ANALYSIS OF FLUIDS USING A SOURCE OF FAST NEUTRONS

Abstract: A method and apparatus are provided for analysing a fluid, especially an oil/water/gas mixture of unknown com­ position flowing in a pipeline (10). The apparatus can operate remotely, for example on the sea bed. A gamma-ray source (12) and a diametrically arranged scintillator (20) enable the density to be determined; while a fast neutron source (14) and a diametrically arranged germanium gamma spectrome­ ter (22) cooled by a cryo-cooler (26) enable the concentrations of the elements present to be determined. The apparatus en­ ables the weight fractions of oil, water and gas to be derived.

Description: The invention relates to an apparatus for analysing a fluid, in particular but not exclusively that flowing from an oil well, to determine its density and composition. The extraction of crude oil from natural reservoirs is normally accompanied by the release of formation water. The relative water output from new wells is usually very low but after a delay, the duration of which is itself of some interest, a significant water content (say 2%) appears. Thereafter this fraction gradually increases to 90% or above. In recent years the use of secondary and tertiary recovery procedures has resulted in a dramatic increase in water content. Thus the fluid emerging from an oil well typically comprises a mixture of crude oil, water, and natural gas (for example methane). The management of an oil well requires that the water, oil and gas fractions should all be known and, if separate wells feed into a common pipeline, measurement of the water, oil and gas fractions at each well is required for effective control to be exercised. For example, the output of oil and gas determine the economics of the well, whereas rapid increases in the water content may indicate a need to modify the perforation pattern in the well. The gas to oil ratio is normally constant over a period of years in a water-controlled well, but in a gas-controlled well it can change rapidly, by a factor of more than ten over a period of five years. For under-sea wells the measurement of the water, oil and gas ratios can be made by providing a pipeline from each well to a nearby platform, but this can be expensive. Hence an aim of the present invention is to enable those ratios to be measured at the well-head, even where the well-head is under the sea. Nuclear techniques are known for analysis of materials. For example JP 59-77346A (Nippon) describes an analytic method in which gamma-ray permeability of a material is used to assess its bulk density; fast neutron transmission through the material is used to indicate water content; and thermalized neutrons diffuse into the material, the detection of capture gamma-rays enabling the concentrations of each element to be determined. An article by J.B. Nelson in Advances in Instrumentation 40 (1985) describes the use of neutron activation for on-stream elemental analysis. Both delayed gamma emission and prompt gamma emission techniques are described, used for assessing the silicon concentration in iron ore, calcium (oxide) concentration in limestone, and the carbon and sulphur concentrations in coal, amongst others, and with a liquid nitrogen cooled, high purity germanium detector to observe the gamma spectrum. In every case however the quantity to be determined is an elemental concentration; the article states that this technique cannot be used to measure the moisture content of coal, because the element hydrogen occurs both in the water and in the coal itself. US 4 314 155 (A.A.E.C./Sowerby) describes such a neutron activation technique used for elemental analysis of coal, and asserts that moisture content can be discovered from the intensities of gamma rays emitted by carbon and by hydrogen, by assuming a value for the carbon/hydrogen ratio in the coal matter itself. The use of neutron irradiation and gamma spectroscopy to determine elemental ratios in a two component oil/water mixture is mentioned in EP 0 007 759. In the present context however, elemental concentrations themselves are not of interest; the proportions of oil, water and natural gas are to be determined, yet there is an overlap of elements between these three components, with hydrogen occurring in all three components, and carbon in two components. The present invention accordingly provides an apparatus as defined in claim 1. Preferred embodiments of the invention are defined in the dependant claims. An apparatus of the invention may be operated remotely for a year or more, and so may be installed at a remote or under-sea well-head. The invention will now be described by way of example only, and with reference to the accompanying drawing, which shows a diagrammatic view of an apparatus for analysing a fluid in a pipe. Referring to the drawing, there is shown a 200 mm diameter pipeline 10 through which an oil/water/gas mixture is flowing in the direction of the arrows. Mounted on the outside of the pipeline 10, but spaced apart from each other, are a ¹³⁷Cs gamma ray source 12, a ²⁴¹Am/Be fast neutron source 14, and a pulsed neutron tube 16 connected to a power supply unit 18. Diametrically opposite the gamma ray source 12 is a sodium iodide scintillator 20; and diametrically opposite the fast neutron source 14 is a high-purity, n-type germanium, high-resolution, gamma spectrometer 22. The gamma source 12 and the scintillator 20 are located by a two-part close-fitting collar (not shown) around the pipeline 10; while the fast neutron source 14 and the spectrometer 22 are located by a similar close-fitting collar (not shown) which excludes sea-water from the region undergoing analysis, and which contains a moderator for the fast neutrons, so as to provide some thermal neutrons. Signals from the spectrometer 22 and the scintillator 20 are supplied to a microcomputer 24. The spectrometer 22 can be cooled, during operation, by means of a heat pump 26 (shown diagrammatically in broken lines) to a temperature of about 100K. It can also be heated to a temperature of about 370K, at intervals, by an electric heater 28 connected to a power supply 30 and controlled by the microcomputer 24. A second sodium iodide scintillator 32 is arranged two metres downstream of the neutron tube 16, and is connected to a velocity-calculating microcomputer 34. A multiplex unit 36 receives output signals from the two microcomputers 24 and 34, and transmits that data about every 15 minutes. Operation of the apparatus is as follows. The ¹³⁷Cs gamma source (whose half-life is 30 years) emits gamma rays which pass through the pipeline 10 and its contents. The scintillator 20 provides an indication of the intensity of gamma rays after their passage through the pipeline 10 and its contents, and the attenuation brought about by the pipe contents enables the density of the pipe contents to be determined by the microcomputer 24. The ²⁴¹Am/Be neutron source 14 (whose half-life is over 400 years) emits fast neutrons of energy up to about 10 MeV. The pipeline 10 and its contents are therefore irradiated by fast neutrons and by thermal neutrons, as some of the neutrons are moderated by their passage through the moderator in the collar, and indeed through the pipe wall and the oil/water/gas mixture. Fast neutrons bring about activation of oxygen and carbon atoms by (n, n' γ ) reactions, and gamma rays of respective energies 6.1 MeV and 4.4 MeV are consequently emitted. Hydrogen atoms may undergo thermal neutron capture, that is an (n, γ ) reaction, with the consequential emission of a gamma ray of energy 2.2 MeV. The germanium spectrometer 22 is held at a temperature of 100 K by the heat pump 26 which is based on the Gifford-McMahon closed-cycle cooler. This cryo-cooler operates in a cyclic thermodynamic process and employs a compressor with an after-cooler heat exchanger which compresses the working gas and then removes the heat of compression, together with an expander with a reciprocating displacer element and a regenerative heat exchanger that removes heat from the spectrometer 22. The intensity of gamma rays of energies 6.1 MeV, 4.4 MeV and 2.2 MeV as detected by the spectrometer 22, along with the known cross-sections for the excitation reactions, enable the elemental concentrations of oxygen, carbon and hydrogen respectively to be calculated by the microcomputer 24. Along with the measured value of density, and the known stoichiometry of the oil and gas, these enable the proportions of oil, water and gas to be determined. This data is provided as an output signal to the multiplex unit 36. The microcomputer 24 also monitors the energy resolution of the spectrometer 22, which gradually decreases due to the damage caused by the fast neutron flux to which the spectrometer 22 is itself exposed. When it reaches an unacceptably low level, the heat pump 26 is switched off and the power supply 30 energised to heat the germanium spectrometer 22 to about 370K for a period of a few tens of hours. This re-anneals the germanium. While re-annealing is being performed no analysis of the pipe contents can be carried out, but re-annealing need only be performed once for several months of operation. The neutron tube 16 is energized about once every 15 minutes (deuterons being accelerated through 70 kV onto a tritium-in-titanium target) to emit a pulse of very fast neutrons of energy 14 MeV. These irradiate the contents of the pipeline 10, and in particular bring about ¹⁶O (n,p) ¹⁶N reactions, for which the threshold energy is 10 MeV. The nitrogen-16 atoms decay with a half-life of a few seconds and emission of gamma rays; the time interval between energizing the tube 16, and the detection of a pulse of gamma rays by the scintillator 32 enables the flow velocity of the fluid to be detected. The distance that the scintillator 32 is downstream of the neutron tube 16 is arranged so that, at normal flow-rates, the pipe contents take about two seconds to flow from the neutron tube 16 to the scintillator 32. The flow velocity is calculated by the microcomputer 34 and provided as an output signal to the multiplex unit 36. Thus the data transmitted by the multiplex unit 36 consist of the values of density, velocity, and oil/water/gas proportions measured every 15 minutes. It will be appreciated that because of the long range of neutrons and gamma-rays, the techniques utilized by this apparatus are sensitive to the total volume within pipes having diameters up to about 250 mm so that the problem of achieving a representative sample is largely removed. Except when the highest accuracies are required, the sampled volume is reasonable even when the water is in a non-dispersed state in the oil. Higher accuracies can be achieved if some dispersion of the fluid components can be induced, and for pipes of diameter greater than 250 mm the fluid components must be dispersed to ensure that the sampled volume is representative. Furthermore, because the fast neutron source 14/spectrometer 22/microcomputer 24 assembly enables elemental concentrations to be derived, the effects of interfering elements, such as carbon in the steel pipe can be allowed for. Measurement of other elements, such as silicon and calcium, can be used to give information on the transmission or build-up of solid material in the pipe. Chlorine, sulphur and nitrogen concentrations can also be derived, if desired. In a modified form of the apparatus (not shown) two scintillators 32 are provided, spaced along the pipeline 10. The signals from the two scintillators are cross-correlated. This enables the flow velocity to be measured while using a lower intensity neutron pulse from from the tube 16, and this can be expected to increase the lifetime of the tube 16.

Claims: An apparatus for determining the proportions of oil, water and natural gas flowing in a pipeline (10) in a marine environment, comprising means for attaching a source (12) of gamma-rays to the pipeline, means for attaching a gamma-ray detector (20) to the pipeline (10) opposite the source (12) of gamma-rays, means (20) for producing a first electrical signal indicative of the attenuation of the gamma-rays by the fluid flowing through the pipeline, and hence indicative of the density of the fluid, means for attaching a source (14) of fast and thermal neutrons to the pipeline (10), means for attaching a gamma-ray spectrometer (22) to the pipeline, the gamma-ray spectrometer (22) being adapted to measure the intensities of prompt gamma-rays having energies of 2.2, 4.4 and 6.13 MeV arising from the excitation of hydrogen, carbon and oxygen, respectively, within the fluids in the pipeline (10), and providing a second electrical signal related thereto, the gamma ray spectrometer (22) comprising a high-resolution semiconductor spectrometer, and the apparatus including a heat pump (26) to cool the spectrometer to about 100 K during operation, the means for attaching the neutron source (14) and the gamma-ray spectrometer (22) to the pipeline (10) being adapted to exclude water from the region outside the pipeline adjacent to the neutron source and to the gamma-ray spectrometer, and means (24) responsive to the first and second electrical signals to determine, from the said signals and from the stoichiometry of the oil, the water, and the gas, the proportions of oil, water and gas in the fluid flowing through the pipeline (10). An apparatus according to claim 1 wherein there is included means for measuring the velocity of the fluid flowing in the pipeline, comprising means (16) for providing brief pulses of very fast neutrons at regular intervals for exciting atoms in the fluid in the pipeline and a gamma-ray detector (32) situated downstream of the source (16) of neutron pulses by a known distance and arranged to detect gamma-rays from the excited atoms, so as to measure the time taken for the excited atoms to flow from adjacent the source (16) to adjacent the detector (32), and hence to determine the velocity of the fluid flowing in the pipeline (10). An apparatus according to claim 1 or claim 2 wherein the gamma-spectrometer (22) comprises a germanium spectrometer and there is also provided means (28) for periodically heating the spectrometer to a temperature of about 370 K to anneal it.

Inventors: CLAYTON, COLIN GEOFFREY
Applicants: UNITED KINGDOM ATOMIC ENERGY AUTHORITY
Application year: 1986
Main classification: G01N23
IPC codes: G01N23, G01N9, G01F1, G01P5