Note: Descriptions are shown in the official language in which they were submitted.
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ENHANCED COOLING FOR DOWNHOLE MOTORS
BACKGROUND
Field of Invention
The present disclosure relates to downhole pumping systems submersible in well
bore
fluids. More specifically, the present disclosure concerns an improved method
of
cooling pump motors used to drive the submersible pumping systems. Yet more
specifically, the present disclosure involves enhancing the surface area of
the pump
motor for increasing the heat transfer between the pump motor and the well
bore fluid
flowing across the surface of the pump motor.
Description of Prior Art
Submersible purnping systems are often used in hydrocarbon producing wells for
pumping fluids from within the well bore to the surface. These fluids are
generally
liquids and include produced liquid hydrocarbon as well as water. One type of
system
used in this application employs an electrical submersible pump (ESP). ESP's
are
typically disposed at the end of a length of production tubing and have an
electrically
powered motor. Often, electrical power may be supplied to the pump motor via
an
electrical cable. Typically, the pumping unit is disposed within the well bore
above
where perforations are made into a hydrocarbon producing zone. This placement
thereby allows the produced fluids to flow past the outer surface of the
pumping motor
and provide a cooling effect.
With reference r.iow to Figure 1, an example of a submersible ESP disposed in
a well
bore is provided in a partial cross sectional view. In this embodiment, a
downhole
pumping system 12 is shown within a cased well bore 10 suspended within the
well
bore 10 on production tubing 34. The downhole pumping system 12 comprises a
pump
section 14, a seal section 18, and a motor 24. The seal section 18 forms an
upper
portion of the motor 24 and is used for equalizing lubricant pressure in the
motor 24
with the wellbore hydrostatic pressure. Energizing the motor 24 then drives a
shaft (not
shown) coupled between the motor 24 and the pump section 14. Impellers are
coaxially disposed on the shaft and rotate with the shaft within respective
diffusers
formed into the pump body 16. As is known, the centrifugal action of the
impellers
produces a localized reduction in pressure in the diffuser thereby inducing
fluid flow
into the diffuser. In this embodiment, a series of inlets 30 are provided on
the pump
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housing wherein formation fluid can be drawn into the inlets and into the pump
section
14. The source of the formation fluid, which is shown by the arrows, are
perforations
26 formed through the casing 10 of the well bore and into a surrounding
hydrocarbon
producing formation 28. Thus the fluid flows from the formation 28, past the
motor 24
on its way to the inlets 30. The flowing fluid contacts the housing of the
motor 24 and
draws heat from the motor 24.
In spite of the heat transfer between the fluid and the motor 24, over a
period of time
the motor 24 may become overheated. This is especially a problem when the
fluid has
a high viscosity, a low specific heat, and a low thermal conductivity. This is
typical of
highly viscous crude oils. The motor 24 may be forced to operate at an
elevated
temperature, past its normal operating temperature, in order to reject the
internally
generated heat. 'This temperature upset condition can reduce motor life and
results in a
reduction in operational times of the pumping system.
SUMMARY OF INVENTION
The present disclosure includes a downhole submersible pumping system
comprising, a
pump, a pump motor coupled to the pump, and a heat transfer member disposed on
the
pump motor outer surface. The pumping system is configured for being disposed
within a well bore. The pumping system may further comprise a fluid intake,
wherein
the fluid intake is configured to receive downhole fluid and is disposed
adjacent the
pump motor. The downhole fluid received by the intake may create a flowpath
flowing
across the heat transfer member that absorbs thermal energy from the heat
transfer
member. In one embodiment, the entire outer surface of the heat transfer
member is
fully contactable by wellbore fluid. The heat transfer member may have a
substantially
rectangular cross section, a"T" shaped cross section, or it may be elongated
and
disposed substantially parallel to the pumping system axis. Optionally, the
heat transfer
member may be disposed at an angle to the pumping system axis. The system may
further comprise a multiplicity of elongated heat transfer members disposed
substantially parallel to the pumping system axis.
The present disclosure may include another embodiment of a wellbore pumping
system
submersible in a downhole fluid, where the system comprises a housing, a
pumping
device disposed in the housing, an intake in fluid communication with the
housing,
wherein the intake provides fluid communication with the outside of the
housing and
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the pumping device inlet, a motor disposed in the housing mechanically
coupling to the
pumping device and a heat conducting fin disposed on the housing adjacent to
the
motor, wherein the fin freely extends away from the housing wherein its entire
outer
surface is in contact with the downhole fluid. The wellbore pumping system may
have
a pump discharge that communicates with production tubing.
BRIEF DESCRIPTION OF DRAWINGS
Some of the features and benefits of the present invention having been stated,
others
will become apparent as the description proceeds when taken in conjunction
with the
accompanying drawings, in which:
Figure 1 shows a prior art downhole submersible system in a partial cross
sectional
view.
Figure 2 shows a side view of a pumping system in accordance with the present
disclosure disposed within a cased well bore.
Figure 3 provides a schematic cross sectional view of a portion of the pumping
system
having a heat transfer member extending therefrom.
Figure 4 shows a side view of a portion of the pumping system of the present
disclosure
illustrating fluid flow over a heat transfer member.
Figure 5 is a cross sectional view of an embodiment of a heat transfer member.
Figure 6 is a cross sectional view of an alternative embodiment of a heat
transfer
member.
Figure 7 is an overhead view of an alternative view of a heat transfer member.
Figure 8 is a side view of an embodiment of a pumping system having laterally
disposed fins.
While the invention will be described in connection with the preferred
embodiments, it
will be understood that it is not intended to limit the invention to that
embodiment. On
the contrary, it is intended to cover all alternatives, modifications, and
equivalents, as
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may be included within the spirit and scope of the invention as defined by the
appended
claims.
DETAILED DESCRIPTION OF INVENTION
The present invention will now be described more fully hereinafter with
reference to
the accompanying drawings in which embodiments of the invention are shown.
This
invention may, however, be embodied in many different forms and should not be
construed as liniited to the illustrated embodiments set forth herein; rather,
these
embodiments are provided so that this disclosure will be through and complete,
and
will fully convey the scope of the invention to those skilled in the art. Like
numbers
refer to like elements throughout.
The present disclosure provides embodiments of a downhole submersible pumping
system for producing fluids from within a well bore up to the surface. One
embodiment of the pumping system disclosed herein includes a pump, an intake
system
for providing fluid intake to the pump, and a motor for providing a mode of
force for
the pump. The cooling system described herein is a largely passive system that
can
maximize the heat transfer surface area on the outer body of the submersible
motor.
Examples of a passive system include a heat transfer member, such as a fin,
extending
along a portion of the length of the housing of the motor.
In Figure 2, one embodiment of a pumping system with enhanced cooling is
provided
in a side view. In this embodiment, the pumping system 40 comprises a pump
section
42, an inlet section 44, and a motor section 48. The pump section includes a
pump 43
shown in a dashed outline. Formed in the inlet section 44 are inlets 46 for
providing a
fluid inlet path 1:o the pump 43. Examples of pumps useful in this system
include
centrifugal pumps, positive displacement pumps, progressing cavity pumps as
well as
multi-stage centrifugal pumps. With regard to the inlet section 44, the
specific inlets 46
may comprise the circular orifices as shown, other embodiments may be
included, such
as elongated slits and other shaped orifices allowing fluid flow into the
pumping unit.
In this embodiment production tubing 56 is included, thereby enabling fluid
communication between the pumping system 40 and the surface.
With regard to the motor section 48 of Figure 2, it comprises a motor housing
50 that
surrounds and protects a motor disposed therein. Provided on the outer surface
in the
housing 50 are a series of heat transfer members 52 for increasing the
effective heat
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transfer surface area of the motor housing 50. Maximizing this heat transfer
surface
area thereby maximizes the heat transfer from the motor through the housing 50
into
the fluid flowing past these heat transfer members 52. In this embodiment, the
fluid is
shown flowing into the well bore via perforations 54 formed through the
wellbore
casing 39. Formation fluid from the formation 55 flows through the
perforations 54
into the wellbore 38. The heat transfer members 52 of Figure 2 are shown as
elongated
fins, however as will be discussed below, the members 52 can take on many
forms and
are not limited in scope to the embodiment illustrated.
Heat transfer from the motor housing 50 to the flowing fluid can be modeled
with the
following equation: Q=hcA(Ts-Tf). Here, Q equals the rate of heat transfer; hc
equals
the heat transfer coefficient; A equals the surface area; TS equals the
temperature of
surface; and Tf equals the temperature of the fluid. For a given amount of
heat
generated by the motor, increasing the surface area and/or the heat transfer
coefficient
can lower the operating temperature of the motor within the housing. The heat
transfer
coefficient represents the complex interaction of the fluid thermophysical
properties,
the temperature differentials, the velocity of flow and, and the geometry of
the flow
path. The thermophysical properties of a fluid at any given temperature are
relatively
fixed and unalterable. Increasing the velocity of flow has only a small effect
on the
heat transfer coefficient of highly viscous fluids.
In one embodiment of the heat transfer member disclosed herein, the member 52
outer
surface is fully contacted by the fluid flowing past the member 52. Thus in
this
embodiment a single flow of fluid is in contact with the member and receives
thermal
energy from the member 52, and thus the pump motor 48. This configuration is
also
referred to herein as a heat transfer member that freely extends from the
housing into
the cooling fluid. The motor housing is normally formed of a steel material
that is
machined from a cylinder. The members 52 (or fins) may also be of steel or
another
material. Preferably the fins are a contiguous part of the motor housing 50.
Alternatively the fins could be machined into the housing if the housing
initial
configuration has extra thick walls. The number of fins, their length,
protrusion,
configuration etc., are determined by a combination of fluid mechanics
considerations,
the space available and heat transfer analysis. It is within the capabilities
of those
skilled in the art to determine fin number and configuration. In general the
annular
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space between the motor housing and the casing inner diameter determines the
protrusion. In one embodiment, the fin length will be substantially equal to
the motor
housing length.
Figure 3 schematically illustrates an embodiment of a section of the pumping
system
having a single freely extending heat transfer member 52a rather than the
plurality of
fins shown in Figure 2. This portion shown in Figure 3 is a cross sectional
axial view
of a semi-circular section of the motor section 48a with the heat transfer
member 52a
also shown in cross section. The heat transfer member 52a extends along a
radial plane
of the axis of the motor housing 50a. In this embodiment, the heat transfer
member 52a
has a substantially rectangular cross section. Fluid flowing along the axis of
the
pumping system 40a is illustrated by a series of dots 58. Arrows are shown
illustrating
the flow of thermal energy from within the motor, through the heat transfer
member
52a, and out into the fluid 58. This provides one illustration of how the
surface area of
an added heat transfer member can increase heat transfer away from a motor 49.
Figure 4, which illustrates an embodiment of the pumping section of Figure 3
from a
side view, also illustrates heat transfer from the motor section 48 into a
surrounding
fluid. In this embodiment, arrow Ai illustrates fluid flow over a heat
transfer member
52a. A series of arrows, represented by AQ illustrate thermal energy flowing
from the
motor section 48 into the heat transfer member 52. The continuous flow of
thermal
energy is further illustrated by arrows AQ1 being directed from the heat
transfer member
52 into the flow of fluid. Preferably the heat transfer member 52a extends
substantially
along the full length of the motor 48.
Figures 5 through 5c illustrate some other alternative embodiments of heat
transfer
members. Figure 5 is a cross sectional view looking axially along the length
of a heat
transfer member 52b and the motor housing 50b. In this embodiment, the heat
transfer
member 52b has a largely rectangular base with a tapered top terminating into
an outer
edge 60. Such a taper may be useful in reducing dynamic frictional drag losses
along
the length of the motor section.
Figure 6 illustrates an alternative embodiment, where the heat transfer member
52c has
a largely T-shaped cross section for further maximizing motor housing surface
area and
thereby heat transfer. The heat transfer member 52c comprises a web 62
extending
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from the motor housing 50c that supports a flange 64 perpendicularly disposed
on its
terminal end.
Figure 7 shows an overhead view of one section of a heat transfer member 52d.
In this
embodiment, the leading edge 66 (lower portion) and trailing edge 68 (upper
portion)
of the heat transfer member 52d is tapered, as well as its outer terminal edge
60a, in an
attempt to reduce dynamic pressure losses across the heat transfer member. The
heat
transfer member 52d is shown disposed on an embodiment of the motor housing
50.
It should be pointed out however that the arrangement of the heat transfer
member can
include any number of heat conducting elements extending out from the body of
the
pumping system 40. These members are not limited to being located on the motor
section but can be included along any portion, or just a single portion of the
pumping
system 40. Moreover, the arrangement is not limited to a series of elongated
fins on the
outer surface of the motor housing 50, but can be a series of relatively
shortened
members having a matrix like pattern along the length of the housing. The
arrangement
of the heat transfer members (fins) is not limited to being substantially
aligned with the
pumping system axis, but can take a helical arrangement around the body of the
motor
or can simply be at some lateral angle with respect to the length of the axis.
Optionally,
protrusions 53 may be included with any embodiment of the fins herein for
creating a
turbulent boundary layer adjacent the fin surface for increasing heat
transfer.
Figure 8 illustrates an alternative embodiment of a heat transfer member 52e
being
disposed at an angle with respect to the axis of the motor section 48b. This
angle can
range from substantially coaxial and to substantially perpendicular to the
axis of the
motor section.
In one example of use of the present system of concept fins in accordance with
the
embodiment of Figure 2, were added to an electrical submersible pump motor.
Temperature results of the finned motor were tested and compared with
temperature
results of an unfinned pumping system. Mathematical heat transfer modeling and
actual physical testing was performed. The results of this analysis are
outlined in the
following tables.
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EXAMPLE 1
In one example, electrical submersible pumps with finned and unfinned motors
were
analyzed in a flowing fluid, wherein the fluid had the following properties, a
density of
62.0 lb/fe, a viscosity of 0.00458 Ibm/ft sec, and a flow rate of 969.7
lbm/min. The
flow velocity in the finned annulus was 1.04 ft/sec and 0.928 ft/sec in the un-
finned
annulus. Each motor outside diameter was 7.25 inch outside diameter with a
10.2 inch
casing inner diameter. The analysis assumed 45 fins on the finned motor, each
fin
being 82 inches long, 0.525 inches in height, and 0.187 inches thick. The
calculated
temperature rise for the finned motor was 27.67 F and 91.78 F for the
unfinned
motor.
EXAMPLE 2
In another example, two electrical submersible pumps having finned and an
unfinned
motors were analyzed in a flowing fluid having a temperature of 40 F, density
of 61.2
lb/ft3, a viscosity of 1.344 Ibm/ft sec, a specific heat of 0.48 btu/lbm F,
thermal
conductivity of 0.075 but/hr ft F, with a flow rate of 2386.2 lbm/min. The
fluid used
in this example was oil. The flow velocity in the finned annulus was 2.89
ft/sec and
2.46 ft/sec in the un-finned annulus. Each motor outside diameter was 7.25
inch
outside diameter with a 10.2 inch casing inner diameter. The motor horsepower
was
1500 hp. The analysis assumed 57 fins on the finned motor, each fin being 816
inches
long, 0.5 inches in height, and 0.2 inches thick. The calculated internal
temperature for
the finned motor was 193.56 F with an external temperature of 94.82 F, the
calculated
intemal temperature was 577.77 F for the unfinned motor with an external
temperature
of 479.04 F.
EXAMPLE 3
In another example, two electrical submersible pumps having finned and
unfinned
motors were analyzed in a flowing fluid having a temperature of 174 F,
density of 61.2
lb/ft3, a viscosity of 0.15456 Ibm/ft sec, a specific heat of 0.48 btu/lbm F,
thermal
conductivity of 0.075 but/hr ft F, with a flow rate of 2386.2 lbm/min. The
fluid used
in this example was oil. The flow velocity in the finned annulus was 2.89
ft/sec and
2.46 ft/sec in the un-finned annulus. Each motor outside diameter was 7.25
inch
outside diameter with a 10.2 inch casing inner diameter. The motor horsepower
was
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1500 hp. The analysis assumed 57 fins on the fmned motor, each fin being 816
inches
long, 0.5 inches in height, and 0.2 inches thick. The calculated internal
temperature for
the finned motor was 327.56 F with an external temperature of 228.82 F, the
calculated internal temperature was 711.77 F for the unfinned motor with an
external
temperature of 613.04 F.
EXAMPLE 4
Table 1 illustrates a comparison of simulated electrical submersible pump
temperature
increases versus actual measured temperature increases. Two electrical
submersible
pumps were analyzed, one with a finned motor and one without.
Horse Powe Fin? Velocity Calculated Measured
(hp) (ft/sec) temperature rise temperature rise
( F) ( F)
50 Yes 2 3.4 4
50 No 2 6.3 9
75 Yes 2 5.1 5
75 No 2 9.5 12.5
100 Yes 2 6.8 5
100 No 2 12.6 15
130 Yes 2 8.9 8
130 No 2 16.4 19
Table 1
The results provided in Table 1 demonstrate good agreement between the
calculated
and measured temperature rises. Additionally, these results listed in this
table further
illustrate the advantages of using a finned motor over an unfinned motor with
an
electrical submersible pump for the purposes of lowering motor temperature.
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Figure 6 is a plot illustrating respective temperatures rises of finned and.
unfinned
motors versus the horsepower (HPf) the motor dissipates as heat. The analysis
used to
create the graphed values assumed a 7.25 inch motor outside diameter, 45 fins
being
0.525 inch high, 0.187 inches wide, and 82 inches long. The analysis further
assumed a
2 rotor motor with 100 hp, a flowrate of 117 gpm inside of a 10.2 inch inner
diameter
casing. The HPf values shown cover a range of motor loading from 46% to 132%
all at
84.8% motor efficiency.
It is to be understood that the invention 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 of the
invention
and, although specific terms are employed, they are used in a generic and
descriptive
sense only and not for the purpose of limitation. Accordingly, the invention
is therefore
to be limited only by the scope of the appended claims.
