GB2364777A

GB2364777A - An improved capacitance measurement probe

Info

Publication number: GB2364777A
Application number: GB0017163A
Authority: GB
Inventors: William Peter Stuart-Bruges; Guy Harvey Mason; Robert Lecore Holding
Original assignee: Sondex Ltd
Current assignee: Sondex Ltd
Priority date: 2000-07-12
Filing date: 2000-07-12
Publication date: 2002-02-06
Anticipated expiration: 2020-07-12
Also published as: GB2364777B; GB0017163D0

Abstract

A guarded capacitance measurement probe for use in bore-hole logging operations such as those in the oil industry, comprises a body (30) on which an outer capacitor plate (20) is mounted on supporting fins (36), an inner cylindrical sensor electrode (22), mounted concentrically with the outer plate (20) and which, in conjunction with it, forms the second plate (24) of a capacitor. A sample of borehole fluid is located between the capacitor plates and a measurement of one or more of the capacitor characteristics, with the fluid acting as dielectric, is made in order to determine the permittivity of the fluid. The permittivity measurement is used to determine the proportion and type of constituent fluids (oil, water, gas) within the sample fluid. The sensor (22), which forms one of the capacitor plates, has guard electrodes (42) and (44) adjacent its edges, which at least partially screen it from leakage currents and edge effects that might disrupt the measurement.

Description

<Desc/Clms Page number 1> An Improved Capacitance Measurement Probe Technical Background to the Invention This application relates to capacitance measurement probes, also called water hold-up meters, used in the oil industry. Such devices measure the capacitance of a sample of mixed fluids between measurement electrodes inserted into a container, pipe or bore-hole.

Fluids from an oil well can comprise a mixture of hydrocarbons, gas and water. In order to assess the productivity of a bore-hole in use in the oil industry, many techniques have been developed to determine the composition of the fluids being obtained from the well. One such technique, to deduce the fraction and type of constituent fluids contained within the borehole, is to allow a fluid sample to flow between the capacitor plates of a sensor probe and measure its permittivity. The relative permittivity of oil hydrocarbons is approximately 10; water on the other hand has a relative permittivity of about 80 and gases have a relative permittivity of about 3 depending on pressure and temperature. Providing the permittivity of the constituent fluids are sufficiently different, the fraction-and type of each fluid present in the sample can be deduced from a measurement of the capacitance across the plates with the fluid sample acting as dielectric.

A typical capacitance probe tool, intended for down- hole use, has an outer cylindrical capacitor plate mounted on and electrically connected to an elongate, rod-like sensor body 30. The inner plate of the capacitor is a cylindrical electrode and is situated co-axially inside the outer plate. The tool and capacitor geometry are cylindrical to allow the sensor to be deployed within a narrow bore-hole, and to allow the bore-hole fluid to flow easily between the plates of the capacitor. Fluid, whose permittivity is to be measured is allowed to enter the

space between the capacitor plates via vents 38 provided where the cylindrical outer capacitor plate is mounted on the tool body.

The inner capacitor plate or electrode is isolated from the sensor body and from the fluid. The body of the sensor and therefore the outer plate of the capacitor, are typically at earth potential.

The determination of permittivity that the sensor makes is based on a measurement of the electric field between the capacitor plates. However, edge effects of the capacitor plates or other external factors may adversely affect the electric field geometry and make the sensor measurements unreliable.

A common problem with sensors of this kind is that at the high densities and pressures experienced in the down- hole environment, certain fluids, water for example, may exhibit low surface tension and form a layer on the plates of the capacitor making them 'wet'.

Wetting by conductive fluids is a particular problem if a layer of an ionically conducting fluid forms on the inner electrode and makes a direct connection to the tool body. Since the tool body is typically at earth potential, the wetting layer may act to short circuit parts of the electric field around the inner electrode, thereby seriously compromising the sensor's operation.

We have appreciated, therefore, that it would be desirable to provide a sensor probe which has means to, at least partially, alleviate the disruption caused to the electric field between the plates of the capacitor probe by edge effects or by a wetting layer.

Summary of the Invention The present invention is described by the independent claims to which reference should now be made. Advantageous features of the invention are set forth in the appendant claims.

The preferred sensor probe, as described below, comprises first and second cylindrical plates which together form the cylindrical capacitor of the sensor probe. The outer cylindrical capacitor plate is connected to the tool body and is held at ground potential. The innermost capacitor plate is driven at some other potential such that an electric field is established between the plates. The test fluid whose permittivity is to be determined is caused to occupy the space between the capacitor plates so that the capacitance of the capacitor is changed in accordance with the fluid permittivity. Adverse effects on the electric field between the capacitor plates which may compromise reliable sensor operation are alleviated in the preferred sensor probe embodiment by guard electrodes, which are disposed to entirely surround the circumference of the innermost of the two capacitor plates so that they at least partially protect the electric field between the capacitor plates from edge effects or short circuiting due to leakage currents. The test fluid may flow through the capacitor plates during sensor operation.

In alternative embodiments other capacitor geometries are employed.

Brief Description of the Drawings The present invention will now be discussed in more detail, and with reference to the drawings, in which: Figure 1 illustrates the preferred tool housing for a sensor probe according to an embodiment of the present invention.

Figure 2 illustrates a section through the sensor casing of the preferred embodiment, showing the arrangement of capacitor plates and guard circuit Figure 3 shows the sensor circuitry and power supply for the capacitor of the sensor probe in the preferred embodiment.

- 4 - Figure 4 shows the amplifier circuit driving the guard electrodes in the preferred embodiment.

Figure 5 shows the electric field geometry between the plates of the capacitor in a) the ideal case, b) in practice, and c) with guard electrodes present.

Figure 6a shows the electric field geometry as it would be if short circuited by a leakage current.

Figure 6b shows the electric field geometry, in the case of a leakage current, restored by the guard electrodes.

Figure 7 shows an alternative arrangement of capacitor plates and guard circuit, according to an alternative embodiment of the invention.

Figure 8 shows another alternative arrangement of capacitor plates and guard circuit, according to an alternative embodiment of the invention.

Description of the Preferred Embodiment Figure 1 shows an external view of the sensor housing 10 of the preferred embodiment. The sensor is designed for use in pipes, containers and boreholes and, therefore, has a substantially elongate shape of narrow diameter. The sensor housing 10 comprises a cylindrical tool body 30 with end caps 32 and 34 disposed at its two ends. In normal down-hole use, the sensor is just one of many different tools linked together to form a multipurpose tool-string for use inside an oil well bore-hole. Mechanical and electrical connection of the sensor 10 to adjacent sensors in the tool-string is achieved by connectors (not shown) situated underneath the end caps 32 and 34. The end caps are attached when the sensor is being stored, in order to protect the connectors, and are shown here in Figure 1.

Mounted concentrically on the tool body 30 at an approximately central location along its length is cylindrical outer capacitor plate 20. The capacitor plate is held in position around the inner tool body 30, which

narrows at this point, by supporting fins 36. Tool body 30, supporting fins 36 and outer capacitor plate 20 are all made of durable, conductive material and are held at earth potential. In practice, however, the tool body, fins and outer capacitor plate may have a potential that floats.

Figure 2 shows a lateral cross-section through the tool body 30 and the capacitor plate 20. The diameter of the tool body narrows as it approaches the cylindrical capacitor plate 20, such that where it lies inside the capacitor plate it has a smaller diameter than elsewhere. The portion of the tool body contained within the outer capacitor plate 20 has a hollow, cylindrical, insulated section 40 which bears a first inner cylindrical electrode 22 on its inner cylindrical surface, and second and third inner cylindrical electrodes 42 and 44 on its outer cylindrical surface. The second and third electrodes 42 and 44 or guard electrodes are disposed one on either side of the first electrode.

The thickness of the insulating section 40 lying above the inner cylindrical electrode 22 may be made sufficiently thin so that its contribution to the capacitance of the sensor capacitor is negligible.

Both first 22, second 42 and third 44 electrodes are insulated from one another by insulating section 40 and are situated concentrically with respect to the outer capacitor plate 30.

The first inner electrode 22 or sensor electrode is connected by wire 50 to sensing circuitry 60 and power supply, as shown in Figure 3, housed within the tool body 30. The configuration of the preferred sensor circuit 60 will be self-evident from Figure 3 to which reference should now be made. In the preferred embodiment shown in Figure 3, sensing circuitry 60 comprise 555 Timer IC 62. The sensor electrode 22, is covered by insulating section 40, and together with outer capacitor plate 20, forms the capacitor 26 of the sensor probe.

Guard electrodes 42 and 44, are connected to an amplifier circuit 80, by wire 52. The amplifier circuit, shown in Figure 4, has a high input impedance and a high output current capacity, and maintains the voltage of the guard electrodes 42 and 44 at substantially the same voltage as the sensor electrode 22. In conjunction with the outer capacitor plate 20, guard electrodes 42 and 44 form a guard circuit for the capacitor 26.

The operation of the preferred embodiment will next be described in more detail, with reference to the drawings. The sensor is immersed in the fluid whose composition is to be determined so that the fluid may occupy the space between the outer capacitor plate 20 and the inner capacitor plate 30, and thereby act as the dielectric of the sensor capacitor 26. The fluid may be relatively static within the capacitor, or may flow through it, as for example will occur if the fluid in the well naturally flows or if the sensor tool is drawn through a bore-hole on a tool string. The fluid can be either static or in motion during sensor operation.

on inserting the sensor 10 into a bore-hole, fluid may flow through the space between the outer plate of the capacitor and the tool body by entering through vents 38 between supporting fins 36. The region of the sensor in which a measurement is made is the region defined between the sensor electrode 22 and the outer capacitor plate 20. Sensor electrode 22 is insulated from the fluid in this region by insulating section 40, should the sample fluid itself be conductive.

A dielectric between the plates of the capacitor affects the strength of the electric field between the capacitor plates according to its permittivity, and therefore, consequently affects the capacitance of the capacitor.

The preferred sensor circuitry 60 measures the permittivity of the test fluid by determining the

capacitance of the sensor probe's capacitor in the manner described below and with reference to Figure 3.

Figure 3 shows the 555 Timer IC configured, according to the manufacturer's specifications such that the capacitor formed by the tool body and sensor electrode forms part of an RC charging/discharging circuit around the IC.

As the sensor electrode charges via Ra and Rb the potential at 'Trigger' is ramped up until the predetermined threshold value of 2/3 Vcc is reached at the threshold. At this point the IC switches the 'Discharge' pin to 'common' and the IQ' pin to Vcc potential. The capacitor then discharges via Rb into the 'Discharge' pin, until the lower predetermined threshold of 1/3 Vcc is reached at the 'threshold' pin. At this point the IC 60 isolates the 'discharge' pin and flips the IQ' pin to Icoml potential. Hence a square wave is generated at the IQ' pin with a frequency that is related to the choice of Ra and Rb and the capacitance of the sensor probe capacitor. Ra and Rb are selected in order to maintain a practicably wide range of frequencies, given the geometry of the sensor electrode, the tool body, the range of permittivities likely to be encountered and the capabilities of the frequency counter circuit or PC connected to the sensor circuit. Hence, by measuring the frequency of the square wave, and by knowing thevalues of Ra and Rb, the capacitance of the sensor probe capacitor and therefore the permittivity of the test fluid can be deduced. Once the permittivity of the fluid is known, its composition, namely what constituent fluids are present and in what proportion, can be deduced.

The value of the permittivity dependent capacitor voltage is dependent on the electric field that exists in the region between the two capacitor plates. Ideally, the electric field lines have a straight line geometry between the two plates so that the field strength is uniform, as shown in Figure 5a. However, in practice the electric field lines at the edges of the capacitor plates bend,

causing the electric field strength to vary in a lateral direction, and assume a geometry like that shown in Figure 5b. This edge effect is undesirable since it affects the capacitor plate voltage in a way that cannot be easily quantified, and thus introduces a source of uncertainty in the determination of dielectric permittivity.

In addition to edge effects, short circuiting from leakage currents can affect the electric field between the capacitor plates and compromise the measurement. Leakage currents can occur if a layer of conductive fluid forms on the insulator 40 above the inner capacitor plate 22 in the measurement region of the sensor and makes contact with the grounded tool body. Leakage currents between the tool body and measuring region can short circuit at least part of the electric field in the measuring region thereby making accurate readings of permittivity difficult if not impossible. Figure 6a shows a view of the electric field lines between the capacitor plates in the event of short circuiting from a wetting layer. The field lines are caused by the wetting layer to bend away from the straight trajectories that they should follow between the plates of the capacitor back onto the tool body.

In order to compensate for the effects mentioned above the preferred sensor embodiment employs guard electrodes, disposed one either side of the inner electrode, such that the electric field created by the guard electrodes 42 and 44 both contains entirely and extends that created by the sensor electrode 22, and that thereby moves the edge of the electric field away from the region in which measurement occurs. Any adverse edge effects that might disrupt the electric field and the determination of the sensor are therefore made less consequential since they are placed at a distance from the measurement region, as shown in Figure 5c.

In order to do this, the guard electrodes are driven, by amplifier circuit 70, at substantially the same voltage as the sensor electrode relative to the tool body. The

separation between the guard electrodes 42 and 44 and the first electrode 22 is therefore substantially transparent to the electric field created between the plates, since both guard electrodes 4.2 and 44 and sensor electrode 22 are driven at substantially the same voltage.

In the preferred embodiment the guard electrodes are exposed to the test fluid, and AC coupled to the circuit via a capacitor. This capacitor prevents DC current leakage and reduces the power consumption of the circuit.

The Operational Amplifier IC, shown in Figure 4, is configured to maintain the guard electrodes connected to the Op-Amp 'Output' Pin at the same potential as the sensor electrodes connected to the Op-Amp '+Input' Pin. If a perturbation in the fluid occurs which decrease the impedance of the guard electrodes, the 'Output' pin of the Op-Amp supplies more current in order to keep the guard electrode voltage from falling. On the other hand if a change occurs which increases the guard circuit impedance the Op-Amp will supply less current from the 'Output' to keep the guard electrode voltage from rising.

The guard electrodes 42 and 44 also protect the sensor electrode 22 from partial short-circuiting from leakage currents.

The guard electrodes 42 and 44, adjacent either edge of the sensor electrode, are maintained at sensor electrode voltage even in the case that a substantial short circuit between the guard electrodes and the tool body occurs. Although the current in the guard electrodes rises rapidly in such an event, the potential at the guard electrode is maintained by the amplifier circuit, so that the strength and geometry of the electric field sensor electrode are not disrupted. This can be seen in Figure 6b, which shows the electric field lines restored to their straight trajectory. In the case where the sensor is to be used in fluids from which only a non-conductive wetting layer may form the guard electrodes do not need to compensate for leakage currents and therefore may be themselves placed under the

insulating layer so that they are insulated from the fluid between the capacitor plates.

Furthermore, although in the preferred embodiment the first inner cylindrical electrode 22 is described as being disposed on the inner surface of section 40, it will be understood that were the fluid in which the sensor is immersed to be non-conducting, the first inner electrode 22 may be disposed on the outer surface of section 40 and therefore be in contact with the fluid.

In the preferred embodiment only the inner plate of the capacitor is driven by an electrode; the outer plate is part of the grounded tool body. In this case, only the inner plate requires guard electrodes.

However, in an alternative embodiment the outer plate of the capacitor 20 may also be provided with a sensor electrode and guard electrodes and may or may not be insulated from the test fluid. Or, additionally, only the outer capacitor plate 20 may be provided with a sensor electrode, and guard electrodes.

We have appreciated that although the geometry of the preferred embodiment is cylindrical and intended for use in down-hole environments, the advantageous features described herein apply equally to other designs or geometries and to tools for use in other industries or areas of expertise that may or may not be connected with the oil industry.

For example, the sensor may also be employed in water bore- holes, or in chemical process pipelines. Figures 7 and Figures 8, by way of example, show two alternative capacitor geometries, in which the capacitor plates are parallel.

Figures 7 and 8 respectively show a circular and a rectangular parallel plate capacitor arrangement, according to an embodiment of the present invention, from two views; in the left hand view (a) the two parallel plates 20 and 24 of the capacitor 26 are shown from the side; in the right hand view (b) the inner capacitor plate 24 is shown,

revealing sensor electrode 22, insulating section 40 and single guard electrode 42.

The guard electrodes shown in the preferred embodiment because of the cylindrical geometry of the sensor are situated one on either side of the sensor electrode. In the more general case, it will be understood that the circumference of the sensor electrode will be entirely bounded by a guard electrode or electrodes.

Claims

CLAIMS 1. A sensor probe for measuring fluid permittivity comprising: first and second plates, defining between them a space that a fluid, whose permittivity is to be determined, is allowed to occupy, wherein at least an area of the first plate defines a first plate of a capacitor, and at least an area of the second plate defines the second plate of a capacitor, such that, by determining one or more properties of the capacitor formed by the corresponding areas of the first and second plates, the permittivity of the fluid can be determined; and one or more guard electrodes; wherein one or more guard electrodes are mounted on either the first, the second, or both plates, such that they circumscribe the area of the plate that defines a plate of the capacitor and such that they are insulated from it; the guard electrodes being held at substantially the same voltage as the area of the plate that defines a plate of the capacitor, whereby the electric field created by the guard electrodes is such that it, at least partially, screens the capacitor from electrical effects that may influence the one or more properties of the capacitor that are used in the determination of fluid permittivity.
2. A sensor probe according to claim 1 in which both first and second plates are cylindrical and thereby define a capacitor with cylindrical geometry.
3. A sensor probe according to claim 1 in which both first and second plates are planar and thereby define a parallel plate capacitor.

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4. A sensor probe according to any preceding claim in which the one or more guard electrodes are insulated from the fluid occupying the space between the first and second plates.
5. A sensor probe according to any preceding claim in which the capacitor plates are insulated from the fluid occupying the space between them.
6. A sensor probe for measuring fluid permittivity comprising: first and second plates, defining between them a space that a fluid, whose permittivity is to be determined, is allowed to occupy, wherein at least an area of the first plate defines a first plate of a capacitor and is held at ground potential, and an electrode, mounted on an area of the second plate, defines the second plate of a capacitor, such that, by determining one or more properties of the capacitor formed by the corresponding areas of the first and second plates, the permittivity of the fluid can be determined; and one or more guard electrodes; wherein one or more guard electrodes are mounted on the second plate, such that they circumscribe the area of the plate on which the electrode is mounted and such that they are insulated from the electrode; the guard electrodes being held at substantially the same voltage as the electrode, whereby the electric field created by the guard electrodes is such that it, at least partially, screens the capacitor from electrical effects that may influence the one or more properties of the capacitor that are used in the determination of fluid permittivity.
7. A sensor probe according to claim 6 in which both first and second plates are cylindrical and thereby define a capacitor with cylindrical geometry.

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8. A sensor probe according to claim 6 in which both first and second plates are planar and thereby define a parallel plate capacitor.
9. A sensor probe according to any of claims 6, 7 or 8, in which the one or more guard electrodes are insulated from the fluid occupying the space between the first and second plates.
10. A sensor probe according to any of claims 6, 7, 8 or 9, in which the electrode is insulated from the fluid occupying the space between the first and second capacitor plates.
11. A sensor probe for measuring fluid permittivity comprising: first and second capacitor plates, defining between them a space that a fluid, whose permittivity is to be determined, is allowed to occupy, the capacitor plates being insulated from the fluid, such that, by determining one or more properties of the capacitor the permittivity of the fluid can be determined; and one or more guard electrodes; wherein the circumference of at least one of the capacitor plates is entirely bounded by a guard electrode, the guard electrode being held at substantially the same voltage as the capacitor plate, whereby the electric field created by the guard electrode is such that it, at least partially, screens the capacitor from electrical effects that may influence the one or more properties of the capacitor that are used in the determination of fluid permittivity.
12. A sensor probe according to claim 12, in which the capacitor plates are insulated from the fluid occupying the space between them.

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13. A sensor probe according to any preceding claim in which the fluid whose permittivity is to be determined is allowed to flow through the space defined by the first and second plates.
14. A guarded capacitance measurement probe according to any preceding claim in which the first capacitor plate is substantially larger than the second capacitor plate.
15. A guarded capacitance measurement probe, as described herein with reference to Figures 1 to 8 of the drawings.