GB2359435A

GB2359435A - Microwave Doppler Flowmeter

Info

Publication number: GB2359435A
Application number: GB0003431A
Authority: GB
Inventors: Zhipeng Wu; Cheng-Gang Xie; De Chizelle Yan Khun
Original assignee: Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 2000-02-16
Filing date: 2000-02-16
Publication date: 2001-08-22
Anticipated expiration: 2020-02-16
Also published as: AU2001232096A1; GB0003431D0; GB2359435B; WO2001061283A1

Abstract

A microwave antenna, 12-1, mounted on the side of a conduit, 11, transmits a microwave signal at a first frequency. The reflected signal is picked up by the antenna and transceiver, and the power and frequency determined. The Doppler shift can be used to establish the rate of flow and the power or strength of the reflected signal can be used to determine the volume fraction of a component, for example water in an oil/water/gas multi-phase flow. The system may determine the holdup of one component of the fluid. The system may use additional antennas preferably mounted between 30 and 60 degrees to the conduit axis. The additional antennas may be diametrically opposed or juxtaposed, and filled with a solid dielectric to provide a smooth face to the edge of the pipe.

Description

2359435 1 - Microwave Doppler Flowmeter for Multiphase Flow The present

invention relates to novel flowmeter and methods to measure the velocity of gas-oil-water multiphase flows in

oilfields, and the water volume fraction in the flows. More particular, it relates to such meters and methods as based on microwave Doppler measurements.

BACKGROUND OF THE INVENTION

To accurately measure the flowrates of gas-oil-water multiphase flows in oilfields, the velocity and the volume fraction of each phase has to be determined. Techniques implemented for in-line multiphase flow-velocity measurement on the market today are mostly based on cross-correlation methods, involving axially installing on the flow pipe dual set of, for example, capacitance sensors, microwave sensors or nuclear densitometers. Water fraction of multiphase flows is usually measured using microwave transmission and gamma-ray attenuation techniques.

Cross -correlation methods have shown to give biased indications of gas or liquid velocity at certain conditions and installations, and two sets of sensor systems (for one velocity measurement) have to be used. The accuracy of the water fraction measurement using microwave transmission is often largely affected by salinity.

Ultrasonic and microwave Doppler velocimeters have been used widely in blood flow and traffic flow measurements, respectively; often involving the use of one transceiver. For applications in gas-liquid (oil-water) multiphase-flow measurement, the use of range-gated ultrasonic Doppler system has problems in medium to high gas flows; the presence of large amount of gas in the pipe blocks the ultrasonic energy transmitting into the pipe, failing to deliver meaningful flow velocity-profile. The use of microwave Doppler systems for 2 multiphase fluids has been described for example in the United States Patents Nos 5,792,962 and 5,793.216.

The '962 patent fails to disclose a specific example of an microwavebased flowmeter. The 1216 patent describes a microwave flowmeter mounted on a horizontal flow constriction of rectangular crosssection. The preferred frequency range for the flowmeter is stated as 2 to 8 GHz. In one of its specific examples, the '216 patent refers to the use of the Doppler effect for determining the velocity of each phase of the multiphase flow by means of a frequency analysis. To determine the proportion of each phase in stratified flows, the 1216 patent measures the attenuation and phase changes of microwave radiation between transmitter/receiver pairs across the width of the flow pipe. The proportion of each phase is then determined from a data table. The method is potentially very inaccurate.

The present invention aims at improving the existing microwave flowmeter designs employing Doppler-based methods. It is an object of the invention to provide a Doppler-based microwave flowmeter having a broader and more versatile range of application in measuring liquid velocity and water volume fractions in multiphase flows in the oil and gas industry.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided methods and apparatus for determining the properties of multiphase flows, particularly flow that are produced from a subterranean wellbore. Such wellbore fluids often contain oil, water and gas phases in varying proportions.

According to the present invention the methods and apparatus 35 comprise the use of at least one microwave antenna mounted on a 3 conduit carrying a fluid; an oscillating unit generating a microwave signal at first frequency; a transceiver unit directing said generated microwave signal to said at least one antenna for emission into said fluid and receiving microwave signals reflected within said fluid, said reflected microwave signals having a spectrum of frequencies shifted with respect to said first frequency; and a signal processing unit for determining a frequency shift (Doppler frequency) spectrum and for determining the amount of power of the reflected microwave 10 signal.

The frequency of the microwave signal lies preferably in the range of 2 to 15 GHz, more preferably in the range of 2 to 4GHz and 8 to 12 GHz.

The amount of power of the reflected energy is defined as any parameter from which the strength of the reflected microwave signal can be derived.

Preferably, the antenna is or the antennas are mounted on the conduit with an inclination angle of 30 to 60 degrees with respect to the axis of the conduit. This inclination is the same for any pair of antennas used to transmit and receive microwaves, respectively. Those pairs of antennas are preferably 25 located diametrically opposed to each other or juxtaposed.

The apparatus can be mounted on vertically oriented conduits where there is no stratification of the flow into layers of separated phases.

The antennas are preferably filled with a solid dielectric material and matched to a component of the fluid. When mounted onto the pipe it is preferred to give the filler a smooth face leaving no discontinuities between the antenna and the surrounding wall of the conduit. More preferably the mounted 4 - flush with the surrounding wall, %%invisible" to the flow inside the conduit.

It may be advantageous to use a plurality of antennas employing for example a multiplexer and/or using microwave signals with different (central) frequencies.

The microwave flowmeter in accordance with the invention uses the Doppler effect to derive a velocity spectrum for components of the multiphase f low. Advantageously, the strength or amount of reflected microwave signal is employed to determine a parameter representing the volume fraction of a component, e.g., water, of the fluid.

The apparatus can be mounted on conduits and pipes of nonrectangular cross-sections and for plastic and metal pipes, thus enhancing greatly the applicability of the new device.

These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the following detailed description and drawings.

DRAWINGS FIG. 1A is a schematic cross-section showing a microwave antenna with a flush aperture to a vertical pipe of circular cross-section carrying a multiphase flow (monostatic case); FIG. 1B is a schematic crosssection showing three microwave antennas with flush apertures to a pipe carrying a multiphase flow (monostatic and bistatic case); FIG. 2 shows basic elements of the signal generation and processing circuit for a microwave flowmeter according to the present invention; FIG. 3 illustrates various monostatic configurations for a microwave flowmeter according to the present invention; FIG. 4 illustrates various bistatic configurations for a microwave flowmeter according to the present invention; FIG. 5 compares the time-averaged (over about 12s) microwave Doppler velocity with the time-averaged velocity (also over about 12s) near the pipe wall measured by a commercial range-gated ultrasonic Doppler velocimeter; FIG. 6 compares the microwave Doppler (time-averaged) velocity with that measured by the ultrasonic Doppler system range-gated near the pipe wall, and with the liquid velocity predicted by a venturi model; FIG. 7 is a plot of the (time-averaged) reflected power at increasing gas volume fractions (GVF); FIG. 8 is a plot of the time-averaged gas holdup, as derived from measurements of the reflected power against the gas-cut (GVF); FIG. 9 is a plot of the estimated water flowrates derived from the microwave Doppler velocity and fraction measurements and the reference water flowrates of the flow-loop for increasing gas volume fractions (GVF); - 6 FIG. 10 is a plot of the microwave Doppler velocity for oil/ water two phase flows vs the reference homogeneous liquid velocity; FIG. 11 is a plot of the microwave Doppler velocity for oil/gas/water three phase flows vs the reference liquid velocity; and FIG. 12 is a plot of the measured water holdup using microwave Doppler technique vs the reference water holdup for various oil/gas/water three phase flows.

MODE(S) FOR CARRYING OUT THE INVENTION With reference to FIG. 1A, there is shown a vertical metal pipe 11 with one microwave antenna 12-1. Through the pipe 11, formation fluids are pumped from a subterranean wellbore to the surface. The fluid comprises a liquid (water/oil) phase 111 and 20 a discontinuous (bubbly) gas phase 112.

The microwave antenna 12 has a dielectric aperture, which is flush with the inner pipe wall of the metal pipe and thus in contact with the gasliquid (oil-water) mixture. In the absence of flow mixture, the antenna 12 has a main beam angle 0 of 45 degrees with respect to the flow axis. This angle can however vary between 30 and 60 degrees depending on design considerations and flow conditions (flow rate, absorption etc.) In the illustrated configuration the system employs a single frequency operation and uses the single antenna 12 for both transmitting and receiving. The generation of the microwave and the processing of the returning signals are performed by equipment described in greater details when referring to FIGs. 24.

7 other possible antenna arrangements are illustrated in FIG. 1B. In FIG. 1B further antennas 12-2 and 12-3 are shown located either in adjacent to antenna 12-1 (as is the case with antenna 12-2) or diametrically opposed (12-3). It is noteworthy that are tilted by the same angle with respect to the pipe 11. All antennas can be operated as transceivers.

At a fixed operating frequency in a single frequency operation, or for each of the operating frequencies in a multiple or variable frequency operation, each antenna transmits electromagnetic signal (field) El(f,)) at a frequency fo into the gas-liquid (oil-water) flow. The reflected electromagnetic signal (field) E(f,), with a differing frequency f, shifted due to the Doppler effect produced by the moving electromagnetic reflectors (or interfaces produced by the gas, oil, water discontinuities in the flow stream) at a velocity v, is received by the same antenna 12-1 in which is referred to as "monostatic11 case or by a closely mounted antenna 12-2 on the same circumferential location (equivalent to the monostatic case), or by a different antenna 12-3 located diametrically opposite (with the same energy angle 0) referred to as "bistatic" case.

The Doppler shift frequency fd is related to the reflector velocity v through the following relationship:

fd = fr - f. - 2vf,, cos 0 c where c is the speed of the applied electromagnetic wave. When the components in the mixture flow at different or variable velocities, the Doppler shift frequency occupies a spectrum. However, equation [1] still holds for the relation between the Doppler frequency and its corresponding velocity. In such a 8 case, dominant flow velocities may be observed and an average flow velocity can be obtained.

Basic elements of the signal generation and processing circuit are illustrated by FIG. 2.

In the system shown in FIG. 2, a power supply 21 provides electrical energy to a microwave oscillator 22. The oscillator 22 generates a microwave signal E(fo) having a center frequency fo. A directional coupler 23 or a power splitter separates the microwave signal into two parts E, (f 0) and E2 (f 0). The f ormer part E, (f c)) is directed into a multiplexer 24 switching the energy between the two antennas 20-1, 20-2 for emission into the fluid flow. The latter part of the oscillator signal E2(fO) is mixed with the received reflected signal by means of a microwave mixer 25 to produce a signal E(fd) containing the information of the Doppler shift frequency fd, and magnitude and hence the power of the reflection.

At this stage the signal is usually processed on a processing board 26 within a data acquisition system or computer. The signal is amplified using an adjustable /programmable gain amplifier PGA 261 (and with lowpass filtering if necessary), the signal E(fd) is digitized by the Analogto-Digi tal converter (ADC) 262 and processed using an Fast -Fourier-Trans form (FFT) based signal processing algorithm 263 to obtain the frequency spectrum of the mixed signal E(fd), and further, using equation [1], the velocity spectrum of the multiphase flow in real time. The signal processing algorithm for velocity determination is based on Short Time (e.g 64 ms with 4 kHz data-sampling rate) FFT with an option of long time (e.g. 1 second) averaging. An average flow velocity of liquid phase is obtained by weighting each velocity component with its corresponding strength. As the reflected power depends strongly on the inhomogeneity of the flow mixture, its measurement in real time (e.g. 64 ms) can 9 provide an accurate indication of the water fraction of oil-gaswater three-phase. Both velocity and water fraction are displayed via a display unit 27 continuously in real time. The results of the data acquisition and processing can be used in subsequent operations such as well control or fluid separation.

In FIG. 3 various monostatic configurations are shown with single or multiple oscillators, single or multiple antennas; including following possible operational cases: single transceiver, single antenna, which is the simplest configuration; single transceiver, multiple antennas (multiplexed) which is used to obtain averaged measurement for circumferentially non- uniform flows; multiple transceivers (multiplexed), single antenna, which is used for large pipe diameters; multiple transceivers, multiple antennas (one to one fixed, one to one multiplexed), which is used to obtain averaged measurement for circumferentially non-uniform flows and pipes with large diameters.

The system shown in FIG. 3, the transceiver unit TR1 may include an oscillator 32 generating signals at a single or with variable frequencies. A control unit 364 controls the oscillator. A directional coupler 33 or power splitter splits the signal into two signals. The first of the two signals is applied to one port of a three port circulator 37 and directed into the matrix switching or multiplexing unit 34 via a bandpass filter 38. The multiplexer switches between the various antennas 30 (i.e. depending on the configuration to just one antenna 30-1 or to any combination of antennas 30-1 to 30-n) Any returning signals picked up by the antenna or antennas 30-1 to 30-n is passed back to the circulator 37 from where it is directed to the mixer 35. The mixer 35 receives as second input part of the originally generated signals as split by the directional coupler 33. The output of the mixer 35 consists of the signal E(fd)with the Doppler frequency fd.

Again depending on the configuration, a plurality of transceiver units TR1,..., TRn may be connected to the matrix switch or multiplexer 34.

The bistatic configuration is only applicable to high gas flows when the microwave transmission loss through the mixture is small. FIG. 4 illustrates the bistatic setup in more detail, also for single or multiple oscillators, single or multiple antennas. The possible operational modes are: single transceiver, two antennas (one for transmitting, the other for receiving); single transceiver, one antenna for transmitting and multiple antennas (multiplexed) for receiving; multiple transceivers (multiplexed), two antennas (one for transmitting, the other for receiving); multiple transceivers, a set of antennas for transmitting and another set for receiving (all multiplexed).

The main alteration of FIG. 4 as compared to FIG. 3 lies in the separation of emitting and receiving circuit. The separation obviates the need for a circulator. Instead the signal received by the set of receiving antennas 40-Rl to 40-Rn is directly guided through a second bandpass filter 48R into the mixer 45. The multiplexing unit 44 consists of a first section that is used to switch between the transmitting antennas 40-Tl to 40-Tn. A second section of the multiplexing unit 44 is used to switch the receiving antennas 40-Rl to 40-Rn to the transceiver unit TR1. The output of the transceiver unit TR1 connects to a processing unit (not shown) as described above.

11 As above, a plurality of transceiver units TR1,..., TRn may be connected to the matrix switch or multiplexer 34.

In the following there are presented some experimental results obtained for multiphase flows in vertically oriented plastic and metal pipes.

In FIG. 5, the time-averaged (over about 12s) microwave Doppler velocity is compared with the time-averaged velocity (also over about 12s) near the pipe wall measured by a commercial rangegated ultrasonic Doppler velocimeter. In this case, a single 10 GHz air-filled antenna was mounted externally outside a 2a ID plastic pipe with concurrent vertically-upward fresh-water/air flows. The antenna was matched to air/gas component and had a relatively shallow penetration depth into the water, and thus measured the velocity of the water near the antenna, leading to the velocity measurement of the carrying water phase near the wall. As shown in FIG. 5, a good agreement between the microwave Doppler velocity and the ultrasonic Doppler velocity (range- gated near the inner pipe wall close to the ultrasonic probe) has been obtained for gas volume fractions (GVF) up to 99% (for liquid volume flowrate qL from 5 to 20 m'/hr).

In metal pipes with flush-mounted antennas the microwave energy is directly coupled to the fluid and propagates within the metal pipe. Therefore, unlike an antenna mounted outside a dielectric (e.g. plastic) pipe, the adverse effects of the external movement (such as pipe vibration) and the external electromagnetic interference on the transmission and reflection of the microwave energy is eliminated. The reflected microwave energy from the moving fluid interface is also contained in the metal pipe and picked up by the microwave transceiver, leading to a more reliable reflection measurement. On the other hand, a metal pipe can form a microwave waveguide (in oil-continuous flows where the loss of the mixture is small) and thus to some 12 - extent affects the transmitting and thus the reflecting paths of the microwave energy, particularly at lower microwave frequencies used. As a result, the Doppler shift frequency and thus velocity and component fraction measurement may be affected. This requires the design of novel antennas and the development of advanced signal processing techniques to minimize the effects.

In the experimental setup, a single 10 GHz epoxy-filled antenna was flush with the inner wall of a 2" ID metallic pipe with concurrent vertically-upward fresh-water/air flows. The antenna for metal pipes was a circular waveguide of an inner diameter of 15 millimeters filled with epoxy (dielectric constant e=2.7). It was excited at the non-aperture end of the guide by a dipole with horizontal polarization. Details of the dipole antenna are described in 5,485,743, itself referring to U.S. Pat. Nos.5,243,290 and 5,434,507.

The metallic pipe has the same dimensions (in terms of diameters and length scales) as the plastic pipe used above. Compared with the airfilled antenna for use with the plastic pipe, the antenna installed in the metal pipe was better matched to the water component ensuring an efficient radiation into the flow. The antenna thus had a relatively larger penetration depth into the flow, and therefore is able to measure the flow velocity further away from the antenna (or pipe wall). As a result, under the similar flow conditions (liquid flowrates and gas volume fractions), the velocity measured in the metal pipe is generally higher than those in the plastic pipe.

FIG. 6 compares the microwave Doppler (time-averaged) velocity with that measured by the ultrasonic Doppler system range-gated near the pipe wall, and with the liquid velocity predicted by a venturi model (both Doppler systems were installed at the throat of a metal venturi of 2" diameter). The homogeneous velocity in - 13 the pipe is also given (for GVF well above 90%, the homogeneous velocity is very close to the gas velocity). FIG. 6 indicates that, for GVF below 80%, the microwave Doppler velocity follows very closely to the liquid velocity predicted by the (venturi) model, and is generally higher than the ultrasonic Doppler velocity (only measured reliably close to the metal pipe wall) For high GVF (e.g. over 80%), the accuracy could be improved by using an air-matched antenna.

Another experiment relates to the reflected power P, The reflection of the microwave Doppler energy was typically sampled at 4 kHz, and the reflected power was averaged over 256 samples to obtain a near instantaneous value Pr(nAt) (At = 64 ms, n = 0 to N). It was found that the reflected power Pr (nAt) becomes more sluggish as GVF increases, and the time-average of Pr(nAt) increases with GVF.

The time-averaged Pr(nAt) (over about 12s) at increasing GVF in for a water-air flow is given in FIG. 7. As shown in FIG. 7, for both metal and plastic vertical pipes, the time-averaged reflected power increases with GVF, and the dependence of the averaged reflected power on the GVF is regular as a result of eliminated environmental interference.

As observed for vertical metal-pipe flows (FIG. 7), because of the existence of the liquid-phase continuity (near the pipewall) over a wide range of GVFs, the reflected power has a good correlation with the variation in the water fraction in the pipe. This correlation can be exploited in the following way:

The instantaneous reflected power Pr(nAt) is firstly normalized max by the peak power Pr at very high gas conditions (for the set of air/water two-phase experiments conducted as shown in Figure 14 8, prmax -: 2 0 f or qw = 5 M3 /h and GVF at 99%). The instantaneous water holdup (Xw(nAt) is estimated from the normalized power by the following relation (established for gas-liquid statisticall and cross- sectional ly homogeneous, but time variant flow 5 mixtures):

[21 (Xw (nAt) 2 1 + -Pr(nAt)/prmax -,2 1 1 The time-averaged gas holdup ((Q = 1 - ((xw) of a water/air flow is plotted in FIG. 8 against the gas-cut (GVF), together with gas holdup estimated from the venturi flow model (FIG. 8 indicates the velocity slippage effect between the gas and the liquid phases). As shown, the two gas holdups are in a good agreement. Combining the average liquid velocity (UI) measured from the microwave Doppler effect (FIG. 6) with the water holdup ((XW from the normalized reflected power, the average water flowrate (%) can be estimated from (q. = (%XUL)A (A being the cross-section of the pipe). The comparison with the respective flow-loop references is generally good as shown in FIG. 9 which plots the estimated water flowrates (qw from the microwave Doppler velocity and fraction measurement in comparison with the reference water flowrates of the flow-loop for increasing gas volume fractions (GVF).

The above examples show that the system is applicable to vertical pipes for liquid velocity, water fraction and volumetric flow measurements, due to the time-averaged flow homogeneity.

The Doppler signal received by the system is mainly due to the reflection from phase/component discontinuities, in particular gas-water (or gas-oil, oil-water) interfaces. Unlike transmission measurement, the reflection measurement is less affected by variations in water salinity as the reflection is dominated by the real part of the dielectric constant, rather than the imaginary part where the salinity effect comes in. Significant reflection can be generated in gas-water (or oilwater) interfaces. In the cases of two-phase flows such as oil/gas, gas/water or oil/water flows, a gas- or oil-matched antenna should be able to pickup the reflection caused by the discontinuous phase, and thus measure the flow velocity and phase fractions.

Further experiment on the velocity measurements of oil/water two-phase flows is presented below.

In FIG.10, the measured microwave Doppler velocity is compared with the reference homogeneous velocity of water/oil liquid mixture on the vertically oriented 2" metal pipe for various values of water cut.

In the following, there are presenting some experimental results obtained using the system for oil/gas/water three phase flows in the vertically oriented 20 ID metal pipe.

In FIG.11, the measured microwave Doppler velocity is compared with the reference liquid velocity of oil/gas/water three phase flows.

The above experiments show that the system is applicable to vertical pipes for liquid velocity measurement of not only gas/water, but also oil/water, and oil/gas/water three phase flows.

16 A further experiment relates to the reflected power on oil/gas/water three phase flows.

For a particularly matched antenna for use with water-oil-gas three-phase flows, the reflected power is also primarily a function of effective dielectric constant of the water-oil liquid mixture (for vertical pipes the liquid mixture is often statistically homogeneous), which is dependent on the water cut and the phase (oil or water) continuous state. For a water- matched antenna for use in water-oil-gas three-phase flows with GVF > 10%, the water holdup can be determined using Equation [21.

In FIG.12, the average value of the measured water holdup (%) over 15 seconds is compared with the reference water holdup measured using gamma-ray technique to establish a mixture density and a reference water cut.

When the Doppler flowmeter is used with a density- sensitive devices such as gamma-ray densitometer for measuring the mixture density PM, in oil/gas/water multiphase flows with GVF>10%, the water cut YW can be determined using Equation [3] 31 VW -9w- = f((xw I PM) (XL The oil fraction can be determined using Equation [4], [ 41 (CCO) - 1 - YW (%) Yw The average water flowrate (q.) can be estimated from (q. = ((XJU> and the oil flow rate qo using (qo = ((X0(ul)A (A being the cross-section of the pipe and UL the average liquid velocity).

The frequency used for the experimental studies is around 10 GHz for a vertical 2 inch (diameter) pipe. For larger pipes, lower microwave frequency can be employed so that microwave energy can be launched to the center of the pipe. Microwave penetration depth into water at high frequency and high salinity is very small. For water-continuous flows of high salinity/conductivity, the penetration depth of the microwave system is also increased by using lower microwave frequencies.

Claims

An microwave flowmeter comprising at least one microwave antenna mounted on a conduit carrying a fluid; an oscillating unit generating a microwave signal at first frequency; a transceiver unit directing said generated microwave signal to said at least one antenna for emission into said fluid and receiving microwave signals reflected within said fluid, said reflected microwave signals having a frequency shifted with respect to said first frequency; and a signal processing unit for determining the spectrum of frequency shift (Doppler frequency) between microwave signals emitted and for determining the amount of power of the reflected microwave signal.
The f lowmeter of claim 1 wherein the at least one antenna is mounted on the conduit with an inclination angle of 30 to 60 degrees with respect to the axis of the conduit.
3.

The flowmeter of claim 1 having at least two antennas, said at least two antennas mounted on the conduit with an inclination angle of 30 to 60 degrees with respect to the axis of the conduit.
4. The flowmeter of claim 3 wherein the at least two antennas are located diametrically opposed to each other.
5. The flowmeter of claim 3 wherein the at least two antennas are juxtaposed.
6. The flowmeter of claim 1 wherein the transceiver unit comprises a directional coupler or power splitter to generate a reference signal communicated to a mixer, said 19 - mixer further receiving the reflected microwave signal and generating an output representing the signal strength of the reflected microwave signal and the frequency shift.
7.

The flowmeter of claim 1 further comprising a multiplexing unit to direct microwave signal to and from a plurality of antennas.
The flowmeter of claim 1 wherein the oscillator unit is adapted to generate microwave signals of more that one (center) f requency.
The f lowmeter of claim 1 wherein the at least one antenna is filled with a solid dielectric material and matched to a component of the fluid, ensuring an efficient radiation into the fluid.
10. The flowmeter of claim 9 wherein the filling material is applied so as to provide a smooth face essentially free of 20 discontinuities to the flow.
11. The flowmeter of claim 1 wherein the processing unit comprises a unit for processing information relating to the frequency shift to determine a velocity spectrum of the 25 fluid.
The flowmeter of claim 11 wherein the unit for determining a velocity spectrum of the fluid comprises a processor unit performing a Fourier transformation and a spectral analysis of received signals.
13. The flowmeter of claim 11 wherein the processing unit further comprises a unit for determining the average velocity of the fluid.
14. The flowmeter of claim 1 wherein the processing unit comprises a unit for processing information relating to the strength of the reflected microwave signal to determine a parameter representing a holdup of one component of the f luid.
15. The flowmeter of claim 1 wherein the processing unit comprises a unit for processing information relating to a normalized power of the reflected microwave signal to determine a parameter representing a holdup of a component of the fluid.
16. The f lowmeter of claim 1 wherein the conduit has a circular crosssection at the location of the at least one antenna of the microwave flowmeter.
17. The f lowmeter of claim 1 wherein the conduit has a vertical orientation at the location of the at least one antenna of the microwave flowmeter.
18. The flowmeter of claim 1 wherein the fluid comprises gas and liquid phases.
19. The flowmeter of claim 1 wherein the fluid comprises a gas, an oil and an aqueous phase.
20. The flowmeter of claim 1 wherein the fluid is produced from a subterranean wellbore.
21. A method of determining characteristic parameter of a multiphase flow', comprising the steps of mounting at least one microwave antenna on a conduit carrying i fluid;. generating a microwave signal at first frequency; directing said generated microwave signal to said at least one antenna for emission into said fluid; receiving microwave signals reflected within said fluid and having a second frequency shifted with respect to said first frequency; determining a frequency shift (Doppler frequency) between the second and the first frequency; and determining the amount of power of the reflected microwave signal.
22. The method of claim 21 wherein the at least one antenna is mounted on the conduit with an inclination angle of 30 to 60 degrees with respect to the axis of the conduit.
23. The method of claim 21 comprising the steps of splitting the generated microwave signals to generate a reference signals and mixing said reference signal with the reflected microwave signal received thereby generating an output representing the signal strength of the reflected microwave signal and the frequency shift.
24. The method of claim 21 further comprising the step of multiplexing microwave signals to and from a plurality of antennas.
25. The method of claim 21 further comprising the step of generating microwave signals of more that one (center) frequency.
26. The method of claim 21 further comprising the steps of filling the at least one antenna with a solid dielectri material and matching the antenna to a component of the fluid ensuring an efficient radiation into the fluid.

c 22 -
27. The method of claim 21 further comprising the step of processing information relating to the frequency shift to determine a velocity spectrum of the fluid.
28. The method of claim 21 further comprising the step of processing information relating to the strength of the reflected microwave signal to determine a parameter representing a holdup, of a component of the fluid.
29. The method of claim 21 further comprising the step of processing information relating to a normalized power of the reflected microwave signal to determine a parameter representing a holdup of a component of the fluid.