WHEN NOT IN OPERATION

FIELD OF THE INVENTION

[001] This invention relates generally to the field of submersible pumping systems, and more particularly, but not by way of limitation, to a method for heating a downhole motor.

BACKGROUND

[002] Submersible pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, the submersible pumping system includes a number of components, including one or more electric motors coupled to one or more high performance pumps. Each of the components and sub-components in a submersible pumping system is engineered to withstand the inhospitable downhole environment, which includes wide ranges of temperature, pressure and corrosive well fluids.

[003] The electric motor used to drive the submersible pump is typically an oil-filled, high capacity electric motor that can vary in length from a few feet to nearly one hundred feet, and may be rated up to hundreds of horsepower. Prior art motors often include a fixed stator assembly that surrounds a rotor assembly. The stator includes a series of windings that are placed into electrical connection with a surface-based power supply.

The rotor assembly rotates within the stator assembly in response to the sequential application of electric current through different portions of the stator assembly.

[004] During installation and operation, the electric motor may undergo significant thermal swings. As the motor increases in temperature, the expansion of the internal fluid lubricant is accommodated by an adjacent motor protector or seal section. The seal section provides a mechanism for the expanding motor lubricant while providing a barrier against corrosive wellbore fluids. As the motor cools, the lubricants contract and the fluid separation mechanisms in the seal section respond to return the contracting fluid to the motor.

[005] In certain applications the expansion and contraction of the motor lubricant is exaggerated by very large thermal swings. For example, in steam-assisted gravity drainage (SAGD) applications, the motor is first subjected to periods of extreme heating as the reservoir is heated to encourage the release of hydrocarbons. The motor is then exposed to periods of cooling as the motor is turned off and the reservoir returns to its naturally cool temperature. In such applications, the seal sections must be made very large and even then the fluid isolation mechanisms are stressed and present a potential point of failure. Accordingly, to reduce the demands and failure risk of seal sections, there is a need for a method for efficiently controlling the temperature of an electric motor within a submersible pumping system when the motor is not in use.

SUMMARY OF THE INVENTION

[006] In presently preferred embodiments, the present invention includes a method for heating an electric motor within a submersible pumping system that includes the step of applying direct current (DC) power to the motor. The method may also include the additional steps of establishing a desired motor temperature range, measuring the temperature of the motor, comparing the temperature of the motor to the desired motor temperature range, and applying DC power to the motor to raise the temperature of the motor. [007] In an alternate preferred embodiment, the present invention includes a method for heating an electric motor within a submersible pumping system that includes the step of applying alternating current (AC) power to the motor, wherein the AC power has a voltage that is insufficient to cause the motor to rotate. The method may also include the additional steps of establishing a desired motor temperature range, measuring the temperature of the motor, comparing the temperature of the motor to the desired motor temperature range and applying AC power to the motor to raise the temperature of the motor.

[008] In yet another aspect, the preferred embodiments include a method for initiating the operation of a submersible pumping system that includes an electric motor and a submersible pump. The method includes the steps of applying power having a first set of characteristics to the electric motor to preheat the electric motor and then applying power having a second set of characteristics to place the motor into operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[009] FIG. 1 is a perspective view of a submersible pumping system constructed in accordance with a presently preferred embodiment.

[010] FIG. 2 is a wiring diagram of a motor constructed in accordance with a preferred embodiment.

[011] FIG. 3 is a process flow diagram of a preferred method of heating a motor when not in use.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[012] In accordance with a preferred embodiment of the present invention, FIG. 1 shows a perspective view of a pumping system 100 attached to production tubing 102. The pumping system 100 and production tubing 102 are disposed in a wellbore 104, which is drilled for the production of a fluid such as water or petroleum. As used herein, the term "petroleum" refers broadly to all mineral hydrocarbons, such as crude oil, gas and combinations of oil and gas. The production tubing 102 connects the pumping system 100 to a wellhead 106 located on the surface. Although the pumping system 100 is primarily designed to pump petroleum products, it will be understood that the present invention can also be used to move other fluids. It will also be understood that, although the pumping system 100 of FIG. 1 is depicted in a deviated or non- vertical wellbore 104, the pumping system 100 and methods disclosed herein will find also utility in traditional vertical wellbores.

[013] The pumping system 100 preferably includes a pump 108, a motor 110 and a seal section 112. The motor 110 is an electric motor that receives power from surface facilities 114 through a power cable 116. When energized, the motor 110 drives a shaft (not shown) that causes the pump 108 to operate. The seal section 112 shields the motor 110 from mechanical thrust produced by the pump 108 and provides for the expansion of motor lubricants during operation. The seal section 112 also isolates the motor 110 from the wellbore fluids passing through the pump 108.

[014] The pumping system 100 optionally includes a sensor module 118. The sensor module 118 measures and generates signals representative of conditions on the pumping system 100 and in the wellbore 104. Such measurements may include, for example, wellbore temperature, pressure, gas fraction, vibration, and internal motor temperature. Data from the sensor module 118 may be carried to the surface facilities 114 through the power cable 116. Thus, in addition to electric power, the power cable 116 may include signal lines that carry data to and from the motor 110 and the surface facilities 114. The data may be carried over the three-phase conductors or through independent signal conducting lines.

[015] Although only one of each component is shown, it will be understood that more can be connected when appropriate. It may be desirable to use tandem-motor combinations, multiple seal sections, multiple pump assemblies or other downhole components not shown in FIG. 1. For example, in certain applications it may be desirable to place a seal section or pressure compensating chamber below the motor 110.

[016] The surface facilities 114 provide power and control to the motor 110. The surface facilities 114 preferably include a power source 120, a variable speed drive (VSD) 122, and a transformer 124. The power source 120 preferably includes one or both of a public electric utility 126 and an independent electrical generator 128. Electricity is fed by the power source 120 to the variable speed drive 122. During normal operation, the variable speed drive 122 controls the output frequency of the alternating current supplied to the motor 110 to adjust the operating speed in the motor 110. The transformer 124 modifies the output voltage from the variable speed drive 122 to the design voltage range of the motor 110.

[017] Turning to FIG. 2, shown therein is a wiring diagram for the motor 110 constructed in accordance with a presently preferred embodiment. Although the scope of preferred embodiments is not so limited, in a particularly preferred embodiment the motor 110 is a two-pole, three-phase motor in which each phase is offset by 120°. It will be understood, however, that the method of the preferred embodiments will find utility in motors with different structural and functional configurations or characteristics. [018] The motor 1 10 generally includes a stator 130, a rotor 132 and a shaft 134. The shaft 134 is keyed or otherwise connected to the rotor 132 and configured for rotation with the rotor 132. Each of the components within the motor 110 is contained within a motor housing (not shown in FIG. 2). In the particularly preferred embodiment depicted in FIG. 2, the stator 130 includes three phase windings 136A, 136B and 136C that extend through a stator core and are connected with an internal wye connector 138. The stator core is preferably formed by compressing a plurality of stator laminates together to form a substantially unitary body through which the phase windings are passed.

[019] Although a wye (or star) connector is presently preferred, it will be appreciated that the use of alternate wiring connections is also within the scope of the preferred embodiments. For example, it may be desirable to use a delta-type connection between the phase windings 136A, 136B and 136C. The remaining end of each phase winding 136A, 136B and 136C is then brought out externally from the motor 110 and connected to corresponding phase conductors 140 A, 140B and 140C within the power cable 116.

[020] Phased alternating current (AC) flowing through the phase windings 136A, 136B and 136C according to different commutation states creates a rotating magnetic field, which causes the rotor 132 to rotate. This, in turn, rotates the shaft 134. It will be appreciated that a threshold amount of current is required to pass through the phase windings 136A, 136B and 136C to generate sufficient torque to rotate the shaft 134.

[021] Turning to FIG. 3, shown therein is a process flow diagram depicting a preferred method 200 for heating the motor 110 when the motor 110 is not in use. In the preferred embodiments, the motor 110 is maintained within a desired temperature range to minimize thermal swings during periods of operation and non-operation. [022] In preferred embodiments, the method 200 begins at step 202 with the evaluation of the temperature in the motor 110. The temperature at the motor 110 can be evaluated with a temperature sensor within the sensor module 118 or otherwise positioned inside or near the motor 110. Alternatively, the temperature of the motor can be evaluated by measuring the resistance of the phase windings 136A, 136B and 136C. Because electrical resistance varies with temperature, a correlation can be established to estimate the internal temperature of the motor 110 according to the measured resistance of the phase windings 136A, 136B, and 136C. The resistance of the phase windings 136A, 136B and 136C can be determined by the variable speed drive 122.

[023] Once the temperature of the motor 110 has been established, the method 200 proceeds to step 204 to determine if the motor temperature is within a prescribed temperature range. The desired temperature range can be established during manufacture, during installation or during operation of the pumping system 100 and changed as needed. If the temperature of the motor 110 falls within or above the prescribed range, the process returns to step 202 and the temperature of the motor 110 is continually or periodically measured and reassessed for qualification within the prescribed range at step 204. If the temperature of the motor 110 falls below the prescribed range, the process moves to step 206 where electric power is applied to the motor 110 to increase the temperature at the motor 110. The application of the electrical power causes heat in the motor 110 due to the resistance of the phase windings 136A, 136B and 136C.

[024] In a first preferred embodiment, direct current (DC) power is supplied to the motor 110 through the three-phase power cable 116. The DC power can be applied to one of the phase conductors (e.g., phase conductor 140A) while grounding a second of the phase conductors (e.g., phase conductor 140B) at the surface facilities 114. In this mode of operation, the DC current will travel through two of the three phase windings (e.g., phase windings 136A, 136B) through the wye connection 138. In a particularly preferred embodiment, the DC power is sequentially switched so that it passes through all of the phase windings 136A, 136B and 136C within the motor 110. DC current traveling through the phase windings 136A, 136B and 136C will generate heat inside the phase windings 136A, 136B and 136C to increase the temperature of the motor 110. The sequential switching of current to the phase windings 136A, 136B and 136C ensures an equal distribution of heat inside the motor 110.

[025] In a second mode of operation, DC power is applied to two phase conductors (e.g., phase conductors 140A, 140B) while grounding the third phase conductor (e.g., phase conductor 140C). In this mode of operation, the DC current will travel through a two phase windings (e.g., phase windings 136A, 136B) to the wye connection 138 and through the third phase windings (e.g., phase windings 136C) on return from the motor 110. Switching the application of current and grounding between the phase conductors 140a, 140B and 140C will again ensure an equal distribution of heating throughout the motor 110.

[026] In a third mode of operation, DC power is applied to one phase conductor (e.g., phase conductor 140A) while grounding the remaining two phase conductors (e.g., 140B, 140C). In this mode of operation, the DC current will travel through one set of phase windings (e.g., phase winding 136A) to the wye connection 138 and through the second and third phase windings (e.g., phase windings 136B, 136C) on return from the motor 110. Switching the application of current and grounding between the phase conductors 140 A, 140B and 140C will again ensure an equal distribution of heating throughout the motor 110.

[027] In these modes of operation in which DC power is applied to heat the motor 110, the variable speed drive 122 can be programmed so that it enters manually or automatically into a "heater mode" where it applies DC power to the motor 110 for the purpose of heating the motor 110 according to the method 200 of FIG. 3. If the surface facilities 114 include a voltage transformer 124 between the variable speed drive and the downhole motor 110, it is desirable to bypass the transformer 124 to allow current to flow from the variable speed drive 122 directly to the motor 110. In these embodiments, the motor 110 is placed into the "heater mode" of operation by applying electric power having a first set of characteristics. The motor 110 can then be placed into a normal mode of operation by applying electric power having a second set of characteristics. The electric power having the first set of characteristics is characterized by having direct current (DC) and the electric power that has the second set of characteristics AC is characterized by having alternating current (AC) with sufficient voltage to cause the motor 110 to drive the pump 108.

[028] In an alternative preferred embodiment, alternating current (AC) power is applied from the variable speed drive 122 with a reduced voltage that is insufficient to cause rotation within the motor 110. Thus, the motor 110 is heated by the AC power passing through the phase windings 136A, 136B and 136C, but the power is insufficient to cause rotation in the motor 110. Thus, AC power with a first set of characteristics is applied to heat the motor 110 without placing the motor 110 into a normal mode of operation. To place the motor 110 into a normal mode of operation, AC power with a second set of characteristics is applied to the motor. The AC power having the second set of characteristics has a higher voltage than the AC power having the first set of characteristics.

[029] In particularly preferred embodiments, the method 200 of heating the motor 110 is automated and controlled by a feedback loop or other control schemes. Temperature measurements are determined at step 202 and fed to the variable speed drive 122 or other controller and the application of power to the motor 110 at step 206 is controlled to establish the temperature of the motor 110 within the predefined temperature range.

[030] In preferred embodiments, the steps of applying power 206 to the motor 110, measuring motor temperature 202, and comparing motor temperature against the predetermined range 204 are carried out simultaneously. As power is applied to the motor 110 at step 206, the temperature of the motor 110 is reported at step 202 and compared against the desired range at step 204. Alternatively, the method 200 can be carried out in stages in which power is applied to the motor 110 at step 206 for a preset period, the temperature of the motor is then measured at step 202 and compared at step 204, before a subsequent application of power to the motor 110 at step 206. For example, it may be desirable to measure the temperature of the motor 110 as the application of DC current to the motor 110 is switched between phase conductors 140 A, 140B and 140C. Although the method 200 is preferably carried out in an automated control system, it will be appreciated that the method 200 for heating the motor 110 can also be carried out manually without the use of process controls. [031] Although the method 200 for heating the motor 110 finds particular utility in minimizing the magnitude of thermal cycles, it will be appreciated that the method 200 will also find utility in other applications. For example, in certain applications in which the wellbore 104 is cool, it may be desirable to pre-heat the motor 110 before operation to reduce the viscosity of the internal motor lubricants. Reducing the viscosity of the motor lubricants before the motor 1 10 is rotated and subjected to torque will reduce wear within the motor 110.

[032] It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functions of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts and steps within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.