Patent Description:
Electrical submersible pumps (ESP) are commonly used in hydrocarbon producing wells. An ESP includes a pump driven by an electrical motor. The pump is often a centrifugal pump having impellers rotated by a shaft assembly extending from the motor. The well fluid may include pockets of gas, which can cause gas locking of the pump. The pump may lose its prime when gas locked, preventing it from pumping the liquid portions of the well fluid. Continued rotation of the impellers while gas locked can cause overheating of the ESP.

A variety of techniques may be employed to cause the pump to overcome a gas-locked condition. If the motor is powered by an electrical variable speed drive, the drive may change the frequencies of the three-phase power being supplied in order to cause the pump to again begin pumping. Many ESPs are driven at a constant speed however, rather than by a variable speed drive.

Some ESPs employ an external tube alongside the pump and motor to divert a portion of the well fluid being discharged to a point below the motor for cooling the motor. The diverted well fluid discharged by the external tube does not assist in re-priming of the pump if gas-locked.

<CIT> discloses another gas-lock re-priming solution having the features of the preamble of claim <NUM>. In that technique, a valve at an upper end of the pump shifts to deliver well fluid from the production tubing back down an external tube alongside the pump to the intake of the pump.

In accordance with an aspect of the present invention, an apparatus for pumping well fluid from a well is provided as defined in claim <NUM>. In accordance with another aspect of the present invention, a method for pumping well fluid from a well is provided as defined in claim <NUM>.

In the embodiments shown the outlet port is adjacent the inlet of the impeller closest to the intake. In some of the embodiments, another one of the outlet ports is at an inlet of another one of the impellers. Also, in the embodiments shown, the shaft passage has a second end that is closed.

In one embodiment, the pump assembly comprises a first pump connected in tandem to a second pump. The shaft comprises a first shaft section in the first pump, a second shaft section in the second pump, and a coupling connecting the first shaft section to the second shaft section. The shaft passage extends through the first shaft section into the second shaft section. At least one outlet port is located adjacent an inlet of one of the impellers in the second pump.

A seal member in the shaft passage at a second end of the first shaft section and a first end of the second shaft section seals the shaft passage at a junction between the shaft passage in the first shaft section and the shaft passage in the second shaft section.

In some of the embodiments, the re-priming device comprises a pressure controlled valve in the shaft passage that is configured to close if a discharge pressure of the well fluid in the discharge adapter is above a selected level, blocking flow through the shaft passage. The valve opens if the discharge pressure is below the selected level. The valve may have a valve seat and an axially movable valve element relative to the shaft. A spring biases the valve away from the seat. The valve element may be positioned closer to the first end of the shaft than the valve seat.

Rather than a valve, the re-priming device may be an orifice member in the shaft passage. The orifice member has an open orifice passage for continuously diverting a selected portion of the well fluid in the discharge adapter through the shaft passage and out the outlet port.

While the disclosure will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the disclosure to that embodiment. On the contrary, the invention is defined by the appended claims.

The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. In an embodiment, usage of the term "about" includes +/- <NUM>% of the cited magnitude. In an embodiment, usage of the term "substantially" includes +/- <NUM>% of the cited magnitude.

It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation.

<FIG> illustrates a cased well <NUM> having an electrical submersible well pump (ESP) <NUM> of a type commonly used to lift hydrocarbon production fluids from wells. ESP <NUM> has an electrical motor <NUM> coupled by a seal section <NUM> to a centrifugal pump <NUM>. Pump <NUM> has intake ports <NUM> for drawing in well fluid and pumping it to a wellhead at the upper end of well <NUM>. The terms "upward," "downward," "above," "below" and the like are used only for convenience as ESP <NUM> may be operated in other orientations, such as horizontal. In this example, pump <NUM> discharges into production tubing <NUM>, which supports ESP <NUM> in well <NUM>. Alternately, ESP <NUM> could be secured to a string of coiled tubing located within a production conduit. In that event, pump <NUM> would discharge into an annulus surrounding the coiled tubing within the production conduit.

In this example, power cable <NUM> extends downward alongside production tubing <NUM> to a splice <NUM> with a motor lead <NUM>. Motor lead <NUM> has a connector <NUM> on its lower end that connects to motor <NUM>. If the ESP is installed on coiled tubing, the power cable would be inside the coiled tubing and the motor would normally be above the pump.

Motor <NUM> contains a dielectric motor lubricant for lubricating the bearings within. A pressure equalizer communicates with the lubricant in motor <NUM> and with the well fluid for reducing a pressure differential between the lubricant in motor <NUM> and the exterior well fluid. In this example, the pressure equalizer is contained within seal section <NUM>. Alternately, the pressure equalizer could be located below motor <NUM>, and other portions of seal section <NUM> could be above motor <NUM>.

Referring to <FIG>, pump <NUM> is a centrifugal type having a tubular housing <NUM> with a longitudinal axis <NUM>. An upper drive shaft section <NUM>, which is part of a shaft assembly extending from motor <NUM>, extends through housing <NUM> along axis <NUM>. Pump <NUM> has an upper adapter <NUM> secured to an upper end of housing <NUM>. Upper adapter <NUM> connects to and discharges well fluid into production tubing <NUM> in this example. Upper adapter <NUM> has a discharge bore <NUM> through which well fluid is discharged. Bore <NUM> may be conical and converging in an upward direction. A lower adapter <NUM> connects to a lower end of housing <NUM> for connecting pump <NUM> to a lower module, which is seal section <NUM> in this example. Intake ports <NUM> are located in lower adapter <NUM>; alternately, they could be within a lower module, such as a gas separator. Upper and lower adapters <NUM>, <NUM> are illustrated to have external flanges with bolt holes; alternately, they could have rotatable threaded collars.

Pump <NUM> is centrifugal type having a large number of stages. Each stage has an impeller <NUM> with impeller passages <NUM> extending upward and outward. Each impeller <NUM> has a hub <NUM> through which shaft <NUM> extends. A key and slot arrangement (not shown) between impeller hubs <NUM> and shaft <NUM> causes impellers <NUM> to rotate in unison with shaft <NUM>. In this embodiment, each impeller <NUM> is free to slide upward and downward short distances on shaft <NUM> in response to up thrust and down thrust. Hubs <NUM> are illustrated to be able to abut each other; alternately, spacer rings (not shown) could be located between the adjacent hubs <NUM>.

Each impeller <NUM> locates between two diffusers <NUM>. Diffusers <NUM> are mounted in a stack within housing <NUM> so as to be non-rotatable relative to housing <NUM>. Each diffuser <NUM> has diffuser passages <NUM> that extend upward and inward for receiving well fluid from a next lower or upstream impeller <NUM> and discharging it into a next upper or downstream impeller <NUM>. <FIG> illustrates impeller passages <NUM> and diffuser passages <NUM> to be a mixed flow type extending generally radially and upward. Alternately, impeller passage <NUM> and diffuser passages <NUM> could be a radial flow type with passages <NUM>, <NUM> being primarily radially oriented.

Upper and lower bearings <NUM> provide radial support to shaft <NUM>. The upper bearing <NUM> is above the uppermost diffuser <NUM>, and the lower bearing <NUM> is below the lowermost impeller <NUM>. Each bearing <NUM> has slots or passages to allow the upward flow of well fluid. The upper end of shaft <NUM> is located within upper adapter bore <NUM>. A lower end of shaft <NUM> has an externally splined lower end <NUM> that protrudes a short distance below lower adapter <NUM> for connecting via a coupling <NUM> to a shaft (not shown) of the next lower module, which is seal section <NUM> in this example.

Shaft <NUM> has a shaft passage <NUM> that is coaxial and has an open upper end within discharge bore <NUM>. Shaft passage <NUM> extends below the lowermost or first impeller <NUM>, and in the embodiment of <FIG>, has a closed bottom <NUM> above splined lower end <NUM>. One or more outlet ports <NUM> in shaft <NUM> extend laterally from shaft passage <NUM> near closed bottom <NUM> to a point near the inlet or low pressure area of the lowermost impeller <NUM>. The upper end of shaft passage <NUM> has a normally open pressure controlled valve <NUM> in this embodiment.

Valve <NUM> will close during normal operation while the well fluid flowing into and being discharged from pump <NUM> is mostly liquid. During normal operation, the discharge pressure of pump <NUM> within discharge bore <NUM> will be sufficient to cause valve <NUM> to close. While valve <NUM> is closed, none of the well fluid being discharged by pump <NUM> into bore <NUM> will flow through shaft passage <NUM>. Outlet port <NUM> at the lower end of shaft passage <NUM> will always be open; however, the pressure of well fluid in the inlet area of the lowermost impeller <NUM> will be much lower than the discharge pressure in bore <NUM> and will not cause valve <NUM> to move to its normally open position.

If a gas slug or pocket occurs in the incoming well fluid, it could result in the lower impellers <NUM> not pumping the gas pocket up through the stages of pump <NUM>. The presence of the gas slug at the inlets of the lower impellers <NUM> could result in gas locking. When partly or fully gas locked, the pressure in discharge bore <NUM> drops even though shaft <NUM> continues to rotate. The drop in discharge pressure below a set pressure for valve <NUM> will cause valve <NUM> to move to the open position. When valve <NUM> is open, some of the well fluid in discharge bore <NUM> and in production tubing <NUM> will be diverted downward through shaft passage <NUM>. The diverted well fluid, which is mostly liquid, flows through outlet port <NUM> into the inlet area of the lowermost impeller <NUM>. The diverted well fluid displaces the accumulated gas in the inlet area of the lowermost impeller <NUM> and re-primes pump <NUM>. Pump <NUM> will again begin pumping well fluid without having to slow the speed of shaft <NUM> or stop it from rotating in order to re-prime.

If fully gas locked, the downward flow in shaft passage <NUM> will come from the well fluid in discharge bore <NUM> and production tubing <NUM> flowing downward by gravity. If only partially gas locked and while valve <NUM> is open because of the low discharge pressure, pump <NUM> may simultaneously continue to pump some well fluid out discharge bore <NUM> and up production tubing <NUM>. As an example only, valve <NUM> may be preset to close when the pressure in discharge bore <NUM> above valve <NUM> is <NUM> psi greater than the pressure in shaft passage <NUM> directly below valve <NUM>.

Valve <NUM> may have a variety of configurations. In the schematic example of <FIG>, valve <NUM> includes a seat <NUM> secured in shaft passage <NUM>. Seat <NUM> has an orifice <NUM> that may be partially spherical with a larger diameter on its upper end. An axially movable valve element <NUM> engages seat <NUM> to block flow through orifice <NUM> while in the closed position shown in <FIG>. Valve element <NUM> is illustrated as having a semi-spherical lower portion and a cylindrical upper portion, but it may have different shapes. A retainer <NUM> having slots <NUM> through it is secured above valve element <NUM>. A spring <NUM> is stretched between retainer <NUM> and the upper end of valve element <NUM>.

In this example, spring <NUM> will be in tension while in the closed position of <FIG>. Spring <NUM> thus exerts an upward pulling force on valve element <NUM> toward the retracted position of <FIG>. The discharge pressure in bore <NUM> (<FIG>) will overcome the bias of spring <NUM> when above the spring set point, causing valve element to close, as shown in <FIG>. When the discharge pressure drops below the spring set point, the bias of spring <NUM> causes it to retract, lifting valve <NUM> from seat <NUM> and opening valve <NUM>. The set point may be adjusted either by using different strengths for spring <NUM> or by changing the distance between retainer <NUM> and seat <NUM>. In this example, the spring set point will be fixed prior to installing ESP <NUM> in well <NUM>.

<FIG> shows an embodiment where the ESP has an upper pump <NUM> and a lower pump <NUM> connected in tandem. Upper pump <NUM> and lower pump <NUM> are also centrifugal pumps having stages of impellers and diffusers. In this example, intake ports <NUM> (<FIG>) will only be present at the lower end of lower pump <NUM>, or in a gas separator secured to the lower end of lower pump <NUM>. Upper pump <NUM> has an upper shaft section <NUM> with an upper shaft passage <NUM> that extends completely through the length of upper shaft section <NUM>. Valve <NUM> (<FIG>) will be located at the upper end of upper shaft passage <NUM>. When valve <NUM> is open, well fluid will be free to flow downward through the open lower end of upper shaft passage <NUM>. In this example, there is no outlet port for upper shaft passage <NUM>, such as outlet port <NUM> (<FIG>), located above the lower end of upper shaft section <NUM>.

An internally splined coupling <NUM> joins the externally splined lower end of upper shaft section <NUM> to the externally splined upper end of a lower shaft section <NUM> in lower pump <NUM>. Lower shaft section <NUM> has an axially extending lower shaft passage <NUM>. A seal member <NUM> fits within coupling <NUM> and seals a junction of the abutting upper and lower shaft passages <NUM>, <NUM>. Seal member <NUM> has external seal rings <NUM> that seal to the inner walls of upper and lower shaft passages <NUM>, <NUM>. Seal member <NUM> may also have an external flange <NUM> that is clamped between the lower end of upper shaft section <NUM> and the upper end of lower shaft section <NUM>.

Although not shown, an outlet port for lower shaft passage <NUM> will be at an inlet area of the lowermost impeller within lower pump <NUM>. The outlet port would be similar to or the same as outlet port <NUM> (<FIG>). Additional outlet ports could be in either shaft section <NUM>, <NUM> above the lowermost outlet port. Similar to the <FIG> embodiment, lower shaft passage <NUM> has a closed bottom below the lowermost outlet port. A pressure controlled valve (not shown) similar to valve <NUM> (<FIG>) may be located at the upper end of upper shaft passage <NUM>. The valve will open to admit well fluid to flow down shaft passages <NUM>, <NUM> when gas locking conditions occur.

Referring to the embodiment of <FIG>, centrifugal pump <NUM> also has numerous stages, each having an impeller <NUM> with impeller passages <NUM> that extend upward and outward. Each impeller has a hub <NUM>. Hubs <NUM> of adjacent impellers <NUM> may abut either other, as shown, or they may be separated from each other by spacer sleeves (not shown). Each impeller <NUM> locates between two diffusers <NUM>. Each diffuser <NUM> is non-rotating and has diffuser passages <NUM> that extend upward and inward. A drive shaft <NUM> extends through impeller hubs <NUM>, which rotate with shaft <NUM>. Shaft <NUM> has an axially extending shaft passage <NUM>.

Rather than only one outlet port from shaft passage <NUM>, as in the embodiment of <FIG>, there will be at least two shaft outlet ports <NUM>, as shown, and possibly many more. Shaft outlet ports <NUM> could be axially spaced along substantially a full length of the stages of pump <NUM>. Each shaft outlet port <NUM> is located in a different pump stage. In this example, three pump stages are illustrated, with shaft <NUM> having shaft outlet ports <NUM> in the lower and the upper stages, but not the intermediate stage.

A hub port <NUM> within at least some of the impeller hubs <NUM> aligns with each shaft outlet port <NUM> to simultaneously deliver re-priming well fluid from shaft passage <NUM> to the intake areas of more than one impeller <NUM>. Hub ports <NUM> could alternately be located in spacer tubes between impeller hubs <NUM>. Hub ports <NUM> may be axially elongated slots, having an axial length longer than the diameter of outlet ports <NUM>, to accommodate axial floating movement of impeller hubs <NUM> on shaft <NUM> during up thrust and down thrust. Alternately, in order to maintain each hub port <NUM> in alignment with one of the shaft outlet ports <NUM>, it may be necessary to rigidly secure impellers <NUM> to shaft <NUM> to prevent axial movement along shaft <NUM>. If so, unlike pump <NUM> of <FIG>, impellers <NUM> would not be able to float or move axially small increments to transfer thrust to an adjacent diffuser <NUM>. Rather all of the up thrust and down thrust of each impeller <NUM> would transfer to shaft <NUM>.

Pump <NUM> could be either a single pump within the ESP, as shown in <FIG>, or it could be one or both of the tandem pumps, as shown in <FIG>. If pump <NUM> is not mounted in tandem to another pump below it, shaft passage <NUM> would have a closed bottom below the lowermost impeller <NUM>. A pressure controlled normally open valve similar to valve <NUM> (<FIG>) would be at the upper end of shaft passage <NUM>. If pump <NUM> is a lower tandem pump, such as pump <NUM> (<FIG>), it would contain multiple outlet ports <NUM> and hub ports <NUM>. The upper tandem pump in such an arrangement may not have any shaft outlet ports <NUM> and hub ports <NUM>. Optionally, the upper tandem pump could also have one or more shaft and hub outlet ports <NUM>, <NUM>.

Referring to <FIG>, in this embodiment pump <NUM> has a shaft <NUM> supported at its upper end by a top bearing <NUM>, as in the other embodiments. A stack of pump stages <NUM> are located below top bearing <NUM>, each pump stage having an impeller and a diffuser. A discharge adaptor <NUM> above top bearing <NUM> has a converging bore <NUM> through which the well fluid pumped by pump stages <NUM> flows. Shaft <NUM> has a shaft passage <NUM> with an open upper end in discharge adapter bore <NUM>. As in the embodiment of <FIG>, shaft passage <NUM> an outlet port for diverting well fluid from discharge adapter bore <NUM> out of shaft passage <NUM> adjacent the inlet of the lowermost pump stage <NUM>. Unless coupled to a lower tandem pump or to a gas separator, shaft passage <NUM> would have a closed bottom above the outlet port.

Rather than a valve, as in the other embodiments, the gas lock re-prime device in this embodiment comprises an orifice member <NUM> installed in the open upper end of shaft passage <NUM>. Orifice member <NUM> has an orifice passage <NUM> extending axially through it with open upper and lower ends. Orifice member <NUM> may have threads <NUM> on its outer diameter for engaging mating threads provided in shaft passage <NUM>. Other ways to install orifice member <NUM> in shaft passage <NUM> are feasible. Prior to installing pump <NUM>, an operator may select and install an orifice member <NUM> with a desired diameter of orifice passages <NUM> based on the capacity of the particular pump <NUM>. Orifice member <NUM> may be formed from a variety of materials.

Orifice passage <NUM> is much smaller in diameter than shaft passage <NUM>, and it is continuously open. For example, the diameter of orifice passage <NUM> may be selected to allow a flow rate of well fluid from discharge adapter bore <NUM> down orifice passage <NUM> that will not exceed five percent of the flow rate of well fluid from pump stages <NUM> up discharge bore <NUM>.

An outlet port, such as outlet port <NUM> in <FIG>, is also continuously open. Consequently, orifice member <NUM> recirculates a small portion of well fluid being discharged into discharge adapter bore <NUM> to one or more outlet ports <NUM> (<FIG>). In the event of a gas slug entering pump stages <NUM>, the continuously recirculated well fluid assists in re-priming of the impeller of the lowest pump stage <NUM>. Orifice member <NUM> may be substituted for valve <NUM> in all of the embodiments described above.

Claim 1:
An apparatus (<NUM>) for pumping well fluid from a well, comprising:
a centrifugal pump assembly (<NUM>) having a shaft (<NUM>), impellers (<NUM>) mounted to the shaft for rotation therewith, an intake (<NUM>) at a second end of the pump assembly, and a discharge adapter (<NUM>) at a first end of the pump assembly into which well fluid discharged by the impellers flows, characterized by:
a shaft passage (<NUM>) extending into the shaft along a longitudinal axis (<NUM>) of the pump assembly, the shaft passage having a first end in fluid communication with well fluid in the discharge adapter;
at least one outlet port (<NUM>) extending laterally from the shaft passage to an inlet of at least one of the impellers; and
a gas-lock re-priming device (<NUM>, <NUM>) in the shaft passage configured for diverting a portion of well fluid in the discharge adapter through the shaft passage and out the outlet port, wherein the re-priming device is located in the shaft passage closer the discharge adapter than any of the impellers.