Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/704—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow using marked regions or existing inhomogeneities within the fluid stream, e.g. statistically occurring variations in a fluid parameter
  • G01F1/7042—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow using marked regions or existing inhomogeneities within the fluid stream, e.g. statistically occurring variations in a fluid parameter using radioactive tracers
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/74—Devices for measuring flow of a fluid or flow of a fluent solid material in suspension in another fluid

Definitions

• the present invention relates to a method of monitoring a fluid flow within a vessel, particularly in a vessel containing a multiphase fluid such as an oil separator.
• Oil/water/gas separation can be a very complex process because the fluid's separation properties vary from field to field and from well to well. It is therefore a great challenge to fully understand the fluid flow process in oil separators. Tracer techniques and other diagnostic methods have been on the market for a long time, and they help the process team to understand what is going on in the separator. A tracer study is typically carried out by placing several radiation detectors outside the separator unit being investigated. A radioactive tracer is then injected upstream of the separator during normal operation. By plotting the radiation intensity detected by each detector as a function of time, one can get information about how the tracer flows through the system while the process is running as normal.
• WO-A-0022387 describes a method of measuring a density profile of a multi-phase system and measurement systems for determining the boundaries between phases, especially where the boundaries between at least three phases have to be located such as gas-oil and oil-water boundaries in separation vessels in oil production installations.
• the measurement systems comprise a density profiler comprising an axially distributed array of radiation sources capable of providing at least 10 collimated beams of ionising radiation; an axially distributed array of radiation detectors, each detector being associated in use with a respective one of the said beams of ionising radiation and producing an output signal in response to the incidence of the ionising radiation; and means for analysing the detector output signals to determine the density of the medium traversed by the beams of radiation in passing from the source array to the detector array.
• the radiation sources are normally a relatively low-energy gamma source, such as 241 Am so that the distance between the source and detector may be kept relatively small (typically up to 15 cm) allowing the apparatus to be installed within a vessel through an existing port.
• This method and apparatus is particularly useful for measuring a density profile of the multi-phase liquids within an oil separator of the type used to separate oil, water and gas in the oil and gas recovery industry.
• the apparatus is deployed in use with the array of sources and respective array of detectors within the fluids in the vessel so that the detectors can monitor the radiation from a respective source which traverses the fluid between the source and detectors.
• the radiation is attenuated by the fluid to an extent which is proportional to the density of the fluid.
• the density profiler described in WO-A-0022387 is effective in providing a measurement of the fluid density and locating the fluid interphase boundaries in a multi-phase fluid. It cannot, however provide detailed information about the flow of fluid within a vessel. In many situations it is desirable to monitor the flow of fluids within vessels and pipelines. In oil separation vessels, for example, information about the flow patterns in a multi-phase oil, water and gas mixture may be used to control the fluid input and output or phase separation methods such as the use of foam- breaking or emulsion-breaking chemicals to optimise the efficiency and speed of separation.
• a method of monitoring a fluid flow within a vessel comprises adding a radioactive tracer compound to said fluid and monitoring the radiation emitted from said tracer by detecting the radiation using at least one radiation detector disposed within said vessel.
• the fluid may comprise a single phase or more than one phase.
• the method of the invention is useful for monitoring fluid flows in multiphase fluid mixtures.
• the fluid comprises a multi-phase mixture of oil, water and gas.
• the radioactive tracer material may be miscible or immiscible with the fluid.
• the tracer material When the tracer is immiscible with the fluid then it is of a suitable particle size to be dispersed within the fluid to be monitored in such a way that the particles of tracer are carried with the fluid flow.
• the tracer material may be treated with a surface modifying composition in order to modify the dispersibility or miscibility of the tracer material with the fluid.
• a separate tracer material may be provided to be miscible with each of two or more of the phases. In this case, the tracer materials may be separately treated each to be miscible with a selected one fluid phase. More than one tracer material may be added to a single fluid if required.
• the tracer materials may be provided in a tracer composition containing more than one radioactive tracer material.
• the tracer composition preferably contains said more than one tracer material in a known ratio so that the composition provides an identifiable tracer mixture.
• Suitable radioactive tracer materials include H-3, Na-24, Ar-41 , Kr-79, Kr-85, Br-82, La-140 and Sn/ln-113m.
• the tracers used preferably have a relatively short half-life so that their environmental impact is limited so far as possible.
• the energies and half-lives of the preferred isotopes is:- H-3 (0.019 MeV, 12.33 years, beta particle), Na-24 (1.37 and 2.75 MeV, 15 hours, gamma), Ar-41 (1.29 MeV, 110 minutes, gamma), Kr-79 (0.26 and 0.39 and 0.6 MeV, 36 hours, gamma), Kr-85 (0.51 MeV, 10.7 years, gamma), Br-82 (0.55, 0.62 and 0.78 MeV, 35 hours, gamma), La-140 (47 and 10 keV, 22.3 years, gamma) and Sn/ln-113m (0.39 MeV, 99.5 minutes, gamma).
• tracers are all well known for use in the oil industry.
• the tracers (other than the noble gases) are typically formulated as compounds for adding to the process stream to be monitored.
• Suitable tracer compounds for the gas phase include tritiated water, krypton gas and argon gas.
• Suitable tracer compounds for aqueous fluids include include potassium bromide, sodium carbonate, and tin / indium generator indium trichloride.
• Organic phase-compatible tracers include bromododecane, 1 ,4 dibromobenzene, and 9 -bromophenanthrene.
• Lanthanum chloride may be used as a solid tracer.
• the appropriate tracer compound is selected according to the phase in which the tracer is intended to flow.
• the tracer compounds are generally made by irradiating non-radioactive compounds in a known manner.
• the tracer material(s) may be added to the fluid within the vessel or before the fluid enters the vessel.
• the tracers may be added to the fluid in the vessel or upstream of the vessel.
• the tracer compounds are added to the fluid according to known procedures.
• the tracer materials may be added manually or, preferably, using a fluid injection means for dispensing a pre-determined amount of tracer material into the fluid at the desired location.
• the injection means preferably comprises a double block and bleed arrangement and preferably is fitted with a non-return valve to ensure operator safety.
• the tracers may be dispensed into a dispensing cylinder which is pressurised and connected to the injection port together with a supply of high pressure nitrogen. The selected quantity of tracer compound is then blown into the fluid line at an elevated pressure using the nitrogen supply.
• the vessel may be a container such as a tank, having at least one fluid in-flow means and at least one fluid out-flow means.
• the vessel may comprise a storage vessel or a pipeline through which the fluid to be monitored is flowing.
• the vessel comprises an oil separator having at least one in-flow port through which a mixture of oil, gas and water (and usually solids such as sand) may flow into the separator and more than one out-flow port for the flow of one or more phases from the separator.
• the oil- separator vessel is usually a pressure-vessel, and normally incorporates baffles or other physical means for controlling the flow of fluid, to slow the flow and reduce turbulence thereby encouraging the separation of the oil and gas from the water.
• oil separators there are typically at least three phases present: oil, gas and an aqueous phase (sometimes brine) and often in effect a fourth phase of sand and/or relatively high molecular weight and density bituminous hydrocarbons commonly called asphaltines which can form a sludge at the bottom of the separator.
• asphaltines which can form a sludge at the bottom of the separator.
• asphaltines relatively high molecular weight and density bituminous hydrocarbons commonly called asphaltines which can form a sludge at the bottom of the separator.
• Oil separators may be located near the well-head, which may be on the sea-bed. Alternatively they may be located a greater distance from the well-head, e.g. at the surface such as on a platform, i.e. top-side.
• the radiation detector comprises any suitable type of radiation detector such as a Geiger-Muller (GM) tube or a scintillation counter, optionally linked with a photomultiplier.
• GM tubes are particularly convenient, because they are electrically and thermally robust and are available in mechanically robust forms.
• scintillation detectors linked to counters by fibre-optic links are particularly useful.
• the total electrical energy and power associated with the detector(s) is sufficiently low as not to be a significant source of ignition in the event of system failure (particularly resulting in direct contact between combustible or explosive materials and any electrically live components).
• the detector should be suitable for immersion in the fluid to be monitored or suitably protected from contact with the fluid.
• more than one detector is placed in the vessel.
• an array of detectors is used, preferably arranged in a linear spaced-apart relationship. Such an array may be positioned within the vessel along an axis of the vessel or otherwise. More than one array of detectors may be used within the vessel.
• the detectors are located within the body of the fluid to be monitored so that they may detect radiation from the radioactive tracer as it flows with the fluid in the vicinity of the detector. In some circumstances a detector may not be located within the body of the fluid, for example when the vessel is empty or not filled with fluid to a level at which the detector is immersed. It is, however, intended that at least one detector is immersed within the fluid.
• the detector(s) is usually placed in at least one dip tube or housing to provide a mechanical (pressure), chemical and, particularly for electrically powered detectors, an electrically insulating barrier between the components of the detector and the material being profiled.
• the housing may incorporate a portion which is made from a radiation transparent (or nearly transparent) material.
• the housing may include, as its radiation transparent portion (or “window") a portion made of relatively thin material compared with the material from which the "non-window" parts of the housing are formed.
• the housing may be a dip-tube.
• the material of the dip tubes will be chosen to have sufficient strength and chemical resistance and to be suitably transparent to the ionising radiation.
• a preferred material is titanium or an alloy thereof which is sufficiently strong at a thickness which is suitably transparent to the radiation. Where electrically powered detectors are used and the material of the dip tube is metallic a separate electrically insulating barrier will generally also be provided.
• the detectors incorporate or are coupled to a means for generating a signal, preferably an electrical signal to indicate the detection of radiation.
• the signal is processed by a suitable apparatus, i.e. a signal processing means, which is capable of converting the electrical signal to an output means for indicating the amount of radiation detected by any one detector.
• the apparatus may be programmable to calculate certain parameters of fluid flow in the vicinity of the detector(s) from the signal it receives and to provide output in the form of numerical data or diagrams, e.g. of flow patterns.
• the radiation detector(s) form a part of a density measurement apparatus, more preferably of the type described in WO-A-0022387.
• the density measurement apparatus comprises at least one radiation source and at least one radiation detector, the or each source being capable of providing at least one collimated beam of ionising radiation; the or each radiation detector being associated in use with a respective one of the said beams of ionising radiation and producing an output signal in response to the incidence of the ionising radiation, usually associated with means for analysing the detector output signals to determine the density of the medium traversed by the beams of radiation in passing from a source to a respective detector.
• the density measurement apparatus preferably comprises a plurality of radiation sources and a plurality of radiation detectors, the sources preferably being axially distributed in an array and capable of providing at least 10 collimated beams of ionising radiation; the radiation detectors preferably being axially distributed as an array, each detector being associated in use with a respective one of the said beams of ionising radiation and producing an output signal in response to the incidence of the ionising radiation together with means for analysing the detector output signals to determine the density of the medium traversed by the beams of radiation in passing from the source array to the detector array.
• This form of density measurement apparatus is useful to deploy within a vessel containing a multi-phase fluid, arranged so that the source and detector arrays extend through the phases of the multi-phase fluid so that monitoring the radiation received by the detectors provides a density profile of the multi-phase fluid.
• the density measurement apparatus may comprise more than one array of detectors.
• the detector array may include at least two columns of detectors, the columns of detectors preferably being radially angularly displaced from each other.
• the columns of detectors are located radially substantially equidistant from the array of radiation sources.
• the sources and detectors are placed in dip tubes, preferably made of titanium, and having a wall thickness of from 1 to 3 mm.
• the beam length i.e. the linear spacing between each detector and the corresponding collimated source through the medium whose density is being measured or profiled
• the minimum and maximum path lengths will also be determined by the operating environment.
• the minimum beam length will generally be about 2.5 cm to minimise the risk of blockage of the source/detector gap and the maximum beam length is not likely to be more than about 1.5 m or the profiler will be too large for practical use in in-line separators.
• the maximum beam length is limited by the need to have a detected signal above the noise floor of the system (dependent on source energy and intensity) and the minimum beam length by obtaining sufficient absorption to resolve density differences adequately (dependent primarily on beam energy).
• a high energy beam needs a longer minimum beam length than a less energetic beam.
• the beam length is from 3 to 15, more usually from 5 to 10, cm although longer beam lengths can be used.
• Compact equipment has a particular advantage in fitting to pressurised equipment in that it requires less extensive access through the wall of the pressure vessel. Also, the use of lower energy (less penetrating) radiation reduces the risk of radiation exposure. For these reasons at least, a beam length of between 3 and 15cm is preferred.
• the energy of the source radiation of the density measurement apparatus is typically not more than about 750 keV and is desirably lower than this.
• the source can be a radioactive isotope as is used in conventional (single source/ detector) density gauges where the radiation source is commonly the 661 keV gamma radiation from 137 Cs.
• the use of a lower energy source is preferred and energies of less than 500 keV, particularly less than 300 keV and optimally less than 100 keV, are more preferred.
• the minimum energy of the radiation is about 20 keV because less energetic radiation will generally have too short an effective path length to be useful, and more desirably the source energy is at least about 40 keV.
• Preferred sources include 133 Ba which is a 356 and 80 keV gamma source and, particularly desirably, 241 Am which is a 60 keV gamma source.
• 241 Am source enables the use of a path length of from 5 to 10 cm so that a density measurement apparatus can be installed through a single 15 cm port in the vessel wall.
• the source radiation could also be X-rays and, although robust compact sources are not easy to engineer, for such sources, intrinsic source half life is not a problem.
• the source radiation referred to in this and the succeeding paragraph refers to the radiation sources forming a part of a density measurement apparatus and not to the radioactive tracer materials.
• the source intensity is at least about 4x10 7 more usually from 4x10 8 to 4x10 9 , Becquerel (Bq).
• Bq Becquerel
• the use of sources with lower intensity may require unduly long integration times to obtain adequately precise results (signal to noise ratio) and more intense sources are relatively expensive and/or may lead to swamping of the detectors.
• 241 Am sources having an intensity of about 1.7x10 9 Bq are readily commercially available and are suitable.
• a further benefit from using multiple detector columns is that where electrically powered detectors are used, the reduction in the number of detectors in each column reduces the power supplied to each column making it easier to comply with safety requirements in when dealing with highly combustible oil/gas systems.
• Using commercially available 12.5 mm GM tubes it is practical to fabricate arrays containing up to about 32 detectors, or even up to about 48, whilst restricting the total power in the detector array so that it satisfies the "intrinsically safe" rating for use in combustible or explosive environments as found in oil/gas extraction.
• using un-powered scintillation detectors with fibre-optic links is even safer as there are no electrical components necessary in the detector array.
• the density measurement apparatus most preferably includes a means by which the radiation emitted from the sources may be shielded from the detector(s) in order that the detectors may detect radiation from the tracer material in the absence of radiation from the density profiler radiation sources. This is conveniently achieved by providing the density measurement apparatus with a combined source holder and beam collimator which can also act as a source shield.
• the density measurement apparatus therefore preferably comprises a combined source holder and collimator wherein the source holder comprises a rod adapted to receive at least one radiation source and the collimator is a tube made of radiation absorbent material, arranged in use to fit coaxially over the source holder, and having at least one transmission hole (or relatively radiation transparent "window") which, in use, is arranged so that a source has aligned with it one or more transmission holes which act to transmit, collimate and direct the radiation towards a detector and wherein the rod and tube are relatively moveable so that in a first position at least one collimated beam is generated from each source and in a second position each source is shielded by a portion of the tube.
• radiation transparent we mean a material which allows the transmission of a substantial portion of the radiation which would be blocked by the radiation absorbent material.
• the source holder is typically a solid rod, e.g. of stainless steel, typically having a diameter of from 10 to 20 mm, having a plurality of longitudinally spaced radial holes adapted to receive radiation sources.
• the collimator is a tube made of radiation absorbent material, typically arranged in use to fit coaxially over the source holder, and having transmission holes in it which in use are arranged so that each source has aligned with it one or more holes which act to transmit, collimate and direct the radiation towards the detectors.
• the rod and tube may be relatively moveable so that in a first position at least one collimated beam is generated from each source and in a second position each source is masked by a portion of the tube so that the bulk of the radiation from the source is absorbed or scattered and no collimated beam of radiation is generated.
• the relative movement may be axial or rotational. Where a single beam is produced from each source, the or each hole in the wall of the collimator tube/shield is usually horizontal. When more than one beam is generated from a single source, the (or at least some of the) holes may extend at angle(s) above and/or below horizontal.
• the alignment of sources and collimator holes are at suitable radial (and if necessary vertical) angles to project beams toward the detectors.
• This form of combined source holder and collimator is described in WO-A-0022387.
• Alternative means of shielding the sources may be used, e.g. by providing shutters to cover the source(s) when required.
• the detectors are preferably arrayed substantially linearly and disposed within a multi-phase fluid such that the array crosses the fluid phase boundaries.
• the detectors may be arranged in a vertical or near vertical line but the linear arrangement of detectors is not critical to this method and other arrangements may be used if convenient.
• the detectors are preferably arranged so that at least one (and preferably more than one) individual detector of the array is located in each of the gas, water, oil, emulsion, foam and optionally solids phase.
• the drawing shows an oil separator vessel 10, having an inlet port 12 for a fluid mixture of gas + oil-i- water and three outlet ports 14, 15 and 16 for separated gas (30) oil (32) and water (34) phases respectively.
• a further port may be provided in the base of the vessel to remove sand/sludge.
• the mixture flows around baffles 20 through the main body of the vessel and out of the ports as shown by arrows.
• the vessel is fitted with a density measurement apparatus 24 having dip tubes fitted with a vertical array of sources 26 and a corresponding array of radiation detectors 28.
• An injection port 22 is located upstream of the separator vessel.
• the detectors of the density measurement apparatus detect radiation emitted by the sources 26.
• the associated electronics and software indicates the density of the fluid between a source and an associated detector by the amount of radiation detected by the detector.
• a profile of the fluid density along the arrayed detectors is obtained.
• the sources 26 are shielded so that they do not emit radiation into the fluid.
• a tracer material is injected through port 22 into the fluid entering the vessel.
• the detectors are then monitored to detect the radiation from the tracer material so that the time of flow to the location of the detector and the level of the tracer within the vessel may be determined.
• a density measurement apparatus may be used to detect tracers flowing with the fluid to provide information concerning the fluid flow in addition to information about the density profile within the vessel.

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• Physics & Mathematics (AREA)
• Fluid Mechanics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Volume Flow (AREA)
• Measurement Of Radiation (AREA)
• Analysing Materials By The Use Of Radiation (AREA)

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• 2005
  • 2005-11-18 GB GB0523518A patent/GB0523518D0/en active Pending
• 2006
  • 2006-11-16 WO PCT/GB2006/050391 patent/WO2007057708A2/en not_active Ceased
  • 2006-11-16 EP EP06808755A patent/EP1949043A2/de not_active Withdrawn
• 2008
  • 2008-05-27 NO NO20082429A patent/NO20082429L/no not_active Application Discontinuation

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