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Analyzer Technical
Information
Frequently Asked Questions
1. How does it all work?
The electrical
characteristics of the water (68-80) and the oil (2.5) are very different
and this provides the means to clearly determine the water content. An
electrical signal is sent from the electronics on the end of the
measurement section down through the fluids. This generates a standing wave
similar to the vibrations of a rubber band held at both ends and plucked.
This standing wave changes position within the section as the water content
changes. The change in position is automatically detected by the microwave
oscillator that originally sent out the signal and it changes its basic
frequency depending upon how much water is in the section. In summary, the
sending and receiving portion of the electronics are the same and they
change the frequency with respect to water content.
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2. Microwave measurement - the old way
Microwave energy
provides for a more complex measurement and provides enough information to
improve the measurement to include absorptive crudes. Once the absorption
becomes too great (such as in the water continuous phase), the measurement
once again degrades to a point where the resolution of the parameters are
too small to be effective.
An additional
problem with standard microwave equipment is the complexity of the
equipment required to make the measurement. A stable oscillator with amplifiers
is used to obtain the frequency and microwave energy for the measurement. A
sample must be obtained of the energy leaving the system to compare to the
energy transmitted or reflected in the measurement section. The energy must
then be transmitted across a barrier (a seal plug or the like) into the
fluids under measurement. The introduction of this energy must be done in
accordance with electrical design constraints in order to actually get it
into the fluids instead of reflected back into the source. This is
comparable to the lens of a camera. The more expensive camera lens has
special coatings which prevent reflections and therefore get more light
through the lens and onto the film.
The energy then
enters the fluids and propagates along the structure of the measurement
section which is again designed to allow proper electrical properties to
contain and control the energy. Typically, an additional barrier (microwave
window and seal plug) and the associated structure to allow the energy to
be received must be introduced into the measurement section. This allows
the energy which has now been affected by the fluids under measurement to
be sent back to the electronics and measured and compared to the energy
originally sent out.
The measurement at
this point takes the energy incident upon (introduced into) the measurement
section from the oscillator and amplifiers and compares it to the received
energy. This is done with additional amplifiers, amplitude detectors, phase
detectors, mixers and local oscillators for the down conversion. This is
technology which is standard in many military and commercial communication
systems. The problem here is twofold; one is the complexity of the system,
and the second is that all of this still doesn't give the required sensitivity
for obtaining a good measurement in the water phase and in other
situations.
The complexity of standard microwave systems lead to:
A)
short time between failures
B)
difficulty of maintenance (most electronic technicians cannot troubleshoot
such a system)
C)
expense of the parts
D)
the requirements for two microwave "windows" into the fluid
E)
sensitivity is still reduced from what is required.
Typically, these systems obtain
increased sensitivities by using a resonant structure (similar to an organ
pipe, the length and diameters determine the note or resonance). The
microwave energy is introduced into the fluids and the measurement section
is designed as the resonant structure. The problem with a resonant
structure is that it goes away when you introduce absorptive materials
(such as salt water or some crudes). This is like placing a rubber plug
into the organ pipe, the strength of the vibrations is vastly reduced and
the clarity of the tone is gone.
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3. What benefits does
Oscillator Load Pull have?
What is required to make a good
measurement comes down to reduction of the complexity and improvement of
the sensitivity. Oscillator load pull is a parameter of an oscillator which
is a measure of how much the oscillator changes its frequency when the load
that is connected to it changes. This is a parameter which is usually
designed out by preventing the oscillator from ever seeing the changing
load through the use of isolation means. Typically amplifiers will be used
after the oscillator to isolate the oscillator from the load. This is like
a guard rail around the pipe organ's pipes to prevent people from touching
the pipes and therefore changing their tone. Interestingly, no one ever
used load pull for a measurement before, they just always knew that they
needed to design it out. Therefore, a patent was issued on this technique.
The advantages of load pull are
simplicity; the source of the microwave frequency and energy is both the
transmitter and the receiver. In addition, the use of oscillator load pull
increases the sensitivity to the measurement parameters by 100 to 1000
times over conventional microwave measurement techniques. This increase in
sensitivity also brings the capability to measure highly absorptive fluids
such as 28% salt in water. Because of simplicity the entire system is very
easy to troubleshoot and is very stable and reliable.
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4. What makes Phase Dynamics
different from other technologies?
The patented measurement method
which allows the oscillator to change frequency with the changing water cut
provides for up to 1000 times the sensitivity than other technologies. This
is the reason that water continuous measurements in the high water cut
region can be done. Prior methods just did not have the sensitivity to
water in the 60-90% water cut regions. This same method gives long term
reproducibility and stability of measurement because of the simplicity of
the circuitry.
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5. What can the user check on
the Measurement Section?
The only items that can be checked
on the measurement section is the tightness of the oscillator housing, the
RF front connector, and the RTD temperature probe connections. The
oscillator housing has brackets on both sides to prevent vibration from
loosening the RF Connector. The Measurement Section has the serial number
on the end metal plug, the oscillator has a separate and different serial
number on the end of its housing. Please use the serial number on the pipe
section end when referring to the unit in correspondence, by fax, or
service call.
The microwave electronics is a
sealed box in order to prevent moisture from the air from getting inside.
The circuitry is surface mount components because of the high frequencies
involved and therefore, is not user serviceable. The measurement section
and the oscillator are calibrated together and are considered a single
unit. A unique EPROM is created for each unit with the calibration curves
generated at the factory. If the measurement section (and electronics) has
some technical problem, the main electronics can be used with another
section if the replacement unit's EPROM is changed out also. The
measurement section has a serial number stamped on the blank end which is
also noted on the EPROM. These must match.
In addition to the electronics, a
100 ohm RTD platinum temperature probe is placed into the fluids just below
the flange. This is connected to the electronics through a terminal block.
The Swage Lock fittings are packed with sealant. The entire RTD and Swage
system is typically replaced as a unit. If the RTD is shorted a 100 ohm
resistor can be placed between P2 and P3 with jumpers between P+ and P2,
and another between P3 and P4 to obtain temperature while the assembly is
obtained and replaced. A suitable value may be used to obtain a close to
fluid temperature reading to maintain temperature compensation. Installing
a temporary resistor also verifies that the analog input board is operating
correctly.
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6. What is the calibration and
how is it done at the factory?
The electronics and the measurement
section are mated and placed on a fluid loop where the calibration curves
are generated. To obtain the oil continuous phase curves (water droplets
are surrounded by oil as the continuous phase), the loop is loaded with oil
and brought to temperature. Water is injected into the recirculating oil at
a known rate with a computer system keeping track of both water cut and the
specific frequencies generated by the oscillator. To obtain the water
curves (oil droplets surrounded by water as the continuous phase), the loop
is loaded with water of a given salinity and then oil in injected at a
known rate. The loop computer again keeps track of the water cut and the
frequency of the oscillator at each water cut. Many runs are made with
various salinities in order to fully characterize the pipe and the
electronics mated with it. This data is then curve fit into various curves
of water cut, frequency, salinity, and temperature and then placed into
EPROM as polynomial equations. This is the data set that is selected when a
salinity calibration is performed in the field. In the field, inversion
from oil continuous phase to the water continuous phase occurs between 60%
and 90% water content and depends on the salinity, temperature, and makeup
of the crude oil.
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7. What are the considerations
in mounting and use?
1) Flow Rates and Mounting of
the Measurement Section
The orientation does not matter if
the flow rate is above approximately 2-3 feet/second. This assures a
homogeneous mixture in the measurement section. If there is gas present,
the preferred orientation is with the electronics end down and the flow
from the bottom to the top. This assures that the free gas is removed from
the section instead of accumulating it at one end. If the flow rate is very
low (below 1 ft/sec), then the preferred orientation is the electronics
upward and the flow from the top to the bottom. This is so that the water
does not accumulate in the section. If the section is mounted horizontally
and the flow rate is low sand may also accumulate in the section. Flow in
the main line must be high enough to prevent separation for a low water cut
unit where a hot tap on the side of a pipeline with a pitot tube in the
upstream section is used. The measurement section should not be at the
lowest or the highest place in a piping line unless due consideration is
given to the hold up of water, oil and gas (by maintaining high flow
rates).
2) High Temperature Units
For high temperature (>100º C
fluid temperature) operation, the electronics should be mounted football
down due to the necessity of keeping the electronics portion below 120º F
ambient. Otherwise, the heat rises and prevents the aluminum explosion
proof box from radiating the heat to the ambient air. The electronics
includes a very small built in heater to keep the electronics, which are
temperature sensitive, at a temperature of 160º F. As the enclosure
temperature (around the electronics) gets above approximately 120º F, the
heater starts to regulate less. Above 140º F, the heater must almost shut
down causing greater temperature changes at the circuit and therefore,
there is a greater error in the measurement. There are several versions of
the Phase Dynamics measurement sections for various temperature ranges.
When the fully brazed system is selected for very high temperatures, the
electronics on the end of the measurement section should be air purged with
dry instrument air in order to maintain the oscillator at a reasonable
temperature. Contact the factory about the requirements for air purging.
3) Electrical Considerations
Cables and Grounding: Included with
each system are a system cable and a green grounding wire. The system cable
must be pulled from the measurement section end back to the main
electronics due to the military type connector on one end. The green ground
wire is to meet CSA requirements and also to assure a good earth ground is
obtained between the measurement section and the electronics. This prevents
ground loops from being carried in the system cable's shields. In all
cases, the AC Input board should have an earth ground wire connected. In
addition to the safety aspects, this assures a solid instrument ground.
Flow Meter Input Connections:
Pulsed flow inputs need to be at least 0.030 Volts and a maximum of 15
Volts. A magnetic pickup can be directly tied in without a preamp if the
cables are run in conduit and are not excessive in length. A solid ground
connection must be made at the flow meter and at the Phase Dynamics'
electronics in order to assure extra pulses are not obtained. Current flow
inputs can be used in conjunction with other devices in the current loop
only if the Phase Dynamics is the last instrument on the loop's path
nearest to the ground leg.
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8. What input and output
options are available?
1) Digital Communications
The RS-422 option can include a
multidrop software as a purchased option so that one RS-422 line can talk
to multiple units. This must be specified at the time of purchase since the
standard unit does not include multidrop capabilities. When the multidrop
option is implemented it becomes an RS-485. The RS-422 is implemented with
ASCII protocol which is included at the back of the user's manual.
2) 0-20 mA and 4-20 mA Outputs
Standard as an output is the 0-20
mA or the 4-20 mA output of water cut. Totalization of the net oil and
water can only be obtained from the digital communications buss.
3) Well Select OPTIONAL Input
using 0-20 mA or 4-20 mA
An option for use of the flow 4-20
mA input as a Well Select is available. This option gives well selection up
to 50 wells. This does preclude use of a current flow input for net oil.
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9. Water Cut measurement and
salinity of the water
In order to make any electrical
measurement of water containing salts, the salinity MUST be considered if
any measurement of consequence is done. Simply speaking, the
reproducibility and accuracy of the measurement is completely compromised
without salinity compensation. If the fluids are in the oil continuous
phase - oil surrounds the water droplets at water cuts typically less
than 60% - salinity does not affect the measurement. Salinity
becomes an issue in the water continuous phase (above about 60% water).
From the early 1950's, it has been
shown and known that measurements in the oil-in-water emulsion phase are
made difficult by the high conductivity of the salt water. In this phase,
the oil droplets appear as voids in the conductive water and therefore, the
nature of the oil does not affect the measurement. Any change in the
salinity makes a large difference in the conductivity. This is in parallel
with the measuring capacitance of standard capacitance and absorption
probes and prevents an effective measurement from being performed. Although
some probes claim to work on attenuation of the signal, the water is so
conductive that the measurement is very minimal at best in the water
continuous phase. A good measurement must include the effects of the salt.
The 1958 Oil and Gas Journal paper
by Mr. Robert S. Wood, Capacitance-type B.S. and W. Recorder Features
Automatic, Continuous Operation, says that when the water continuous
phase is entered, the "resistance of the emulsion has dropped so low
that the probe is practically short-circuited." He gives the
resistance (one divided by the conductivity) for various salinity waters
versus water cut. It is obvious where the water continuous phase begins.
Other literature gives the equations for the conductivity vs salt and
percent water.
Another paper from the IEEE Trans.
on Microwave Theory and Techniques, August, 1971, which gives the response
for salt in water is by A. Stogryn and is titled Equations for
Calculating the Dielectric Constant of Saline Water. Again it is
obvious that the parameters of measurement are all dependent upon both
temperature and salinity to a high degree. Salinity and temperature MUST be
considered and corrected in the measurement for accuracy and
reproducibility.
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10. Phase Dynamics salinity
calibration methods
The Phase
Dynamics Water Cut Analyzer provides two manual and one automatic means by
which to enter and determine the salinity of the produced water.
10.1. Heuristic Salinity™ Routine
which is a patented process by which the salinity is determined versus time. Typically the proper salinity is selected
rapidly when the fluids become water continuous. This routine has been proven in many
fields across the world and is based on statistics and the fact that a real
calibration curve exists for salinity and water percentages. This totally eliminates any operator
intervention.
10.2. Use of the known salinity from laboratory reports. Salinity (equivalent NaCl) given in the
water reports for wells and is typically easily available. Direct entry of the salinity will provide
a reasonable beginning salt point.
The problem with this is that different salts of various species
have different conductivity so it is a relative number.
10.3. Salinity Calibration Mode of the Phase Dynamics
The
preferred method to determine and enter salinity is through the
"Salinity Calibration" which is in the Phase Dynamics' Menu. A "Standard Salinity
Calibration" allows the calculation of the salinity by knowledge of
the water cut of the well. The refractometer measurement of salinity is
used to compare the values obtained from the "Standard" salt
calibration to check both the instrument's operation and the validity of
the sample you will pull during the calibration. Provision for the pressing
of the "Enter" push button is made while a sample is being pulled
from the stream. This push button is
again pressed when the sample is complete.
Once the sample has been analyzed for water cut through distillation
or centrifuge, the resultant water cut is entered into the Phase Dynamics
unit. This allows the Phase Dynamics
system to then calculate the salinity from the frequency and temperature
obtained during the sample pull. The water cut entered by the operator then
completes the required information to calculate the apparent salinity.
The most
certain method is to raise the test separator level to obtain a true 100%
water dump through the analyzer.
This is easily confirmed by looking at the sample. Only a 5-10 second dump is required to
select the proper time frame to determine salinity using the Standard
Salinity Calibration.
10.4. Obtaining a Valid Sample in the Water Continuous
Phase
Typically, the manner in which the sample was obtained is the source
of the greatest error at the higher water cuts.
a.)
The upstream sample port should be ahead of a mixer or have many elbows
with high flow rates to assure mixing.
b.)
A pitot or quill should be installed in the sample port to obtain a sample
out of the center of the flow.
Samples
obtained from the sides or bottom of the pipe may be biased either way
depending upon flow and oil conditions.
If the
sample port is located some distance in front or in back of the Phase
Dynamics Measurement Section a biased sample may be obtained for two
reasons: 1.) the operator cannot read the Phase Dynamics Display during the
sampling to assure a constant water cut reading 2.) the process may be dynamic and due to
flow conditions the sample time frame was too short to encompass the
changes in water cut. This would lead to another problem where the water
cut is changing rapidly and therefore an average sample would be difficult
to obtain.
The Phase Dynamics' system continually displays the instantaneous
water cut at the previous entered salinity during the sample time
frame. This should be within the
operator's line of sight while the sample is pulled to observe the change
in water vs time. The best opportunity to obtain a valid sample is to raise
the two phase separator's fluid level to get a water dump which will last
for 5-20 seconds. Then a sample can
be easily obtained and validated that it was 100% water during the
"Enter to Start" and "Enter to End" sample time frame.
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11. Capacitance measurements -
What is the problem?
The key factor missing in prior
technology was enough sensitivity to effectively make a good measurement.
Capacitance measurements are good enough for 0.5% to 15% measurements as
long as the medium under measurement does not absorb much of the low frequency
energy. Some crudes do absorb energy and therefore the measurement becomes
more difficult for standard technology since this further reduces the
sensitivity. Capacitance measurement is a single factor measurement, it
just measures the amount of energy stored between two metal plates. This
limits the capability of this type of measurement.
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12. Phase Dynamics compared to
Capacitance probes
Capacitance probes have been used
for measurements of fluids for well testing for over 40 years. These probes
are used for level detection, water content determination, and other
measurements where accuracy and reproducibility have not been an issue. The
basis of measurement is that of the ability of a material to store
electrical energy. This is typically done at low frequencies from 100 KHz
to 10 MHz. Standard off-the-shelf electronics are available to make this
measurement inexpensively. These probes are limited to approximately 30%
water cut.
Measurements requiring resolutions below
1% water in oil have been accomplished with capacitance probes but, the
lower the water content the less the reproducibility. Long term drift has
been a problem with these instruments from the start. Since it is
fundamentally an electrical measurement at low frequencies, the length of
the probe is not critical if the flow is homogeneous. Absorptive
measurements (where the radio frequency energy is absorbed by the fluids)
are extremely compromised using capacitance probes.
Phase Dynamics uses high frequencies
where the length of the measurement section approaches a portion of a
wavelength or longer. This provides a full cross sectional mesurement of
the process fluids. The higher frequencies of operation give greater
resolution and repeatability. The measurement is based upon a patented
technology called "load pull" that provides sensitivity
improvement of 100 to 1000 times over previous technology used in microwave
receivers. The basic benefits that a microwave measurement brings to the
process analyzer are; simplicity, molecular level measurement, smaller
size, no sampling or preconditioning, and a long time before recalibration
or maintenance.
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13. Phase Dynamics compared to
Coriolis technology
These two technologies differ in
method and parameter of measurement for determination of water content in a
crude oil stream. The Coriolis method has an advantage in the fact that it
can measure both water cut and mass flow rate.
The Coriolis method compares the
density of the emulsion to that of oil and water. The density of water is
0.997 grams/cc at 77 degree F while the oil’s density can
approximately range from 0.85 to 0.95 grams/cc depending on the crude type.
Through electro-mechanical means the tubes in the Coriolis meter are set
into motion and the resultant frequency of vibration is proportional to the
density of the fluids passing through the tubes. To calculate water cut the
60 degree F density of both the oil and the water of the particular well in
test must be known. The water density varies due to salinity variations,
while the crude density is related to the makeup of light and heavy ends
contained within the oil. The mass flow rate is determined by the Coriolis
effect between the two tubes in motion.
Since there is only a small
difference between the densities of the water and oil, knowledge of the
densities to the 4th decimal place for both oil and water are
required. This requires a laboratory measurement of each well’s water
and oil densities for calibration. If the oil density is low, assume 0.85
grams/cc, and the water is assumed to be 1.0 grams/cc, then the difference
of 0.15 grams/cc represents 100% change in water cut. If the wells in test
are water flood wells, salinity may change with time which affects the
density. This technology cannot determine the phase of the fluids (water or
oil continuous) since it is only a density measurement and therefore,
cannot distinguish emulsion types.
Entrained gas will dramatically affect
the density of a petroleum stream. Since most gases have a typical density
of 0.003 grams/cc and in the case stated above 1% water cut equals 0.0015
grams/cc change, gas has a large effect on the results. For a 1% gas
content the density is changed at 0.0% water cut by 1%*0.85 grams/cc or
0.0085 grams/cc, which represents a 0.0085/0.0015 = 5.6% water cut error.
At 5% gas content the error is up to 28%.
Phase Dynamics’ technology
uses the permittivity of the oil and water at 120 MHz for reproducible water
cut determination. The permittivities for water and oil are approximately
2.4 and 80, respectively. This large difference provides a very accurate
determination of the water cut. Unlike capacitance probes which cannot
measure in the water continuous phase, the Phase Dynamics patented
technology provides for operation from 0% to 100% water cut and in
addition, gives the operator of the field information as to the fluid phase
of the well under test. By providing the phase information well reservoir
management can be impacted both in the well’s production lifetime and
profits. When a breakthrough from an injection well occurs, a previously
oil continuous well will become water continuous. Thus, the operator will
know to take action before irreversible damage is done. Additional issues
in reservoir management can be addressed through knowing the phase of the
fluids.
Entrained gas affects the Phase
Dynamics meter much less than in the Coriolis technology. For a 0.0% water
cut, the error for 5% gas content is only 0.16%. This is due to the large
difference in the measured parameters (permittivity of water = 80 and oil =
2.5, gas = 1.0). For 90 — 100% water cut the same 5% gas content
gives an equivalent error in water cut since it looks like oil in the measurement
method for water continuous emulsions. Usually, the very high water cuts do
not have as much gas since these wells are typically water floods.
Coriolis ^ 0.0015 gm/cc = ^ 1% w/c
Phase
Dynamics ^ 2 MHz = ^ 1% w/c
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14. The science of Oscillator
Load Pull
The key development distinguishing
Phase Dynamics' Analyzer from prior
technology is the use of "Oscillator Load Pull". This microwave
parameter had been understood by defense engineers since the 1940s, but
never used as a measurement parameter. Phase Dynamics’ breakthrough
was precisely to use it for measurement, on-line and in real-time, of
product compositions in solutions and emulsions, specifically of water in
oil in the petroleum industry. This "Oscillator Load Pull"
technology provides for a sensitivity increase of up to 1000 times that of
other measurement methods. Phase Dynamics’ achievement was recognized
in 1992 by the R&D100 Award for Innovative Technology.
An unbuffered oscillator changes
its frequency when the complex electrical impedance at its output changes.
This is known as "Oscillator Load Pull". In Phase Dynamics’
Analyzer the complex impedance is determined by the dimension of the
measurement section and the permittivity of the material. When the permittivity
changes, the complex impedance changes and the frequency of the unbuffered
oscillator changes precisely, predictably and repeatably. Permittivity is
related to the molecular dipole moment, which represents energy storage,
and the relaxation time which represents energy loss. These are also known
as relative dielectric constant and absorption characteristics,
respectively.
Microwave permittivity depends
strongly upon the molecular structure of a material. Polar moments are
created by molecular asymmetry, ionic bond positions, or additional
intrinsic details of the molecule and its environment. This gives rise to
the microwave permittivity. Permittivity of a molecular structure will be
changed by slight position or bonding changes.
Microwave energy will rotate polar
molecules about their electrical axes within a small microwave field.
Molecules are rocked individually or as groups which have orientational or
interfacial polarizations. It is this effect that gives rise to a change in
the oscillator frequency and gives Phase Dynamics’ Analyzers their
excellent sensitivity. As molecular structures are changed or as free ions
exist during the course of reaction, the microwave oscillator exhibits a
unique response which encompasses the above phenomena.
Water is an example of a compound
with a large permittivity because of its natural polar moment. Since the
two hydrogen atoms occupy a non-symmetrical position about the oxygen, the
moment about the electrical center of the molecule is large.
This means that water is a polar
liquid. It has a relative permittivity of approximately 80.
Benzene is an example of a compound
with a very low relative permittivity because of its symmetrical structure.
Benzene is a non- polar liquid. It has a relative permittivity of approximately
2.
Measurements based on permittivity
allow a wide range of relative dielectric constants and absorption
characteristics. However measurements based on capacitance are limited to
materials with low absorption characteristics.
**Note: Although others fail
to distinguish the difference, Phase Dynamics is the sole inventor and only
user of this unique method of measurement.
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15. Testing the effects of API
gravity and of sulfur content
A study on the effect of sulfur and
a change in the API gravity on Phase Dynamic’s water cut measurement
is outlined below. Eight samples of different crude oils with varying
gravity and sulfur were supplied by ARCO Pipeline for this evaluation. The
gravity varied from API 22 to API 40, and the sulfur from 0.343 to 2.17%.
The test procedure included the
following steps:
Measure
API gravity and correct for temperature.
Load a
flow loop including a Phase Dynamics with a sample of oil and circulate.
Pull a representative
sample and determine the water cut by distillation.
Inject at
a constant rate from the initial water cut to 4% total volume of water into
the loop, while measuring the apparent water cut with the Phase Dynamics.
Figure 1 shows the calibration factor
required to zero out the water cut change vs. API gravity and a best-fit
straight line. From this data a factor of —0.174% change in water cut
offset per +1 degree API gravity can be determined. The sulfur data
deviation from a straight line vs. sulfur content appeared to be
statistically randomly scattered. It can be concluded that the sulfur
content is not a function of water cut error.
The calibration factor mentioned
above is used as an offset from a single point Karl Fischer or distillation
where the crude oil API gravity is also known. With this single point
calibration, when another crude oil with a known API is to be measured, the
correct calibration offset can be determined for accurate readings.
For example: A single point
calibration was performed and gave a 0.25% water cut and an API gravity of
30. If the water cut read on the Phase Dynamics was 0.3% when the
calibration sample was pulled, then an offset of —0.05% is placed
into the memory. If the next crude type comes on line and its API is 22,
then
(Cal API — New API) x
(-0.174% factor) + (Cal
offset) = New Offset
(30-22) x (-0.174%) + (-0.05%) =
-1.34% New Offset.
The Phase Dynamics Analyzer can be
corrected for varying API gravities once a calibration for a given crude is
performed. Other crude oils of known API gravity are easily corrected by
entering a water cut offset using the above quotation. Sulfur content has
no effect on the measurement.
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16. Effects of gas on Phase
Dynamics
Gas carryover in two-phase or
three-phase separators can occur if the flow rate is too high to give
enough residence time, or if the separator is partially filled with sand or
silt. Another case which will release gas from solution is where there is a
pressure drop large enough to cause flashing of the light ends. In these
instances, the entrained gas can be carried over into the measurement
instrumentation, and the effect of this free gas needs to be evaluated.
In the Phase Dynamics Analyzer, the
measurement is made using the microwave permittivities of the gas, the oil,
and the water, which are 1.0, 2.5, and 80 respectively. Since the gas and
oil have low permittivity, the gas looks like extra oil and therefore, the
effect at lower water cuts is to reduce the measured water content. As the
water percentage goes above 50% the gas effect is increased, since the gas
is replacing a volume in the fluid of which water is a great part. As the
fluid enters the water-continuous phase (typically between 60 and 90% water
content), the error in water cut due to the gas is approximately equal to
the volume content of gas.
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17. Emulsion phase and
electrical measurement
Literature since the early 1920s describes
the two physical phases of an oil and water emulsion. One is similar to the
turkey gravy — cornstarch helps to maintain a viscous water-in-oil
phase. If just water were added to the turkey oil, it would settle out into
the two discreet phases very easily. An oil-in-water emulsion exists and is
physically described as oil surrounded by water.
If we use a coaxial line to
establish the microwave energy waves with a bare center rod and a metal
outer pipe, this becomes the electromagnetic structure to make our water
cut measurement.
The oil-continuous emulsion phase
(water-in-oil) is non-conductive since the oil is insulating. The saltwater
bubbles are uniformly spaced throughout the oil, and the salinity does not
matter at the frequencies Phase Dynamics operates (100 to 300 MHz).
When the emulsion inverts to the
water-continuous phase (oil-in-water), then a path can be drawn from the
center conductor of the coaxial line to the outer pipe wall. Since
saltwater is conductive and the measurement is electromagnetic in nature, the conductive
liquid attenuates the electrical energy. A correction for the conductivity
must be made to know where 100% water is on the electrical curve.
Phase Dynamics uses the fact that
when the emulsion is in the water-continuous phase (where salinity does
matter), the microwave energy is absorbed to a much higher degree than in
the oil-continuous phase. This information is used to determine the phase
of the emulsion. The difference in attenuation of the signal between the
oil-continuous phase and the water-continuous phase is great enough that a
line of demarcation can be drawn between the two attenuation curves for all
salinities of the water.
Phase Dynamics senses the phase of
the emulsion at each measurement of 2 seconds in time. It will respond to
emulsion phase changes at any water cut (from approximately 30% to 100%
water cut). The calibration on a flow loop is done for both emulsion phases
across the entire salinity calibration range. This is true even though the
calibration runs go only to 50% water cut for water-continuous emulsions.
The calibration runs stop at 50% water cut only due to the stability of the
water emulsion with the oil used to make this phase’s calibration.
Phase Dynamics has proven that the correct phase will be selected from the
attenuation power curves generated during the water-continuous
calibrations, even for emulsions below 50%.
The Analyzer furnished by Phase
Dynamics makes a continuous emulsion phase determination at every
measurement point. This is unlike other vendors where the emulsion phase is
determined by a fixed value of water cut to select between the curves. Only
Phase Dynamics’s dynamic determination of the emulsion phase will
give accurate water cut measurements. The emulsion phase is a dynamic
event, which will change at each cycle of the well and with any change in
temperature, salinity, or oil properties. Instrumentation used must be
capable of instantaneous determination of the phase of the emulsion for an
accurate measurement.
Pulling samples to test
emulsion phase
The emulsion phase cannot be
determined easily by examination of the fluid. Once a sample is pulled from
the flowing stream, the emulsion phase becomes unknown due to the change in
temperature and flow rate. Temperature directly determines phase because it
is energy added or substracted; flow rate is an indirect contributor.
Flow rate contributes energy to the
emulsion depending upon the Reynolds Number of the flow region, which
contributes to the shear of the fluid. Shear adds energy as mechanical and
thermal energy.
If a good sample can be obtained,
the phase is determined by a microscopic examination of the emulsion
itself, or by viscosity comparisons to known emulsions of the same makeup.
Extreme care must be taken to assure not to disturb the emulsion type
during preparation and examination. Phase Dynamics gives an unusual insight
into the emulsion type of the fluid at a temperature and actual flowing
conditions. This information can be used to aid in the evaluation and control
of the reservoir.
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