WO2016050787A1

WO2016050787A1 - Method and apparatus for monitoring of the multiphase flow in a pipe

Info

Publication number: WO2016050787A1
Application number: PCT/EP2015/072463
Authority: WO
Inventors: Andrew Hunt; Malcolm Byars; Dominic Patrick MCCANN
Original assignee: Iphase Limited
Priority date: 2014-09-29
Filing date: 2015-09-29
Publication date: 2016-04-07
Also published as: GB2534337B; GB201417174D0; WO2016050792A1; GB201507135D0; GB2530601A; GB2534337A; GB2530601B

Abstract

A monitoring apparatus for monitoring a multiphase flow in a pipe using magnetic induction tomography, the apparatus comprising: at least one annular array of elements disposed around a pipe, each element being adapted to transmit a magnetic field when energized by an input electrical signal and/or to receive a magnetically-induced electrical field and generate an output electrical signal, and at least one screening device for screening at least one of the elements of at least one of the annular arrays from an interfering magnetic field emitted from at least one other element. Also disclosed is a method of monitoring a multiphase flow in a pipe using magnetic induction tomography.

Description

Method and apparatus for monitoring of the multiphase flow in a pipe

The present invention relates to a method of, and a monitoring apparatus for, monitoring a multiphase flow in a pipe using magnetic induction tomography. The multiphase flow comprises fluids, and may comprise a mixture of liquids, or one or more liquids in a mixture with solids and/or gases. This invention relates to a multiphase flow metering apparatus and method which has a number of applications, in particular within the oil and gas exploration and production industry. The method and apparatus is typically an arrangement of coils suitable for improving the use on Magnetic Induction Tomography, either used alone or in conjunction with other techniques such as those mentioned in patent application GB 1307785.4.

In the current state of the art the optimisation of production from subsea wells is difficult because flow from multiple wells is often comingled in subsea manifolds and transferred to surface through a single flowliee. As a result, the flow from any one well is not measured and so cannot be optimised by means of artificial lift or other techniques. For example, if there is an increase in the production of water at surface then it is unknown from which well it is coming. The multiphase flowmeters in the market today are expensive and may not be reliable enough to be placed on each wellhead.

Multi-component flows are often loosely called multiphase. For example, a mixed flow of oil and water is not multiphase (it is one phase - liquid) but it is multi-component (two components - oil and water). A typical oilfield flow of oil, gas and water may often contain solids (for example, sand or hydrates) and thus have four components but only three phases. Throughout this specification, the same loose convention as in many industries is adopted and the terms multi-component and multiphase are used interchangeably to mean the same thing - a mixture of fluids and solids flowing in a pipe.

In the case of a multiphase flow with several components, the operator (for example, an oil company) requirement may be the volume or mass flowrate of some or all of components. In a typical oilfield flow the operator requires the measurement of the mass flow of gas, oil and often the water, but typically not specifically the solids. Although measurement of the flowrate or concentration of solids is a useful additional measurement and can help determine the health of the downhole sand screens or gravel packs. Early detection of the potential failure of these elements of the system will help reduce failure of other components due to, for example, erosion.

There are many applications of multiphase flowmeters in the oil industry for flows of gas, oil, water and solids: downhole, wellhead, platform, pipelines, subsea, wet gas, heavy oil, gas lift, tar sands etc. The further upstream in the process the more complex and demanding the conditions, so subsea and downhole are the most difficult.

An oil well starts life producing mainly oil, but as the oil depressurizes along the flow line gas is liberated, so at the wellhead there is almost always some gas present. In addition, most wells produce some water and the amount increases through the life of the well until by the end of its life the well may be producing mostly water. Because of the long flow path even small quantities of gas may cause the well to slug - oscillating between high liquid and high gas states.

A gas well starts life producing mainly gas, but frequently this is associated with the production of light oil known as condensate and again later in life some water is likely to be produced.

Therefore both oil and gas wells generate multiphase flows with gas, oil and water almost always present in highly variable quantities, and in addition many reservoir formations produce sand as a natural part of production, and any workover of the reservoir will often leave some solids to be cleaned out over the succeeding days or weeks.

In this specification and claims the term 'oil well' will be used to represent any kind of well drilled for oil and gas exploration and/or production, including injection wells that can be used for the purposes of production enhancement.

It is very beneficial for both the reservoir and production engineers to have reliable measurements of the multiple phases in the production from a well. In addition to the quantitative measurements of the volume or mass flowrates of the individual components, it is also very beneficial to determine the flow regime, that is, how the different phases are distributed in the flow. For example, the same volume of gas arriving at surface as slugs rather than evenly distributed in the flow represents very different production scenarios and poses different problems to the production engineer. Real-time determination of these different and changing flow regimes would offer reservoir and production engineers a deeper insight into production and so allow improved optimization. The historical 'normal* method for measuring fio rate of oil, gas and water is to use a separator that separates the input flow into output flows of oil, gas and water with three independent single-phase flowmeters to measure each. In production this is still the prime measurement technique - here the flow needs to be separated anyway for use in the downstream process. For well testing, well monitoring and subsea completions however the separator is a large, expensive and not very accurate unit and is steadily being replaced by multiphase flowmeters. However, as already mentioned, multiphase flowmeters available in the market have many drawbacks that have limited their application and the invention here disclosed addresses these shortcomings. In particular, multiphase flowmeters on the market today widely use a nuclear source (sometimes more than one) and restrict the flow by the use of a Venturi element to the meter.

The oilfield environment is physically demanding - high pressure (up to 1000 bar), with high temperatures of the fluids (up to 250 degrees Celsius), variation of physical properties of the oil and gas (PVT), variation in salinity of the produced water, issues of H₂S production (known as sour gas), subsea or downhole access etc. This has led to various meter designs involving multiple technologies in order to address each challenge. As a result the cost base is high, independent of the technologies used.

A list of the main multiphase flowmeters available can be found in oil industry catalopes (see for example MPFM Handbook Revision2 2005 ISB -82-91341 -89-3) along with the technologies used in each. The essence of many of them is that the overall mass flow is estimated by a Venturi meter, in most the density is estimated using a gamma density meter and then some sort of electrical method is used to estimate the oil/water ratio. The common use of a gamma ray nuclear source is one particular requirement of devices that the industry has wanted to remove for sometime. Obviously the use of a nuclear source brings issues related to health and safety but also security in some situations. The common use of a Venturi has also lead to reliability issues. This is due to the fact that the pressure in the meter can be very high (1000s of PSI) but pressure drop across the Venturi is typically less than 0.1 PSI. As a result it is typical to have a delta pressure (dP) sensor rather than two absolute pressure sensors, one each side of the Venturi. It should also be noted that the Venturi imposes a restriction in the flow. Unfortunately, dP sensors can be a source of reliability issues, for example, a blockage or restriction of the pressure feed on one side of the sensor causes an overpressure resulting in the sensor failing. Large pressure transients cross the Venturi can have a similar result. It is not uncommon for these dP gauges to fail within a year or two of operation.

Another issue with solutions in the prior art is the fact that average densities and velocities are generally estimated across the meter, for example, the nuclear absorption provides an average density estimation across the meter. Also, it is well known that in many situations the velocity of the different phases can be quite different, for example, the velocity of the gas bubbles can be very different to the velocity of the oil or water in which they travel. The difference is often called the 'slip velocity' of one phase relative to another. Because the various fluids are highly fluctuating in both space and time, it can be shown that there is an unbounded error if the average phase concentration is multiplied by the average phase velocity to get average phase volumetric flowrate, this error may easily be 50% of the reading, see Hunt 2012. In fact, the phase velocity and concentration must be multiplied together before integration across the flow to get the correct answer. However, generally, current multiphase flowmeters do the multiplication incorrectly because they are based on independent devices that average across the flow first.

Corrections may be attempted by using slip velocity models. However the fundamental problem of the incorrect integration process means that the correction is often large and uncertain.

The only way to reduce the slip velocity to zero is to completely homogenize the different phases before metering but this would necessitate significant separation problems downstream, otherwise accurate multiphase flowrate measurements must start with independent estimates of velocity and concentration of each component across the flow.

Another issue with the solutions presently available is that phase concentrations are averages and it is unknown how these phases are distributed in the flow. For example, a meter may indicate that the flow stream contains 70% oil and 30% gas. However, it is not necessarily known if the gas is distributed in small bubbles in the stream or in larger bubbles or even a single bubble.

Finally, some prior art has attempted to address some of these issues, for example, see EP 2379990 A l Multiphase flowmeter. However, the resulting solution involves splitting the flow such that the meter has an obstruction in the flow stream. In many applications the measurement must be non-intrusive so that access to the pipeline is not impeded. The addition of an obstruction in the meter has significant disadvantages. For example, anything directly in the flow path has a tendency to erode, leading to early failure. The increased pressure drop can also impact production performance.

Three-phase flow has nine variables: the velocity, density and concentration (often call holdup) of each phase. If the pipe contains only the three phases then the sum of concentrations = 100%. Therefore, in principle eight measurements are required. In instruments available today there are generally less than 8 measurements (often using different technologies) and so assumptions are required. For example, phase densities are measured using samples of fluids and are considered constant between samples, slip velocities are calculated using models or all phases are pre-mixed before passing through the meter and it is assumed all 3 velocities are equal. Any of these assumptions can introduce significant errors in the measurements obtained.

It is understood that electromagnetic energy can provide information related to certain physical properties of materials exposed to this type of energy. Well known examples include the electromagnetic flowmeter, electrical capacitance tomography (ECT), electrical resistance tomography (ERT) and magnetic inductance tomography (MIT). In each case a varying electric or magnetic field can be applied across the material and measurements of voltage, current and magnetic field can be used to measure certain physical parameters of the constituent components.

The present invention provides a monitoring apparatus for monitoring a multiphase flow in a pipe using magnetic induction tomography, the apparatus comprising: at least one annular array of elements disposed around a pipe, each element being adapted to transmit a magnetic field when energized by an input electrical signal and/or to receive a magnetically-induced electrical field and generate an output electrical signal, and at least one screening device for screening at least one of the elements of at least one of the annular arrays from an interfering magnetic field emitted from at least one other element.

Optionally, each element comprises a magnet or a coil, and preferably each element comprises a coil. Preferably, a plurality of the annular arrays are mutually separated along the pipe. Each of the annular arrays is preferably controllable to define a respective magnetic field zone within an interior of the pipe, the magnetic field zone extending along a respective axial portion of the pipe.

When there is a plurality of the annular arrays of elements disposed around the pipe, the apparatus preferably further comprises a controller coupled to each of the annular arrays, wherein the controller is adapted selectively to drive at least one of the annular arrays in a predetermined transmission and receiving mode to comprise a monitoring array and to drive at least one of the annular arrays in a predetermined transmission mode to comprise a guarding array, the guarding array comprising the screening device.

Preferably, the or each monitoring array transmits a magnetic field into, and receives a magnetically-induced electrical field from, a respective axial portion of the pipe and the axial extent of the axial portion is defined by at least one screening magnetic field from at least one guarding array.

Preferably, at least two monitoring arrays are provided, the monitoring arrays being mutually separated by a respective guarding array whereby each monitoring array is adapted independently to measure at least one property of the multiphase flow in a respective axial portion of the pipe.

Preferably, two of the least two monitoring arrays are adapted independently to measure at least one property of multiphase flow in a respective axial portion of the pipe, and the controller is adapted to determine a parameter derived from a comparison of the respective measurements of the respective monitoring arrays.

Preferably, the controller is adapted to determine a velocity of the multiphase flow derived from a comparison of the respective measurements of the respective monitoring arrays.

Preferably, the controller is adapted to generate a display signal pictorially representing a velocity profile of the multiphase flow in at least one respective axial portion of the pipe and/or across at least a portion of a cross-section of the pipe. Preferably, the controller is adapted to determine any change in a measured parameter of the multiphase flow, the change occurring between the respective axial portions of the pipe, derived from a comparison of the respective measurements of the respective monitoring arrays.

The apparatus preferably further comprises a display device for displaying a representation of the parameter, or a derivative thereof, determined from a comparison of the respective measurements of the respective monitoring arrays.

Preferably, in the predetermined transmission and receiving mode in at least one of the monitoring arrays at least one preselected first element of the respective annular array is energized to transmit a magnetic field and at least one preselected second element of the respective annular array receives the magnetic field and generates an output electrical signal.

Preferably, the predetermined transmission and receiving mode in the respective at least one monitoring array comprises a sequence of transmission and receiving cycles, in each cycle at least one respective element comprising the first element energized to transmit a magnetic field and at least one other respective element comprising the second element to receive the magnetic field and generate an output electrical signal, the first and second elements differing in successive cycles.

Preferably, in the predetermined transmission mode in at least one of the guarding arrays at least one preselected element of the respective annular array is energized to transmit a magnetic field.

Preferably, the predetermined transmission mode in the respective at least one guarding array comprises a sequence of transmission cycles, in each cycle at least one respective element being energized to transmit a magnetic field, the energized elements differing in successive cycles.

Preferably, the apparatus comprises five of the annular arrays which are mutually separated along the pipe. More preferably, the five annular arrays comprise, in sequence along the pipe, a first end array, a first intermediate array, a central array, a second intermediate array and a second end array. Preferably, the controller is adapted selectively to drive the first end array, the central array and the second end array as guarding arrays and the first intermediate array and the second intermediate array as monitoring arrays. The present invention further provides a method of monitoring a multiphase flow in a pipe using magnetic induction tomography, the method comprising the steps of: a, providing at least one annular array of elements disposed around a pipe, each element being adapted to transmit a magnetic field when energized by an input electrical signal and/or to receive a magnetically-induced electrical field and generate an output electrical signal;

b, flowing a multiphase flow along the pipe;

c, transmitting a magnetic field from a first element into the multiphase flow; d, receiving by a second element a magnetically-induced electrical field from the multiphase flow and generating an output electrical signal therefrom; and e, screening, during at least step (d), the second element from an interfering magnetic field emitted from at least one other element of the at least one annular array of elements.

Optionally, each element comprises a magnet or a coil. Preferably, each element comprises a coil .

Preferably, the at least one annular array of elements comprises a plurality of the annular arrays which are mutually separated along the pipe.

Preferably, each of the annular arrays is controlled to define a respective magnetic field zone within an interior of the pipe, the magnetic field zone extending along a respective axial portion of the pipe.

Preferably, there is a plurality of the annular arrays o elements disposed around the pipe, and in steps (c) and (d) at least one first annular arrays is selectively driven in a predetermined transmission and receiving mode to comprise a monitoring array to provide the transmitting and receiving of steps (c) and (d) and at least one second annular array is selectively driven in a predetermined transmission mode to comprise a guarding array, the guarding array comprising a screening device for providing the screening of step (e).

Preferably, at least two monitoring arrays are provided, the monitoring arrays being mutually separated by a respective guarding array whereby each monitoring array is independently measures at least one property of the multiphase flow in a respective axial portion of the pipe.

Preferably, two of the least two monitoring arrays independently measure at least one property of multiphase flow in a respective axial portion of the pipe, and the method further comprises the step (fj of determining a parameter derived from a comparison of the respective measurements of the respective monitoring arrays.

Preferably, in step (f) a velocity of the multiphase flow is determined.

Preferably, the method further comprises the step of (g) generating a display signal pictorially representing a velocity profile of the multiphase flow in at least one respective axial portion of the pipe and/or across at least a portion of a cross-section of the pipe.

Preferably, the method further comprises the step of (h) determining any change in a measured parameter of the multiphase flow, the change occurring between the respective axial portions of the pipe, derived from a comparison of the respective measurements of the respective monitoring arrays.

Preferably, the method further comprises the step of (i) displaying a representation of the parameter, or a derivative thereof, determined from a comparison of the respective measurements of the respective monitoring arrays.

Preferably, in the predetermined transmission and receiving mode in the respective at least one monitoring array comprises a sequence of transmission and receiving cycles, in each cycle at least one respective element comprising the first element energized to transmit a magnetic field and at least one other respective element comprising the second element to receive the magnetic field and generate an output electrical signal, the first and second elements differing in successive cycles.

Preferably, the predetermined transmission mode in the respective at least one guarding array- comprises a sequence of transmission cycles, in each cycle at least one respective element being energized to transmit a magnetic field, the energized elements differing in successive cycles.

Preferably, the at least one annular array of elements comprises five of the annular arrays which are mutually separated along the pipe.

Preferably, the five annular arrays comprise, in sequence along the pipe, a first end array, a first intermediate array, a central array, a second intermediate array and a second end array.

Preferably, the method further comprises the step of (j) selectively driving the first end array, the central array and the second end array as guarding arrays and the first intermediate array and the second intermediate array as monitoring arrays.

The present invention relates specifically to an improved method and apparatus for the use of MIT (Magnetic Induction Tomography), in particular in the appl ication of MIT to measuring multiphase flows in the oil and gas and other industries. The principle of MIT is that electric coils are excited with alternating current that results in the coils producing varying magnetic fields. However, magnets may alternatively be used to produce the required magnetic fields.

The object of interest is placed within these fields and the varying field induces varying currents within the object that is dependant on the conductivity of the object. The varying currents in the object produce secondary magnetic fields that can be received by the same or other coils. The received secondary magnetic field in conjunction with the primary imposed magnetic field can use be used to compute the conductivity contrast between the object and the material that surrounds it. See for example EP 2044470 A l and US 20080258717. Magnetic induction has been used to measure components of a multiphase flow, see US7276916B2, but this application makes only one measurement across the flow. MIT is mentioned as one of three combination elements in patent application GB 1307785.4. The present invention relates to an apparatus to improve the measurements of such a system.

The preferred embodiments of this invention disclose a method to measure the flow of mixtures of fluids from a well or group of wells during oil and gas exploration, production or transportation operations. Through these listed aspects of this invention, the inventors have provided different embodiments, which cover some of the potential applications of the multiphase flowmeter described. However, it is understood that this is a subset of the potential applications and those skilled in the art will appreciate that there can be many others which are additionally provided in this invention.

These and other aspects of the present invention will now be described, by way of examples, with reference to the accompanying drawings, in which:

Figure 1 illustrates multiphase flow through a pipeline;

Figures 2a, 2b and 2c show schematics of an electromagnetic measurement that is in accordance with an embodiment of the state of the art;

Figure 3 shows the approximate magnetic field lines of the coils used in the state of the art;

Figure 4 shows a schematic of an arrangement of coils in accordance with an embodiment of the present invention;

Figure 5 shows the approximate magnetic field lines of the coils used in an embodiment of the present invention.

Hereinafter, the present invention will now be described in more detail with reference to the accompanying figures, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

In all of the illustrated apparatus, including the embodiments of the invention, coils are utilised selectively (i) to transmit a magnetic field when energized by an input electrical signal and/or (ii) to receive a magnetically-induced electrical field to generate an output electrical signal. However, any such coil may alternatively be replaced by a magnet which is switchable, correspondingly selectively to transmit a magnetic field when energized by an input electrical signal and/or to receive a magnetically-induced electrical field to generate an output electrical signal. Any mixture of such coils and magnets may be employed in any of the embodiments of the invention.

Referring to figure 1 there is shown a schematic of a multiphase flow, 1 1, in a pipeline 10. In figure 1 , 12, illustrates the primary or continuous phase of the flow, e.g., oil, water or gas. Within this primary phase there is schematically shown two other constituents to the flow labelled 13 and 14. Solid (e.g., sand) in the flow is illustrated as labelled 15. The figure illustrates that the flow in the pipeline, 10, has multiple phases including solids. Clearly this figure is a simplistic and the distribution of these phases can vary significantly depending on the concentrations of each phase and the flow regime. The structure of such multiphase flows can be very complex and there are many industry papers that attempt to explain this complexity and better understand these various flow regimes, the reasons for their existence and how they affect overall production performance, see for example Hunt et al 2010.

In figures 2a, 2b and 2c is shown a schematic of a measurement element, 30, representing the state of the art. Figure 2a shows a cross-section that is taken perpendicular to the flow and figure 2b is a view parallel to the flow. Referring to figure 2a there are a plurality of coils, 36, arranged around the circumference of the instrument. In figure 2a the continuous or primary fluid phase or constituent is labelled 12, e.g., oil. Two other phases or constituents are shown diagrammatically as 13 and 14; these could be gas and water, respectively. Each of the antennae 36 can act as either a transmitting or receiving coil and can change between the two modes. In figure 2a antennae 313 is shown as a transmitter. A varying electric current is passed through the coil 313 as illustrated by the sign wave 37. Although this varying signal is shown as a sine wave it could be of another form, e.g., square wave and all other potential forms are provided in this invention. The varying electric current passing through the coil 313 will generate a varying magnetic flux through the multiphase fluid 1 1 that is within the pipe. The magnetic flux lines are schematically shown and one is labelled 314 for illustration purposes. Depending on the physical properties of the different phases that the flux lines interrogate and in particular the conductivity contrast between, e.g., phases 12 and 14, a varying current is induced in the second phase, 14. This is shown schematically and labelled 315 in figure 2a. This induced current will in turn generate a secondary varying magnetic field that will propagate through the pipe where it will be pickup by the other antennae that are used as receivers. This secondary varying magnetic field is shown as dashed lines and labelled 316 in figure 2a and will induce varying currents in the receiver coils. This is shown schematically in one coil on figure 2a and labelled 38. Comparing 37 and 38 with appropriate processing, for example, the phase shift between the signals, allows the conductivity contrast between the materials, for example, 12, 13 and 14, to be computed.

At any point in time there is one coil that is transmitting and all of the others are receiving. Once all the receiver coils signals have been processed, one or more of the other coils becomes the transmitter and again the remainder are receivers and so forth. As an example, 313 is the first transmitter coil then the coil immediately next to it going clockwise becomes the next transmitter. After that, the next coil immediately next to and clockwise to it becomes the transmitter. This continues around all the coils and eventually 313 will become the transmitter coil once more. In figure 2a it will be appreciated by those skilled in the art that the sequencing can take place in any order and that a complete cycle of measurements, that is, where every coil has been the transmitter once, can occur very rapidly with, e.g., 500 to 5000 measurement cycles every second. This frequency being primarily limited only by the processing power available that can be scaled as needed. It will also be appreciated by those skilled in the art that after one complete cycle of measurements a mesh of properties is produced that can be processed to provide a mesh or image of the fluids phases across the section of the pipe. Although this description describes each coil being either a transmitter or receiver, clearly, a configuration can be provided whereby certain coils are always transmitters and others are always receivers. In other embodiments coils can be enclosed within other coils so that dedicated transmitter and receiver coils are at the same location. Those skilled in the art will appreciate that many combinations are possible and all such combinations are provided in this invention.

Referring to figure 2b, it is shown that there are 2 sets of coils 36 and 317 that are separated by a known fixed distance 39. Both sets operate in the same fashion and provide independent meshes or images of the flow at two points along the pipe 10. It is possible to cross correlate the measurements from these two sets, 36 and 317, in order to establish the time-of- flight of features that represent different phases in the multiphase flow 1 1. This is illustrated in figure 2c where 31 1 shows two curves; one showing a feature passing coils 36 and the second the same feature passing electrodes 317. The time difference between the features provides the time it takes for this phase to travel from 36 to 317, that is, the distance 39. Those skilled in the art will appreciate that the velocity of this phase is easily computed from this information. In figure 2c, 312 illustrates features that result from a different phase in the flow and in this case the time difference is longer illustrating that this phase is travelling slower than the first phase shown by 31 1 . It will be understood by those skilled in the art that the features shown by 31 1 and 3 12 could in fact be derived by cross correlating the mesh elements at the same location in the cross section of the pipe such that a velocity profile across the cross section of the pipe is obtained. That is, a mesh or image of velocities is produced that can be used to establish the velocity differences between the primary /continuous phase, phase labelled 13 and the phase labelled 14. It will be appreciated that while figure 2a shows 3 phases ( 12, 13 and 14); it is possible that more can be present and in particular solids (e.g. sand) can also be present. Also, these velocities can be obtained when the primary or continuous phase is either conducting (e.g. water) or nonconducting fluid (e.g. oil).

The electromagnetic measurement, 30, as described above will provide measurements where there is a conductivity contrast between the phases. This is possible when the different phases or constituents are flowing in a predominately conducting (e.g. water) or non-conducting (e.g. oil) primary phase 12.

Although the apparatus representing the state of the art shown in Figures 2a, 2b and 2c can make the measurements described there are significant limitations in some aspects of the measurement. These restrictions will be discussed with regard to Figure 3.

Figure 3 shows the same embodiment as in Figures 2a, 2b and 2c, but simplified and indicating the approximate magnetic field lines 320 when coil 36 is energised as a transmitter and coils 317 are receivers. As before transmitter 313 is a transmitter for a period of time, then the next coil of the array 36 is energised as transmitter in sequence around the pipe. In each case the magnetic field lines seen from a side view will have a similar form to 320. As stated above, each coil 36, 317 or some coils 36, 3 17 may alternatively be a magnet. A significant limitation of this embodiment of the state of the art is that the magnetic field lines 320 extend for an unlimited length of the pipe so that an element of multiphase flow, such as a bubble of water, represented by 321, will influence the measurement as it cuts the magnetic field lines 320 even though it is nominally 'outside' of the sensor. A similar element such as 13 will have a larger effect on the measurement as the magnetic field lines are closer together towards the centre of the sensor, but the measurement cannot differentiate between the effects of the two elements of multiphase flow.

In Figure 4 is shown a preferred embodiment of the present invention, whereby the array of transmitting coils 36, where 313 is the coil presently active, and receiver coils 317 is now part of an array containing two sets of guard coils 42 (including a presently active coil 44) and 43 Including a presently active coil 45) and another set of receiver coils 41. In this embodiment coils 313, 44, and 45 are all driven with the same signal, and in turn each of the azimuthal electrodes in arrays 42, 36, and 43 become the active transmitters for a period of time. At each point the active coils 313, 44, and 45 are driven with the same signal and are along the same axial line on the pipe. As stated above, each coil 36, 3 13, 42, 43 or some coils 36, 313, 42, 43 may alternatively be a magnet.

In Figure 5 are shown the approximate magnetic field lines generated when coils 3 13, 44, and 45 are for the time being the active transmitter coils. The major advantage o this arrangement can now be seen: multiphase flow element 321 does not cut any of the magnetic field lines passing through the measurement coils 317, and so does not contribute to the measurement received at that array. In addition multiphase elements such as 13 only contribute to measurements at array 41 and elements such as 46 do not contribute to the measurement at all. As stated above, each coil 31 3, 317, 44, 45 or some coils 313, 317, 44, 45 may alternatively be a magnet.

The embodiment shown in Figure 5 thus effectively divides the flow into 4 zones, as shown in figure 5. In zone 1 to the left o the centreline of array 42 (including 44) there is no sensitivity to the flow, in zone 4 to the right of the centreline of array 43 (including 45) there is no sensitivity to the flow. Flow elements present in zone 2 between the centrelines of array 42 and array 36 (including 3 13) are measured at array 41 while flow elements present in zone 3 between the centrelines of array 43 and array 36 (including 313) are measured at array 317. The control of the sensitivity zones along the axis of the pipe given by the present invention will lead to clearer and more precise than those possible by the current state of the art.

The preferred embodiment of the present invention shown in Figure 5 makes possible a method of measuring the transit velocity of elements of the multiphase flow by cross-correlating measurements made at array 41 and array 317. Because of the closer control of sensitivity, such a method will give clearer cross-correlogram peaks than that described in the state of the art described in patent application GB 1307785.4 and hence lead to improved maps of velocity across the flow.

Claims

1. A monitoring apparatus for monitoring a multiphase flow in a pipe using magnetic induction tomography, the apparatus comprising: at least one annular array of elements disposed around a pipe, each element being adapted to transmit a magnetic field when energized by an input electrical signal and/or to receive a magnetically- induced electrical field and generate an output electrical signal, wherein the apparatus comprises a plurality of the annular arrays which are mutually separated along the pipe, and a controller coupled to each of the annular arrays, wherein the controller is adapted selectively to drive at least one of the annular arrays in a predetermined transmission and receiving mode to comprise a monitoring array and to drive at least one of the annular arrays in a predetermined transmission mode to comprise a guarding array, the guarding array comprising a screening device.

2. An apparatus according to claim 1 wherein each element comprises a magnet or a coil.

3. An apparatus according to claim 2 wherein each element comprises a coil.

4. An apparatus according to any one of claims 1 to 3 wherein each of the annular arrays is controllable to define a respective magnetic field zone within an interior of the pipe, the magnetic field zone extending along a respective axial portion of the pipe.

5. An apparatus according to any foregoing claim wherein the or each monitoring array transmits a magnetic field into, and receives a magnetically-induced electrical field from, a respective axial portion of the pipe and the axial extent of the axial portion is defined by at least one screening magnetic field from at least one guarding array.

6. An apparatus according to any foregoing claim wherein at least two monitoring arrays are provided, the monitoring arrays being mutually separated by a respective guarding array whereby each monitoring array is adapted independently to measure at least one property of the multiphase flow in a respective axial portion of the pipe.

7. An apparatus according to claim 6 wherein two of the least two monitoring arrays are adapted independently to measure at least one property of multiphase flow in a respective axial portion of the pipe, and the controller is adapted to determine a parameter derived from a comparison of the respective measurements of the respective monitoring arrays.

8. An apparatus according to claim 7 wherein the controller is adapted to determine a velocity of the multiphase flow derived from a comparison of the respective measurements of the respective monitoring arrays.

9. An apparatus according to claim 8 wherein the controller is adapted to generate a display signal pictorially representing a velocity profile of the multiphase flow in at least one respective axial portion of the pipe and/or across at least a portion of a cross-section of the pipe.

10. An apparatus according to any one of claims 7 to 9 wherein the controller is adapted to determine any change in a measured parameter of the multiphase flow, the change occurring between the respective axial portions of the pipe, derived from a comparison of the respective measurements of the respective monitoring arrays.

1 1. An apparatus according to any one of claims 7 to 10 further comprising a display device for displaying a representation of the parameter, or a derivative thereof, determined from a comparison of the respective measurements of the respective monitoring arrays.

12. An apparatus according to any foregoing claim wherein in the predetermined transmission and receiving mode in at least one of the monitoring arrays at least one preselected first element of the respective annular array is energized to transmit a magnetic field and at least one preselected second element of the respective annular array receives the magnetic field and generates an output electrical signal.

13. An apparatus according to claim 12 wherein the predetermined transmission and receiving mode in the respective at least one monitoring array comprises a sequence of transmission and receiving cycles, in each cycle at least one respective element comprising the first element energized to transmit a magnetic field and at least one other respective element comprising the second element to receive the magnetic field and generate an output electrical signal, the first and second elements differing in successive cycles.

14. An apparatus according to any foregoing claim wherein in the predetermined transmission mode in at least one of the guarding arrays at least one preselected element of the respective annular array is energized to transmit a magnetic field.

15. An apparatus according to claim 14 wherein the predetermined transmission mode in the respective at least one guarding array comprises a sequence of transmission cycles, in each cycle at least one respective element being energized to transmit a magnetic field, the energized elements differing in successive cycles.

16. An apparatus according to any foregoing claim which comprises five of the annular arrays which are mutually separated along the pipe.

17. An apparatus according to claim 16 wherein the five annular arrays comprise, in sequence along the pipe, a first end array, a first intermediate array, a central array, a second intermediate array and a second end array.

18. An apparatus according to claim 17 wherein the controller is adapted selectively to drive the first end array, the central array and the second end array as guarding arrays and the first intermediate array and the second intermediate array as monitoring arrays.

19. A method of monitoring a multiphase flow in a pipe using magnetic induction tomography, the method comprising the steps of:

a. providing at least one annular array of elements disposed around a pipe, each element being adapted to transmit a magnetic field when energized by an input electrical signal and/or to receive a magnetically-induced electrical field and generate an output electrical signal, wherein the at least one annular array of elements comprises a plurality of the annular arrays which are mutually separated along the pipe;

b. flowing a multiphase flow along the pipe;

c. transmitting a magnetic field from a first element into the multiphase flow; d. receiving by a second element a magnetically-induced electrical field from the multiphase flow and generating an output electrical signal therefrom; and e. screening, during at least step (d), the second element from an interfering magnetically-induced electrical field from the multiphase flow;

wherein in steps (c) and (d) at least one first annular arrays is selectively driven in a predetermined transmission and receiving mode to comprise a monitoring array to provide the transmitting and receiving of steps (c) and (d) and at least one second annular array is selectively driven in a predetermined transmission mode to comprise a guarding array, the guarding array comprising a screening device for providing the screening of step (e).

20. A method according to claim 1 wherein each element comprises a magnet or a coil.

21. A method according to claim 20 wherein each element comprises a coil.

22. A method according to any one of claims 19 to 21 wherein each of the annular arrays is controlled to define a respective magnetic field zone within an interior of the pipe, the magnetic field zone extending along a respective axial portion of the pipe.

23. A method according to any one of claims 1 to 22 wherein the or each monitoring array transmits a magnetic field into, and receives a magnetically-induced electrical field from, a respective axial portion of the pipe and the axial extent of the axial portion is defined by at least one screening magnetic field from at least one guarding array.

24. A method according to any one of claims 1 to 23 wherein at least two monitoring arrays are provided, the monitoring arrays being mutually separated by a respective guarding array whereby each monitoring array is independently measures at least one property of the multiphase flow in a respective axial portion of the pipe.

25. A method according to claim 24 wherein two of the least two monitoring arrays independently measure at least one property of multiphase flow in a respective axial portion of the pipe, and the method further comprises the step (f) of determining a parameter derived from a comparison of the respective measurements of the respective monitoring arrays.

26. A method according to claim 25 wherein in step (f) a velocity of the multiphase flow is determined.

27. A method according to claim 26 further comprising the step of (g) generating a display signal pictorially representing a velocity profile of the multiphase flow in at least one respective axial portion of the pipe and/or across at least a portion of a cross-section of the pipe.

28. A method according to any one of claims 25 to 27 further comprising the step of (h) determining any change in a measured parameter of the multiphase flow, the change occurring between the respective axial portions of the pipe, derived from a comparison of the respective measurements of the respective monitoring arrays.

29. A method according to any one of claims 25 to 28 further comprising the step of (i) displaying a representation of the parameter, or a derivative thereof, determined from a comparison of the respective measurements of the respective monitoring arrays.

30. A method according to any one of claims 19 to 29 wherein in the predetermined transmission and receiving mode in at least one of the monitoring arrays at least one preselected first element of the respective annular array is energized to transmit a magnetic field and at least one preselected second element of the respective annular array receives the magnetic field and generates an output electrical signal.

3 1. A method according to claim 30 wherein the predetermined transmission and receiving mode in the respective at least one monitoring array comprises a sequence of transmission and receiving cycles, in each cycle at least one respective element comprising the first element energized to transmit a magnetic field and at least one other respective element comprising the second element to receive the magnetic field and generate an output electrical signal, the first and second elements differing in successive cycles.

32. A method according to any one of claims 19 to 3 1 wherein in the predetermined transmission mode in at least one of the guarding arrays at least one preselected element of the respective annular array is energized to transmit a magnetic field.

33. A method according to claim 32 wherein the predetermined transmission mode in the respective at least one guarding array comprises a sequence of transmission cycles, in each cycle at least one respective element being energized to transmit a magnetic field, the energized elements differing in successive cycles.

34. A method according to any one of claims 1 to 33 wherein the at least one annular array of elements comprises five of the annular arrays which are mutually separated along the pipe,

35. A method according to claim 34 wherein the five annular arrays comprise, in sequence along the pipe, a first end array, a first intermediate array, a central array, a second intermediate array and a second end array. , A method according to claim 35 further comprising the step (j) of selectively driving the first end array, the central array and the second end array as guarding arrays and the first intermediate array and the second intermediate array as monitoring arrays.