Abstrict An insert device for narrowing a throat of a Venturi based multiphase
flow meter is shaped to be inserted in the throat. The insert comprises
an entrance window and an exit window for a radiation beam generated
at a periphery of the throat. The insert allows pressure takeoff
to be made in a conventional way. Several examples of insert device
are described including a rod, a tubular and a tuning fork insert.
Claims 15. An insert device for narrowing a throat of a Venturi based
multiphase flow meter, the insert device being shaped to be inserted
in the throat and the insert comprising an entrance window means
and an exit window means for a radiation beam generated at a periphery
of the throat.
16. The insert device of claim 15 rod shaped and dimensioned to
be introduced in the throat in order to obtain a narrowing for a
flow passing through the throat and along the insert device.
17. The insert device of claim 16 hollow and further comprising
windows transparent to the radiation beam.
18. The insert device of claim 16 hollow and further comprising
an annular section transparent to the radiation beam.
19. The insert device of claim 15 wherein an extremity of the
insert device to be introduced in the throat has a rounded cone
shape to improve the flow passing through the throat.
20. The insert device of claim 15 tube-shaped with an outside
diameter substantially the same as a diameter of the throat such
that the insert device may be positioned in the throat by sliding,
and further comprising at an extremity to be introduced in the throat
an opening through which the flow may enter in a cavity of the tube
shaped insert device.
21. The insert device of claim 20 wherein the entrance and exit
windows are hollow.
22. The insert device of claim 20 wherein the entrance and exit
windows are made out of material transparent to the radiation beam.
23. The insert device of claim 15 having a tuning fork shaped
structure comprising a U-shaped extremity and a holder, the U-shaped
extremity comprising two lateral walls to be positioned in intimate
contact with the throat of the Venturi, the lateral walls defining
an opening for a flow passing through the Venturi.
24. The insert device of claim 23 wherein the entrance and exit
windows are defined as lateral sides of the opening between the
lateral walls oriented towards the periphery of the throat.
25. The insert device of claim 23 wherein each lateral wall has
an extremity at a flow inlet with a shape adapted to optimise the
flow entering the insert device.
26. The insert device of claim 23 wherein the holder comprises
a cavity to receive a flow passing between the lateral walls.
27. The insert device of claim 26 wherein the holder comprises
at least one lateral aperture to evacuate the flow out of the cavity.
28. A Venturi based multiphase flow meter comprising an insert
device for narrowing a throat of the Venturi based multiphase flow
meter, the insert device being shaped to be inserted in the throat
and the insert comprising an entrance window means and an exit window
means for a radiation beam generated at a periphery of the throat.
29. The Venturi multiphase flow meter of claim 28 further comprising
at one of its extremities a connection to a pipe forming an assembly,
the connection-pipe assembly comprising an opening through which
the insert device may be introduced and positioned in the throat
of the flow meter.
30. A method of measuring multiphase flows comprising passing a
flow through a Venturi based multiphase flow meter and inserting
an insert device into a throat of the Venturi for narrowing the
throat and increasing a pressure drop between an inlet of the Venturi
and the throat, said insert device being shaped to be inserted in
the throat and the insert comprising an entrance window means and
an exit window means for a radiation beam generated at a periphery
of the throat.
Description BACKGROUND OF THE INVENTION
[0001] The invention relates in general to measurements intended
to determine at least one characteristic of oil well effluents made
up of multiphase fluids, typically comprising three phases: two
liquid phases--crude oil and water--and one gas phase. In particular
the invention relates to such measurements performed using a composition
meter associated or not with a Venturi based flow meter.
[0002] The ability of the oil industry to optimise production of
a reservoir relies on the possibility of evaluating the well effluent
at regular intervals, in terms of quantity (flow rate) and of composition
(the proportions of the various phases). This makes it possible
to determine what corrective action may need to be taken. However,
measuring the flow rate of oil well effluent is a problem that is
complex because of the way effluents are usually made up of three
phases, and because of the changes in flow conditions to which they
are subject (flow rates, fluid fractions, pressure, upstream pipe
geometry). These factors give rise to a wide variety of flow regimes
being observed, including some regimes of highly non-uniform and
unstable character, with the proportions of the phases in the fluid
mixture being capable of varying very considerably both in the flow
direction (i.e. over time) and across the flow direction, in the
form of phase stratification across the flow section.
[0003] Numerous proposals based on a Venturi type flow meter have
been made to evaluate the well effluent. Amongst those proposals,
the international patent application WO99/10712 provides for a Venturi
and a gamma ray density meter placed at the throat of the Venturi.
The effluent is passed through the Venturi in which it is subjected
to a pressure drop. A mean value of the pressure drop is determined
over a period of time using pressure sensors and a mean value is
determined for the density of the fluid mixture at the throat of
the Venturi using the gamma ray density meter. The mean values are
used to deduce a total mass flow rate value. Finally further measurements
and calculations allow to obtain oil, water and gas flow rates.
[0004] While the proposed Venturi flow meter offers a reliable
performance in most encountered environments, it becomes difficult
to obtain good results at relatively low flow rates. Indeed at low
flow rates, the pressure drop measured between the inlet and the
throat of the Venturi is becoming too small to provide the flow
rate expected accuracy.
[0005] Low flow rates may occur when a well produces fewer amounts
than expected. This may occur right at the start of measurements
or in the course of life of the well.
[0006] One solution to the problem of a decreasing pressure drop
would be to replace the Venturi flow meter with another Venturi
flow meter having a smaller throat diameter. Hence the pressure
drop would increase and measurements become more accurate. However
the replacement of the Venturi requires to disassemble parts of
the Venturi including the gamma ray source, the photomultiplier
used to measure the gamma rays and the pressure lines at the inlet
and outlet of the Venturi. This makes the replacement a hazardous,
expensive and time consuming operation.
SUMMARY OF THE INVENTION
[0007] In a first aspect the invention provides an insert device
for narrowing a throat of a Venturi based multiphase flow meter.
The insert device is shaped to be inserted in the throat. The insert
comprises an entrance window and an exit window means for a radiation
beam generated at a periphery of the throat.
[0008] Appropriately the insert device may be rod shaped and dimensioned
to be introduced in the throat in order to obtain a narrowing for
a flow-passing through the throat and along the insert device.
[0009] Appropriately, the insert device may be tube shaped with
an outside diameter substantially the same as a diameter of the
throat such that the insert device may be positioned in the throat
by sliding. The insert device comprises at an extremity to be introduced
in the throat an opening through which the flow may enter in a cavity
of the tube shaped insert device.
[0010] Appropriately the entrance and exit windows are either hollow
or out of material transparent to the radiation beam.
[0011] Appropriately the insert device has a tuning fork shaped
structure comprising a U shaped extremity and a holder. The U shaped
extremity comprises two lateral walls to be positioned in intimate
contact with the throat of the Venturi. The lateral walls define
an opening for a flow passing through the Venturi.
[0012] Appropriately the entrance and exit windows are defined
as lateral sides of the opening between the lateral walls oriented
towards the periphery of the throat.
[0013] Appropriately the holder comprises a cavity to receive a
flow passing between the lateral walls.
[0014] In a further aspect the invention provides a Venturi based
multiphase flow meter which comprises an insert device.
[0015] In yet a further aspect the invention provides a Venturi
based multiphase flow meter for use with an insert device, comprising
at one of its extremities a connection to a pipe forming an assembly.
The assembly comprises an opening through which an insert device
may be introduced and positioned in the throat of the flow meter.
[0016] In a further aspect the invention provides a method for
measuring multiphase flows, comprising passing a flow through a
Venturi base multiphase flow meter, and inserting an insert device
into a throat of the Venturi for narrowing the throat and increasing
a pressure drop between an inlet of the Venturi and the throat.
BRIEF DESCRIPTION OF THE FIGURES
[0017] The invention will now be described in greater detail with
reference to the accompanying drawings, in which:
[0018] FIG. 1 is a diagram of a flow measuring apparatus known
from prior art;
[0019] FIG. 2 is a diagram of an insert according to the invention;
[0020] FIG. 3 is a diagram of an insert according to the invention;
[0021] FIG. 4 is a diagram of an insert according to the invention;
[0022] FIG. 5 is a diagram of a 3-dimensional insert view according
to the invention.
EVALUATION OF THE WELL EFFLUENT IN THE VENTURI FLOW METER
[0023] Oil effluents are usually made up of a multiphase mixture
of liquid oil, of gas (hydrocarbons), and of water. Below we use
the following notations: the symbols Q and q designate mass flow
rates and volume flow rates respectively; the symbol .rho. designates
density; the symbols .alpha. and .gamma. designate the static and
dynamic proportions of the various phases ; and the indices o, w,
g and l refer respectively to the oil, water, gas and liquid phases
(where the liquid phase is the oil and the water taken together),
while the index m designates the fluid mixture.
[0024] Referring now to FIG. 1 in which a Venturi based flow meter
as known from prior art is represented, the device comprises a pipe
section 10 comprising a convergent Venturi 110 whose narrowest portion
120 is referred to as the throat. In the shown example, the section
of the pipe 100 is disposed vertically and the effluent flows upwards,
as symbolized by arrow F.
[0025] The constriction of the flow section in the Venturi induces
a pressure drop .DELTA.p between level 130 situated upstream from
the Venturi at the inlet to the measurement section, and the throat
120. This pressure drop is associated with the total mass flow rate
Q and with the density .rho..sub.m by the following equation: 1
p = K Q 2 m + m g h V ( 1 )
[0026] where g is the acceleration due to gravity, h.sub.v is the
distance between the upstream level 130 and the throat 120 and
K is a constant associated essentially with the geometry of the
Venturi, and which is given by: 2 K = 1 - 4 2 C 2 A 2
[0027] where .beta. is the constriction ration of the Venturi,
i.e. the ration between the diameter of the throat and the upstream
diameter of the Venturi, C is the discharge coefficient, and A is
the section of the throat. The term .rho..sub.m .multidot.g.multidot.h.sub.v
is generally small or negligible. By writing .DELTA.p*=.DELTA.p-.rho..sub.m.multidot.g-
.multidot.h.sub.v, equation (1) becomes:
Q=k(.DELTA.p*.multidot..rho..sub.m).sup.1/2 (2)
[0028] where k=K.sup.-1/2.
[0029] In a preferred embodiment, the ratio .beta. is 0.5. With
a pipe having a diameter of 10 cm, the diameter of the throat is
5 cm. The discharge coefficient C is about 1. This coefficient depends
to a small extend and in predictable manner on the properties of
the fluid. Traditionally, this corrective effect is taken into account
by the Reynolds number.
[0030] The pressure drop .DELTA.p is measured by means of a differential
pressure sensor 150 connected to two pressure takeoffs 160 and 170
opening out into the measurement section respectively at the upstream
level 130 and in the throat 120 of the Venturi. In a variant, the
measurement may also be performed by means of two absolute pressure
sensors connected to the pressure takeoffs 160 and 170 respectively.
[0031] The density .rho..sub.m of the fluid mixture is determined
by means of a sensor which measures the attenuation of gamma rays,
by using a source 100 and a detector 101 placed on opposite sides
of the Venturi throat 120. The throat is provided with "windows"
of a material that shows low absorption of photons at the energy
levels, referred to below as the "high energy" level and
the "low energy" level. The detector 101 which comprises
in conventional manner a scintillator crystal such as NaI and a
photomultiplier produces two series of signals W.sub.hi and W.sub.lo
referred to as count rates, representative of the numbers of photons
detected per sampling period in the energy ranges bracketing the
above-mentioned levels respectively.
[0032] These energy levels are such that the high energy count
rate W.sub.hi is essentially sensitive to the density .rho..sub.m
of the fluid mixture, while the low energy count rate W.sub.lo is
also sensitive to the composition thereof, thus making it possible
to determine the water content of the liquid phase.
[0033] FIG. 1 also shows a pressure sensor 102 connected to a pressure
takeoff 103 opening out into the throat 120 of the Venturi, which
sensor produces signals representative of the pressure p.sub.v in
the throat of the Venturi, and a temperature sensor 104 producing
signals T representative of the temperature of the fluid mixture.
The data p.sub.v and T is used in particular for determining gas
density .rho..sub.g under the flow rate conditions and gas flow
rate q.sub.g under normal conditions of pressure and temperature
on the basis of the value for the flow rate under flow rate conditions,
determined in a manner described below. In this respect, it is preferable
for the pressure to be measured at the throat of the Venturi. In
contrast, it does not matter too much where temperature is measured.
[0034] The information coming from the above-mentioned sensors
is applied to a data processing unit 109 constituted by a computer
running a program for delivering the looked-for results by performing
various treatments.
[0035] The principle underlying the treatments performed by data
processing unit 109 are explained in detail in document WO99/10712
and will not be described here.
[0036] Simply, it appears from equation (1) which allows to calculate
the differential pressure .DELTA.p, that when the mass flow rate
Q is divided by a factor of 10 the differential pressure is divided
by one hundred. Hence measuring the differential pressure becomes
subject to a relatively low accuracy, and as a consequence e.g.
the mass flow rate may not anymore be determined with sufficient
accuracy when using equation (2).
[0037] The accuracy may not be improved by simply rescaling the
differential pressure sensor 150.
[0038] Venturi Insert Devices
[0039] Several examples of insert devices according to the invention
will be described in the following. These include a rod, a tubular
and a tuning fork insert.
[0040] For the purpose of comparison between the different options
presented here, the original throat diameter is assumed to be 52
mm, and an equivalent diameter with any insert is 30 mm. The flow
is assumed to be vertical upwards as shown in each Figure by an
arrow F.
[0041] The Rod Insert
[0042] Making reference to FIG. 2 a convergent Venturi 210 is
represented whose narrowest portion 220 is referred to as the throat.
Two pressure takeoffs 260 and 270 open up into the measurement section
and allow to gather pressure data to determine the pressure drop
.DELTA.p between the inlet of the Venturi and the throat 220. A
gamma ray source 200 emits photon in the throat 220 which may be
measured by a detector 201 placed on the opposite side of the throat
220 facing the source 200.
[0043] A rod shaped insert device 250 is inserted in the throat
220. A magnified portion 251 of the Venturi shows the inserted rod
250 centrally positioned in the throat 220. As mentioned above,
the diameter of the throat was chosen to be 52 mm as an example
and for reasons of comparison between all devices presented here.
In order to have a flow section equivalent to a Venturi with a 30
mm diameter throat, the outside diameter of the rod shaped insert
device 250 has to be 42.48 mm as indicated in FIG. 2.
[0044] In a preferred embodiment the rod shaped insert 250 is hollow.
An entrance window 252 and an exit window 253 allow the photons
emitted by the source 200 to travel across a diameter of the insert
in order to be measured by the detector 201. The windows 252 and
253 may be realised out of material which possesses acceptable transparency
for the photons emitted by the source 200. The windows may be delimited
portions of the rod insert's wall positioned to correspond to the
source 200 and the detector 201 or alternatively have the shape
of an annular section of the rod insert.
[0045] The rod shaped insert is terminated in a rounded cone shape
254. This contributes to improve the geometry of the convergent
section in the Venturi, hence optimising the flow passing through
the throat.
[0046] It is apparent from FIG. 2 that the pressure takeoffs 260
and 270 may continue to be used when the rod shaped insert is present.
[0047] The Tubular Insert
[0048] FIG. 3 represents a convergent Venturi 210 equivalent to
the one shown in FIG. 2. Same reference numbers will be used throughout
the description for same elements showed in different Figures.
[0049] A tubular insert device 350 is inserted into the throat
220. The tubular insert device 350 has substantially the shape of
a tube with an outside diameter of 52 mm, i.e. a diameter equal
to the original throat diameter. This way the tubular insert 350
may be positioned in the throat by sliding. The flow F enters in
a cavity 352 of the tubular insert 350 through an opening 351 at
an extremity of the tubular insert 350. The flow finally exits the
cavity 352 through lateral apertures 353 operated in walls of the
tubular insert and continues in an outlet 354 of the Venturi.
[0050] A magnified view 355 of the Venturi Throat 220 shows the
outside diameter of the tubular insert 350 and the inside diameter
which measures 30 mm. Entrance and exit windows 356 and 357 are
positioned in front of the gamma ray source 200 and detector 201
respectively. The windows 356 and 357 may be made either out of
a material which shows transparency to the used radiation or just
be hollow, i.e. holes in the wall of the tubular insert 350.
[0051] It may be necessary to make openings in the wall of the
tubular insert 350 in order to connect, for example, the pressure
takeoff 358 with the cavity 352 of the tubular insert.
[0052] The Tuning Fork Insert
[0053] FIG. 4 represents a convergent Venturi 210 in which a tuning
fork insert 450 has been positioned.
[0054] The tuning fork insert 450 is so called because its appearance
reminds of the shape of a musical tuning fork, as can be seen in
the magnified view 451 of a longitudinal section through the Venturi
210 along the axis A in a plane perpendicular to the drawing. The
magnified view 451 shows the U-shaped extremity of the tuning fork
insert 450 with lateral walls 452 and 453 of the U-shape.
[0055] The lateral walls 452 and 453 of the tuning fork insert
450 comprise outer surfaces which are in intimate contact with the
walls of the Throat 220.
[0056] A section view 455 which is defined by a plan passing through
axis B of the view 451 and perpendicular to the drawing shows the
walls of the throat 220 and the lateral walls 452 and 453 of the
tuning fork insert 450. The lateral walls 452 and 453 have been
dimensioned to create a rectangular opening which offers an opening
to the flow F which is equivalent to a circular opening of 30 mm
diameter, i.e. a rectangle sized 13.6.times.52 mm.sup.2. The throat
section of the tuning fork insert 450 is designed in such a way
to stabilise the flow and minimise a pressure drop gradient between
the lateral walls 452 and 453 of the tuning fork insert.
[0057] The opening created by the lateral walls 452 and 453 between
the gamma ray source 200 and the detector 201 may be used as a hollow
window between the gamma ray source 200 and the detector 201. The
same hollow window allows the pressure takeoffs located in the throat
section, e.g. pressure takeoff 358 to be directly in contact with
the flow.
[0058] The extremities of the lateral walls 452 and 453 at the
flow inlet may have various shapes in order to optimise the convergent
section to different kinds of encountered flows: viscous flow, low
flow rate, high gas volume fraction, wet gas . . . .
[0059] The divergent section of the tuning fork insert 450 i.e.
the section at which the flows exits from between the lateral walls
452 and 453 may be shaped as an abrupt edge. In another embodiment
the divergent section may be shaped into a smooth profile to optimise
the flow (not shown in FIG. 4).
[0060] Referring again to view 451 a holder 454 of the tuning fork
insert 450 is shown. The holder 454 is used to position the tuning
fork insert 450 fork inside the throat 220. The lateral walls 452
and 453 of tuning fork insert 450 may be mounted on the holder 450
or be an integral part with this one. The size and more particularly
the outside diameter of the holder 454 is shown to be 52 mm as an
example only. It may have a diameter smaller than the diameter of
the throat.
[0061] The flow F thus enters the tuning fork insert 450 through
the rectangular opening between the lateral walls 452 and 453 arrives
into a cavity 456 of the holder 454 and leaves the cavity 456 through
lateral apertures 457 operated in walls of the holder 454 to continue
in the outlet 354 of the Venturi 210.
[0062] FIG. 5 represents a 3-dimensional view of a tuning fork
insert in which the lateral wall's inner surfaces are each terminated
by halves of concave rounded cone shapes. FIG. 5 also shows 4 lateral
apertures through which a flow may exit the cavity of the holder.
[0063] A possible gap between the outer diameter of the tuning
fork insert and the throat diameter needs to be machined precisely
in order to avoid any significant leak and any interference with
the pressure measurements that would affect the accuracy of the
flow rates calculation. In a preferred embodiment positive seals
(not represented in the Figures) are applied to prevent leakage.
[0064] Common Features to Rod, Tubular and Tuning Fork Insert Devices
[0065] All described examples of insert devices may be installed
in the Venturi flow meter at the well site. The insert devices may
appropriately be installed under pressure conditions. The insert
devices are inserted into the throat of the Venturi through an opening
600 (See FIGS. 2-4) located in an elbow of the Venturi outlet.
[0066] It is well understood that a Venturi flow meter may also
be a cylindrically shaped tube in which the throat is obtained by
inserting an insert device. The insertion of the insert device creates
a throat, i.e. a narrowing for the flow. |