Abstrict A cross correlation fluid flow meter including a body having first
and second sensor devices positioned such that in use they are spaced
apart a known distance in the direction of fluid flow, each sensor
device having a signal transmitter and a signal receiver. According
to one embodiment of the flow meter, the signal transmitter of the
first sensor device is operable to transmit signals at first phase
of a first frequency and the signal receiver of the first sensor
device is arranged to receive signals at the first phase of the
first frequency. The signal transmitter of the second sensor device
is operable to transmit a second signal at a second phase of the
first frequency and the signal receiver of the second device is
operable to receive signals at the second phase of the first frequency,
the first and second phases being out of phase with one another.
The first sensor is non-responsive to the second phase and the second
sensor is non-responsive to the first phase. In another embodiment,
the signal transmitter of the first sensor transmits a signal at
a first frequency and the signal transmitter of the second sensor
transmits a signal a second frequency different that the first frequency.
First and second receivers are adapted to receive signals at the
respective first and second frequencies, but not to respond to the
other of the frequencies. In either embodiment, cross correlation
of the received signals provides an indication of the fluid flow
rate.
Claims We claim:
1. A cross correlation fluid flow meter for measuring the rate
of flow of a fluid in an upstream to downstream direction, comprising:
a body having a first sensor means and a second sensor means spaced
a known distance in the upstream to downstream direction from said
first sensor means,
said first sensor means having a first transmitter means for transmitting
a first electrical signal of a first frequency at a first phase
as transmitted, and a first receiver means for receiving said first
electrical signal in substantially said first phase, and
said second sensor means having a second transmitter means for
transmitting a second electrical signal of said first frequency
at a second phase out of phase with said first electrical signal
as transmitted, and a second receiver means for receiving said second
electrical signal in substantially said second phase,
said first receiver means not being responsive to said second electrical
signal, and said second receiver not being responsive to said first
electrical signal,
wherein a cross correlation of said first electrical signal as
received by said first receiving means and said second electrical
signal as received by said second receiving means provides an indication
of the rate of the fluid flow the fluid.
2. A cross correlation fluid flow meter according to claim 1 wherein:
said second electrical signal has a quadrature phase difference
from said first electrical signal.
3. A cross correlation fluid flow meter according to claim 1 further
comprising:
an array of sensors, said first and second sensors being selected
from said array.
4. A cross correlation fluid flow meter according to claim 1 wherein:
the fluid flows in an upstream to downstream direction through
a chamber, and said body is adapted to fit within the chamber.
5. A cross correlation fluid flow meter for measuring the rate
of flow of a fluid in an upstream to downstream direction, comprising:
a body having a first sensor means and a second sensor means spaced
a known distance in the upstream to downstream direction from said
first sensor means,
said first sensor means having a first transmitter means for transmitting
a first electrical signal of a first frequency at a first phase
as transmitted, and a first receiver means for receiving said first
electrical signal and for providing an indication of a first fluid
property of the fluid, and
said second sensor means having a second transmitter means for
transmitting a second electrical signal of said first frequency
at a second phase out of phase with said first electrical signal
as transmitted, and a second receiver means for receiving said second
electrical signal and for providing an indication of a second fluid
property of the fluid,
said first receiver means not being responsive to said second electrical
signal, and said second receiver not being responsive to said first
electrical signal,
wherein a cross correlation of said indications of said first and
second fluid properties provides an indication of the rate of the
fluid flow of the fluid.
6. A cross correlation fluid flow meter according to claim 5 wherein:
said first and second fluid properties are respective first and
second multiphase variations of the fluid.
7. A cross correlation fluid flow meter according to claim 5 wherein:
said first and second fluid properties are respective first and
second electrical impedance measurements of said fluid at said first
and second sensor means.
8. A cross correlation fluid flow meter according to claim 5 wherein:
said first and second indications are respective amplitude modulations
of said first and second electrical signals as modulated by said
respective first and second fluid properties and received by said
first and second receiver means.
9. A cross correlation fluid flow meter according to claim 5 wherein:
said second electrical signal has a quadrature phase difference
from said first electrical signal.
10. A cross correlation fluid flow meter for measuring the rate
of flow of a fluid in an upstream to downstream direction, comprising:
a body having a first sensor means and a second sensor means spaced
a known distance in the upstream to downstream direction from said
first sensor means,
said first sensor means having a first transmitter means for transmitting
a first electrical signal having a first phase and first frequency,
as transmitted, and a first receiver means for receiving said first
electrical signal and for providing a first indication of a first
fluid property, and
said second sensor means having a second transmitter means for
transmitting a second electrical signal having a second phase and
a second frequency, as transmitted, and a second receiver means
for receiving said second electrical signal and for providing a
second indication of a second fluid property, wherein said second
phase is out-of-phase with said first phase,
said first receiver means not being responsive to said second electrical
signal, and said second receiver not being responsive to said first
electrical signal,
wherein a cross correlation of said first and second indications
provides an indication of the rate of the fluid flow the fluid.
11. A cross correlation fluid flow meter according to claim 10
wherein:
said second frequency is different than said first frequency, as
transmitted.
12. A cross correlation fluid flow meter according to claim 11
wherein:
said first frequency and said second frequency are substantially
in a range of 5-7 kHz.
13. A cross correlation fluid flow meter according to claim 11
wherein:
said first frequency has a frequency difference of approximately
1 kHz from said second frequency.
14. A cross correlation fluid flow meter according to claim 11
wherein:
said first frequency is selected such that it is outside a passband
of circuitry in said second sensor.
15. A method of measuring a rate of fluid flow in an upstream to
downstream direction, comprising:
a) providing a first sensor means and a second sensor means spaced
a known distance in the upstream to downstream direction from the
first sensor means;
b) transmitting, at the first sensor means, a first electrical
signal of a first frequency at a first phase as transmitted;
c) receiving, at the first sensor means, the first electrical signal,
wherein the first electrical signal, as received, provides a first
indication of a first fluid property of the fluid;
d) transmitting, at the second sensor means, a second electrical
signal different in phase than the first electrical signal, as transmitted;
e) receiving, at the second sensor means, the second electrical
signal, wherein said second electrical signal, as received, provides
a second indication of a second fluid property of the fluid, said
first sensor means being non-responsive to said second electrical
signal and said second sensor means being non-responsive to said
first electrical signal; and
f) cross correlating the first and second indications to determine
an indication of the fluid flow rate.
16. A method according to claim 15 wherein:
the first signal is of a first frequency at a first phase as transmitted,
and the second signal is of the first frequency at a second phase
out of phase with the first electrical signal.
17. A method according to claim 16 wherein:
said first and second signals have a quadrature phase difference
relative to each other.
18. A method according to claim 15 wherein:
the first signal is of a first frequency, and the second signal
is of a second frequency different than the first frequency.
19. A method according to claim 18 wherein:
said first and second frequencies are substantially within the
range of 5-7 kHz with a difference of approximately 1 kHz relative
to each other.
Description The present invention relates to a flow meter for measuring fluid
flows and to a method of measuring such flows. In particular, the
invention relates to a cross-correlation flow meter and method which
is useful for measuring multiphase flows of liquid(s) and gas(es).
Cross-correlation flow metering relies on measurements of the flowing
fluid at two different places positioned a known distance apart
along the flow. By cross correlating the measurements from the two
places the time for corresponding events to pass between the two
places can be calculated, and an indication of the flow rate can
be obtained. Previous cross-correlation flow meters have relied
on the injection of a marker or tracer into the flow which can be
detected by the spaced sensors. Such a system is described in GB-A
1001022 in which a marker such as common salt or radioactive materials
is introduced into the flow to be measured. As an alternative to
the tracer or marker method suggested in GB-A 1001022 it is also
possible to measure events which are inherent in the flow being
measured, especially in multiphase flows. Such events might be the
passage of groups of bubbles in a liquid/gas flow, or droplets in
a liquid/liquid flow such as oil in water. The events can be detected
by various techniques depending on the nature of the fluids concerned.
Typically, changes in fluid density, electrical impedance, resistivity
optical or ultrasonic properties of the flow can be sensed (see
GB-A 2057141 GB-A 2227841 GB-A 2194058).
One particular application of cross-correlation flow metering is
in the production logging of oil wells. In production logging, it
is desirable to determine the flow rates of fluids being produced
at different locations in the well. In order to achieve this, a
logging tool comprising a flow meter is lowered into the well by
means of a wireline cable, and flow measurements are made at the
location of interest. In cases where oil is the continuous phase,
the impedance of the flow can be measured to identify the events
(see GB-A 2227841 aforementioned,. GB-A 2253907 and our co-pending
British patent application no: 92/04854.5 [now GB-A 2265009])
describe systems for use when water is the continuous phase, and
the resistivity is measured.
The problem of cross-talk is common in many measurement systems.
A system might employ two pairs of electrodes, each pair comprising
a transmitter electrode and a receiver electrode, one pair being
situated downstream of the other, the object is to measure, say,
the resistivity between the two electrodes comprising the first
pair and correlate the measurements with those obtained at the second
pair. However, in certain circumstances the signal from the transmitter
of one pair might be received at the receiver of the other pair,
creating an interfering signal which makes cross correlation increasingly
difficult. Cross-talk can also occur in the electronic circuitry
used to effect the detection and cross-correlation.
It is an object of the present invention to provide a cross-correlation
flow measurement system in which the problem of cross-talk is reduced.
In accordance with a first aspect of the present invention, there
is provided a cross correlation fluid flow meter comprising a body
having first and second sensor means positioned such that in use
they are spaced apart a known distance in the direction of fluid
flow, each sensor means comprising a signal transmitter and a signal
receiver, the signal transmitter of the first sensor means being
operable at a first frequency and the signal receiver of the first
sensor means being arranged to receive signals only at the first
frequency and the signal transmitter of the second sensor means
being operable at a second frequency and the signal receiver of
the second sensor means being arranged to receive signals only at
the second frequency, which flow metre is characterised in that
the first and second frequencies are close but different, that the
difference is such that a signal at one frequency is substantially
undetectable at a receiver operating at the other frequency yet
the response of the fluid to the transmitted signals at both frequencies
is substantially the same, and that the two frequencies are not
harmonically related.
In accordance with a second aspect of the present invention, there
is provided a cross-correlation fluid flow meter comprising a body
having first and second sensor means positioned such that in use
they are spaced apart in the direction of fluid flow, each sensor
means comprising a signal transmitter operable at a given frequency
and a signal receiver, which flow metre is characterised in that
the signal transmitters of the first and second sensors means are
operable at said frequency but with a phase difference therebetween,
and the signal receivers of the first and second sensor means are
arranged to receive signals at said frequency only when the signals
have the corresponding phase to that of the relevant transmitter.
The invention also provides methods of measuring fluid flow using
one or other of the cross-correlation flow meters of the invention.
Thus:
In accordance with a third aspect of the present invention, there
is provided a method of measuring fluid flow comprising:
a) providing first and second sensor means at first and second
locations spaced at a known distance in the direction of fluid flow,
the sensor means each comprising a signal transmitter and a signal
receiver;
b) transmitting a signal at a first frequency from the signal transmitter
of the first sensor means, and detecting only signals at the first
frequency at the signal receiver of the first sensor means so as
to produce a first sensor signal;
c) transmitting a signal at a second frequency, which is different
from the first frequency, from the signal transmitter of the second
sensor means, and detecting only signals at the second frequency
at the signal receiver of the second sensor means so as to produce
a second sensor signal; and
d) cross correlating the first and second sensor signals to determine
the fluid flow rate;
which method is characterised in that the first and second frequencies
are close but sufficiently different that a signal at one frequency
is substantially undetectable at a receiver operating at the other
frequency yet sufficiently similar that the response of the fluid
to the transmitted signals at both frequencies is substantially
the same; and the two frequencies are not harmonically related.
In accordance with a fourth aspect of the present invention, there
is provided a method of measuring fluid flow comprising:
a) providing first and second sensor means at first and second
locations spaced at a known distance apart in the direction of fluid
flow, the sensor means each comprising a signal transmitter and
a signal receiver;
which method is characterised by
b) transmitting a signal at a given frequency and phase from the
signal transmitter of the first sensor means, and detecting only
signals at such given frequency and phase at the signal receiver
of the first sensor means so as to produce a first sensor signal;
c) transmitting a signal at said frequency with a phase signal
which is different from that of the first transmitter from the signal
transmitter of the second sensor means, and detecting only signals
at the given frequency with said different phase at the signal receiver
of the second sensor means so as to produce a second sensor signal;
and
d) cross correlating the first and second sensor signals to determine
the fluid flow rate.
When differing frequencies are used the first and second frequencies
are sufficiently different that a signal at one frequency is substantially
undetectable at a receiver operating at the other frequency and
yet are sufficiently similar that the response of the fluid to the
transmitted signals at both frequencies is substantially the same.
The two frequencies should not be harmonically related, i.e., the
frequency of operation of one sensor are also not be a multiple
of the frequency of the other. For example, when measuring a water
continuous flow, the first and second frequencies might be in the
range 5-7 kHz with a difference of about 1 kHz.
It is preferred that the frequency for a sensor is selected such
that it is outside the passband of circuitry used for the other
sensor. Each receiver is conveniently connected to a lock-in amplifier
input, the amplifier operating at the appropriate frequency. The
signal can be low-pass filtered and A/D processed.
Where a phase difference is used, it is preferred that there is
a quadrature phase difference between the signals for the two sensors.
The flow meter according to the first aspect of the present invention
conveniently forms part of an oilfield production logging tool such
as that described in the aforementioned GB-A 2265009 (incorporated
herein by reference).
The present invention will now be described by way of example,
with reference to the accompanying drawings, in which:
FIG. 1 shows a diagrammatic side view of a resistivity cross correlation
production logging tool;
FIG. 2 shows the existence current paths available with the tool
of FIG. 1;
FIG. 3 shows the equivalent electric circuit to the tool of FIG.
1;
FIG. 4 shows a block diagram of the electronics for a tool according
to a first aspect of the invention;
FIG. 5a shows a block diagram of the electronics for a tool according
to a second aspect of the invention;
FIG. 5b shows a block diagram of a system for quadrative phase
signals;
FIG. 6 shows a cross correlation for a prior art tool;
FIG. 7 shows a cross correlation for a tool according to a first
aspect of the present invention;
FIG. 8 shows an alternative sensor circuit to that shown in FIG.
3; and
FIG. 9 shows an alternative tool configuration.
Referring now to the drawings, FIG. 1 shows a diagrammatic view
of a resistivity cross correlation flow measuring tool of the type
described in British Patent Application 9204854.5. The tool generally
comprises an insulating tool body 10 having two sensors 12 14 spaced
a known distance apart therein. Each of the sensors comprises a
transmitter electrode 16a, 18a and a receiver electrode 16b, 18b
spaced therefrom. In use, the transmitter electrodes 16a, 18a transmit
an oscillating signal which is received by the associated receiver
electrodes 16b, 18b via the fluid in which the tool is suspended.
Since the tool is measuring the resistivity of the fluid in which
it is suspended, it is necessary that, for a multiphase flow, the
continuous phase is conductive, ie water when measuring flow in
an oil well. It is intended that the current flow to be measured
by the tool is between the transmitter and receiver electrodes of
a given sensor, shown on FIG. 2 as paths RX.sub.1 and RX.sub.2 which
are indicated as resistors since it is the resistivity between these
two points which is being measured. It will be apparent that other
parasitic current paths exist in the fluid between the electrodes
on the tool. Parasitic current paths are undesirable because they
cross couple with the sensor signals. These paths are shown on FIG.
2 as resistive paths R1 R2 R3 R4.
The equivalent electrical circuit is shown in FIG. 3 with resistances
and operational amplifiers OP.sub.o and OP.sub.i indicated. The
reference input signal voltage is shown as V.sub.i and the reference
input resistance as R.sub.i. The input/output channel 0 corresponds
to sensor 12 and channel 1 corresponds to sensor 14. The existence
of the parasitic current paths means that any signal detected is
not merely the result of the appropriate transmitter signal propagating
through the desired portion of the flowing fluid, but also includes
interfering contributions from the other sensors. Since path R3
is between points of essentially identical potential and since path
R4 is between the driver output of the op amps (low impedance sources),
the contribution from these current paths can be ignored. However,
this still leaves paths R1 and R2.
The present invention seeks to minimise the effect of current paths
R1 and R2 by matching the receiver of one sensor to a characteristic
of its associated transmitter, either frequency or phase, which
is different and separable from that of the transmitter of the other
sensor.
In accordance with a first aspect of the invention, two different
excitation frequencies are used for the two channels. FIG. 4 shows
this situation where two oscillators are used to provide each channel
with a different excitation frequency, for example 6.73 kHz for
channel 0 and 5.6 kHz for channel 1. The two frequencies are chosen
to be not too dissimilar since measured sensor conductance changes
with frequency in certain frequency ranges. With about 1 kHz difference
in the range of 5 to 7 kHz, it can be assumed that the measured
resistances of the two channels are reasonably close. Due to the
cross talk, at the pre-amplifier output of each channel, there are
signals of both 6.73 kHz and 5.6 kHz. In each channel a bandpass
filter which is tuned to the excitation frequency of its own channel,
is used. The filter typically has a Q value of about 3 and is used
to provide some attenuation to the cross talk frequency at an early
stage to avoid large cross talk amplitude which may cause circuit
saturation. The filter also serves as a phase trimmer for the phase
sensitive detector. By using the channel excitation as the reference
signal, the phase sensitive demodulator (PSD) behaves like a bandpass
filter with its centre frequency locked onto the channel excitation.
In one embodiment of this invention, the PSD is implemented using
an AD630 balanced demodulator chip from Analog Devices. The bandwidth
of the PSD is determined by the lowpass filter following it. With
a second order lowpass filter of 100 Hz bandwidth, the PSD provides
40 dB attenuation to the frequency of the other channel (about 1
kHz difference). The output of the lowpass filter represents the
standing value of the measured resistance, and a highpass filter
with a pass band of 0.1 Hz is used to extract the fluctuating signal,
which can be used for cross correlation. Thus the bandwidth of the
measurement system is 0.1-100 Hz which acts as a bandpass filter.
In accordance with a second aspect of the invention, both sensors
use the same frequency but the excitation of one channel is made
in quadrature phase with respect to that of the other. With the
excitation of its own channel as the reference, the PSD rejects
the cross talk signal which is 90 degrees out of phase. FIG. 5a
shows the principle of the quadrature phase method. The circuit
differs from the dual frequency method only in the excitation source,
where a quadrature oscillator replaces the two oscillators of different
frequencies. In industrial implementation, the quadrature phase
signals can be synthesised from a higher frequency square wave through
some simple digital circuitry and bandpass filters, as illustrated
in FIG. 5b, this technique may be more convenient than using two
sinusoidal oscillators.
The cross talk problem can be further reduced by adapting the circuit
arrangement shown in FIG. 8. This can be contrasted with the corresponding
FIG. 3 which is discussed above. In that case, the unknown resistances
RX.sub.1 and RX.sub.2 are connected across the op amps and the parasitic
resistances R1 and R2 cause cross-channel gain modulations which
cannot be completely removed due to the operation of the amplifiers.
In FIG. 8 the unknown resistances RX.sub.1 and RX.sub.2 precede
the op amps and a fixed resistor is connected across the op amps.
Thus the parasitic current paths are removed from the op amp functioning
and the problem of cross talk is reduced.
The effect of the present invention can be seen by comparing FIGS.
6 and 7. FIG. 6 shows the cross correlation plot from a tool of
the type generally described above where both sensors are run with
the same frequency and phase of excitation signal. These results
are obtained in a flow loop through which a mixed flow of oil and
water is passed, both the oil and water having a flow rate of 5
m.sup.3 /hr and the loop being inclined at an angle of 30.degree.
from vertical. FIG. 7 shows the cross correlation for the same experiment
set up but with the sensors operated at 6.73 kHz and 5.6 kHz respectively.
As will be seen, the maximum correlation is more easily identified.
An alternative version of a tool incorporating the present invention
is shown in FIG. 9. In this case, several sensors 20a, b, c, d .
. . are shown, each sensor comprising a pair of electrodes. The
signals from any two sensors can be cross-correlated. This allows
various operations to be investigated and the optimum chosen for
a given set of flow conditions. A degree of redundancy is also provided
in such an arrangement. It will be appreciated that signals from
more than two sensors can be cross-correlated if required. In any
of these cases, the sensors are driven at different, separable frequencies
as described above to avoid the cross talk problem. |