Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- 1 207 4017
DENSITY COMPENSATED PIPELINE MONITOR
INTRODUCTION
This invention relates to a cut monitor and,
more particularly, to a cut monitor which utilises density
compensation to obtain the water content of a hydrocarbon
liquid and water mixture.
BACKGROUND OF THE INVENTION
It is necessary in many applications to obtain
an indication of the water content of a hydrocarbon liquid
and water mixture such as a petroleum and water mixture.
For example, pipeline operators will ordinarily wish to
ensure that the oil carried by such pipelines has a water
content which does not exceed a certain value. This is so
because the pipeline companies wish to ensure they are not
purchasing water and paying for oil. This is also so
because water can corrode the pipeline which can result in
premature pipeline failure. Water can also freeze in the
pipeline and block flow. This has the potential to close
the pipeline and shut in the oil being carried by the
pipeline.
The maximum amount of water generally allowed in
oil carried by a pipeline is 0.5% of the gross volume of
the oil. In the event such a percentage is exceeded, the
producer may be refused access to the pipeline or
penalised accordingly.
Similarly, oil producers have reasons for
ensuring they too have information concerning the water
content of the oil which they have produced. First, they
may wish to corroborate the water content figures obtained
by the pipeline operator. Second, they may wish to
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monitor performance of their water removal equipment and,
third, they may wish to optimize their water removal
processes and equipment so that they meet the standards
required by the pipeline operator but do not substantially
exceed them which can be more profitable for their
operations.
Several previous instruments have been used to
measure the water content of a liquid hydrocarbon and
water mixture but each suffers from various disadvantages.
Such instruments include the net oil coriolis meter type
instrument which does not have sufficient accuracy for
measuring small amounts of water because it depends on the
accuracy of mass, density and temperature measurements.
It likewise depends on the hydrocarbon and water densities
remaining constant during operation and that such
densities be established prior to the initiation of
operation of the device. The errors in the readings
obtained by net oil coriolis meter devices can be in the
order of 0.5% water, which errors are in the range or
even greater than the amount of water in the oil-water
mixture which is permitted by the pipeline operator.
Further instruments used in the measurement of
water content in an oil-water mixture include the cut
monitor or basal sediment and water ("BS & W") type
instrument which utilises capacitance to determine the
volume of water in the oil. Such instruments are far more
accurate than the coriolis type instruments for measuring
small percentages of water but they too depend on the
density of the hydrocarbon mixture and temperature
remaining constant although some such instruments do
utilise temperature compensation.
Manual sampling is also used but it suffers from
clear disadvantages, perhaps the greatest of which is that
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it is not a continuous sampling on-line technique.
Automatic sampling gives an average rather than an
instantaneous water cut. Both manual and automatic
sampling require that the sample obtained of the oil and
water mixture be a representative sample.
Instruments known as temperature compensated
basal sediment and water ("BS & W") monitors are also
utilised. A resistance temperature device ("RTD") is used
with the BS & W instrument. The RTD is inserted in and
measures the temperature of the fluid stream. It
calculates the corrected water content using a linear
relationship between the temperature and the dielectric
constant of the oil. In addition to the measurement of
the water content for a mixture of specific density, the
instrument allows four(4) different mixture densities to
be measured by utilising a linear relationship between the
capacitance and the water content. However, it also
suffers limitations in that the density of the mixture
must be known prior to the startup of the apparatus and it
is assumed that the density is constant over time which
may not be correct. The four(4) densities must be close
to each other for good accuracy and an external switch is
required to select the calibrated density closest to that
of the mixture being tested. However, errors still arise
when the density of the mixture differs from the
densities for which the apparatus is calibrated.
To improve the correlation between the
capacitance measurement of the liquid hydrocarbon and
water mixture obtained by the BS & W monitor and the
corresponding value for water content percentage of the
mixture being measured, it is noted that errors can
arise. For example, if the liquid hydrocarbon and water
mixture is inserted into a capacitance measuring device
and the capacitance measurement obtained is "Yl", the
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percentage water content of the mixture "Xl" is directlyobtained from the linear relationship shown therein.
However, if the density of a second sample of
liquid hydrocarbon changes from the density of the first
sample of liquid hydrocarbon being measured with reference
to Figure 3 as would be the case, for example, where oil
from a different field is being sampled and even though
the water content may be precisely the same in the second
sample, the capacitance can and will change to, say, "Y2".
Correlating the capacitance value of "Y2" to the water
content value will give an erroneous reading of "X2".
Techniques have been adapted in an attempt to
minimize the erroneous readings. For example, different
sets for capacitance tables can be generated depending on
the density of the liquid hydrocarbon intended to be
carried. Again with reference to Figure 3, a second
sample of known density will result in a capacitance
reading of "Y2" and in correlating this value to water
content, it will be found that the correct value of "xl"
will be obtained assuming that the two samples are as
described above; that is, that the two samples have the
same water content. This techn;que, however, is clearly
disadvantageous when the density of the liquid
hydrocarbon-water mixture changes unbeknownst to the
operators.
In capacitance type instruments such as cut
monitor instruments, three variables effect the
capacitance reading, namely the area of the plates between
which the capacitance is taken, the distance between the
plates and the dielectric constant of the material between
the plates. Since the area of the plates and the distance
between them can be held constant by the layout of the
instrumentation, the only remaining factor affecting
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_ 5 _ 2079 01 7
capacitance is the dielectric constant of the material
being measured between the plates.
The dielectric constant for petroleum changes
depending upon the density of the oil and the temperature
of the oil. As the density of the oil increases, the
dielectric constant increases and as the density
decreases, the dielectric constant decreases. Likewise,
as the temperature of the oil increases,the density
decreases and the dielectric constant therefore decreases.
Since the dielectric constant for oil is
approximately 2.0, depending upon its density and the
dielectric constant for water is approximately 80, as the
water content of the oil increases, the dielectric
constant will also increase.
However, while some previous instruments have
provided compensation for dielectric changes due to
temperature changes and while some instruments have
provided for manual calibration of the instrument
depending on the density of the oil and water mixture
carried by the pipeline, none have provided on-line
compensation for dielectric changes due to density changes
in the oil-water mixture which results by differing oil
compositions. This is clearly disadvantageous since the
product being carried by the pipeline may change
significantly over time thereby resulting in incorrect
readings for the water content of the oil.
A further problem which produced incorrect
readings for the instrument resulted from the previously
incorrect understanding that the capacitance of the oil
was directly proportional to the density. In fact, it has
been found that the relationship is non-linear with the
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result that the oil dielectric constant can be more
accurately determined than previously.
Yet a further problem is set forth herein to
assist a full understanding of the invention. This
problem relates to the dual effect of dielectric constant
changes due to changes in density and temperature. It was
previously thought that two compensations were needed to
obtain capacitance, namely that compensation due to
temperature change and that compensation due to density
change. However, it has been found that temperature
compensation need not be performed for the instrument of
the present invention. Rather, as the density changes in
the oil-water mixture and as the dielectric constant
likewise changes as a result of such density changes, it
appears that temperature compensation itself need not be
performed. It is emphasized that, at the present time,
not enough is known about this phenomena to ensure that
the statements made herein are correct without
qualification but, based upon results to date, it does
appear as if such temperature compensation need not be
made.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there
is provided apparatus for determining the water content of
a liquid hydrocarbon and water mixture comprising means to
measure the density of said liquid hydrocarbon and water
mixture, means to correlate said density measurement of
said liquid hydrocarbon and water mixture to a first
dielectric constant for said liquid hydrocarbon, means to
measure the capacitance of said liquid hydrocarbon and
water mixture and to convert said capacitance to a second
dielectric constant of said liquid hydrocarbon and water
mixture, means to obtain the difference between said first
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and second dielectric constants and means to convert said
difference between said dielectric constants to a reading
indicating the water content of said liquid hydrocarbon
and water mixture.
According to a further aspect of the invention,
there is provided a method of measuring the water content
of a liquid hydrocarbon and water mixture comprising the
steps of determining the density of said liquid
hydrocarbon and water mixture, converting said density
determination to a first dielectric constant for said
liquid hydrocarbon, determining the capacitance of said
liquid hydrocarbon and water mixture, converting said
capacitance to a second dielectric constant for said
liquid hydrocarbon and water mixture, measuring the
difference between said first and second dielectric
constants and converting said difference to a value for
the water content of said liquid hydrocarbon and water
mixture.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
A specific embodiment of the invention will now
be described, by way of example only, with the use of
drawings in which:
Figure 1 is a view of the apparatus according to
the invention in its operating relationship with the
pipeline being monitored;
Figure 2 is a diagrammatic schematic view of the
operation of the water cut monitor according to the
invention;
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Figure 3 is a diagram illustrating the
capacitance of a liquid hydrocarbon and water mixture for
a first and second sample;
Figure 4 is a non-linear curve obtained
experimentally and cross-referencing the dielectric
constant against the density of dry oil measured by the
densitometer;
Figure 5 is a cross-reference chart which
correlates the change in dielectric constant to the actual
water content of the oil; and
Figure 6 is a non-linear curve obtained
experimentally which correlates the change in dielectric
constant to the actual water content of the oil for high
percentages of water in the oil.
DESCRIPTION OF SPECIFIC EMBODIMENT
Reference is made to the drawings and, in
particular to Figure 1 which illustrates a pipeline 10
with a liquid hydrocarbon, conveniently oil, flowing
therein. A branch line 11 extends from the pipeline 10
and a densitometer 12 is connected to the branch line 11.
A pump 13 is connected downstream from the densitometer 12
and a basal sediment and water ("BS & W")instrument 14 is
mounted downstream of pump 13 and acts to measure the
capacitance of the oil and water mixture flowing
therethrough. The BS & W instrument 14 returns the oil
and water mixture to the pipeline 10 by way of return line
20. A resistance temperature device ("RTD") 21 may be
utilised to measure the temperature of the oil and water
mixture passing through the BS and W instrument 14 if
desired or necessary.
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OPERATION
In operation and with reference to Figures 1 and
2, an oil and water mixture will generally enter the
densitometer 12 from pipeline 10 through branch line 11.
The densitometer 12 will measure the density of the oil
and water mixture and this result, say d, will then be
correlated with the corresponding value of dielectric
constant, say rd, by the use of the experimentally
generated data illustrated in Figure 4. It will be
noticed that while previously, it was thought that the
relationship between the density of the mixture and the
dielectric constant was linear; that is, a straight line
as opposed to the "two-stage" linear relationship
illustrated in Figure 4, empirical data suggest this is
not the case and, indeed, that considerable error can
arise if a linear relationship is assumed. It will also
be noted that the chart of Figure 4 correlates the density
with the dielectric constant for dry oil; that is, for oil
without measurable water content. While this does
introduce a small error as will be shown in greater detail
hereafter, it has been assumed that the density does not
significantly differ with or without water in the mixture
and that, accordingly, that the value obtained for rd is
appropriate for the applications under which the
instrument is intended to be used and, in fact, this has
been shown to be correct.
The water and oil mixture from densitometer 12
is then moved downstream by the use of pump 13 and into
the BS and W instrument 14 where the capacitance of the
oil and water mixture is measured in the normal way and a
value for the dielectric, rmiX~ is obtained. The value
for rmiX has been found to be the sum of the dielectric of
the oil, rd plus an additional amount, ~ rw, due to the
contribution of the water content as expressed below:
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rmix = rd + ~ rw and, therefore,
~ rW = rmix ~ rd
Since the value for rd is known from reading the density
"d" from densitometer 12 and correlating the density
reading to a value for the dielectric constant rd by
utilising Figure 4 through linearization device 23, the
difference between the two measurements is obtained within
computing device 24 which provides a final value for
change in dielectric ~ rw which is due to the water
content of the oil and water mixture. This value is then
correlated through the use of Figure 5 utilizing
linearization device 23, to obtain the percentage water
content of the oil and water mixture by volume and may be
displayed or recorded as desired by the operator.
In the event the densitometer 12 measures the
density and temperature of the mixture passing
therethrough, and from these measurements internally
calculates a compensated density at a specified reference
temperature, the BS & W monitor 14 will also need to be
temperature compensated. For this purpose, a resistance
temperature device 21 will also be provided to take the
temperature of the fluid stream from the BS & W monitor
14. Temperature compensation utilizing the RTD 21 may be
switched on and off depending upon whether it is to be
used or not.
It will be noted that there is an error
introduced in the measuring process described by the use
of the data appearing on Figure 4 which correlates the
density reading of dry oil against its dielectric
constant. However, it has also been found that the error
at low concentrations of water is insignificant.
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For example, and with reference to Figure 2, it
will be assumed that the density of the oil in the oil
water mixture being measured by the densitometer 12 is 0.8
g/cc and that the density of the water in the oil water
mixture being measured by the densitometer 12 is 1.0 g/cc.
It is assumed that the density of the oil-water mixture
does not change significantly from the density of dry oil
and, accordingly, the densitometer should measure the
density of the oil-water mixture at approximately .8.
With reference now to Figure 4, it will be seen that a
value for the density of 0.8 on the abscissa of Figure 4
correlates to a dielectric constant, rd, of approximately
1.79. This value for rd, therefore, will be entered into
the computing device 13.
Meanwhile, the dielectric constant of the oil
water mixture will also be measured by the BS & W
instrument 14 and this measurement is assumed to be for
the purposes of this example to be 1.83. The value of
1.83 is then taken to be the value for rmiX and by the use
of the equation, ~ rw = rmiX ~ rd, the value for ~ rw is
obtained as follows:
~ rw = 1.83 - 1.79 = .04
The value of the dielectric of .04 is correlated
to water content through the use of Figure 5 thereby
obtaining a value of water content of approximately .72 %
water by volume.
It will be assumed that the density of the
mixture dmiX can be computed as follows:
dmiX = (percent oil) (oil density) +
(percent water) (water density)
= (100% - 0.72%) (0.8) + (0.72~) (l.Oo) = .8014
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Thus, there is a slight error that does arise
but the assumption that the change is small is a valid
one.
It will be further noted that there is an error
introduced in the measuring process by use of the
correlation appearing on Figure 5 which correlates
linearly the water component of the dielectric of the
mixture, ~ rw against its water content. It is known that
this relationship is non-linear and exponential in nature
as illustrated in Figure 6. However, it has been found
that the error at low concentrations of water is
insignificant.
While a specific embodiment of the invention has
been described, many modifications will readily occur to
those skilled in the art to which the invention relates
and such description should be taken as illustrative of
the invention only and not as limiting its scope as
defined in accordance with the accompanying claims.
