 1 is a diagram illustrating a conventional ESP system installed in a well.

FIG. 2 is a diagram illustrating an ESP system with a CW and CCW pump.

FIG. 3a and 3b is a diagram illustrating the CCW and CW impellers.

FIG. 4 is a longitudinal sectional view of the ESP in one mode of operation.

FIG. 5 is a longitudinal sectional view of the ESP in another mode of operation.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIM ENTS

One or more embodiments of the invention are described below. It should be noted that these and any other embodiments described below are exemplary and are intended to be illustrative of the invention rather than limiting.

As described herein, various embodiments of the invention comprise systems and methods for preventing rotation of an ESP motor when the motor is not powered on, thereby preventing the motor from acting as a generator when fluid flowing through the pump section of the ESP applies a torque to the motor. Other embodiments of the invention expand the operating range of the ESP system or extend the useful life of the ESP system.

As noted above, a permanent magnet motor in an ESP can act as a generator when fluid flowing through the pump causes the motor to rotate. This may occur in a number of different circumstances. For example, during certain well operations such as running the equipment in the well, or pulling the equipment from the well, fluid may be forced through the pump. Fluid flowing through the pump may also result from well conditions such as natural flow of well fluids, the fluid column in the tubing above the pump falling back through the pump, or fluids "bull heading" through the pump. During one of these occurrences, the permanent magnet motor can generate enough electricity to be fatal. It should be noted that, since induction motors do not use magnets on their rotors, the rotation of these motors is not likely to present as much of a hazard as permanent magnet motors. Induction motors can, however, have some residual magnetism that may cause them to generate electricity when rotated by fluid flowing through the pump. Embodiments of the invention may therefore be applicable to induction motors and other types of motors that are used in ESPs, even though they do not normally present the same level of hazard as permanent magnet motors.

Because of the electrical hazard presented by permanent magnet ESP motors in conditions where fluids flowing through the pump may rotate the rotor (e.g., while installing, pulling, operating or troubleshooting the ESP), embodiments of the present invention prevent the permanent magnet rotor from rotating when the motor is not being driven (i.e., when the motor is not powered on). By preventing rotation of the rotor, the potential for the motor to generate electricity is eliminated. This provides a key safety advantage over prior art permanent magnet motors. By implementing this feature in a mechanical design, embodiments provide a reliable solution with minor impact to the existing product design.

In an exemplary embodiment, the ESP system includes a multistage centrifugal pump section, a seal section, an ESP motor, and a power cable which transfers electrical power from an electric drive at the surface of the well to the motor, which is downhole in the well. The motor is a three-phase permanent magnet motor. The motor consists of a stator assembly (which may be referred to herein simply as a stator) and a permanent-magnet- type rotor that is affixed to a motor shaft. Support bearings are provided along the shaft and at the ends of the shaft.

When power is applied to the stator magnetic fields will rotate the rotor and shaft in a direction determined by the design of the motor (this direction may be clockwise CW or counterclockwise CCW, depending on the pump design). Mechanical power is transferred through the seal and to the pump. This is the desired operation condition.

When power is not applied, the permanent magnet motor has the potential to become a generator when turned. Liquid passing through the pump (in either direction) in a conventional ESP system will cause the pump stages to rotate, and the rotation will be transferred to the motor from the pump and seals section shaft. The rotation direction will depend on the direction of flow through the pump. The motor may potentially generate a high voltage, depending on the stator design and the rotational speed of the rotor. The electricity that is generated by the motor will be transferred back to the surface through the cable. This is an undesirable condition due to the potential to generate electrical energy in the system, such that unprotected personnel or equipment can be harmed or killed by the electricity.

Referring to FIG. 1, a diagram illustrating an ESP system is shown. In this embodiment, an ESP system is installed in a well for the purpose of producing oil, gas or other fluids. The ESP 120 is coupled to the end of tubing string 150, and the ESP and tubing string are lowered into the wellbore to position the ESP in a producing portion of the well (as indicated by the dashed lines at the bottom of the wellbore). Surface equipment which includes an electric drive system 110 is positioned at the surface of the well. Drive system 110 is coupled to ESP 120 by power cable 112, which runs down the wellbore along tubing string 150.

In this embodiment, ESP 120 includes a motor section 121, a seal section 122, and a pump section 123. ESP 120 may include various other components, such as gauge packages, which will not be described in detail here because they are well known in the art and are not important to a discussion of the invention. Motor section 121 is coupled to power cable 112, and is driven by AC power (typically three-phase AC waveforms) that are received from drive system 110 through the cable. Motor section 121 drives pump section 123, thereby pumping the oil or other fluid through the tubing string and out of the well. Power for the non-motor components of the system (e.g., gauges, telemetry communication systems, etc.) may be provided from motor section 121 to the other components. Seal section 122 is provided between motor section 121 and pump section 123 for purposes including equalizing the pressure between the motor interior and the well bore and allowing the oil within the motor to expand and contract.

Referring to FIG. 2, a diagram illustrating an ESP system with a CW and CCW pump is shown. In this embodiment, CW Pump 123 connects to a CCW pump 124 and seal section 122.

As with the FIG. 1, ESP 120 includes a motor section 121, a seal section 122, and a pump section 123. ESP 120 may include various other components, such as gauge packages, which will not be described in detail here because they are well known in the art and are not important to a discussion of the invention. Motor section 121 is coupled to power cable 112, and is driven by AC power (typically three-phase AC waveforms) that are received from drive system 110 through the cable. Motor section 121 drives pump section 123, thereby pumping the oil or other fluid through the tubing string and out of the well. Power for the non-motor components of the system (e.g., gauges, telemetry communication systems, etc.) may be provided from motor section 121 to the other components. Seal section 122 is provided between motor section 121 and pump section 123 for purposes including equalizing the pressure between the motor interior and the well bore and allowing the oil within the motor to expand and contract.

ESP systems are available with pumps designed with either CW or CCW pumps. Combining them to address issues with unintended power generation, extending ESP system hydraulic operating range, and extending the useful life is novel.

Referring to figures 3a, 3b and 4, the pump section 123 of an ESP comprises a first set of CCW impellors 160 and a second set of CW impellers 162, set on a rotating shaft 165. The shaft 162 will conveniently be comprised of two shafts joined by a coupling 163, however the shafts are torsionally linked so that the entire shaft rotates as a single member in the same rotational direction. When the shaft 162 is rotated in a clockwise manner as indicated by arrows rl, both sets of impellers 160, 162 rotate in a clockwise manner. Each of the impellers 160, 162 is housed in a casing which includes conventional diffusers 167, 168, the first upper set of diffusers 168 being optimised for the CCW impellers and the second lower set of diffusers 167 being optimised for the CW impellers. To operate in a normal mode, the shaft 162 is rotated clockwise by the ESP motor as indicated by arrows rl, and the rotation of the first and second sets of impellers urges the well fluid as indicated by arrows fl. The second set of CW impellors 162 operate at good efficiency, while the first set of CCW impellors 160 may operate at a reduced efficiency in comparison to the second CW set 162, but still urge fluid radially outwards and so contribute to the production of well fluid.

It will also be noted that the shaft 165 could be rotated counter-clockwise, so that the first set of CCW impellors 160 operate at good efficiency, while the second set of CW impellors 162 may operate at a reduced efficiency in comparison to the first CCW set 160, with both sets contributing to the production of well fluid in the direction of arrows fl. Of course, the positions of the CW impellers and the CCW could be swapped with CW impellors and diffusers set above CCW impellors and diffusers, or the impellers configured or interspersed in some other manner.

Referring to figure 5, in fallback mode the driven rotation of the shaft 165 is ceased. As fluid falls as indicated by the arrows f2, the first set of impellers 160 and second set of impellers 162 experience opposite rotational forces as described above, and ideally these rotational forces balance or substantially balance.

The ESP configuration also extends the useful life of the downhole pump system by utilizing two centrifugal pumps with differing vane designs connected to a common shaft. One pump is designed to operate in a clockwise direction with a larger flow rate and another pump designed to operate in a counterclockwise direction with a lower flow rate. Alternatively, the larger flow rate pump could also operate in the counterclockwise direction and the lower rate pump operate in the clockwise direction. As a well loses production capability the ESP system will be operated in the opposite rotational direction with the lower rate pump.

The ESP configuration extends the useful life of the downhole pump system by utilizing two centrifugal pumps with differing vane designs connected to a common shaft. One pump is designed to operate in a clockwise direction and another pump designed to operate in a counterclockwise direction with the same or similar flow rate. Periodical operation in clockwise and counterclockwise directions will vary wear patterns extending system life.

A related ESP configuration also extends the useful life of the downhole pump system by utilizing two centrifugal pumps with differing vane designs connected to a common shaft, where one pump is designed to operate in a clockwise direction with a larger flow rate and another pump designed to operate in a counterclockwise direction with a lower flow rate. Alternatively, the larger flow rate pump could also operate in the counterclockwise direction and the lower rate pump operate in the clockwise direction. As a well loses production capability the ESP system will be operated in the opposite rotational direction with the lower rate pump.