GB2276237A - Multiphase flow monitor - Google Patents

Abstract

An apparatus and a method are provided for monitoring the contents and flow rate of a multiphase oil pipeline. The apparatus (10) includes a neutron tube (14) as a controllable source of fast neutrons, and two NaI(TI) scintillator units (24, 18), one to detect prompt gamma rays, and one to detect gamma rays from <16>N nuclei generated by activation of oxygen, the latter being several metres downstream. The tube (14) operates in two modes: continuous, to enable the spectrum of prompt gamma rays to be measured; and brief operation, to activate a discrete portion of fluid and so to enable the velocity and oxygen concentration to be determined from signals from the downstream detector (18, 21, 22). The latter signals are used to compensate for the poor resolution of the scintillator (24) used to measure the gamma spectrum during continuous operation, thus allowing the composition of the fluid to be determined accurately. <IMAGE>

Description

Multiphase Flow Monitor This invention relates to an apparatus and to a method for monitoring a multiphase flow.

The use of neutron interrogation and/or neutron activation, in conjunction with gamma detection, has been suggested for monitoring the contents of multiphase oil pipelines, and for flow measurement in such pipelines.

For example EP-A-O 007 759 suggests the use of neutron irradiation, and measurement of gamma rays with a NaI(Tl) scintillation detector, to assess the water content of crude oil. It suggests the detection of prompt y-rays arising from inelastic scattering of neutrons by 12C at 4.43 MeV, y-rays from neutron capture by 1H at 2.223 MeV, y-rays of energy 6.13 MeV due to 160 (either arising promptly from inelastic scattering, or as a result of the decay of 16N nuclei produced by fast neutron capture), and 7-rays arising promptly from the capture of thermal neutrons by 35C1 nuclei at 6.111 MeV.

Such measurements are not generally satisfactory if more than one element is to be detected because the poor resolution of a scintillation detector means in particular that 7-rays from oxygen and chlorine at 6.13 and 6.11 MeV cannot be distinguished, and indeed these spectral lines may overlap. J.P. Nelson in "Neutron Activation for On-Stream Elemental Analysis" (Advances in Instrumentation 40 (1985), pp.1407-1433) discusses the use of neutron activation and prompt neutron analysis techniques. Where the concentration of a single element is to be measured, neutron activation and subsequent gamma ray detection with a NaI(Tl) detector may be used.

The other technique involves detection of prompt gamma rays using high resolution detectors which can measure the complex gamma ray emission spectra. The specified detector is a liquid-nitrogen cooled, high-purity germanium detector, and it is said to be preferred over a NaI detector because of its ability to resolve gamma rays with only slightly different energies. Such HPGe detectors are said to be used in applications where it is necessary to determine several elements whose gamma emissions are close in energy. Analysis of limestone and of coal is discussed. US 4 795 903 suggests using in the context of oil pipeline analysis, a cooled, high-purity, n-type germanium spectrometer; this may be expected to overcome the problem of lack of resolution, however the requirement to cool the spectrometer continuously is a considerable complication.US 4 795 903 also teaches the use of a pulsed fast neutron source in conjunction with a gamma detector spaced along the pipeline to measure the flow velocity of the fluid within the pipeline. The above techniques of determining the pipeline contents have thus not proved entirely satisfactory, and a reliable technique to achieve this end would be advantageous.

According to the present invention there is provided a method for monitoring the contents of a pipeline carrying a fluid, the method comprising irradiating the fluid at a region of the pipeline with fast neutrons from a fast neutron generating tube so as to generate at least one discrete portion of the fluid containing activated nuclei, detecting with a scintillation detector gamma rays emitted by the activated nuclei from a region of the pipeline downstream of the irradiation region, and providing first signals representing the detected gamma rays; irradiating the fluid with fast neutrons from a fast neutron generating tube, detecting with a scintillation detector prompt gamma rays emitted from fluid in the irradiation region during a prolonged irradiation time, and providing second signals representing the energy spectrum of the prompt gamma rays; using the first signals to determine the velocity of the discrete portion of the fluid; and using the first signals in combination with the second signals to determine the composition of the fluid.

In the preferred method the fluid may comprise oil, water and gas, the method using a single fast-neutron generating tube, and two scintillation detectors adjacent to the pipeline to detect gamma rays emitted by nuclei within the fluid, the first detector being spaced along the pipeline from the irradiation region in the downstream direction so as to provide the first signals, and the second detector being close enough to the irradiation region to detect prompt gamma rays, and so to provide the second signals; the method involving determining from the first signals the velocity of the discrete portion of the fluid, and also the concentration of oxygen nuclei in the fluid; and determining from the second signals and also from the oxygen nuclei concentration the concentrations of carbon and hydrogen nuclei in the fluid.

The prolonged irradiation time is preferably at least one hour, and may be as long as 15 or 24 hours, most preferably between 5 and 15 hours; these times presume a mean neutron flux from the tube of 108 sec-l, and for a more intense neutron flux, shorter time periods can be used. During the prolonged irradiation it is thus preferred that the neutron tube should irradiate the pipeline with, in total, at least 3.6 x 1011 neutrons, and more preferably between 1.8 and 5.4 x 1012 neutrons. The spectrum of gamma energies is hence related to the average concentrations of the relevant nuclei over the said prolonged time, so it will be appreciated that this approach prevents rapid fluctuations in concentrations from being detected.Furthermore the counting statistics are considerably improved by measuring over a longer period, as the numbers of gamma rays at each energy are thereby increased. The prolonged irradiation time may be achieved by operating the neutron tube continuously for the said time, or may comprise a plurality of periods of continuous irradiation separated by periods in which no irradiation occurs. Indeed the neutron tube may operate in a pulsed mode throughout the prolonged irradiation, for example generating 1000 pulses per second, each of duration 25 Rs, and 105 neutrons per pulse. Because of the improved counting statistics, the poor resolution of the scintillation detectors is at least partly overcome.

The discrete portion of fluid containing activated nuclei can be produced by operating the neutron tube for just a brief interval, for example between O.ls and 4s; the preferred arrangement is to operate it for between 0.5s and l.Os at 108 neutrons per sec. As exemplified above, the neutron tube may operate in a pulsed mode throughout this brief interval. Since the fluid velocity is to be determined from the time of receipt by the first detector of gamma rays from the activated nuclei, this irradiation interval must be short compared to the time interval between the irradiation and the subsequent detection. It will also be appreciated that the counting statistics can be improved by combining the first signals corresponding to a plurality of successive brief irradiations.

The most significant elements in the fluid are hydrogen, carbon, oxygen and, if the water is saline, chlorine. The effects of neutron irradiation on these elements are indicated in Table 1. Those reactions requiring thermal neutrons are indicated by T after the value of the cross-section; those requiring fast neutrons are indicated by an F, and in these cases the stated cross-sections are those at 14 MeV.

Table 1 Nuclear Detection Reactions

Cross Gamma Gamma Decay Element Reaction Section Energy Fraction Half (mb) (MeV) (%) Life Hydrogen 1H(n,y)2H 332 T 2.223 100 prompt Carbon 12C(n,)13C 3.4 T 4.945 68 prompt 3.684 26 prompt 12C(n,n')12C 250 F 4.439 100 prompt prompt Oxygen 16O(n,p)16N 35 F 6.130 68 7.2 s 7.117 5 7.2 s 16O(n,n')16O 250 F 6.130 68 prompt 7.117 5 prompt Chlorine 35C1(n,y)36C1 43000 T 6.111 20 prompt 6.620 8 prompt 7.414 10 prompt 7.790 8 prompt The first detector need only provide an output signal corresponding to the 6.13 MeV gamma rays from 16N, and this may be assured by use of a single-channel pulseamplitude selector. The second detector is required to determine the intensities of gamma lines produced promptly, for example those at 2.223 MeV, 4.439 MeV, 6.110 MeV and 6.130 MeV emitted by hydrogen, carbon, chlorine and oxygen respectively.

The prompt gamma spectrum is rendered more complex by additional lines, which are a measurement artefact due to single escape and double escape, at 0.511 MeV less and at 1.022 MeV less than the values of gamma energy listed in the Table; there are also other emission lines not listed in the Table. All the lines appear above a background generated by Compton scattering of gamma rays of higher energy. The carbon line at 4.439 MeV is difficult to analyse because the line is broad because of the Doppler broadening which arises from the comparatively short decay time of the excitation state, and also because chlorine lines at 4.473 MeV and 4.491 MeV are close to it. The scintillation detector cannot distinguish these lines. Equally the chlorine line at 6.110 MeV cannot be distinguished by the scintillation detector from the oxygen line at 6.130 MeV.

The neutron tube preferably generates fast neutrons of energy above 5 MeV, more preferably above 10 MeV, for example of energy 14 MeV. The tube may be provided with a moderator (though not one containing any of the elements of interest in the pipeline) e.g. beryllium, to ensure the pipeline contents are irradiated by both fast and thermal neutrons. The fluid in the pipeline also acts as an effective moderator, although it is clear that its effect on the neutron spectrum within the pipeline depends on its composition: the liquid phases are much better moderators than the gas phase, and although oil and water have similar moderating properties the thermal neutron flux is strongly depressed in saline water by the large thermal neutron capture cross section of chlorine.

It will also be appreciated that the cross-sections of the reactions whereby gamma rays are emitted vary with the neutron energy; for example the cross-sections of the two oxygen reactions in the Table are significantly larger at 14 MeV than at 5 MeV.

The number of 6.13 MeV gamma rays detected by the first detector, as a result of passage of a discrete portion of fluid containing 16N nuclei, depends on the water content (and so the concentration of oxygen nuclei) and also on the flow velocity. At low velocities this discrete portion of fluid will take longer to move past the first detector than it will at higher velocities, which tends to increase the number of gamma rays detected; however at low velocities it will take longer to flow along the pipeline to reach the vicinity of the first detector, so more of the 16N nuclei will have decayed before they reach the detector, which tends to reduce the number of gamma rays detected.Nevertheless from measurements of the time of receipt of these gamma rays and from the number detected it is possible both to determine the velocity of the portion of the fluid, and to obtain an indication of the oxygen concentration.

The present invention also provides an apparatus for performing the method of the invention.

The invention will now be further described by way of example only and with reference to the accompanying drawing which shows a block diagram of an apparatus 10 for analysing the contents of a pipeline 12 carrying a flow of crude oil, connate water (which is saline), and natural gas (principally methane), flowing in the direction of the arrow.

The apparatus 10 includes a 14 MeV neutron generator 14 connected to a power supply unit 16. By way of example a suitable generator 14 is the Sodern GNT 02, in which the generator 14 is connected by a 15 m long cable to the power supply/control unit 16. The generator 14 is arranged so as to irradiate the pipeline 12 and its contents with 14 MeV fast neutrons.

Several metres downstream of the irradiated region of the pipe 12 is a gamma detector unit 18 consisting of six cylindrical 3 inch x 3 inch (75 mm x 75 mm) thallium doped sodium iodide scintillators 19 placed close to the pipeline 12 with their longitudinal axes parallel to the longitudinal axis of the pipeline 12, arranged as three pairs; in each pair the two scintillators 19 are end to end with their flat surfaces touching, and each scintillator 19 having a photomultiplier 20 at its other end. The photomultipliers 20 of the detector unit 18 supply electric signals via a single channel pulse amplitude selector 21, to a multi-channel analyser operated in multi-channel scaler (MCS) mode 22. The selector 21 allows only those signals corresponding to a gamma ray energy of 6.130 MeV to pass.The multichannel scaler 22 operates as a counter and timer recording the numbers of gamma counts received in each of 4096 successive intervals, referred to as dwell times. The scaler 22 may be arranged to start operation in synchronism with the neutron generator 14; and the dwell time can be adjusted so that for example gamma rays from the irradiated fluid are received in about the 2000th interval.

Adjacent to the part of the pipeline 12 irradiated by the neutron generator 14 is another array of three NaI(Tl) scintillators 24; they have their longitudinal axes extending radially, in a plane at right angles to the neutron flux, and are shielded from any direct gamma rays emitted by the neutron generator 14 by a 150 mm thick lead shield 26. The photomultipliers associated with the NaI(T1) scintillators 24 provide signals to a pulse height multichannel analyser 28. This determines the pulse height for each signal (which corresponds to the energy of the detected gamma ray), and records the numbers of signals of each height. This is equivalent to a pulse height (or gamma energy) spectrum. Data from the multichannel scaler 22 and from the pulse height analyser 28 are supplied to a microcomputer 30.

The apparatus 10 is arranged to operate in two different modes, and the change between one mode and the other may be in response to control signals from the microcomputer 30 to control operation of the power supply 16, and of the scaler 22 and the analyser 28. In the first mode the neutron tube 14 is energised for one second, so as to activate nuclei in a discrete portion of the contents of the pipeline 12. The neutron tube 14 might for example, during that one second, produce 500 pulses of 14 MeV neutrons each of duration 100 Rs, with 2 x 105 neutrons per pulse. These fast neutrons lead to activation of the water phase by the 160 (n,p)16N reaction. As the fluid flows along the pipeline 12 the activated nuclei decay with a half-life of 7.2 s; the gamma rays emitted during decay are detected by the unit 18 as the activated portion of fluid passes it.The data recorded by the multichannel scaler 22 are indicative of both the flow rate of the water, and also of the oxygen concentration. The data is transmitted to the microcomputer 30.

It will be appreciated that the accuracy of the data may be improved by repeating this measurement. The data for several successive brief activations of the fluid may be accumulated in the scaler 22; or alternatively the scaler 22 may transmit its data to the microcomputer 30 after each measurement, and be reset to zero. It is important that the scaler 22 should start to record data in synchronism with each one second brief generation of neutrons (or at any rate at a known time after generation of the neutrons). This may be achieved by control signals; alternatively it has been found that the intensity of prompt gamma rays emitted during neutron irradiation is so large that a clear signal is emitted by the detector unit 18, and this signal may be recorded by the scaler 22 and used as a timing reference.

In the second mode of operation the neutron tube 14 is energised for a prolonged period, for example 12 or 15 hours, and produces for example 108 neutrons per second throughout that time. It might for example generate 100 pulses per second, each of duration 10 Rs, with 106 neutrons per pulse. The array of scintillators 24 provides signals to the pulse spectrum analyser 28.

Hence the data supplied to the microcomputer is indicative of the prompt gamma ray spectrum from the fluids in the pipeline 12. Nuclei in the fluid are interrogated both by fast neutrons from the neutron generator 14 and also by thermal neutrons as a result of passage of the fast neutrons through moderating material, in particular the oil and water in the pipeline 12.

Hence, as discussed earlier in relation to the Table, gamma rays of distinctive energies are emitted by hydrogen, oxygen, carbon and chlorine nuclei.

From the signals received from the multichannel analyser 22 the microcomputer 30 derives the water velocity, and hence determines from the total count, the concentration of oxygen in the pipeline 12 (principally the oxygen is in the water phase). This value of oxygen concentration can then be used to interpret the data received from the pulse spectrum analyser 28. The portion of the spectrum in the vicinity of 6.1 MeV is due to oxygen and chlorine; the value of the oxygen concentration enables the chlorine signal strength to be determined. The portion of the spectrum in the vicinity of 4.45 MeV is due to carbon and chlorine; the value of the chlorine signal strength enables the carbon signal strength to be determined. The portion of the spectrum in the vicinity of 2.2 MeV is due to hydrogen. Hence the concentrations of hydrogen, carbon, chlorine and oxygen can be determined.

At least as an initial approximation it can be assumed that the phase configuration in the pipeline 12 does not affect the relationship between the gamma count from an element, and the concentration of that element.

Typically it can be assumed that the oil phase is (CH2) n the gas phase is CH4, the water (H2O) is of known salinity, and the densities of the oil and of the water are known. The concentrations of chlorine and of oxygen are both due to the presence of water, so the volume fraction Vw of water can be determined from say the oxygen concentration and checked from the chlorine concentration.

Then the volume fractions V and densities p of the oil, water and gas (indicated by the subscripts o, w and g respectively) are related to the elemental concentrations m by: mc = VOPO 12 + Vgpg 12 14 16 mH = VoPo 2 + VgPg 4 + Vwpw 2 14 16 18 and 1 = V0 + Vg + Vw Hence the volume fractions of oil and of gas, and the density of the gas, can be calculated. If desired these values can then be used to give better estimates of the neutron spectrum in the pipeline 12 and so of the elemental concentrations. The above-outlined calculations can then be reiterated to improve the accuracy of the determination.

Claims (10)

Claims

1. A method for monitoring the contents of a pipeline carrying a fluid, the method comprising irradiating the fluid at a region of the pipeline with fast neutrons from a fast neutron generating tube so as to generate at least one discrete portion of the fluid containing activated nuclei, detecting with a scintillation detector gamma rays emitted by the activated nuclei from a region of the pipeline downstream of the irradiation region, and providing first signals representing the detected gamma rays; irradiating the fluid with fast neutrons from a fast neutron generating tube, detecting with a scintillation detector prompt gamma rays emitted from fluid in the irradiation region during a prolonged irradiation time, and providing second signals representing the energy spectrum of the prompt gamma rays; using the first signals to determine the velocity of the discrete portion of the fluid;and using the first signals in combination with the second signals to determine the composition of the fluid.

2. A method as claimed in Claim 1 wherein the prolonged irradiation time is at least one hour.

3. A method as claimed in Claim 1 or Claim 2 wherein the prolonged irradiation time is of sufficient duration, and the fast neutron flux of sufficient magnitude, that the pipeline is irradiated with at least 3.6 x 1011 neutrons.

4. A method as claimed in Claim 3 wherein the pipeline is irradiated with between 1.8 and 5.4 x 1012 neutrons.

5. A method as claimed in any one of the preceding Claims wherein the discrete portion of fluid is produced by irradiating the fluid for a brief period, for example between 0.1s and 4.0s.

6. A method as claimed in Claim 5 wherein the brief period is between 0.5s and 1.0s.

7. A method as claimed in any one of the preceding Claims wherein during both the prolonged irradiation, and the generation of the discrete portion of the fluid, the neutron generating tube is operated in a pulsed mode generating between 100 and 10000 pulses per second, for example 1000 pulses per second.

8. A method for monitoring the contents of a pipeline carrying a fluid, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

9. An apparatus for performing the method as claimed in any one of the preceding Claims.

10. An apparatus as claimed in Claim 8 wherein the scintillation detectors comprise thallium-doped sodium iodide scintillators.