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 fiber optic based
flow meter along the conduit for measuring at least one parameter
of the homogenized fluid mixture.
2. The apparatus of claim 1 wherein the at least one fiber optic
based flow meter is coupled to the outside of the conduit.
3. The apparatus of claim 1 wherein the flow meter is selected
from 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 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. 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; and a computer for
receiving the at least one parameter indicative of the flow momentum
to calibrate the at least one flow meter.
13. The apparatus of claim 12 wherein the restriction comprises
a venturi.
14. The apparatus of claim 12 wherein the at least one flow meter
is fiber optic based.
15. The apparatus of claim 14 wherein the at least one fiber optic
based flow meter is coupled to the outside of the conduit.
16. The apparatus of claim 12 wherein the flow meter is selected
from the group consisting of a flow rate meter and a phase fraction
meter.
17. The apparatus of claim 16 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.
18. The apparatus of claim 17 further comprising an additional
sensor along the conduit and selected from the group consisting
of a pressure sensor and a temperature sensor.
19. The apparatus of claim 18 wherein the additional sensor is
used to further calibrate either the flow rate meter or the phase
fraction meter.
20. The apparatus of claim 12 wherein the sensor assembly comprises
pressure sensors.
21. The apparatus of claim 20 wherein the at least one flow meter
is downstream from the restriction in the flowing fluid mixture.
22. 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 the 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; 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.
23. The apparatus of claim 22 further comprising an additional
sensor along the conduit and selected from the group consisting
of a pressure sensor and a temperature sensor.
24. The apparatus of claim 23 wherein the additional sensor is
used to calibrate either the flow rate meter, the phase fraction
meter, or the sensor assembly.
25. The apparatus of claim 22 wherein the flow rate meter, the
phase fraction meter, or the sensor assembly are fiber optic based.
26. The apparatus of claim 25 wherein the flow rate meter, the
phase fraction meter, or the sensor assembly are coupled to the
outside of the conduit.
27. The apparatus of claim 22 wherein the at least one sensor
assembly is located proximate to a restriction within the conduit.
28. The apparatus of claim 27 wherein the restriction comprises
a venturi.
29. The apparatus of claim 27 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.
30. The apparatus of claim 22 wherein the fluid mixture has three
components.
31. The apparatus of claim 30 wherein one of the components is
gaseous.
32. 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
fiber optic based flow meter along the conduit.
33. The method of claim 32 wherein the flow meter is selected
from the group consisting of a flow rate meter and a phase fraction
meter.
34. The method of claim 33 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.
35. The method of claim 32 wherein the at least one fiber optic
based flow meter is coupled to the outside of the conduit.
36. The method of claim 32 further comprising measuring the pressure
or temperature of the fluid mixture.
37. The method of claim 36 further comprising calibrating either
the flow rate meter or the phase fraction meter using the measured
pressure or temperature.
38. The method of claim 32 wherein the restriction comprises a
venturi.
39. The method of claim 32 further comprising measuring at least
one parameter indicative of the flow momentum of the fluid mixture
at the first axial location.
40. The method of claim 39 wherein the method contains two flow
meters.
41. The method of claim 40 wherein the at least one parameters
measured by the flow meters are indicative of the fluid mixture
flow rate and a phase fraction.
42. The method of claim 41 wherein the at least one parameters
indicative of the fluid mixture flow rate and the phase fraction
are used to compute the phase fraction.
43. The method of claim 39 wherein the parameter indicative of
the flow momentum is used to calibrate either the flow rate meter
or the phase fraction meter.
44. 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;
calibrating the at least one flow meter using the at least one parameter
indicative of the flow momentum.
45. The method of claim 44 wherein the parameter indicative of
the flow momentum is measured at a location having a restriction
in the conduit.
46. The method of claim 45 wherein the restriction comprises a
venturi.
47. The method of claim 44 wherein the restriction homogenizes
the fluid mixture before the at least one parameter of the fluid
mixture is measured.
48. The method of claim 44 wherein the flow meter is selected
from the group consisting of a flow rate meter and a phase fraction
meter.
49. The method of claim 48 further comprising measuring the pressure
or temperature of the fluid mixture.
50. The method of claim 49 further comprising calibrating either
the flow rate meter or the phase fraction meter using the measured
pressure or temperature.
51. The method of claim 49 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.
52. The method of claim 44 wherein the at least one flow meter
is coupled to the outside of the conduit.
53. The method of claim 44 wherein the method contains two flow
meters.
54. The method of claim 53 wherein the at least one parameters
measured by the flow meters are indicative of the fluid mixture
flow rate and a phase fraction.
55. The method of claim 54 wherein the parameters indicative of
the fluid mixture flow rate and the phase fraction are used to compute
the phase fraction.
56. 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; 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.
57. The method of claim 56 wherein the parameter indicative of
the flow momentum is measured at a location having a restriction
in the conduit.
58. The method of claim 57 wherein the restriction comprises a
venturi.
59. The method of claim 56 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.
60. The method of claim 56 further comprising measuring the pressure
or temperature of the fluid mixture.
61. The method of claim 60 further comprising calibrating either
the flow rate meter or the phase fraction meter using the measured
pressure or temperature.
62. The method of claim 56 wherein the flow rate meter, the phase
fraction meter, or the sensor assembly are fiber optic based.
63. The method of claim 56 wherein the flow rate meter, the phase
fraction meter, or the sensor assembly are coupled to the outside
of the conduit.
64. The method of claim 56 wherein the fluid mixture has three
components.
65. The method of claim 64 wherein one of the components is gaseous.
Description TECHNICAL FIELD
[0001] 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
[0002] 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. patents: Ser. Nos. 09/740760 filed Nov.
29 2000; 09/344070 filed Jun. 25 1999; 09/346607 filed Jul.
2 1999; 09/344093 filed Jun. 25 1999; 09/345827 filed Jul.
2 1999; 09/519785 filed Mar. 7 2000; 09/346606 filed Jul. 2
1999; 09/346604 filed Jul. 2 1999; 09/346605 filed Jul. 2
1999; 09/344094 filed Jun. 25 1999; 10/010183 filed Nov. 7
2001; and 09/344069 filed Jun. 25 1999.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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
[0010] 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
[0011] FIG. 1 is a side view of a pipe having a venturi placed
upstream of traditional flow meter(s) to improve their performance.
[0012] FIG. 2 is a side view of a pipe with a venturi, illustrating
homogenization of fluid flow.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0013] 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.
[0014] FIGS. 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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, glucing, 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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).
[0025] 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.mu..sub.mix.sup.2
where .rho..sub.mix is the density of the mixture, .mu..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., .mu..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 8 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:
.rho..sub.mix=.phi..sub.1.rho..sub.1+.phi..sub.2.rho..sub.2+.phi..sub.3.rh-
o..sub.3;
1=.phi..sub.1+.phi..sub.2+.phi..sub.3; and
1/(.rho..sub.mixa.sub.mix.sup.2)=.phi..sub.1/(.rho..sub.1a.sub.1.sup.2)+.p-
hi..sub.2/(.rho..sub.2a.sub.2.sup.2)+.phi..sub.3/(.rho..sub.3a.sub.3.sup.2-
).
[0026] (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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] The benefits disclosed herein can be realized independently
of the orientation of the pipe, be it horizontal, vertical, or otherwise.
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