Abstrict A venturi-assisted flow meter arrangement is disclosed. The venturi
is positioned in a pipe or conduit containing the fluid mixture
to be measured upstream of the flow meter. The flow meter is preferably
a flow rate meter and/or a phase fraction meter. When the fluid
mixture is passed through the venturi, it is homogenized or mixed,
which can increase the accuracy of the measurements made by the
downstream flow meter. Additionally, the venturi can be used to
compute the flow momentum of the fluid mixture, which may be used
to calibrate or double check the operation of the flow meter, or
allow it to compute the phase fraction for a three phase mixture.
Claims What is claimed is:
1. An apparatus for determining at least one parameter of a fluid
mixture flowing in a conduit, comprising: a restriction at a first
axial location along the conduit for homogenizing the fluid mixture
downstream from the restriction; and at least one flow meter along
the conduit for measuring at least one parameter of the homogenized
fluid mixture, the flow meter comprising an array of pressure sensors
for measuring unsteady pressure variations within the conduit.
2. The apparatus of claim 1 wherein the at least one flow meter
is non-invasively coupled to the outside of the conduit.
3. The apparatus of claim 1 wherein the flow meter is selected
from at least one member of the group consisting of a flow rate
meter and a phase fraction meter.
4. The apparatus of claim 3 wherein the at least one parameter
measured by the flow rate meter is indicative of the flow rate of
the fluid mixture, and wherein the at least one parameter measured
by the phase fraction meter is indicative of the phase fraction
of the fluid mixture.
5. The apparatus of claim 4 further comprising a sensor along
the conduit and selected from the group consisting of a pressure
sensor and a temperature sensor.
6. The apparatus of claim 5 wherein the sensor is used to calibrate
either the flow rate meter or the phase fraction meter.
7. The apparatus of claim 1 wherein the restriction comprises
a venturi.
8. The apparatus of claim 7 further comprising a sensor assembly
proximate to the venturi that provides at least one parameter indicative
of a flow momentum of the fluid mixture.
9. The apparatus of claim 8 wherein the flow meters are comprised
of a flow rate meter for measuring at least one parameter indicative
of fluid mixture flow rate and a phase fraction meter for measuring
at least one parameter indicative of the phase fractions of the
fluid mixture.
10. The apparatus of claim 9 wherein the parameters indicative
of flow momentum and flow rate are used to compute the phase fraction
of the fluid mixture.
11. The apparatus of claim 9 wherein the at least one parameter
indicative of flow momentum is used to calibrate either the flow
rate meter or the phase fraction meter.
12. The apparatus of claim 1 wherein the pressure sensors are
selected from the group consisting of a fiber optic based sensor,
a piezoelectric sensor, a capacitive strain gauge, piezoresistive
sensor, an accelerometer, and a hydrophone.
13. The apparatus of claim 1 wherein the first axial location
is upstream of the flow meter.
14. The apparatus of claim 1 wherein the at least one parameter
is selected from at least one member of the group consisting of
a speed of sound of the fluid mixture, a velocity of the fluid mixture,
and a phase fraction of the fluid mixture.
15. An apparatus for determining at least one parameter of a fluid
mixture flowing in a conduit, comprising: a restriction at a first
axial location along the conduit; at least one sensor assembly located
at the restriction for providing at least one parameter indicative
of the flow momentum of the fluid mixture; at least one flow meter
at a second axial location along the conduit for providing at least
one parameter indicative of the fluid mixture, the flow meter comprising
an array of pressure sensors for measuring unsteady pressure variations
within the conduit; and a computer for receiving the at least one
parameter indicative of the flow momentum to calibrate the at least
one flow meter.
16. The apparatus of claim 15 wherein the restriction comprises
a venturi.
17. The apparatus of claim 15 wherein the at least one flow meter
is fiber optic based.
18. The apparatus of claim 17 wherein the at least one flow meter
is non-invasively coupled to the outside of the conduit.
19. The apparatus of claim 15 wherein the flow meter is selected
from the group consisting of a flow rate meter and a phase fraction
meter.
20. The apparatus of claim 19 wherein at least one parameter measured
by the flow rate meter is indicative of the flow rate of the fluid
mixture, and wherein the at least one parameter measured by the
phase fraction meter is indicative of the phase fraction of the
fluid mixture.
21. The apparatus of claim 20 further comprising an additional
sensor along the conduit and selected from the group consisting
of a pressure sensor and a temperature sensor.
22. The apparatus of claim 21 wherein the additional sensor is
used to further calibrate either the flow rate meter or the phase
fraction meter.
23. The apparatus of claim 15 wherein the sensor assembly comprises
pressure sensors.
24. The apparatus of claim 23 wherein the at least one flow meter
is downstream from the restriction in the flowing fluid mixture.
25. An apparatus for determining the phase fraction of components
in a fluid mixture flowing in a conduit, comprising: at least one
sensor assembly located at a restriction for providing at least
one parameter indicative of the flow momentum of the fluid mixture;
at least one flow rate meter along the conduit for providing at
least one parameter indicative of the fluid mixture flow rate, the
flow meter comprising an array of pressure sensors for measuring
unsteady pressure variations within the conduit; at least one phase
fraction meter along the conduit for providing at least one parameter
indicative of the density of the fluid mixture; and a computer for
receiving the at least one parameter indicative of the flow momentum,
the at least one parameter indicative of the fluid mixture flow
rate, and the at least one parameter indicative of the density of
the fluid mixture to compute the phase fraction of components in
the fluid mixture.
26. The apparatus of claim 25 further comprising an additional
sensor along the conduit and selected from the group consisting
of a pressure sensor and a temperature sensor.
27. The apparatus of claim 26 wherein the additional sensor is
used to calibrate either the flow rate meter, the phase fraction
meter, or the sensor assembly.
28. The apparatus of claim 25 wherein the flow rate meter, the
phase fraction meter, or the sensor assembly are fiber optic based.
29. The apparatus of claim 28 wherein the flow rate meter, the
phase fraction meter, or the sensor assembly are coupled to the
outside of the conduit.
30. The apparatus of claim 25 wherein the at least one sensor
assembly is located proximate to a restriction within the conduit.
31. The apparatus of claim 30 wherein the restriction comprises
a venturi.
32. The apparatus of claim 30 wherein either the at least one
flow rate meter or the at least one phase fraction meter is downstream
from the restriction in the flowing fluid mixture.
33. The apparatus of claim 25 wherein the fluid mixture has three
components.
34. The apparatus of claim 33 wherein one of the components is
gaseous.
35. A method for determining at least one parameter of a fluid
mixture flowing in a conduit, comprising: homogenizing the fluid
mixture by passing the mixture through a restriction in the conduit
at a first axial location along the conduit; and measuring at least
one parameter of the homogenized fluid mixture using at least one
flow meter along the conduit, the flow meter comprising an array
of pressure sensors for measuring unsteady pressure variations within
the conduit.
36. The method of claim 35 wherein the flow meter is selected
from at least one member of the group consisting of a flow rate
meter and a phase fraction meter.
37. The method of claim 36 wherein the at least one parameter
measured by the flow rate meter is indicative of the flow rate of
the fluid mixture, and wherein the at least one parameter measured
by the phase fraction meter is indicative of the phase fraction
of the fluid mixture.
38. The method of claim 35 wherein the at least one flow meter
is non-invasively coupled to the outside of the conduit.
39. The method of claim 35 further comprising measuring the pressure
or temperature of the fluid mixture.
40. The method of claim 39 further comprising calibrating either
the flow rate meter or the phase fraction meter using the measured
pressure or temperature.
41. The method of claim 35 wherein the restriction comprises a
venturi.
42. The method of claim 35 further comprising measuring at least
one parameter indicative of the flow momentum of the fluid mixture
at the first axial location.
43. The method of claim 42 wherein the method contains two flow
meters.
44. The method of claim 43 wherein the at least one parameters
measured by the flow meters are indicative of the fluid mixture
flow rate and a phase fraction.
45. The method of claim 43 wherein the at least one parameters
indicative of the fluid mixture flow rate and the phase fraction
are used to compute the phase fraction.
46. The method of claim 42 wherein the parameter indicative of
the flow momentum is used to calibrate either the flow rate meter
or the phase fraction meter.
47. The method of claim 35 wherein the pressure sensors are selected
from the group consisting of a fiber optic based sensor, a piezoelectric
sensor, a capacitive stain gauge, piezoresistive sensor, an accelerometer,
and a hydrophone.
48. The method of claim 35 wherein the first axial location is
upstream of the flow meter.
49. The method of claim 35 wherein the at least one parameter
is selected from at least one member of the group consisting of
a speed of sound of the fluid mixture, a velocity of the fluid mixture,
and a phase fraction of the fluid mixture.
50. A method for determining at least one parameter of a fluid
mixture flowing in a conduit, comprising: measuring at least one
parameter indicative of the flow momentum of the fluid mixture using
a sensor assembly along the conduit; measuring at least one parameter
of the fluid mixture using at least one flow meter along the conduit,
the flow meter comprising an array of pressure sensors for measuring
unsteady pressure variations within the conduit; calibrating the
at least one flow meter using the at least one parameter indicative
of the flow momentum.
51. The method of claim 50 wherein the parameter indicative of
the flow momentum is measured at a location having a restriction
in the conduit.
52. The method of claim 51 wherein the restriction comprises a
venturi.
53. The method of claim 50 wherein the restriction homogenizes
the fluid mixture before the at least one parameter of the fluid
mixture is measured.
54. The method of claim 50 wherein the flow meter is selected
from at least one member of the group consisting of a flow rate
meter and a phase fraction meter.
55. The method of claim 54 further comprising measuring the pressure
or temperature of the fluid mixture.
56. The method of claim 55 further comprising calibrating either
the flow rate meter or the phase fraction meter using the measured
pressure or temperature.
57. The method of claim 55 wherein at least one parameter measured
by the flow rate meter is indicative of the flow rate of the fluid
mixture, and wherein the at least one parameter measured by the
phase fraction meter is indicative of the phase fraction of the
fluid mixture.
58. The method of claim 50 wherein the at least one flow meter
is coupled to the outside of the conduit.
59. The method of claim 50 wherein the method contains two flow
meters.
60. The method of claim 59 wherein the at least one parameters
measured by the flow meters are indicative of the fluid mixture
flow rate and a phase fraction.
61. The method of claim 60 wherein the parameters indicative of
the fluid mixture flow rate and the phase fraction are used to compute
the phase fraction.
62. A method for determining the phase fraction of components in
a fluid mixture flowing in a conduit, comprising: measuring at least
one parameter indicative of the flow momentum of the fluid mixture
using a sensor assembly along the conduit; measuring at least one
parameter indicative of the fluid mixture flow rate using at least
one flow rate meter along the conduit, the flow meter comprising
an array of pressure sensors for measuring unsteady pressure variations
within the conduit; measuring at least one parameter indicative
of the density of the fluid mixture using at least one phase fraction
meter along the conduit; and computing the phase fraction of components
in the fluid mixture using the at least one parameter indicative
of the flow momentum, the at least one parameter indicative of the
flow rate, and the at least one parameter indicative of the density.
63. The method of claim 62 wherein the parameter indicative of
the flow momentum is measured at a location having a restriction
in the conduit.
64. The method of claim 63 wherein the restriction comprises a
venturi.
65. The method of claim 62 wherein the restriction homogenizes
the fluid mixture before either the at least one parameter indicative
of the flow rate or the at least one parameter indicative of the
density are measured.
66. The method of claim 62 further comprising measuring the pressure
or temperature of the fluid mixture.
67. The method of claim 66 further comprising calibrating either
the flow rate meter or the phase fraction meter using the measured
pressure or temperature.
68. The method of claim 62 wherein the flow rate meter, the phase
fraction meter, or the sensor assembly are fiber optic based.
69. The method of claim 62 wherein the flow rate meter, the phase
fraction meter, or the sensor assembly are coupled to the outside
of the conduit.
70. The method of claim 62 wherein the fluid mixture has three
components.
71. The method of claim 70 wherein one of the components is gaseous.
Description TECHNICAL FIELD
This invention relates to measuring fluid parameters in pipes and
more particularly to measuring fluid composition, volumetric flow,
or other fluid parameters using a flow meter or meters assisted
by a venturi.
BACKGROUND OF THE INVENTION
In many industries it is desirable to measure various parameters
of fluids or fluid mixtures in pipes, including the temperature,
pressure, composition (i.e., phase fraction, e.g., 10% water, 90%
oil), flow rate, and the speed of sound (SOS) in the fluid or mixture.
(As used herein, "fluid" may refer to a liquid or gas,
and a "fluid mixture" may be mixtures of liquids or gases).
Different sensor arrangements, referred to generically as "flow
meters," can be used to measure these parameters, such as those
that are disclosed in the following U.S. patent applications, which
are incorporated herein by reference in their entireties, and which
may have issued as U.S. patent Ser. No. 09/740760 filed Nov. 29
2000; Ser. No. 09/344070 filed Jun. 25 1999; Ser. No. 09/346607
filed Jul. 2 1999; Ser. No. 09/344093 filed Jun. 25 1999; Ser.
No. 09/345827 filed Jul. 2 1999; Ser. No. 09/519785 filed Mar.
7 2000; Ser. No. 09/346606 filed Jul. 2 1999; Ser. No. 09/346604
filed Jul. 2 1999; Ser. No. 09/346605 filed Jul. 2 1999; Ser.
No. 09/344094 filed Jun. 25 1999; Ser. No. 10/010183 filed
Nov. 7 2001; and Ser. No. 09/344069 filed Jun. 25 1999.
A flow meter typically comprises a sensor, a sensor array, or multiple
sensor arrays. In many of these flow meters, the sensors may comprise
fiber optic sensors, possibly incorporating fiber Bragg gratings
(FBGs), which can be mounted or coiled around the pipe containing
the fluid to be measured. Other flow meters allow optical devices
or other sensing devices to be ported or placed within the pipe
to make the required measurements. When one uses a fiber optic based
flow meter, the fluid or mixture parameters may be measured without
the need to "tap in" to the pipe, as many of these parameters
may be sensed externally to the pipe though the means disclosed
in these incorporated references. Often, these externally mounted
sensors are "passive" sensors in the sense that they do
not require stimulating the fluid or mixture of interest by external
means, but instead make the required measurements simply by sensing
various naturally occurring fluid effects.
In the oil and gas industry, or comparable industries, it is desirable
to measure, in situ, the flow produced from an oil well. Typically
the produced fluid may be comprised of three components or phases,
such as oil, water, and gas, which may additionally contain other
components, such as solids (e.g., rocks or sand) or other liquid
phases. In a production environment, it is often useful to determine
the phase fraction, or composition, of the fluid being measured,
as well as the speed of the flowing mixture.
Techniques for measuring fluid or mixture flow rate exist in the
prior art. For example, in patent application Ser. No. 09/346607
entitled "Flow Rate Measurement Using Unsteady Pressures,"
filed Jul. 2 1999 incorporated herein by reference in its entirety,
there is disclosed a flow rate meter which preferably uses fiber
optic sensors. These fiber optic sensors are disposed at two different
axial locations along the pipe containing the fluid to be measured.
Naturally occurring pressure disturbances in the fluid perturb the
first sensor through the wall of the pipe, creating a time-varying
pressure signal. When the pressure disturbances, or pressure field,
moves from the first sensor to the second sensor, a similar pressure
signal is measured. The two signals from the pressure sensor can
then be cross-correlated using well-known techniques to determine
the time shift in the pressure signals. This time delay, when divided
by the axial distance of the sensor, can be used to determine flow
rate. Optionally, the sensors may comprise filters capable of filtering
out certain undesirable wavelengths, which may constitute a single
sensor or multiple sensors.
Other flow rate techniques using venturis are also known in the
art. For example, U.S. Pat. No. 5591922 entitled "Method
and Apparatus for Measuring Multiphase Flow," issued Jan. 7
1997 and which is incorporated by reference herein in its entirety.
In the '922 patent, a pair of venturis within a pipe are spaced
from one another at an axial distance. As is well known, the venturi
causes a pressure difference (.DELTA.P) at each venturi, which are
measured. These differential pressure signals are cross-correlated
to determine a time delay, which can then be divided by the axial
distance to determine a flow velocity.
Flow meters for determining phase fraction ("phase fraction
meter") in a fluid mixture are also known in the art. For example,
U.S. Pat. No. 6354147 entitled "Fluid Parameter Measurement
in Pipes Using Acoustic Pressures," issued Mar. 12 2002 which
is incorporated by reference herein in its entirety, a spatial array
of pressure sensors, preferably fiber optic sensors, are coupled
to the outside of the pipe. These sensor produce pressure signals,
which are used to determine the speed of sound of the mixture. Because
the speed of sound of a given mixture can be calculated from the
speed of sound of the components in the mixture, the measured speed
of sound can be used to determine the phase fraction of the mixture.
Often these various types of flow meters will be used in conjunction
with each other to measure various fluid parameters of the device.
For example, a flow rate meter may be used on one section of the
pipe, followed downstream by a phase fraction meter, or vice versa.
Or, these flow meters may be combined into an integrated flow meter
apparatus.
The accuracy of these and other prior art flow meters are generally
enhanced when the fluid mixture being measured is relatively well
mixed or "homogenous." Relatively inhomogeneous mixtures,
having larger unmixed portions of the components that constitute
the mixture, may not provide suitable pressure disturbances (i.e.,
acoustic differences) that can be easily resolved by the pressure
sensors that typical constitute a traditional flow meters. Additionally,
prior art flow meters may have difficulties in measuring the parameters
of fluid mixtures having more than two phases. The art would therefore
benefit from ways to improve the performance of these and other
traditional flow meter techniques.
SUMMARY OF THE INVENTION
A venturi-assisted flow meter arrangement is disclosed. The venturi
is positioned in the pipe or conduit containing the fluid mixture
to be measured upstream of the flow meter. The flow meter is preferably
a flow rate meter and/or a phase fraction meter. When the fluid
mixture is passed through the venturi, it is homogenized or mixed,
which can increase the accuracy of the measurements made by the
downstream flow meter. Additionally, the venturi can be used to
compute the flow momentum of the fluid mixture, which may be used
to calibrate or double check the operation of the flow meter, or
allow it to compute the phase fraction for a three phase mixture.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of a pipe having a venturi placed upstream
of traditional flow meter(s) to improve their performance.
FIG. 2 is a side view of a pipe with a venturi, illustrating homogenization
of fluid flow.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
In the disclosure that follows, in the interest of clarity, not
all features of actual commercial implementations of a venturi augmented
flow meter and related techniques are described. It will of course
be appreciated that in the development of any such actual implementation,
as in any such project, numerous engineering and design decisions
must be made to achieve the developers' specific goals, e.g., compliance
with mechanical and business related constraints, which will vary
from one implementation to another. While attention must necessarily
be paid to proper engineering and design practices for the environment
in question, it should be appreciated that development of a venturi
augmented flow meter and related techniques would nevertheless be
a routine undertaking for those of skill in the art given the details
provided by this disclosure, even if such development efforts are
complex and time-consuming.
FIG. 1 shows a cross section of a pipe 1 having a venturi 5 placed
within. The fluid mixture within the pipe flows from left to right
as indicated by the arrow. Downstream from the venturi, in a preferred
embodiment, are a phase fraction meter 7 and a flow rate meter 8.
Any suitable phase fraction meter 7 and/or flow rate meter 8 can
be used in, and would be benefited by, the disclosed configuration.
For example, the phase fraction meter 7 could constitute the meter
disclosed in U.S. Pat. No. 6354147 entitled "Fluid Parameter
Measurement in Pipes Using Acoustic Pressures," issued Mar.
12 2002 which is incorporated by reference herein in its entirety.
The flow meter could constitute the meter disclosed in U.S. patent
application Ser. No. 09/346607 entitled "Flow Rate Measurement
Using Unsteady Pressures," filed Jul. 2 1999 which is incorporated
herein by reference in its entirety. The details of these flow meters
are not disclosed herein, but preferably include passive fiber optic
based sensors employing, or working in conjunction with, fiber Bragg
grating (FBGs) in, for example, a scanning Fabry-Perot, acousto-optic
tuned filter (AOTF), optical filter, or time-of-flight arrangement
having sufficient sensitivity for the particular conditions and
measurements to be made. Such sensors are capable of measuring one
or more of the following parameters: pressure, temperature, speed
of sound, axial momentum, and volumetric flow. The sensors may operate
actively or passively.
While shown at the same location on the pipe, it should be understood
that the meters 7 and 8 could be displaced axially along the pipe,
i.e., in sequence. Moreover, the venturi 5 can be used in conjunction
with the phase fraction meter 7 or flow rate meter 8 individually,
although in a preferred embodiment it is preferred that both meters
be present because in a commercial embodiment it is usually beneficial
to know as much about the flowing fluid mixture as possible. Additionally,
other types of flow meters may be added, and may be multiplexed
together, which is facilitated when optical technologies are used.
Each flow meter 7 and/or 8 can be incorporated into an existing
section of pipe 1 or can be incorporated into a pipe section that
is inserted in line into a production pipe or well casing.
Multiple phase fraction meters 7 and/or flow rate meters 8 could
be employed, with such redundancy providing back up in case of meter
failure or providing additional measurements to aid in measurement
accuracy. The meters 7 and/or 8 may be placed in a housing within
the production pipe or well casing. The housing may comprise a vessel,
such as a pressure vessel, that serves to protect the sensing arrays
and to physically and acoustically isolate them from the outside
environment.
Also disclosed in FIG. 1 are pressure sensors 11 and 12 and a
temperature sensor 6 for respectively measuring the pressure and
temperature of the fluid mixture in the pipe. The use of such additional
sensors may be useful in their own right to determine further parameters
about the fluid, or to calibrate the system. For example, the density
(and hence speed of sound) of a fluid component or mixture, measured
by the phase fraction meter 7 varies as a function of temperature
and pressure, and so the temperature and pressure measured by the
sensors 11 and 6 are useful inputs into the phase fraction meter.
It may also be useful to know the temperature and/or pressure to
determine if materials within the mixture will volatize into a gaseous
state, which again would change the speed of sound and/or affect
the speed of sound/phase fraction calculation made by the phase
fraction meter 7. The temperature and pressure sensors 6 11 and
12 may be incorporated within the phase fraction meter 7 and/or
the flow rate meter 8 or may occur at different locations along
the pipe than those shown in FIG. 1. Additionally, the pressure
sensors 11 and 12 may also be useful in computing the momentum
of the fluid, as will be explained in further detail below.
The pressure sensors 11 and 12 and the temperature sensor 6 may
constitute any several well known sensors, including fiber optic
sensors, electrical sensors, piezoelectric sensors, and the like,
such as those that are disclosed in the incorporated references.
The sensors, depending on their construction, may be placeable on,
around, or within the pipe, and at varying locations along its length.
Moreover, these measurements, and particularly the pressure measurements
by sensors 11 and 12 at the venturi, may be used to further improve
the capabilities of the phase fraction meter 7 and/or the flow rate
meter 8 as will be explained further below. Measuring the temperature
and pressure via sensors 6 11 and 12 may also be useful to calibrate
the actual sensor elements themselves. For example, one skilled
in the art will appreciate that the FBGs used in a preferred system
have optical characteristics that will vary with temperature, and
the temperature measurement may therefore be used to calibrate those
sensor elements.
Venturi 5 could constitute any known material such as aluminum,
iron, stainless steel, plastic, or similar materials, and could
be affixed to the inside of the pipe 1 in any number of ways, including
by brazing, soldering, bolting, glueing, or by a pressure fit. Alternatively,
the venturi need not be a unitary piece but instead could be formed
by deforming the pipe's surface, or could be separately formed and
welded to respective ends of the pipe 1. Additionally, the venturi
can be manufactured using a single solid cylinder, and machining
the cylinder to form the venturi and related contiguous pipe structures.
As one skilled in the art will realize, the composition of the venturi
5 and the means for affixing the venturi to the pipe 1 will necessarily
depend on the composition (and possibly thickness) of the pipe and/or
the environment in question. For example, where the venturi is to
be employed in an oil/gas production pipe, and therefore potentially
subject to corrosive chemicals, high temperatures, and mechanical
disturbances (such as from sand or rock floating in the pipe), it
may be beneficial for the venturi to comprise stainless steel welded
to the inner diameter of the pipe.
The venturi is comprised of three sections: a converging inlet
or approach section 3 a throat section 4 and an outlet or diffuser
section 10. The inlet 3 may be shaped as appropriate, for instance
as a circle, ellipse, or portion thereof. In a preferred embodiment,
it is shaped as a quarter ellipse. The exit section may diverge
at an angle of about 5 to 10 degrees relative to the axis of the
pipe 1 and preferably at an angle of about 6 to 7 degrees. As one
skilled in the art will recognize, the venturi 5 could be designed
in any number of ways and consistently with well-known venturi design
constraints.
In one aspect of the disclosed invention, the venturi 5 assists
the phase fraction meter 7 and/or the flow rate meter 8 by causing
the fluid mixture within the pipe to become more uniform or homogenized.
Referring to FIG. 2 it should be appreciated that the fluid mixture
may not be well mixed (2) prior to its entry into the venturi 5.
For example, large bubbles, or large unmixed sections of oil in
water, may have formed in the mixture as it progresses along the
pipe, perhaps aided by gravity or the natural tendency of such components
to separate. However, once the fluid mixture enters the venturi
5 the fluid is subject to increased velocity in the throat 4 of
the venturi. This tends to mix, or cavitate, the mixed fluid, particularly
after it enters the outlet 10 resulting in a better homogenized
version of the mixed fluid (9). This improved homogenization will
provide a more constant input to both the phase fraction meter 7
and/or the flow rate meter 8 which should improve the accuracy
of those meters. Moreover, the cavitation of the fluid mixture by
the venturi 5 adds additional acoustic energy to the fluid mixture,
making it easier for the meters 7 and/or 8 to passively detect the
dynamics in the mixture. One skilled in the art will appreciate
that because of the large acoustic impedance contrast in gas/liquid
mixtures, fluid mixture mixing in such mixtures is more important
than for liquid/liquid phase fraction determinations, such as for
oil and water.
To realize the full benefit of the mixing that is provided by the
venturi, the phase fraction meter 7 and/or the flow rate meter 8
should be placed within the wake created by the mixture. In other
words, the meters 7 and/or 8 should not be placed so far from the
venturi along the axis of the pipe that the mixture will have the
opportunity to re-settle or re-segregate. On the other hand, the
meters 7 and/or 8 may need to be suitably spaced from the venturi
so that the turbulent effect of the venturi does not so directly
affect the measurements being made or add "noise" to the
measurement. A suitable spacing for the meters 7 and/or 8 will depend
on several factors, such as the expected flow rate of the mixture,
the speed at which the components in the mixture will settle, etc.,
and experimentation may be required in a given commercial setting
to determine the optimal spacing. Depending on the application,
it may not be necessary to place both the phase fraction meter 7
and the flow rate meter downstream from the venturi. For example,
if suitably sensitive, the flow rate meter might be placed upstream
of the venturi.
It is preferred to use a venturi 5 to provide mixing of the fluid
mixture, especially when one considers the related benefits of the
use of the venturi to measure flow momentum, as discussed below.
However, should those additional benefits not be necessary in a
given application, any other well-known means could be used in place
of venturi 5 to mix the mixture. For example, a rotating paddle
wheel, a screen containing holes, or other restriction or means
for mechanically disrupting the mixture could be used. Likewise,
other venturi-like structures could be made which would intrude
into the inner diameter of the pipe to create sidewall discontinuities
or undulations, which could have the same effect on cavitating the
fluid mixture within the pipe.
The venturi is useful in aspects other than in just homogenizing
the mixture. Specifically, and in accordance with another aspect
of the invention, the pressure sensors 11 and 12 can be used to
measure the pressure differential across the venturi 5. (Although
drawn on the outside of the pipe 1 the pressure sensor 12 could
also, and may preferably be, placed in conjunction with the wall
of the venturi 5 at the throat 4 which might improve its dynamics).
This pressure differential measurement allow different, or extra,
information to be determined about the mixture, which can improve
the accuracy of the phase fraction meter 8 and/or the flow rate
meter 7 and which can be particularly useful in determining the
phase fraction of a three-phase fluid mixture (e.g., oil/gas/water).
An example of how the addition of a venturi 5 can assist in determining
the phase fractions in a three phase flow is first discussed. As
is well known to those skilled in the art of fluid dynamics, the
pressure differential (.DELTA.P) measured across the venturi by
sensors 11 and 12 is proportional to the flow momentum of the fluid,
i.e., .DELTA.P=.alpha..rho..sub.mix u.sub.mix.sup.2 where .rho..sub.mix
is the density of the mixture, u.sub.mix is the velocity of the
mixture, and .alpha. is a constant or fitting parameter. Because
the flow rate meter 8 measures the speed of the mixture, i.e., u.sub.mix,
(see U.S. patent application Ser. No. 09/346607 for further details),
the density of the mixture, .rho..sub.mix, can be calculated. Moreover,
the speed of sound In the mixture, a.sub.mix, is measured by the
phase fraction meter 7 as previously explained (see U.S. Pat. No.
6354147 for further details). The densities of the three components,
e.g., oil, gas, and water, (.rho..sub.1 .rho..sub.2 and .rho..sub.3)
are also known, as are the speed of sound in these components (a.sub.1
a.sub.2 and a.sub.3). From this, the phase fraction of the oil,
gas, and water (.phi..sub.1 .phi..sub.2 .phi..sub.3) can be calculated
by the solving for these three variables in the following known
fluid dynamics equations:
and
(The latter of these equations is known in the art as the "Wood
Equation."). Accordingly, while an existing phase fraction
meter has some capability to handle or infer the phase fraction
for more than two-phase fluid mixtures, the accuracy of the phase
fraction determination can be facilitated, and made more accurate,
by the addition of the measurement provided by the venturi 5. Moreover,
the disclosed technique is useful to measure phase fraction of three-liquid-phase
mixtures, or those mixtures incorporating a gaseous phase.
The additional measurement provided by the venturi can also assist
in improving, or calibrating existing phase fraction meters 7 and/or
flow rate meters 8 when dealing with the simpler problem of two-phase
fluid mixtures. Consider for example an oil/water mixture. Fundamental
to the operation of known phase fraction meters such as those disclosed
herein by reference is that the densities of the oil and water be
known within some degree of accuracy. However, suppose the density
of water cannot be accurately known. A practical example of this
would occur if water produced in conjunction with oil from an oil/gas
well inadvertently mixes with water used to pressurize the well.
The mixed waters may have different densities, for example, because
they may differ in salinity content. Therefore, if the produced
water and the pressurizing water become mixed, for example, because
of a breach in the production system, the density of the water may
shift away from its assumed value. By using the venturi measurement,
the overall density of the mixture can be calculated, which measurement
can then be used to adjust or calculate the supposedly known density
of the water in the fluid mixture by manipulating the above equations
and/or other equations as one skilled in the art will appreciate.
In other words, the venturi 5 can be said to "calibrate"
the phase fraction meter 7 by providing additional measurements
useful with respect to the phase fraction meter. In a sense, the
venturi "over constrains" the system by providing additional
measurements that are not necessarily needed by the phase fraction
meter 7 but which can improve its accuracy, or double check the
integrity of the fluidic parameters used by that system. The flow
rate meter 8 can be similarly calibrated (e.g., double-checked or
verified or adjusted), as one skilled in the art will appreciate
by reviewing the incorporated references.
Although it has been disclosed that mixing or "homogenization"
of the fluid mixture is believed advantageous to supply a homogenized
fluid mixture to the phase fraction meter and/or flow rate meter
7 and hence preferable to place these meters downstream from the
venturi 5 this is not necessary in all applications. If the meters
can operate sufficiently in a given application without homogenizing
the fluid mixture, then the venturi, and its accompanying flow momentum
measurement, can be made downstream of the meters 7 and/or 8 by
applying the same mathematical analysis described above. Indeed,
this may be a desirable configuration in some applications if it
is desired to calibrate the meters 7 and/or 8 or if it is desirable
to use the flow momentum measurement to assist in determining the
phase fraction of a three-phase flow, but it is not desirable to
alter the native state of the fluid mixture in the pipe when making
the flow rate and phase fraction measurements.
As one skilled in the art will appreciate, especially in light
of the incorporated references, the various signals from the sensors
and flow meters disclosed herein are preferably sent to computer
to process and evaluate the received data, and to make the necessary
mathematic calculations. If the disclosed sensors and/or flow meters
are fiber optic based, the signals will first be sent to an optoelectric
detector(s) to transform the optical signals into electrical signals
useable by a standard computer, as is well known. Moreover, the
optical devices may be multiplexed together, e.g., by wavelength-division
multiplexing or time-division multiplexing, which would allow a
single fiber optic fiber to carry the signals from the pipe to the
necessary electronics, as is well known. In an oil/gas application,
the pipe will preferably be deployed down the oil well and connected
by a fiber optic cable(s) to the detection electronics and computer(s)
residing on the earth's surface and accessible by an operator.
In summary, the measurement of flow momentum by the venturi 5 improves
the phase fraction and/or flow rate measurement than when the phase
fraction meter 7 and/or the flow rate meter 8 operated independently,
and adds additional performance to the system.
The benefits disclosed herein can be realized independently of
the orientation of the pipe, be it horizontal, vertical, or otherwise. |