Abstrict A flow meter for use in an oil well has a central bore region in
which an annular tube is fixed co-axially with the central bore
region by a ring, which also seals the gap between the inner bore
of the central bore region and the tube at its downstream end, thus
forming a pocket open to the direction of fluid flow through the
central bore region. The pocket brings the fluid to rest, thereby
converting all of the dynamic pressure of the fluid into static
pressure. This has the effect of maximizing the pressure differential
obtainable between the central bore region and a position upstream
of the tube, which allows an electronic calculation to be made to
provide a more accurate flow rate measurement than would have been
possible without the pocket.
Claims I claim:
1. A flow meter comprising a duct having an internal perimeter
and means for indicating fluid flow in the duct, said indicating
means comprising: a pocket open at its upstream end in the duct;
and detection means for detecting a pressure differential between
a first position inside the pocket, and a second position outside
the pocket and deriving from the pressure differential an indication
of the fluid flow within the duct, wherein the pocket extends around
only a segment of said internal perimeter of the duct.
2. The flow meter in accordance with claim 1 in which the pocket
is closed at its downstream end.
3. The flow meter in accordance with claim 1 in which the indication
of the fluid flow is a fluid flow rate measurement.
4. The flow meter in accordance with claim 1 in which said duct
is provided by a central bore region of the flow meter having a
non-circular cross-section.
5. The flow meter in accordance with claim 4 in which the central
bore region has an oval cross-section.
6. The flow meter in accordance with claim 5 in which the segment
corresponds to an apex of-the oval cross-section.
7. The flow meter in accordance with claim 4 in which an effective
diameter of the bore region is no less than an effective diameter
of the bore of each an inlet and an outlet of the flow meter.
8. The flow meter in accordance with claim 4 in which the flow
meter has first and second flow paths allowing communication of
pressure information from respective ones of first and second positions
at its inner surface to a measuring position at its outer surface.
9. The flow meter in accordance with claim 8 in which the first
flow path extends first substantially radially and then substantially
longitudinally.
10. An insert, comprising: a flow meter comprising a duct having
an internal perimeter and means for indicating fluid flow in the
duct, said indicating means comprising: a pocket open at its upstream
end in the duct; and detection means for detecting a pressure differential
between a first position inside the pocket, and a second position
outside the pocket and deriving from the pressure differential an
indication of the fluid flow within the duct, wherein the pocket
extends around only a segment of said internal perimeter of the
duct.
11. The insert according to claim 10 comprising a section of production
tubing.
Description BACKGROUND OF THE INVENTION
The present invention relates to a flow meter for indicating fluid
flow. It is particularly, but not exclusively, related to indicating
or measuring the flow of oil and water mixtures in production tubing
of a well.
A well used to extract hydrocarbons from an underground reservoir
typically comprises a borehole drilled down to the underground reservoir
and lined with a steel casing. The casing is inserted into the borehole
and cemented into place. Production tubing, which is used to convey
hydrocarbons from the reservoir to an upper region, or mouth, of
the borehole, extends from the mouth of the borehole to the reservoir.
The reservoir is typically located near to the bottom of the borehole.
An elongate annular space thus defined between the casing and the
tubing, referred to as the annulus, is usually sealed at points
along its length by devices known as packers.
In operation of the well, hydrocarbons enter the production tubing
either through its open lower end or through a choke device located
at a position along its length between two packers. It has been
proposed to make measurements of hydrocarbons' flow rate, temperature
and pressure in the borehole and use the measurements to control
the choke devices.
Since hydrocarbons may enter the production tubing at different
locations the need arises to measure hydrocarbons' flow rate with
a flow meter at different locations within the production tubing.
A problem found with certain conventional flow meters is that they
reduce the effective diameter of the bore of the production tubing
and must be removed to allow the passage of tools to upstream tubing.
The term effective diameter here means the smallest unobstructed
diameter and can be hypothetically equated to the largest diameter
circular tube which could be passed lengthwise through the insert
I without obstruction. Removal of such flow meters is costly, time
consuming and risky. Another problem is that at low flow rates,
certain conventional flow meters can give less accurate or lower
resolution measurements than is desired.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a flow
meter for indicating fluid flow in a duct, comprising a pocket open
at its upstream end in the duct, detection means for detecting a
pressure differential between a first position inside the pocket
and a second position outside the pocket and means for deriving
from the pressure differential an indication of the fluid flow within
the duct.
At low flow rates, a flow meter according to the invention can
provide more accurate or higher resolution flow indication or measurement
using the same or even inferior accuracy pressure transducers than
has been possible hitherto. An advantage of the invention is that
it need not reduce the effective diameter of a duct in which it
is installed.
The duct may be a conduit, a pipe or some other fluid path.
The pocket may be closed at its downstream end. Alternatively it
may be open at its downstream end and be provided by at least one
hole through which fluid is able to flow.
The pocket may extend continuously around the internal perimeter
of the flow meter. Advantageously, this provides a relatively simple
configuration. Alternatively the pocket extends around a segment
of the internal perimeter of the flow meter. Such a configuration
may be of particular use if the dimensions of the insert are limited
by an external factor.
The flow meter comprises a central bore region. This may have a
bore having a non-circular cross-section. Preferably it is oval.
Use of such a cross-section enables the cross-sectional area of
the bore of the central bore region to be increased relative to
that available if a bore having a circular cross-section is used.
In this event the effective diameter may be limited by features
on the outside surface of the insert. A non-circular bore may be
of particular advantage in boreholes where the outer dimensions
of the flow meter are limited by the inner dimensions of, for example,
a borehole casing.
Preferably, the segment corresponds to an apex of an oval bore.
Here, the effective diameter of the bore of the flow meter may be
greater than or equal to the bore of an inlet or an outlet of the
flow meter for the whole of its length. This may be advantageous
in providing a flow meter which permits the passage of tools and
instrumentation through production tubing in which it is installed.
Preferably the flow meter has a first conduit allowing communication
of pressure information from a first position in its bore to a measuring
position at its outer surface.
It may also have a second conduit allowing communication of pressure
information from a second position in its bore to a measuring position
at its outer surface. This allows pressure measurement transducers
and any associated electronics to be located outside the duct, away
from the fluid flow, to increase its lifetime, reliability, or both.
This is advantageous if the fluid flow provides a harsh environment.
Preferably, the first conduit extends first substantially radially
and then substantially longitudinally. This feature may allow the
flow meter to occupy minimal space radially from the central axis
of the duct. This is of particular significance where the insert
is to be used within a hydrocarbon production borehole. It is also
of significance in a number of narrow bore holes for other purposes.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the present invention will now be described by
way of example only, with reference to the accompanying drawings,
in which:
FIG. 1 shows a longitudinal cross-section of a flow meter in accordance
with the present invention;
FIG. 2 shows cross-section A-A of the FIG. 1 flow meter;
FIG. 3 shows a cross-section of a second embodiment of a flow meter
in accordance with the present invention; and
FIG. 4 shows a cross-section of a third embodiment of a flow meter
in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 an insert I comprises the flow meter having an inlet
1 a tapering bore region 2 a central bore region 3 a tapering
bore region 4 and an outlet 5. In use, fluid flows through the insert
I from right to left in the Figure, the insert thus forming a duct.
The insert I is formed by welding together stainless steel sections
which have previously been machined. The inlet 1 includes a threaded
portion 6 which is threaded into oppositely-threaded upstream production
tubing (not shown). The outlet 5 similarly includes a threaded portion
7 which is threaded into oppositely-threaded downstream production
tubing (not shown). The connections between the inlet 1 and the
outlet 5 and the production tubing provide little or no resistance
to fluid flow through the insert I and to the passage of tools,
instrumentation or both through the insert I.
The inlet 1 and outlet 5 each have bores having a uniform circular
cross-section. The central bore region 3 has a bore having a uniform
circular cross-section which is larger than that of the inlet 1
and outlet 5. In the direction of fluid flow, the tapering bore
region 2 has a bore which tapers outwards uniformly from the cross-section
of the bore of the inlet 1 to the larger circular cross-section
of the bore of the central bore region 3. The tapering region 4
has a bore which tapers inwards uniformly from the bore of the cross-section
of the central bore region 3 to the bore of the circular cross-section
of the outlet 5. Thus, the bore of the insert I does not have a
cross-sectional area at any point along its length which is smaller
than the cross-sectional areas of the inlet 1 and the outlet 5.
A tube section 8 of uniform circular cross-section is coaxially
located in the central bore region 3. An annular space is provided
between the central bore region 3 and the tube section 8. At its
downstream end the tube section 8 has an outwardly extending flange
or ring 9. The ring 9 is welded to the inside surface of the central
bore region 3 and so closes off the annular space at its downstream
end. The tube section 8 the inner surface of the central bore region
3 and the ring 9 thus form a pocket 10 which is open at its upstream
end, that is, the pocket 10 faces upstream. The bore of the tube
section 8 has a cross-sectional area which is equal to or slightly
greater than the cross-sectional areas of those of the inlet 1 and
the outlet 5. Desirably, the effective diameter of the bore of the
insert I is not less than the diameter of the circular cross-section
of the bore of the inlet 1 and the outlet 5.
A first conduit 11 extends radially from the pocket 10 to the outer
surface of the central bore region 3. The first conduit 11 is connected
at its middle to a first end of a second conduit 12. The second
conduit 12 is located in the wall of the insert I and extends parallel
to its longitudinal axis. A first port 13a of a differential pressure
measurement transducer 13 is located at a second end of the second
conduit 12. An end of the first conduit 11 located at the outer
surface of the central bore region 3 is closed off with a bung 14
so as to provide a first fluid contact path 11 12 between the pocket
10 and the transducer port 13a. A second fluid contact path 16 is
provided between the bore of the inlet 1 and a second port 13b of
the transducer 13. The transducer 13 is a high accuracy quartz sensor.
Alternatively it may be a strain gauge measurement transducer. The
transducer 13 is connected to an electronic calculating means 19
by a twisted pair 17. The calculating means 19 is attached to the
outside of the inlet 1. The transducer 13 and the calculating means
19 are protected by a protective cover 20. The arrangement of features
11 to 20 described above is referred to as the first flow measuring
arrangement.
There is a second, similar, flow measuring arrangement provided
on the opposite side of the inlet 1. It has a differential pressure
measurement transducer configured to measure pressure inside the
pocket 10 and in the bore of the inlet 1 and an electronic calculating
means. It is protected by a protective cover 21. This second flow
measuring arrangement is redundant and is to be used in the event
of failure of the first flow measuring arrangement, Such redundancy
is useful in borehole applications, because replacement of a flow
meter may require production tubing to be removed from a well. This
is a costly and time consuming procedure.
The open end or mouth of the pocket 10 is located a distance from
the tapering bore region 2 at which flow is expected to be fully
developed under all expected flow conditions. This occurs when the
velocity profile of the fluid does not change along the direction
of flow. Fully developed flow does not require an absence of turbulence.
The distance provided between the open end of the pocket 10 and
the junction between the central bore region 3 and the tapering
bore region 2 depends on the nature of the fluid, the maximum expected
fluid flow rate, the cross-sectional area of the bore of the inlet
1 the cross-sectional area of the bore of the central bore region
3 the length of the tapering bore region 2 and the accuracy of
flow measurement required.
The open end of the pocket 10 should also be a sufficient distance
away from the junction of the central bore region 3 and the tapering
bore region 2 so that changes in the flow caused by the tube section
8 and the pocket 10 do not significantly change the static pressure
extant in the inlet 1 particularly adjacent the second fluid path
16.
By locating the open end of pocket 10 where fully developed flow
occurs under typical flow conditions, the transducer 13 measures
a substantially constant differential pressure for a constant flow
rate.
Fluid which enters the pocket 10 cannot pass through it and so
is brought to rest. This converts the dynamic pressure of such trapped
fluid into static pressure within the pocket 10. This maximises
the pressure differential measured by the transducer 13 for given
inner dimensions of the insert 1. This enables the calculating means
19 to provide a more accurate flow rate measurement than is possible
in the absence of the pocket 10. This is especially important at
low flow rates and at high common mode pressures where low pressure
differentials have previously given unsatisfactorily inaccurate
or low resolution flow rate measurements, even when high accuracy
pressure transducers are used.
The calculating means 19 comprises detection means to detect the
pressure differential and means to derive from it an indication
of the flow rate of fluid through the insert I. This is achieved
by applying Bernoulli's equation to the detected pressure differential,
the density of the fluid and the known internal dimensions of the
bore the insert. From this indication of flow rate, a measurement
of fluid flow in the duct is obtained. This measurement may be in
the form of flow mass, volume or distance, and may or may not be
related to time. The fluid flow measurement may for example be total
mass of fluid which has flowed past the flow meter since it started
to take measurements.
Referring now to FIG. 2 first and second channels 23 and 24 are
present in the wall of the insert I. The channels 23 and 24 extend
along the extent of the insert I comprising the tapering bore region
2 the central bore region 3 and the tapering bore region 4. The
channels 23 and 24 allow service lines, such as hydraulic power
lines, electrical power conductors or electrical communication conductors
and the like, to by-pass the insert I when it is installed in a
well. In a typical well, the inner diameter of the casing is slightly
larger than the outer diameter 25 of the insert I. The presence
of the channels 23 24 limits the diameter of the bore of the central
bore region 3. The thickness of the walls of the insert I between
the floor of the channels 23 24 and the bore of the central bore
region 3 must be sufficiently large so as not to compromise the
strength of the insert I. Since the diameter of the bore of the
tube section 8 is not less than the diameter of the bore of the
inlet 1 and the outlet 5 the maximum distance between the tube
section 8 and the inner surface of the central bore region 3 is
more or less predetermined by the sizes of the casing and production
tubing which are used.
As the fluid flow rate decreases across the cross-section of a
regular duct from a maximum at its centre to zero at its walls,
the pocket 10 will trap fluids travelling at speeds significantly
slower than the fluids flowing along the central axis of the central
bore region 3. The pocket 10 is unable to trap faster flowing fluids
except by protruding further into the central bore region 3 which
is undesirable as it would reduce the effective diameter, and thus
is likely to hinder the movement of tools through, the insert I.
FIG. 3 shows an alternative embodiment of a flow meter in accordance
with the present invention including an insert I having a bore in
its central region 3 which is oval in cross-section. Both conduits
11 are shown. They are in fluid communication with the oval bore
at its apexes.
First and second pockets 30 and 31 are formed by respective tube
portions 32 and 33 and closures 34 and 35. As can be seen from the
view in FIG. 3 looking downstream along the insert I, the cross-sections
of the tube portions 32 and 33 each form an arc of a circle and
are located so as to be co-axial with the insert I. The arcs of
the tube portions 32 and 33 correspond to a hypothetical circle
having a diameter equal to or slightly greater than that of the
inlet 1 and the outlet 5 the bore of which is shown at 36 in FIG.
3. Thus, although the width of the bore of the central bore region
3 between the channels 23 and 24 has not been increased, the width
of the bore of the central region has been increased between the
conduits 11 that is the bore width is increased in a direction
which is not limited by the presence of the channels 23 and 24 in
the insert I.
In this way, the pockets 30 and 31 are wider in the sense that
the distance between arcs of the tube portions 32 and 33 and opposing
regions of the inner surface of the central bore region 3 is increased
in comparison to the FIG. 2 embodiment. The pockets 30 and 31 thus
formed extend over more of the fluid path in the central bore region
3 and can thus be described as wider than the pocket 10 of the FIG.
2 embodiment. The pockets 30 and 31 thus capture fluids which would
be flowing at speeds, closer to those found along the central axis
of the central bore region 3. As the wider pockets 30 and 31 provide
a larger static pressure than would the pocket 10 for a given flow
rate this second embodiment allows a more accurate fluid flow measurement
to be made by the calculating means 19.
Also, as the pockets 30 and 31 extend only around the two separate
outer regions of the bore (they occupy 100.degree. segments of the
perimeter of the oval cross-section of the central bore region 3),
two other regions of the bore are not occupied by pockets (2.times.80.degree.
segments of its perimeter). The two other regions are at the smallest
radius portions of the cross-section adjacent the channels 23 24.
Therefore, this embodiment provides a greater cross-sectional area
through which fluid can flow compared to the FIG. 1 embodiment.
Since the segments correspond to the apexes of the oval cross-section
of the bore in the central bore section 3 the effective diameter
of the bore of the insert is not reduced to be less than the diameters
of the inlet 1 and the outlet 5.
FIG. 4 shows a third embodiment of a flow meter in accordance with
the present invention including an insert I having pockets 40 and
41 which each comprise a 60.degree. segment of the oval cross-section
or the central bore region 3. The pockets 40 and 41 are formed by
tube portions 42 and 43 and closures 44 and 45.
The pockets 40 and 41 although smaller than the FIG. 3 pockets
30 31 have the same maximum width between the arcs of the tube
segments 42 and 43 and opposing regions of the inner surface of
the central bore region 3.
As more of the cross-sectional area of the central bore region
3 is available for fluid to flow through, the flow is improved in
comparison to the FIG. 3 flow meter and thus provides a lower pressure
drop across its inlet 1 and outlet 5 than the FIG. 3 flow meter.
In a modification to any of the embodiments described above, holes
are provided in the bottoms of, i.e. at the downstream end of, the
pockets. This increases the maximum flow rate which is measurable
by the pressure sensors which necessarily have upper measurement
limits. Since some fluid flows through the holes, not all of the
dynamic pressure is converted into static pressure. In an oil production
well, sand and other abrasive materials in the fluid could enlarge
the holes over a period of time. This can be compensated for by
making arrangements for the calculating means 19 to include a time
variable factor based on prediction of hole enlargement. Alternatively,
the calculating means 19 could be programmable from a surface located
controller to alter or vary its calculations by an amount dependent
on the composition and the amounts and the flow rates of the fluids
which have actually passed through the insert I since its installation.
In another embodiment, the calculating means 19 is remote from
the insert I and located near the mouth of the borehole or on a
platform such as a ship or an oil rig. Here, signals from the transducer
13 travel up the borehole to the calculating means 19. Locating
electronics associated with the calculating means 19 outside of
the environment of the borehole may provide improved reliability
of the flow meter.
Furthermore, instead of detecting a pressure differential between
the first and second positions with a differential pressure measurement
transducer, separate transducers can be used which measure separate
pressures at respective ones of the first and second positions.
A subtraction operation on the pressures provides a pressure differential.
Although in the foregoing the pockets have been described as being
of tubular or arc configurations which are aligned to co-axially
with the longitudinal axis of the insert, the present invention
is not so limited. For example, a pocket which tapers from a large
cross-sectional area at its open, upstream, end to a smaller cross-sectional
area at its opposite, downstream, end may be provided. Such a pocket
need not be closed off at its downstream end thereby forming what
can be described as a narrow necked funnel. Alternatively, a pocket
may be formed by a tube or the like which has one end sealed against
the inner surface of the insert and extends upstream, either axially
or otherwise, and having a free open end such that its mouth is
not in contact at all with the inner surface of the insert. The
main requirements of the pocket are that it captures fluid flowing
through the insert I and that it provides a region of fluid in the
pocket from which worthwhile pressure measurements can be made.
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