Note: Descriptions are shown in the official language in which they were submitted.
CA 02619826 2008-02-05
REAL TIME OPTIMIZATION OF POWER IN ELECTRICAL SUBMERSIBLE PUMP
VARIABLE SPEED APPLICATIONS
BACKGROUND OF THE INVENTION
Field of the Invention
Embodiments of the present invention generally relate to hydrocarbon recovery
and, more particularly, to monitoring and optimizing the operation of a
variable
frequency drive (VFD) in real time to control an electric submersible pump
(ESP), such
as those used in production operations.
Description of the Related Art
An electric submersible pump (ESP) is a type of pump with an electric motor
enclosed in a protective housing such that the assembly may be submerged in
the fluid
to be pumped. A system of mechanical seals may be employed to prevent the
fluid
being pumped from entering the motor and causing a short circuit. When used in
an oil
well, an operating ESP decreases the pressure at the bottom of the well and
allows
significantly more oil to be produced from the well when compared to natural
production.
To control the production rate, the electric motor of the ESP may be driven by
a
variable frequency drive (VFD). Generally, the higher the drive frequency
output by the
VFD, the faster the electric motor rotates. A typical VFD is an electronic
power
conversion device that converts input AC power to DC intermediate power using
a
rectifier circuit and converts the DC intermediate power to quasi-sinusoidal
AC power of
a desired frequency using an inverter switching circuit. The voltage and
current
waveforms output by the VFD are no longer perfect sinusoids, but have a
distorted
appearance. The details of the circuits within a VFD are beyond the scope of
the
invention and are known to those skilled in the art, so they will not be
described herein.
When the motor of an ESP is driven by a VFD, voltage and current harmonics
are generated on the input and output of the VFD indicative of the distorted
output
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CA 02619826 2008-02-05
waveforms. This is because a VFD is a type of nonlinear load and draws current
in a
non-sinusoidal manner. A power system that carries such a nonlinear load may
experience such operational problems as capacitor failures, overheating of
transformers and/or cables, tripping of circuit breakers, power waveform
distortion,
motor failures, programmable logic controller (PLC) failures or malfunction,
and power
generator failures. The more distorted the current waveform is, the hotter the
motor will
be and the more the motor run life will deteriorate. Voltage spikes on the
voltage
waveform from distortion may damage the insulation of the motor, which will
drastically
decrease the motor run life and possibly lead to a premature motor failure.
Because a VFD-ESP system possesses inductance, capacitance, and
resistance due to the long cables (e.g., 10,000 ft) running between the ESP
located
downhole and the VFD typically located at the surface, a natural resonance
condition
occurs. If the VFD is operated at or near the resonant carrier frequency,
voltage peaks
may occur in the output waveforms. These voltage spikes from resonance may
also
reduce motor run life and lead to premature motor failure.
Furthermore, power companies impose fines on operators, such as hydrocarbon
recovery companies, for exceeding the allowable total harmonic distortion
(THD) limit
on the input line of the VFD, thereby creating additional operating
expenditures. In
remote locations, such as the desert, where there may not be an infrastructure
to
supply power lines, excessive harmonics may cause the power generators to fail
and
need to be replaced. Equipment failures and fines combine to diminish the
benefits
from production optimization campaigns carried out for ESP-pumped wells.
Traditionally, a number of techniques have been applied to cope with input and
output harmonics produced by VFD-ESP systems. For example, input harmonics
have
been reduced by placing a line filter on the input of a VFD. However, ESP
motors are
capable of up to 1000 horsepower (hp) @ 60 Hz, and the cost of this type of
filter
increases exponentially with the horsepower, thereby making line filters
prohibitively
expensive for many VFD-ESP systems. Furthermore, there is no guarantee with
the
line filter solution that the input harmonics will be within allowable limits
at all
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operational conditions. Output harmonics have been addressed with a load
filter placed
at the output of the VFD. The purpose of the load filter is to smooth the
waveforms and
remove, or at least reduce, the high frequency spikes. Like the line filters,
load filters
are also often costly, and there is no guarantee that the load filters will
perform equally
well at all operational conditions.
Because operators are fined when exceeding the THD limits imposed by the
power companies on the input line of the VFD, operators may hire a consultant
group to
perform a harmonic study of a violating well. During a harmonic study, the VFD
is
typically shut down and digital oscilloscopes may be coupled to the input and
output of
the VFD. Then, the VFD is restarted, waveforms at the input and output of the
VFD are
stored, and the THD may be recorded. The waveforms may be analyzed at a later
time
to determine how the allowable harmonic levels were exceeded. After some time
(e.g.,
hours, a day, or more), the VFD is shut down, the metering equipment is
removed, and
the consultant group may move on to another VFD.
The main drawbacks of this method include the following: (1) the study is
costly,
(2) production has to be halted during VFD shutdown, (3) during every VFD
restart
there is heightened likelihood of burning an ESP motor, and (4) the consultant
group's
study is limited to a one-time recommendation based on a particular short-term
operational observation of a "VFD/ESP/oil well" system, offering only a
snapshot of the
system. Because a VFD/ESP/oil well system is a highly dynamic system and
operating
parameters may change significantly every few days, this type of study does
not allow
the operator to react to changed operational conditions by fine-tuning well
operation to
reduce harmonics and prolong the life of the motor while maintaining the
highest
possible production level during the entire ESP run life (typically ranging
from 6 months
to 15 years). Such studies also cannot capture and control every possible
change in
operating parameters during ESP run life, archive this data, and correlate
different
modes of system operation to such critical parameters as ESP run life,
generator and
transformer shut-downs, failure of other surface electrical and electronic
equipment,
and loss of oil production associated with such failures to improve efficiency
of future
installations of generators, transformer, VFD, and ESP for the same well or
oil pad.
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Accordingly, what is needed is a method of operating a variable speed ESP such
that hydrocarbon production can be optimized without violating the THD limit
on the
input power line or damaging or prematurely wearing out the equipment.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide methods and apparatus for
operating a variable frequency drive (VFD) coupled to an electric submersible
pump
(ESP) such that hydrocarbon production can be optimized without violating the
THD
limit on the input power line or damaging or prematurely wearing out the
equipment.
Data of the VFD-ESP system may be collected and analyzed in real time such
that
operating parameters of the VFD-ESP system may be fine-tuned in short order in
an
effort to maintain hydrocarbon production equilibrium.
One embodiment of the invention provides a method of adjusting one or more
operating parameters of a variable frequency drive (VFD) coupled to an
electric
submersible pump (ESP) in real time. The method generally includes collecting
data
associated with the VFD and the ESP while operating the ESP, analyzing the
collected
data in real time to determine if the one or more operating parameters can be
adjusted
to achieve a desired operating state, and, in response to determining that the
operating
parameters can be adjusted to achieve the desired operating state, adjusting
the one or
more operating parameters of the VFD.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features of the present
invention
can be understood in detail, a more particular description of the invention,
briefly
summarized above, may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however, that the
appended
drawings illustrate only typical embodiments of this invention and are
therefore not to
be considered limiting of its scope, for the invention may admit to other
equally effective
embodiments.
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FIG. 1 is a block diagram of a well system comprising a variable frequency
drive
(VFD) to power an electrical submersible pump (ESP) in accordance with an
embodiment of the invention;
FIGs. 2 and 2A are block diagrams of a system for measuring and analyzing
properties of the VFD in real-time while data is being transmitted via a
supervisory
control and data acquisition (SCADA) communication system in accordance with
embodiments of the invention;
FIGs. 3A and 3B are a flow diagram illustrating the optimization of a VFD in
real-
time in accordance with an embodiment of the invention;
FIG. 4 illustrates the harmonics that may be present in a VFD waveform in
accordance with an embodiment of the invention;
FIG. 5 illustrates the analysis of data measured from the VFD input and
transmitted via a SCADA communication system in accordance with an embodiment
of
the invention;
FIG. 6 illustrates the analysis of data measured from the VFD output by
software
executed on an office PC (personal computer) and transmitted via the SCADA
system
in accordance with an embodiment of the invention;
FIG. 7 illustrates historical tracking of data measured from the VFD input in
accordance with an embodiment of the invention; and
FIG. 8 illustrates historical tracking of data measured from the VFD output in
accordance with an embodiment of the invention.
DETAILED DESCRIPTION
Embodiments of the present invention provide methods and apparatus for real-
time monitoring of a variable frequency drive (VFD) controlling an electric
submersible
pump (ESP) and for fine-tuning operation of a VFD-ESP system such that oil
production, power, and motor run life are optimized.
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AN EXEMPLARY VFD-ESP SYSTEM
FIG. 1 is a block diagram of a typical VFD-ESP system 100. An ESP 102 may
be located at or near the bottom of a well borehole in an effort to decrease
the pressure
at the bottom of the well and create a form of "artificial lift" for the
production fluid, such
as oil. The borehole may contain well casing 104 lined with cement to keep the
casing
104 in place. The production fluid may travel to the well surface 108 via
production
tubing 106, and the well may be capped by a wellhead 110 having numerous
production control devices.
The electric motor within the ESP 102 may be coupled to a VFD 112 located at
the surface via a power cable 114 run inside the well casing 104. The output
of the
VFD 112 may be coupled to a VFD transformer 116 with various taps to permit
different
voltage amplitudes output from the VFD 112. For some embodiments, the ESP 102
may be coupled to the VFD 112 or the VFD transformer 116 via a junction box
118,
which is meant to vent free gas trapped in a cable line. The input of the VFD
112 may
be coupled to an input power source 120, such as a distribution transformer or
a
generator. The voltage waveform from the input power source 120 operates at a
constant frequency and constant amplitude (e.g., 120 Vrms 60 Hz), while the
output of
the VFD transformer 116 allows for a variable frequency and variable voltage
amplitude. The operating frequency supplied to the ESP 102 is generally
proportional
to the operating speed of the electric motor and, therefore, proportional to
the
production rate. The voltage (and hence, the power) supplied to the ESP 102
may
need to be adjusted as the load changes over time with production.
As described above, power harmonics 122 may be caused by the inverter
switching circuit of the VFD 112 leading to distorted waveforms and/or by
operating with
a carrier frequency near resonance caused by the capacitance and inductance of
the
long power cable 114. On the output side of the VFD 112, these harmonics 122
may
cause overheating of cables, or damage to or reduced run life of the electric
motor of
the ESP 102 from overheating, for example. The harmonics 122 may also be
reflected
by the VFD 112 onto the input side where they may cause disturbances in the
input
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CA 02619826 2008-02-05
power source 120. Such disturbances may lead to failure of the generator or
overheating of the distribution transformer in addition to causing failures of
the
equipment connected to the same power line, all of which explain why power
companies impose fines for exceeding the allowable total harmonic distortion
(THD)
limit.
AN EXEMPLARY VFD MONITORING SYSTEM
FIG. 2 is a block diagram of a system 200 for measuring and analyzing
properties of the VFD in real time according to some embodiments. Both the
input and
the output of the VFD 112 may be coupled to a waveform capture device 202,
with
sufficient sampling speed and a suitable number of channels. For some
embodiments,
the capture device 202 may be a stand-alone unit or a plug-in card for a
computer. The
sampling speed should be more than double that of the greatest harmonic
frequency of
interest according to the Nyquist criterion, as those skilled in the art will
recognize. For
some embodiments, both the voltage and the current may be sampled with a scope
probe and a current probe, respectively, on the input and the output of the
VFD 112,
suggesting at least 4 channels on the capture device 202. Furthermore, since
the
system 200 may be intended for long-term monitoring, the connections for the
capture
device 202 on the VFD 112 should not interfere with normal well operation or
require a
VFD shutdown.
Within the capture device 202, there may be one or more analog-to-digital
converters (ADCs) 204 to sample the incoming waveforms from the VFD 112. The
ADCs 204 may be coupled to a memory management unit (MMU) 206 to govern
memory access, which may be part of a central processing unit (CPU) or a
separate
integrated circuit (IC) coupled to the CPU 208 as shown. The CPU 208 may send
the
sampled data to an interface 210 via an input/output unit (I/O) 212, which may
also be
part of the CPU 208 for some embodiments. The interface 210 may be for
parallel
communication (e.g., general purpose interface bus (GPIB)) or serial
transmission (e.g.,
RS-232) to a remote terminal unit (RTU) 214, such as the CAC EXS-1000 series
from
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CA 02619826 2008-02-05
eProduction Solutions. The RTU 214 may be seated on the oil pad. For some
embodiments, a waveform capture device may be an integral part of the RTU 214.
Among other functions, the RTU 214 may collect the sampled VFD data from the
capture device 202 and make it available to a host computing device, such as a
computer or a supervisory control and data acquisition (SCADA) Human Machine
Interface (HMI) 216, on the network via any suitable I/O interface 218, such
as
ModbusTM RTU, Modbus ASCII, OPC (object linking and embedding (OLE) for
Process
Control, ODBC (Open DataBase for Connectivity), or proprietary protocols. For
some
embodiments, the sampled VFD data may be transmitted via satellite as shown in
FIG.
2 to the HMI 216 located anywhere in the world. The HMI 216 may run stand-
alone or
integrated executable software, such as eProduction Solutions' Life-of-the-
Well Info
System (LOWIS), to analyze the sampled data. For other embodiments, the
sampled
VFD data may be processed by the CPU 208 or the HMI 216 locally, rather than
remotely.
Described in greater detail below, such analysis may include calculating the
THD
at the input and the output of the VFD, comparing the data to other control or
sensed
parameters of the well or VFD, and computing trends or scenarios based on the
historical data so that operating parameters of the well may be optimized. For
example,
by sending signals from the HMI 216 to the RTU 214, the RTU 214 can send
control
signals to the VFD 112 to adjust such operating parameters as the operating
frequency
and the carrier frequency. For some embodiments, the software running on the
HMI
216 may include setpoints for automatically sending such signals. In this
manner, an
operator in any part of the world should be able to access the data and trends
and
monitor in real time how the value of THD and the operation in respect to the
resonant
frequency change as the operating parameters of the VFD 112 are fine-tuned.
Figure 2A shows additional detail of the workings of the capture device 202,
additional detail of the workings of the VFD 112, according to one embodiment
of the
invention. Note that Figure 2A does not show all of the components illustrated
in Figure
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2. Figure 2A illustrates the capture device 202, the VFD 112, and additional
elements
of the system 200 as a frame of reference.
AN EXEMPLARY METHOD FOR OPERATING A VFD-ESP SYSTEM
FIGs. 3A and 3B are a flow diagram 300 illustrating the optimization of the
operating parameters for a VFD, such as the VFD 112 of the typical VFD-ESP
system
100 in FIG. 1, in real time in accordance with an embodiment of the invention.
The
following steps may be executed in the real-time monitoring system 200 of FIG.
2 as a
suitable, although not limiting, example.
In step 302, the initial VFD operating parameters may be established. These
operating parameters may include, but are not limited to, the operating
frequency, the
carrier frequency, the base frequency, the target operating frequency, the
operating
frequency ramp profile and the VFD transformer tap setting. These initial
operating
parameters may be entered into and stored in the Human Machine Interface (HMI)
216.
Once the initial operating parameters have been set, the well may be
commissioned in step 304 by configuring the VFD transformer tap setting
(typically
done manually) and then powering up the VFD 112. The VFD 112 may start with an
initially small operating frequency, such as 30-35 Hz, and then slowly
increase the
operating frequency according to a ramp profile up to the target frequency,
such as 35-
90 Hz. The ramp profile may specify a period of a few days to months to reach
the
target operating frequency depending on the well. As the operating frequency
increases to increase the fluid flow rate, the THD will most likely also
increase at both
the input and output of the VFD 112. FIG. 4 illustrates the harmonics in a
power
spectrum 400 that may be present in a VFD waveform of voltage or current on
the input
or the output of the VFD 112.
Therefore, the voltage and current waveforms at the input and the output of
the
VFD 112, among other data, are sampled by the capture device 202 and
transmitted to
the HMI 216 via the RTU 214 in step 306. As described above for some
embodiments,
the capture device may also be built into the RTU 214. In an effort to make an
informed
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CA 02619826 2008-02-05
decision with respect to adjusting the operating parameters of the VFD 112,
other data
may be collected, as well, typically by the RTU 214. This data may include the
VFD's
operating frequency, carrier frequency, and base frequency, the system power,
the
power factor, the well fluid flow rate, the temperature of the electric motor
of the ESP
102, and the ESP vibration.
The data collected in step 306 may be analyzed in step 308 in real time, and
such analysis may occur in software packages, such as LOWIS or MATLAB ,
executed
by the HMI 216. On the input side of the VFD 112, the voltage and current
waveforms
may be displayed and analyzed. Referring now to the graph 500 in FIG. 5, the
THD
502 (in %), operating frequency 504 (in Hz), power 506 (in kW), power factor
508 (in
%), and fluid flow rate 510 (in stock tank barrels of oil per day (stbd)), for
example, may
be calculated and displayed over a small period of elapsed time. From the
graph 500,
as the operating frequency 504 of the VFD 112 increases, the ESP motor rotates
more
quickly, thereby increasing the flow rate 510. However, the THD 502 and the
power
506 drawn from the input power source 120 increases with the increased VFD
operating frequency 504.
On the output side of the VFD 112, the voltage and current waveforms may also
be displayed and analyzed in real time. Referring now to the graph 600 in FIG.
6,
motor vibration 602 (in g), apparent VFD power 604 (in kVA), real power
consumed by
the system 606 (in kW), motor temperature 608 (in F), total harmonic
distortion of the
voltage (THDv) 610 (in %), total harmonic distortion of the current (THDi) 612
(in %),
flow rate 614 (in stbd), the base frequency 616 (in Hz), operating frequency
618 (in Hz),
carrier frequency 620 (in Hz), motor voltage imbalance 622 (in %), and motor
current
imbalance 624 (in %), for example, may be measured and/or calculated and
displayed
over a small period of elapsed time. Furthermore, the resonant frequency (in
Hz), or
the carrier frequency at which resonance may occur, and the energy (in kWh)
may be
analyzed and displayed on graph 600 or separate graphs in real time.
The data collected in step 306 may also be stored in the memory of the HMI 216
in step 310. Once a number of data gatherings have occurred, this historical
data may
CA 02619826 2008-02-05
be analyzed and displayed for both the input and output side of the VFD 112.
Such
historical data may be useful for analyzing trends, predicting behavior, and
potentially
avoiding THD limit violations and VFD-ESP system damage and wear. New data
gathered in step 306 may be added to the historical data on this or the next
iteration of
step 310. If available, the historical data may also include data collected
before the well
was operated with the real-time monitoring system according to embodiments of
the
invention.
Referring now to FIG. 7, a graph 700 of historical data for the input side of
the
VFD 112 may include data collected over a period of several months for such
operating
data as THDv (in %), THDi 702 (in %), operating frequency 704 (in Hz), total
oil loss
706 (in stock tank barrels (stb)), and numbers of transformer, VFD, and
switchboard
shutdowns 708, 710, 712. In FIG. 8, a graph 800 of historical data for the
output side of
the VFD 112 may include data collected over a period of several months for
such
operating data as energy 802 (in kWh), motor run life 804 (in days), THD 806
(in %),
operating frequency 808 (in Hz), carrier frequency 810 (in Hz), VFD apparent
power
812 (in kVA), motor voltage imbalance 814 (in %), motor current imbalance 816
(in %),
motor vibration 818 (in g), and motor temperature (in F).
In step 312, the analyzed data from step 308 and the historical data from step
310 may be further analyzed to decide if fine-tuning of the VFD operating
parameters is
possible. This decision may be based in part on an economic analysis of the
historical
data. The goal here may be to find equilibrium, where the maximum possible oil
production can be achieved while the cleanest possible voltage and current
waveforms
(i.e., least distorted waveforms) are supplied to the electrical motor of the
ESP 102 and
the lowest possible level of harmonics is generated on the input side of the
VFD 112 to
disturb the input power source 120. In addition, since resonance leads to
instability
thereby making the job of finding equilibrium of the operating parameters much
more
difficult, the VFD 112 should be operated with a carrier frequency outside of
the
resonant area. This approach may allow the operator to gain maximum profit
from
production optimization procedures by incrementally achieving maximum
production
while maintaining a high ESP run life and decreasing expenditures (e.g., fines
imposed
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CA 02619826 2008-02-05
by power companies for distorted power and fees from failed power generators,
motors,
transformers, and other equipment on the power supply line).
If the VFD operating parameters can be fine-tuned to reach a desired
equilibrium, then one or more of these parameters may be adjusted in step 314.
The
VFD operating parameters (e.g. operating frequency and carrier frequency) may
be
adjusted automatically. For example, the VFD 112 may be automatically adjusted
by
sending a signal from the HMI 216 to the RTU 214, which may in turn send a
control
signal to the VFD 112. In some cases, if the analysis suggests that the output
voltage
needs to be optimized and it cannot be done by automatic adjustment of the VFD
base
frequency, a VFD transformer tap setting should be adjusted, and then the VFD
112
may need to be temporarily shut down for manual adjustment of the VFD
transformer
116. Once the adjustment has been made, then data collection may resume at
step
306 in a continuous loop, thereby allowing the operator to monitor the effects
of the
adjustments to the VFD-ESP system in real time. In the case of a VFD shutdown,
however, the process may need to be restarted at step 302 to reinitialize the
VFD
operating parameters and then re-commission the well in step 304.
If the VFD operating parameters cannot be fine-tuned according to the decision
made in step 312, then data collection may resume at step 306 in a loop. Over
time
and as fluid is extracted from the well, conditions may change. For instance,
the
operator may desire to increase the ESP rotational speed 102 as oil is
depleted from
the well in an effort to maintain a certain fluid flow rate. To increase the
pressure, the
electric motor may eventually need to be supplied with a higher operating
frequency
from the VFD 112 in step 314 to increase the motor's rotational speed.
However, this
increase in operating frequency should be balanced against the risk of
increasing the
THD according to embodiments of the invention.
MULTIPLE VFD-ESP SYSTEM OPTIMIZATION
The techniques described above may be extended to finding equilibrium for
multiple wells, each having a VFD-ESP system that may be fine-tuned in an
effort to
achieve the maximum possible oil production from all of the wells, while the
least
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CA 02619826 2008-02-05
distorted waveforms are supplied to the electrical motors of the ESPs and the
lowest
possible level of harmonics is generated on the input side of the VFDs to
disturb the
input power source. In this case, the input power source may be shared by
some, if not
all, of the multiple wells being monitored in real time. Multi-well monitoring
may also
present an advantage over single well monitoring by permitting an operator to
consider
more variables at once and make more informed tradeoffs.
A single Human Machine Interface (e.g., a personal computer), such as the HMI
216 in FIG. 2, may be used to analyze the data collected from the plurality of
wells via
one or more RTUs and decide whether the operating parameters can be fine-
tuned.
The HMI may also send signals to the one or more RTUs to relay control signals
to one
or more of the VFDs to adjust the operating parameters as described above.
Those
skilled in the art will appreciate that other configurations of networked HMIs
and/or
RTUs may be deployed to gather data from and adjust the operating parameters
of the
multiple VFD-ESP systems in an effort to achieve production equilibrium as
described
herein.
CONCLUSION
The use of real time monitoring and analysis on the VFD-ESP system may
provide for fine-tuning of the system such that fluid production, power, and
equipment
run life are optimized simultaneously. Such an approach to oil/water
production may
permit a broader approach to ESP performance optimization, where downhole
parameters are measured and analyzed in conjunction with surface parameters.
Advantages of the techniques described herein may involve optimization of the
equipment operating parameters including, but not limited to, VFD operating
frequency,
carrier frequency, VFD transformer tap settings, and base frequency. Other
advantages may include avoiding or reducing the number of well shutdowns,
lowering
production costs from equipment failures and replacements and fines from the
power
companies, reducing the electric motor and power cable temperatures, and
determining
desired well-commissioning techniques.
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While the foregoing is directed to embodiments of the present invention, other
and further embodiments of the invention may be devised without departing from
the
basic scope thereof, and the scope thereof is determined by the claims that
follow.
14
