Measuring and testing – Volume or rate of flow – By measuring transit time of tracer or tag
Patent
1995-08-29
1998-11-17
Bigel, Ronald L.
Measuring and testing
Volume or rate of flow
By measuring transit time of tracer or tag
G01F 1712
Patent
active
058379029
DESCRIPTION:
BRIEF SUMMARY
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 1,001,022 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 1,001,022, 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 2,057,141, GB-A 2,227,841, GB-A 2,194,058).
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 2,227,841 aforementioned,. GB-A 2,253,907 2,265,009!) 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 flui
REFERENCES:
patent: 4019038 (1977-04-01), Critten et al.
patent: 4583409 (1986-04-01), Lannuzel et al.
patent: 4922750 (1990-05-01), Magori
Huang Songming
Veneruso Anthony Frank
Bigel Ronald L.
Gordon David P.
Lee Peter Y.
Patel Harshad
Schlumberger Technology Corporation
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