GB2365978A

GB2365978A - Measuring relative proportions of dissimilar fluids in a pipe

Info

Publication number: GB2365978A
Application number: GB0020167A
Authority: GB
Inventors: Steven Robert Powell
Original assignee: ABB Offshore Systems Ltd
Current assignee: Baker Hughes International Treasury Services Ltd
Priority date: 2000-08-17
Filing date: 2000-08-17
Publication date: 2002-02-27
Also published as: GB0020167D0; WO2002016931A1; AU2001278636A1

Abstract

Apparatus and method for measuring the relative proportions of dissimilar fluids in a pipe (2) includes a variable frequency source (6) of electromagnetic radiation, arranged to send radiation into a coil resonator (1), which is helically wound around the pipe. A detector (7) detects radiation emitted by the resonator. At least one of the source and the detector is physically connected to the resonator but preferably both are connected, by means of co-axial cables (8, 10), and capacitors (9,11). The provision of direct connection between the source, and/or the detector, and the coil resonator, results in reduced attenuation between input and output and an improved signal-to-noise ratio of the system.

Description

2365978 1 P/62378.6BAMIL MEASURING RELATIVE PROPORTIONS OF DISSIMILAR

FLUIDS IN A PIPE This invention relates to measuring relative proportions of dissimilar fluids in a pipe, for example gas, water and oil.

In a hydrocarbon extraction system where, for instance, crude oil is being pumped from 5 the ground, it is desirable to be able to measure the gas and water content of that crude oil.

A number of methods of measuring the gas and water content of crude oil have been proposed. These proposed methods generally fall into two types. The first type 10 comprises methods involving taking a sample of the oil flow and analysing it. The other type comprises so-called full flow systems, involving the measurement of the aggregate gas and water content of the entire flow.

In general, measurement methods involving sampling have been difficult because the 15 gas and water content of the oil flow tends to be non-homogenous over time, and/or across the width of the pipe. There is, therefore, no guarantee that a sample is representative unless an homogeniser is employed to homogenise the gas, oil and water in the flow. Generally speaking, homogenisers are not completely effective and require power for operation. The need for such power supply can be problematic in subsea 20 environments. As a result, full flow systems are preferred.

2 P/62378.GBA/OIL In the system disclosed in GB 2 271 637, microwave energy is directed into a cavity containing a coil resonator wound around a section of the pipe, The activity of the coil resonator is detected remotely by a probe. This system exploits the fact that oil, water and gas have very different permitivities. However, this system has been found to exhibit unacceptable levels of attenuation between the input and the output.

The invention provides apparatus for measuring the relative proportions of dissimilar fluids in a pipe, the apparatus comprising: a coil resonator, helically wound, in use, around the pipe; a variable frequency source of electromagnetic radiation, arranged to 10 send electromagnetic radiation into the coil; an electromagnetic detector arranged to detect electromagnetic radiation emitted by the resonator, which radiation is indicative of the relative proportions of the fluids in the pipe; character-ised in that one of the source and the detector is physically connected to an end portion of the resonator.

Preferably, both the source and the detector are physically connected to opposite end 15 portions of the coil resonator. The provision of direct connection between the electromagnetic source and the coil, and/or the coil and the detector, results in a significantly reduced attenuation between input and output.

Consequently, an amplifier of lower gain can be employed at the output. Previously, 20 high gain amplifiers needed to be used, which caused problems when the detected signal caused by a slight change in fluid proportions was so small as to be swamped by the noise inherent in the amplifier.

3 P/62378.GBAJOIL The invention is particularly suitable for measuring the relative proportions of oil, gas and water in crude oil being extracted from a hydrocarbon well.

The invention will now be described, by way of example, with reference to the 5 accompanying drawings, in which:- Figure 1 is a partly sectional side view of prior art apparatus for measuring the relative proportions of dissimilar fluids in a pipe;

10 Figure 2 schematically illustrates apparatus constructed according to the invention; Figure 3 is a partly sectional side view of apparatus constructed according to the invention; and 15 Figure 4 is a schematic diagram of a measurement system including the apparatus of Figure 3.

Like reference numerals have been used for like parts throughout the specification.

20 Referring to the prior art apparatus of Figure 1, a helical coil resonator I is shown, which is an electric conductor shaped into a helix. A pipe 2, through which a portion of the fluid of interest is arranged to flow, passes through the coils of the resonator 1. The resonator I and pipe 2 are situated in a conductive metal cavity 3. Microwave energy is introduced into the cavity 3 by means of an input coaxial probe 4 and the resulting

4 P/62378.GBA/OIL activity of the resonator I can be sensed using a remotely located output coaxial probe 5.

The resonant peak of the helical coil resonator I for a given input power vanes, both in 5 frequency and amplitude, in dependence on the material within the pipe 2. The resonant frequency is affected by the dielectric constant of the material within the pipe 2, while the amplitude of the resonant peak is affected by the absorption of energy by the material within the pipe. Thus, the region between the input 4 and output 5 acts as an attenuator, with the degree of attenuation varying in dependence on the relative 10 proportions of the fluid flowing through the pipe 2. In practice, it has been found that the attenuation can be as high as 60-9OdB. Consequently, a high gain amplifier is conventionally employed to amplify the signal from the output probe. However, when the change in relative proportions of the fluids is small and, therefore, the change in output signal is correspondingly small, this signal change can become lost in the noise 15 inherent in such a high gain amplifier. Thus, the sensitivity of the device is low.

Apparatus constructed according to the invention is shown in Figure 2. This apparatus also comprises a coil resonator I helically wound around a section -2 of pipe. There is also a microwave energy source 6 and a microwave output detector 7, which includes an 20 amplifier 13 and analysing circuitry. The output detector 7 processes signals corresponding to the activity of the resonator 1.

In accordance with the invention, at least one of the energy source 6 and output detector 7 are physically connected to the coil resonator itself. In the embodiment shown in 5 P/62378.GBA/OIL Figure 2, both the energy source 6 and output detector 7 are connected to the coil resonator. The energy source 6 is connected to one end portion of the coil resonator I by means of a co-axial cable 8, via a first capacitor 9. The output detector 7 is connected to the other end portion of the resonator I by means of a co- axial cable 10, 5 via a second capacitor 11. In this manner, the attenuation between the input and output is much reduced, typically by at least I OdB. As a consequence, the gain of the amplifier 13, which amplifies the output signal, can be reduced by a similar amount. Thus, the amplifier noise level is reduced, resulting in a much improved output signal-to-noise ratio, This enables much smaller changes of output signal to be measured and results in 10 a device with sufficient dynamic range to allow the desired degree of accuracy of measurement of the fluid characteristics.

The capacitors 9, 11 are located close to opposite end portions of the coil resonator 1. A suitable value for the capacitors is in the range of six picofarads. The co-axial cables 8, 15 10 are screened in the region of the respective capacitors 9, 11. An electrically conductive insulated wire 12 extends between the ends of the cables 8, 10 and is joined to the respective screens. Thus, the apparatus shown in Figure 2 comprises a resonant circuit distinct from the prior art apparatus of Figure 1.

20 Referring now to Figure 3, the pipe 2 is arranged to carry a portion of the dissimilar fluids of interest. The pipe 2 is formed of plastics material, although any electrically insulating material could be used, A suitable plastics material for this application would be polyetheretherketone (PEEK) owing to its high resistance to corrosive substances and stability at high temperatures. The coil resonator I is arranged to be helically 6 P/62378.GBA/OIL wound around a section 14 of the pipe and is co-axial with it.

The pipe section 14 may be machined with a helical groove having the same dimensions as the resonator 1, so that the resonator is located on the outer surface by the groove. A 5 sleeve 15 is arranged to fit over the coil resonator 1. The internal diameter of the sleeve is chosen so that there is a slight interference fit with the external diameter of the resonator 1, thereby ensuring that the resonator is kept in place. This is especially useful in circumstances where the pipe may be susceptible to vibrations.

10 The apparatus of Figure 3) is mounted within a larger fluid extraction pipe (not shown).

Therefore, it is desirable to isolate the resonating circuit from the fluid surrounding the apparatus. Thus, the pipe section 14 incorporating the resonator is held within a metal tube 16. The metal tube 16 screens the apparatus within from external electromagnetic fields. The tube 16 also provides protection for the enclosed apparatus from mechanical

15 damage or corrosion, such as might be encountered in a sub-sea hydrocarbon well. 0 rings 17, 18 provide a seal between the ends of the metal tube 16 and the pipe 2.

The co-axial cables 8, 10 associated with the source of microwave radiation (not shown in this drawing) and the microwave output detector (also not shown in this drawing) are 210 introduced into the metal tube 16 via an aperture 19 sealed by a cable gland 20. A channel in the outer surface of the sleeve 15 is arranged to accommodate the cables 8, and the associated capacitors 9, 11. The insulated wire for connecting the screens of the cables 8, 10 is embedded in the sleeve 15 and is not shown in this drawing, The co axial cable 8 is connected to a first end portion of the resonator I via a first opening in 7 P/62378.GBA/OIL the sleeve 15; a second opening in the sleeve allows the cable 10 to be connected to the other end portion of the resonator 1.

The resonator I and capacitors 9, 11 are shown symbolically in Figure 4. The source 6 5 of microwave radiation and microwave detector 7 including amplifier 13 are also represented in this drawing. The source 6 of microwave radiation is an oscillator having an output variable across a range of frequencies. The microwave radiation is supplied directly to an end of the coil resonator 1 via the capacitor 9, A coupler 21 takes a portion of the input power to the resonator I and supplies it to the microwave detector 7 10 as a reference. The coil resonator 1 is connected, via capacitor 11, to the detector 7, which detects activity of the resonator. In operation in, for example a hydrocarbon well, a mixture of oil, water and gas flows through the pipe, including the section 14 having the coil resonator. A data analyser and controller, included in the circuitry comprising the microwave output detector 7, varies the frequency of the microwave source and 15 plots, for example, the output voltages derived from the output signal of the coil resonator 1. The output detector 7 could alternatively plot the output current, or any other characteristic of the output signal of the coil resonator 1.

The output power of the helical coil resonator I is plotted against input frequency to 20 produce the resonant peak for the resonator. A measurement of the attenuation of microwave energy provides an indication of the relative proportions of oil, water and gas in the fluid extraction pipe because those three fluids have very different permitivities, 8 P/62378.GBA/OIL It has been found that the resonant frequency of a coil resonator is more sensitive to the properties of material in the vicinity of the coil itself than it is to the properties of material in the centre of the pipeline. This is because the strength of the electric fields generated by the coil are stronger proximate the coil. Therefore, an advantageous 5 arrangement is that of a p. arality of coil resonators, such as that illustrated in GB T)71637. Each coil resonator has a larger circumference than the pipe. The coil resonators are arranged around the pipe so that the axis of each coil is offset from that of the pipe. The resonant peaks of the helical coil will be mainly influenced by the part of the fluid flow adjacent each coil and, since each coil is adjacent different part of the 10 fluid flow, this will allow a map of the distribution of oil, water and gas within the pipe to be produced if required. The number of coils used is a trade-off between increasing accuracy and sensitivity of the system by increasing the number of coils used, and the increase in the cost and bulk the system due to the use of more coils.

15 The system as hereinbefore described will only provide information on the relative proportions of gas, oil and water in the pipe. In order to determine the absolute volumes of the three fluids passing through the pipe. additional apparatus to determine the rate of' flow of fluid within the pipe would be needed. Such apparatus is well known in itself and need not be described in detail here.

The microwave oscillator could be replaced by an r.f. oscillator, Helical coil resonators also work at r.f. frequencies (50OMHz to I GHz); the microwave frequency band is from I GHz to 20GHz.

9 P/62378.GBA/OIL

Claims

1. Apparatus for measuring the relative proportions of dissimilar fluids in a pipe, the apparatus comprising: a coil resonator, helically wound, in use, around the pipe; a variable frequency source of electromagnetic radiation, arranged to send electromagnetic radiation into the coil; an electromagnetic detector arranged to detect electromagnetic radiation emitted by the resonator, which radiation is indicative of the relative proportions of the fluids in the pipe; characterised in that one of the source and the detector is physically connected to the resonator.

2. Apparatus as claimed in Claim 1, in which the physical connection is provided by means of a co-axial cable.

3. Apparatus as claimed in Claim 2, in which the end portion of the cable nearest the resonator is screened.

4. Apparatus as claimed in Claim 2 or 3, in which a capacitor is interposed between the co-axial cable and the resonator.

5. Apparatus for measuring the relative proportions of dissimilar fluids in a pipe, the apparatus comprising: a coil resonator, helically wound, in use, around the pipe; a variable frequency source of electromagnetic radiation, arranged to send electromagnetic radiation into the coil; an electromagnetic detector arranged to detect electromagnetic radiation emitted by the resonator, which radiation is indicative of the relative proportions of the fluids in the pipe; characterised in that the source is P/62378.GBA/OIL physically connected to an end portion of the resonator and the detector is physically connected to the other end portion of the resonator.

6. Apparatus as claimed in Claim 5, in which each physical connection is provided by means of a co-axial cable.

7. Apparatus as claimed in Claim 6, in which the end portions of the cables nearest the resonator are screened.

8. Apparatus as claimed in Claim 7, in which the screens of each cable are joined.

9. Apparatus as claimed in Claim 8, in which the screens are Joined by means of an insulated, electrically conductive wire.

10. Apparatus as claimed in any one of Claims 6 to 9, in which capacitors are interposed between the ends of the respective cables and the resonator.

11. Apparatus as claimed in any preceding claim, further comprising a screen for the resonator.

12. Apparatus as claimed in claim 11, in which the screen comprises an electrically conductive tube.

I I Apparatus as claimed in claim 12, in which the end portions of the tube are sealed to the pipe.

11 P/62378.GBA/OIL 14. Apparatus as claimed in any preceding claim, in which the detector includes an amplifier.

15. Apparatus as claimed in any preceding claim, in which the electromagnetic radiation is microwave radiation.

16. Apparatus as claimed in any preceding claim, in which the electromagnetic radiation is r.f. radiation.

17. Apparatus, substantially as hereinbefore described, with reference to, or as illustrated in, Figures 2 to 5 of the accompanying drawings.

18. A pipe incorporating apparatus as claimed in any preceding claim.

19. A hydrocarbon well incorporating apparatus as claimed in any one of Claims I to 18.

20. A method of measuring the relative proportions of dissimilar fluids in a pipe, comprising: sending electromagnetic radiation, by means of a variable frequency source of electromagnetic radiation into a coil resonator helically wound around the pipe; detecting electromagnetic radiation emitted by the resonator, by means of a detector, which radiation is indicative of the relative proportion of fluids in the pipe; wherein at least one of the source and the detector is physically connected to an end portion of the resonator.