U.S. patent application number 09/838744 was filed with the patent office on 2002-10-24 for method for pumping fluids.
Invention is credited to Breidenthal, Robert E., Hartman, Michael G., Moe, Eric M., Rehr, Jesse S..
Application Number | 20020153141 09/838744 |
Document ID | / |
Family ID | 25277938 |
Filed Date | 2002-10-24 |
United States Patent
Application |
20020153141 |
Kind Code |
A1 |
Hartman, Michael G. ; et
al. |
October 24, 2002 |
Method for pumping fluids
Abstract
The production rate, useful life and operating efficiency of
electric submergible pumping systems (ESP) is improved by operating
the system so that the motor is cooled by two passes of production.
The two passes of production fluid are achieved by employing a
shroud with the inlet at the opposite end of the production fluid
intake or by installing the motor below the perforations that allow
production fluids to enter a well. The motor is designed with an
internal conduit through which the production fluids are pumped.
Production fluids remove waste heat from the motor by passing in
fluid contact with the exterior of the motor and again by passing
through the internal conduit of the motor. The improved cooling
permits the motor to be operated at a lower temperature improving
life and efficiency, and/or to be operated at higher power at a
similar temperature. Additionally, oversized and excess equipment
is not required, further improving performance and economics for
the user. In another aspect, the invention is a method of pumping
fluids using a motor having vortex generators on its exterior
surfaces or on the interior surfaces of a surrounding shroud, and a
method for determining optimum well operating conditions.
Inventors: |
Hartman, Michael G.;
(Kirkland, WA) ; Moe, Eric M.; (Seattle, WA)
; Breidenthal, Robert E.; (Seattle, WA) ; Rehr,
Jesse S.; (Seattle, WA) |
Correspondence
Address: |
GARY C COHN, PLLC
4010 LAKE WASHINGTON BLVD., NE
#105
KIRKLAND
WA
98033
|
Family ID: |
25277938 |
Appl. No.: |
09/838744 |
Filed: |
April 19, 2001 |
Current U.S.
Class: |
166/302 ;
166/369; 166/62; 166/66.4 |
Current CPC
Class: |
E21B 43/128
20130101 |
Class at
Publication: |
166/302 ;
166/369; 166/66.4; 166/62 |
International
Class: |
E21B 043/16 |
Claims
What is claimed is:
1. A method of pumping a fluid with a submergible pumping system
including a pump and an electrical motor, wherein (a) the
electrical motor has an internal conduit and said pumping system is
adapted to pump said fluid through said internal conduit, and (b)
the pumping system is submerged in said fluid such that production
fluids entering the pumping system pass in fluid contact with
exterior portions of the electrical motor and remove heat therefrom
before entering the pumping system, and said liquid production
fluids then pass through the internal conduit of the electrical
motor as the fluid is pumped, and remove additional heat from the
motor.
2. A method for producing fluids from a well having a casing that
contains perforations through which production fluids enter the
well, comprising (a) positioning an electrical submergible pumping
system including an electric motor within said casing such that the
electric motor is at or below or downstream of at least some of
said perforations, and (b) pumping production fluids from the well
with said pumping system, wherein (1) the pumping system includes a
pump, an electrical motor having an internal conduit and production
fluid intakes located below said electrical motor, said pumping
system being adapted to pump said production fluids through said
internal conduit of the electrical motor, (2) production fluids
entering the well through at least some of the perforations in the
casing pass in fluid contact with exterior portions of the
electrical motor and remove heat therefrom before entering the
production fluid intakes, and (3) production fluids pass through
the internal conduit of the electrical motor as the fluid travels
to the wellhead, and remove additional heat from the motor.
3. The method of claim 2 wherein the production fluids pass through
the internal conduit of the electrical motor under conditions of
turbulent flow.
4. The method of claim 3 wherein the internal conduit takes the
form of a hollow drive shaft.
5. The method of claim 4 wherein the electrical motor is located
between the wellhead and the pump.
6. The method of claim 5 wherein production fluids entering the
well through the perforations in the casing travel downward in
fluid contact past exterior portions of the electrical motor to an
intake, and the liquid production fluids are subsequently pumped
upwardly through the internal conduit of the electrical motor and
then to the wellhead.
7. The method of claim 6, wherein the flow of production fluid
through the internal conduit is characterized by a Reynolds number
of at least 2300.
8. The method of claim 7, wherein the motor contains a motor fluid
at a positive pressure to the exterior of the motor, and has seals
which allow motor fluid to leak into the production fluid.
9. The method of claim 7, wherein conditions are selected such that
heat transfer from at least one of the internal conduit and the
exterior surfaces of the motor to the production fluid is
characterized by a Nusselt number of at least 10.
10. The method of claim 7 wherein the internal conduit is the drive
shaft of the motor, and conditions are selected so as to generate a
Brinkmann number of less than 2.
11. The method of claim 7 wherein the internal conduit is the drive
shaft of the motor, and conditions are selected so as to generate a
Rossby number of more than 0.5.
12. The method of claim 4 wherein the pump is located between the
wellhead and the electric motor.
13. The method of claim 4 wherein electrical submergible pumping
system contains two pumps, one of which is located above the
electric motor and one of which is located below the electric
motor.
14. A method of pumping fluids with a submergible pumping system
including a pump and an electrical motor, wherein the pumping
system is submerged in production fluids such that said fluids flow
across vortex generators adapted to impart streamwise vorticity to
the production fluids as the pass in fluid contact with the
exterior portions of the electrical motor.
15. The method of claim 14 wherein the vortex generators are
attached to the exterior portions of the electric motor.
16. The method of claim 14 wherein the vortex generators are
attached to the interior surface of a motor shroud.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to methods for pumping fluids,
primarily from wells.
[0002] Water, oil and natural gas are commonly produced in wells.
The wells are formed by drilling a hole into a rock formation to
the depth where the fluid reservoir lies. A well casing is inserted
into the hole following the drilling. If natural gas is present in
the well, it is readily recovered from the well due to its natural
buoyancy, while in most cases water and oil must be pumped to the
surface.
[0003] Various types of pumping systems are used to produce fluids
from wells. Among these are sucker-rod/beam systems, plunger lift
systems, continuous or intermittent gas lift systems, hydraulic
reciprocating pumps, progressing cavity pumps and electrical
submergible pumps (ESPs). An ESP includes an electric motor that
drives a submergible pump that forces fluids from the reservoir to
the surface.
[0004] ESPs are often remotely operated in wells commonly at great
depths, in harsh environmental conditions. A challenge in using
ESPs is effective heat removal. Resistance in the electric motor
windings generates a significant amount of waste heat in operation,
as do mechanical friction and fluid friction. If this waste heat is
not sufficiently removed, the motor temperature can rise
significantly. Increasing the motor temperature leads to a number
of problems. Motor life becomes considerably shorter as
temperatures increase. Motor winding insulation, bearings, seals,
and lubrication are all adversely affected by high temperature. As
a result, ESPs commonly are removed from wells more often than
desired in order to replace or repair the electric motor. In the
oil market in particular, this results in high maintenance and
repair costs as well as significant losses of revenue due to lost
production. Alternatively, to keep motor temperatures within
reasonable limits, sometimes the production is reduced to a rate
lower than desired, equipment is oversized or excess equipment is
used, compromising the efficiency and profitability of the well.
Electrical efficiency also tends to decrease as the temperature
rises and additional horsepower is necessary when excess equipment,
like gas separators or additional seals, are used. This results in
increased energy costs, as more electrical power is needed to
perform a given amount of work.
[0005] Sometimes this problem is exacerbated because of particular
well conditions or operator choices. Well conditions such as high
gas content, high viscosity production fluids, the formation of
emulsions, low flow rates, multiphase flows, steam flood
production, deviated and horizontal wells, increasing well depth
and other factors can contribute to high motor temperatures. Some
of these conditions make it difficult for a well operator to
properly size and operate an ESP.
[0006] The production fluid is often used to remove some of the
waste heat and provide some temperature control. In conventional
ESP installations, production fluids are drawn or pumped past the
motor and remove some heat. This effect is illustrated in FIGS. 1
and 1A, which represents conventional ESP and so-called "inverted
ESP" pumping systems. In FIG. 1, well 1 has casing 2 that has
perforations 7 through which production fluids enter the well. The
ESP system includes pump 3, motor 4 and seal section 5. Production
fluids enter the well at a point below motor 4, pass by motor 4 in
the direction indicated by arrows 8, and then enter pump 3 at pump
intake 6. The production fluids are forced upwardly by pump 3,
exiting at the top of pump 3 and traveling to the surface in the
direction indicated by arrow 9. Cable 10 provides electrical power
to motor 4 from the surface. As shown, a packer 16 may be used in
the assembly.
[0007] In the "inverted ESP" system shown in FIG. 1A, the relative
positions of the pump and motor are reversed. Well 11 has casing 12
and perforations 17 as before. Production fluids enter pump intake
22 in the direction indicated by arrow 18. The production fluids
are pumped upwardly through pump 13, exiting at point 21 and then
flowing upwardly past seal section 15 and motor 14 in the direction
indicated by arrow 19. As before, cable 20 provides electrical
power from the surface. A packer 16 is again illustrated in this
assemblage.
[0008] In the systems shown in FIGS. 1 and 1A, the production
fluids remove some heat as they flow past the motor. However, the
amount of heat that is removed is often insufficient to optimize
motor performance and life. As a result, well operators have
attempted in various ways to improve motor performance and life.
One way of accomplishing this is to use special high temperature
windings in the motor so that it can better tolerate higher
temperatures. Unfortunately, this does nothing to reduce heat in
the bearings, seals, and lubrication, and significantly increases
the cost of the motor. Additionally, running at higher temperatures
makes the equipment more prone to scale and corrosion and lowers
electrical efficiency. Another technique is to add a shroud around
the motor to increase the fluid velocity. Except under conditions
where fluid does not flow across the motor housing, shrouds will
not significantly change the character of the flow (laminar or
turbulent), are expensive and can be difficult or impossible to
install in deviated and slim-hole wells.
[0009] Another attempt to improve production using ESPs is
described in "Operating Electric Submersible Pumps Below
Perforations, J. of Petrol. Techn., p. 742 (July 1997) and by Sison
in "Use of A Motor Cooling By-Pass System in An Electric
Submersible Pump to Increase the Economic Life of Gas Wells", Feb.
28, 2000. In these methods, an ESP system is installed in the well
below the perforations. Production fluids enter the well and the
pump intake without flowing past the motor. Without convective
cooling, a portion of the production fluids must be drawn off from
the pump and recirculated down to the base of the motor and
released to remove heat. This typically requires that a second,
recirculation pump be used together with special recirculation
tubing, and thus is complex, expensive and less efficient than
desired. It is also difficult to effectively recirculate enough
fluid to provide adequate heat removal. Another approach is the
so-called Framco ESP system, which also involves circulating a
cooling fluid through the motor from the wellhead. This is
described in "The Framco ESP System: A New Approach to Downhole
Electric Submersible Pumping", presented by Jon A. Svaeren and
Frank Mohn at the ESP Workshop, Houston, Tex., April 30-May 2,
1990. Again, the complexity and expense of such a system makes it
undesirable.
[0010] Thus, an improved ESP system would be highly desirable,
particularly one which enables more effective and efficient heat
removal from the electrical motor particularly if this can be
achieved without the expense of additional components or
unnecessary features.
SUMMARY OF THE INVENTION
[0011] In one aspect, this invention is a method of pumping fluids
with a submergible pumping system including a pump and an
electrical motor, wherein
[0012] (a) the electrical motor has an internal conduit and said
pumping system is adapted to pump said fluid through said internal
conduit, and
[0013] (b) the pumping system is submerged in said fluid such that
production fluids entering the pumping system pass in fluid contact
with exterior portions of the electrical motor and remove heat
therefrom before entering the production fluid intakes, and said
production fluids then pass through the internal conduit of the
electrical motor as the fluid is pumped, and remove additional heat
from the motor.
[0014] In a second aspect, this invention is a method for pumping
fluids from a well having a casing that contains perforations
through which production fluids enter the well, comprising
[0015] (a) positioning an electrical submergible pumping system
including an electric motor within said casing such that the
electric motor is at or below at least some of said perforations,
and
[0016] (b) pumping production fluids from the well with said
pumping system,
[0017] wherein
[0018] (i) the pumping system includes a pump, an electrical motor
having an internal conduit and production fluid intakes located
below said electrical motor, said pumping system being adapted to
pump said production fluids through said internal conduit of the
electrical motor,
[0019] (ii) liquid production fluids entering the well through at
least some of the perforations in the well casing come into fluid
contact with exterior portions of the electrical motor and remove
heat therefrom before entering the production fluid intakes,
and
[0020] (iii) liquid production fluids pass through the internal
conduit of the electrical motor as the fluid travels toward the
wellhead, and remove additional heat from the motor.
[0021] The method of the invention provides several significant
benefits. Production fluids in laminar or turbulent flow that come
into fluid contact with the exterior of the electrical motor
provide initial heat removal. In gassy wells, the increased flow
rate made possible by operating the ESP below the perforations may
foster turbulent flow in the annulus between the motor housing and
the well casing contributing to a significant increase in heat
transfer. An additional cooling effect is seen when the production
fluids pass through the internal motor conduit; thus, the
production fluids are used to remove heat twice in this invention,
once before entering the pump and a second time as they pass
through the internal motor conduit. The combined cooling effects
help maintain the motor within optimal or desired temperature
limits. This prolongs motor life and reduces maintenance costs and
other expenses attributable to premature motor failure. It also
allows an operator to increase the production by increasing the
horsepower capability of existing equipment without raising the
temperature. The cooling effect can be further facilitated if the
pump and/or internal conduit are designed so that the production
fluids experience turbulent flow as they pass through the internal
motor conduit. The combined cooling from both passes past the motor
provides advantages such as prolonged motor life, increased
horsepower density and/or greater electrical efficiency. It often
provides combinations of these benefits.
[0022] Surprisingly, these benefits are achieved despite the fact
that the production fluids are forced through a reduced diameter
area as they pass through the internal motor conduit. Additional
energy is needed to accomplish this, which normally would be
expected to increase operating costs and contribute to additional
temperature rises. Unexpectedly, it has been found that this
penalty is significantly offset by the benefits of additional heat
removal that is achieved by flowing the production fluids past the
motor twice.
[0023] In a third aspect, this invention is a method of pumping
fluids with a submergible pumping system including a pump and an
electrical motor, wherein the pumping system is submerged in said
fluid such that production fluids entering the pumping system pass
in fluid contact with exterior portions of the electrical motor and
remove heat therefrom before entering the production fluid intakes,
wherein the exterior portions of the electrical motor or the
interior portions of a surrounding shroud include vortex generators
adapted to impart streamwise vorticity to the production fluids as
they pass in fluid contact with the exterior portions of the
electrical motor. This aspect of the invention provides an
economical means for substantially increasing the efficiency of
heat removal from the motor. This aspect of the invention can be
incorporated into the first and second aspects of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic view of two conventional ESP
systems
[0025] FIG. 2 is a cross-sectional view of an embodiment of this
invention.
[0026] FIGS. 3 and 3A represent a schematic view and detail of an
embodiment of a motor having vortex generators for use in preferred
aspects of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] In this invention, a submergible pumping system is used to
pump a fluid. The system includes a pump and an electrical motor.
The electrical motor has an internal conduit and said pumping
system is adapted to pump said fluid through said internal conduit.
The pumping system is submerged in the fluid such that production
fluids entering the pumping system pass exterior portions of the
electrical motor and remove heat therefrom before entering the
production fluid intakes, and said production fluids then pass
through the internal conduit of the electrical motor as the fluid
is pumped, and remove additional heat from the motor. The
conditions of the fluid flow are preferably such that the fluid
flow is turbulent, has streamwise vortices, or both as the fluid
passes the exterior portions of the electrical motor, and is
turbulent as it passes through the internal conduit.
[0028] Typically, the pumping system is installed in a well, or
else has a mechanical means for inducing the fluid to pass in fluid
contact with exterior portions of the electrical motor. An example
of such a mechanical means is a shroud which partially covers the
pumping system, and has an opening to the fluid being pumped at the
opposite end of the pumping system from where the production fluid
intakes are located. This shroud permits the fluids to pass in
fluid contact with the exterior portions of the electric motor and
enter the production fluid intakes. The spacing between the shroud
and the exterior portions of the electrical motor is preferably
such that the fluids exhibit turbulent flow as they pass in fluid
contact with the exterior of the motor and do not impair
installation in the well or create excessive head loss.
Alternatively, or in addition, vortex generators impart streamwise
vortices to the production fluids as they flow through the spacing
between the shroud and the exterior portions of the electrical
motor.
[0029] In preferred embodiments of the invention, the pumping
system is installed in a well. The well may be vertical,
horizontal, or a so-called "divert" type. The well has a casing
that has perforations through which production fluids enter the
well. The pumping system includes an electrical motor having an
internal conduit. In preferred embodiments, the internal conduit
functions as the drive shaft of the motor, rotating when the motor
is operated to drive the pump. The pumping system has production
fluid intakes through which the production fluids enter. These are
located below (in the case of a horizontal well, downstream of)
said electrical motor. The pumping system is adapted to pump said
production fluids through the internal conduit of the electrical
motor and then on to the wellhead.
[0030] By "production fluids", it is meant any fluids which are
desired to be withdrawn from the well. Examples of such fluids
include water, oil, natural gas, and the like, with liquids being
of particular interest and oil being of special interest.
[0031] By "perforations", it is merely meant openings in a well
casing through which the production fluids enter the well. No
particular configuration or method of forming these openings is
critical to the invention. The pumping system is located below at
least some of the perforations in the casing. Thus, production
fluids entering those perforations that are above the pumping
system flow in fluid contact past the exterior portions of the
electrical motor and into the production fluid intakes, by force of
gravity and/or the action of the pumping system. In general, the
pumping system should be below enough of the perforations that
sufficient production fluids pass the exterior of the electrical
motor to provide a cooling effect. Preferably, at least half of the
production fluids will pass by the exterior of the motor before
entering the production fluid intakes. More preferably, the pumping
system is below substantially all of the perforations, and
substantially the entire volume of production fluids passes in
fluid contact with the exterior of the motor. This tends to
maximize the desired cooling effect. Having the pumping system
below substantially all of the perforations also facilitates
separation of gaseous and liquid production fluids before the
liquids enter the pumping system. This will increase the production
rate when the well produces a mixture of gasses and liquids. Having
the gasses circumvent the pump avoids various associated problems
such as gas-lock or pumping inefficiencies that are attributable to
entrained gasses. As gasses have lower heat capacities than
liquids, having the gasses circumvent the motor in this manner
further improves heat transfer and, thus, motor cooling.
Additionally, without oversized equipment or a gas separator
typically used in a system installed above the perforations, no
significant additional power is consumed.
[0032] The pumping system includes an electrical motor and a
submergible pump that is driven by the electrical motor. The motor
and pump may have a unitary structure (i.e., share a common
housing), but it is often preferred that they are separate units
which are connected together directly or indirectly as they are
installed in the well.
[0033] Two embodiments of the process of the invention are
illustrated in FIG. 2. In FIG. 2, well casing 210 extends along the
periphery of well 201, which as shown has been bored through a
producing subterranean rock stratum 440 and into a lower,
non-producing subterranean rock stratum 441. Producing stratum 440
contains production fluids that are to be pumped to the well head.
Casing 210 includes perforations 211 through which the production
fluids are admitted into the well for pumping to the surface.
[0034] Located within casing 210 is a pumping system that includes
motor 220 and pump 230. Both motor 220 and centrifugal pump 230 are
located below perforations 211. Motor 220 is affixed to production
pipe 250, which is shown in section. If desired, production pipe
250 can take the form of a coiled tube. Motor 220 is shown in
section to reveal drive shaft 221, which rotates when motor 220 is
operated to provide motive force to operate pump 230. Motor 220
contains an internal conduit through which the production fluids
pass on their way to the wellhead. In FIG. 2, a preferred type of
conduit is shown, in which drive shaft 221 is hollow to permit the
production fluids to flow through it and then into production pipe
250. Drive shaft 221 is mechanically connected, directly or
indirectly, to pump 230 in a manner such that pump 230 is operated
when drive shaft 221 is rotated. As shown, pump 230 is directly
connected to drive shaft 221, without intermediate apparatus.
However, it may be desirable to include various types of
intermediate apparatus such as a sealing section between motor 220
and pump 230. The manner through which motor 220 is affixed to
production pipe 250 and pump 230 is not critical; a variety of
fasteners, interlocking devices and the like can be used. Power is
provided to the motor from the wellhead through a cable or similar
device, which is not shown.
[0035] Intake 260 is attached to pump 230. When the pumping system
is activated, drive shaft 221 rotates substantially along its
longitudinal axis, driving the action of pump 230. Production
fluids enter the pumping system through intake 260 and enter pump
230. Drive shaft 221 is in liquid communication with production
pipe 250 and pump 230, so that fluids pumped upwardly by pump 230
pass through hollow shaft 221 and then enter production pipe 250
through which they are delivered to the wellhead.
[0036] Because motor 220 is below perforations 211, production
fluids that enter well 201 flow in fluid contact past the exterior
of motor 220 before entering intake 260. This can be due to simple
gravity, the pumping action of pump 230, or some combination of
these. Arrows 290 indicate the direction of flow of the production
fluids. As the production fluids must flow between well casing 210
and motor 220, the motor is somewhat smaller than the diameter of
well casing 210, creating an annulus through which the production
fluids can move. As the production fluids move in fluid contact
past the exterior portion of motor 220, they remove waste heat and
thus provide cooling.
[0037] As the production fluids then are pumped through the conduit
in motor 220 (in the embodiment of FIG. 2, through drive shaft
221), they remove additional waste heat and thus provide additional
cooling. The embodiment of FIG. 2 is a so-called "inverted" pumping
system in which the motor is above the pump. Although this is
preferred, it is not critical to the invention, and FIG. 2A
illustrates an embodiment in which the motor is below the pump. In
FIG. 2A, well 201A has casing 210A. The pumping system includes
production pipe 250A, pump 230A, motor 220A and intake 260A. Motor
220A includes drive shaft 221A that is hollow and thus forms the
internal conduit through which production fluids pass. As before,
drive shaft 221A is in mechanical communication with pump 230A so
that as motor 220A operates, drive shaft 221A causes pump 230A to
operate and pump the production fluids into production pipe 250A
and up to the wellhead. In the embodiment shown in FIG. 2A,
production fluids enter the well through perforations 211A, travel
past the exterior of motor 220A, enter intake 260A, travel upwardly
through the conduit in drive shaft 221A through pump 230A and then
through production pipe 250A to the wellhead. Motor 220A is cooled
twice by the production fluids; once as they pass in fluid contact
with the exterior of motor 220A and again as they pass through the
internal conduit in motor 220A (hollow drive shaft 221A).
[0038] The pumping system may contain additional elements as may be
necessary or desirable in any particular application. For example,
seal chamber sections are often provided in pumping systems for
deep hole wells. Such seal sections may also carry pump thrust and
can perform additional functions as well, as described by Brookbank
in "Inverted Pump Systems Design and Applications", ESP Workshop,
Houston, Apr. 26-28, 2000. As described by Brookbank, the functions
performed by the seal chamber section can be divided among several
pieces of apparatus if desired. In this invention, a seal chamber
section or other device for carrying pump thrust is preferably part
of the pumping system. In so-called "inverted" embodiments, it is
preferred that some means of carrying pump thrust is included in
the pump or motor design or as a separate piece of apparatus above
or below the pump. If desired, seal sections may be located above
the pumping system, or between the motor and the pump. Any
apparatus situated between the motor and pump must be adapted so
that motive force is transmitted through the apparatus from the
motor to the pump, and so that production fluids travel through
internal conduits in the motor, as described above.
[0039] Similarly, apparatus such as sand skirts, packers, various
types of connectors and the like can be incorporated into the
pumping system or used in conjunction with the pumping system. The
pumping system may contain anti-cavitation devices like a primer
pump to prevent cavitation of the fluid in the hollow rotor shaft
or pump. These may be especially useful in configurations where the
pump is above the motor.
[0040] In producing deep bore wells, it is common practice to use a
pumping system that is made up of separate components of a
relatively short length. This approach can be adopted in the
process of this invention as well. The motor, pump, intakes, seal
sections and other apparatus may be constructed as two or more
separate sections that are connected together to form the overall
pumping system.
[0041] A wide variety of electrical motor designs can be used in
the pumping system of the invention, provided that the motor
contains an internal conduit through which the production fluids
can flow to provide cooling and/or vortex generators in the annulus
between the motor and the well casing or shroud. Induction motors
and brushless DC motors are useful, among others. The internal
conduit is preferably a bore in a longitudinal drive shaft, as
shown in FIGS. 1 and 2A. Suitable motors of that type are described
in U.S. Pat. Nos. 5,951,262 and 6,000,915, both to Hartman, both
incorporated herein by reference in their entirety. No special
motor design is required, except that (1) it is adapted for
submergible applications (2) it contains the internal conduit as
described herein and (3) it is of a size and shape to fit within
the well casing and allow production fluids to pass in fluid
contact with the exterior of the motor. Conventional electric
motors as are commonly used for downhole pumping applications can
be used, if they are adapted to provide them with an internal
conduit through which the production fluids can be pumped. In its
simplest case, the motor can be any submergible electric motor
having a longitudinal, rotating drive shaft, in which the drive
shaft has a longitudinal bore. Conventional electric motors having
a longitudinal bore in some cases can be retrofitted for use in
this invention simply by boring out the drive shaft to form the
internal conduit.
[0042] The motor may be a single piece or a tandem configuration.
Multiple motors can be used and, if desired, pumps can be placed
both above and below the motor. Seal sections and other components
may be installed between the motor and pump.
[0043] As the motor will be submerged in the production fluids, it
is preferably adapted to operate in those conditions: The motor
preferably will contain a motor fluid that provides lubrication but
more importantly retards the leakage of production fluids into the
motor. The motor fluid may also contain various well-treating
materials such as scale inhibitors, emulsifiers, anti-emulsion
agents, surfactants, water, and the like. Various types of seals
can be incorporated into the motor to retard the leakage of
production fluids into it, and as mentioned above, seal chamber
sections can be used to accommodate thermal expansion of the motor
fluid and help equalize pressure between the inside and outside of
the motor.
[0044] In a preferred type of motor, the motor fluid is at a
positive pressure relative to the exterior of the motor and has
leaking seals, so that motor fluid slowly leaks from the motor into
the production fluid. A source of fresh motor fluid is provided,
either from a reservoir in the pumping system or through a tube or
capillary system from the wellhead. By maintaining a positive motor
fluid pressure, displacement of the motor fluid by the production
fluids can be reduced or eliminated, and motor life can be
prolonged. A pressure independent modulating flow control valve,
such as a SkoFlo.TM. SubSea valve is suitable for maintaining a
suitable positive pressure and flow of fresh lubrication fluid into
the motor.
[0045] A particularly preferred motor has vortex generators on its
exterior surface, or else is enclosed within a shroud that has
vortex generators on its interior surface. The vortex generators
operate to generate streamwise vortices in the production fluids as
they pass the exterior of the motor. The vortex generators are
preferably static devices having geometry and dimensions such that
when the production fluids flow through and past the generators, a
swirl in the flow is imparted. These streamwise vortices greatly
improve heat transfer from the exterior of the motor to the
production fluids, even further increasing the benefits of this
invention.
[0046] An example of a motor containing vortex generators is
schematically illustrated in FIG. 3. Well 300 has casing 310 and
perforations 311. The internal diameter of well 300 is D. Motor 320
and centrifugal pump 330, each having a diameter d, are disposed in
the well. Motor 320 is affixed to production pipe 350, which
extends to the wellhead. Shaft 321 connects motor 320 and pump 330,
in the same manner as described with respect to FIG. 2. Pump 330
includes intake 360, where production fluids enter the pumping
system. In the embodiment shown, production fluids enter the well
at perforations 311, flow past the exterior of motor 320, into
intake 360, up shaft 321 through motor 320 and up through
production pipe 350 to the well head, as indicated by arrows
390.
[0047] In the embodiment shown in FIG. 3, the exterior surface of
motor 320 has vortex generators. An alternative arrangement would
be to surround motor 320 in a shroud that has vortex generators on
its interior surface. In the embodiment in FIGS. 3 and 3A, the
vortex generators take the form of a plurality of fins 366. Fins
366 are slightly offset to the direction of flow of the production
fluids, and adjacent fins 366 are offset in opposite directions.
Thus, each pair of fins 366 define a gap which narrows in the
direction of flow. The dimensions of fins 366 and the offset angle
are sufficient to create streamwise vortices in the production
fluids as they flow past the fins.
[0048] Suitable offset angles are typically +/- 10-30, preferably
10-20 degrees from the direction of flow of the production fluids.
Suitable dimensions for fins 366 are illustrated in FIG. 3A. In
FIG. 3, the gap between motor 320 and well casing 311 is defined by
(D-d)/2, where D and d are as described before. A suitable fin 366
will extend outwardly from the exterior of motor 320 (or inwardly
from the exterior surface of a surrounding shroud) approximately
1/3-3/4 of the width of the gap between the motor and well casing
or, when a shroud is used, between the motor and interior surface
of the shroud; in FIG. 3A an especially preferred dimension of 1/2
the gap width is illustrated. In FIG. 3, numeral 325 designates
either the exterior surface of the motor or the internal surface of
a surrounding shroud. As shown in FIG. 3A, fin 366 is preferably
beveled with increasing width in the direction of flow, reaching
its maximum width at a point approximately 1/3-3/4 down its length.
However, if desired, leading edge 399 can take other shapes,
including those shown in outline in FIG. 3A. Overall length is
preferably about 1 to about 4, more preferably about 1.5 to about 3
times the width of the gap. In FIG. 3A, the length is shown as 2
times the gap width.
[0049] In the embodiment shown in FIG. 3, two rows of fins 366 are
used. A single row can be used, or greater than two rows can be
used. When multiple rows are used, a preferred spacing between the
rows is about 10-30 times the gap distance. However, the spacing
may be adjusted to trade off pressure loss with heat transfer.
[0050] Other suitable vortex generator designs can be used, such as
are described, for example, by Paulie and Eaton, Report #MD51,
August 1988, "The Fluid Dynamics and Heat Transfer Effect of
Streamwise Vortices Embedded in a Turbulent Boundary Layer".
[0051] The pump itself has no special design requirements, other
than it is adapted to pump production fluids through the internal
conduit of the motor (in an inverted system) or from the internal
conduit of the motor (in a conventional configuration). Generally,
the particulars of the pump design will be selected to fit the
particular application. The pump is typically in liquid
communication with the internal conduit of the motor. This is
accomplished by building the pump and motor as a single unit or
incorporating the pump into the motor, as described in U. S. Pat.
Nos. 5,951,262 and 6,000,915, by designing the pump to mate with
the internal conduit of the motor, or in some other manner. Pumps
of the type conventionally used in ESPs are entirely suitable, and
can easily be adapted for use in this invention through the design
of the connection between the pump outlet (or inlet) and the motor.
Progressive cavity pumps are also preferred types. The pump may be
one piece or in tandem sections. Multiple pumps may be used. If
desired, separate pumps can be provided above and below the
motor.
[0052] The method of this invention is useful in a variety of
wells, including water, oil and natural gas wells. The pumping
method is particularly adapted for wells where, using a
conventional ESP, any flow of production fluids through the annulus
between the motor housing and the well casing would be expected to
be laminar. Wells of this type include those having well conditions
such as high gas content, high viscosity production fluids, the
formation of emulsions, low flow rates, multiphase flows, deviated
and horizontal wells and increasing well depth, especially in the
oil industry. The well operator can take advantage of the enhanced
heat transfer and improved cooling of the motor in several ways. At
equivalent power usage, the motor will be more efficiently cooled,
and the operating temperature will be lower. The operator may
choose to take advantage of this lower operating temperature to
prolong the motor life. The lower temperature also tends to reduce
electrical resistance, thus allowing equivalent work to be done
with less power consumption. Conversely, the operator may elect to
increase the power to the motor so that it runs at higher
temperatures similar to those that would be experienced in prior
art processes. In this case, the operator chooses to forego longer
motor life in return for higher production rates that are achieved
because of the additional power that is used.
[0053] In order to increase cooling efficiency as the production
fluids flow through the motor, it is preferred to operate under
conditions that produce turbulent flow within the internal conduit.
More preferably, there is turbulent flow both inside the conduit,
and turbulent flow or streamwise vortices in the annulus between
the motor housing and the well casing where the production fluid
passes in contact with the exterior of the motor. Turbulent flow
can be expressed in terms of Reynolds number (a dimensionless
parameter), which is a function of the average fluid velocity,
kinematic viscosity of the fluid and diameter of the internal
conduit. A Reynolds number of about 2300 or higher is typically
indicative of turbulent flow. Preferably, flow conditions of the
production fluids through the internal conduit of the motor is such
that the Reynolds number is at least about 3000. A Reynolds number
in excess of 5000 to 10000 is more preferred.
[0054] Another parameter, Nusselt number, is a dimensionless
measure of heat transfer. The Nusselt number is a function of the
Reynolds number, the Prantl number, and the absolute viscosity of
the bulk fluid and fluid at the wall. With turbulent flow, a high
Nusselt number (exceeding 10) represents a high heat transfer rate
for a given temperature difference. The more turbulent the flow the
higher the Reynolds and the Nusselt numbers. With laminar flow, low
Nusselt numbers (below 5) are indicative of poor convective heat
transfer. Enhanced heat transfer from the motor to the production
fluid can be expected when the Nusselt number is at least 10,
preferably at least 50, and the method of the invention is
preferably operated under conditions that achieve such Nusselt
numbers.
[0055] Operating conditions preferably are also chosen so as to
provide a Brinkmann number (another dimensionless parameter) of
less than 2, preferably less than 0.5. The Brinkmann number
indicates the direction of heat transfer within a viscous fluid. It
is a function of the average velocity of the fluid, its absolute
viscosity, the thermal conductivity of the fluid and the
temperature difference between the fluid and the inside wall of the
internal conduit. When conditions are such that the Brinkmann
number is less than 2, heat will travel from the hot motor to the
cooler production fluid and fluid friction will not add additional
heat to the motor. Viscous dissipation effects are negligible, when
the Brinkmann number is less than 0.5.
[0056] When the internal conduit is the drive shaft of the motor,
operating conditions can be further described with reference to a
Rossby number (still another dimensionless parameter). The Rossby
number provides an indication as to whether spinning flow will or
will not dominate axial flow in the internal motor conduit.
Conditions generating a Rossby number of at least about 0.5,
preferably at least about 1.0 are preferred. The Rossby number is a
function of average fluid velocity, fluid angular velocity and the
diameter of the internal conduit.
[0057] Yet another advantage of this invention is that it permits
gaseous production fluids to separate from liquids before entering
the pumping system. Gasses entering the well through perforations
above the pumping system will tend to travel directly upward to the
wellhead without passing through the pumping system, due to the
natural buoyancy of the gasses. Liquids will flow downwardly and
enter the pumping system, without significant entrained gas, for
delivery to the wellhead. Thus, a ready separation of production
gasses and production liquids is made. Because the pumping system
will process less gas, pumping problems and inefficiencies
associated with pumping mixtures of gasses and liquids are largely
mitigated. Moreover, by eliminating the need for oversized
equipment or a gas separator in a typical system installed above
the perforations, no additional failure modes or power losses are
introduced.
* * * * *