GB2491823A

GB2491823A - Redundant downhole sensor system with voltage controlled switching

Info

Publication number: GB2491823A
Application number: GB1109647.6A
Authority: GB
Inventors: David Sirda Shanks
Original assignee: Zenith Oilfield Technology Ltd
Current assignee: Zenith Oilfield Technology Ltd
Priority date: 2011-06-09
Filing date: 2011-06-09
Publication date: 2012-12-19
Anticipated expiration: 2031-06-09
Also published as: GB201109647D0; GB2491823B

Abstract

A redundant sensor system monitoring an electric submersible pump in a wellbore includes a plurality of sensors 9 located downhole, a single power cable 7 to the surface and a voltage detector switch 52 for switching power between the sensors. A power supply 40 at the surface selectively produces a plurality of DC voltages under control of a voltage selector switch 42. Voltage detector switch 52 detects the applied voltage and switches on the appropriate sensor 9. If the sensor fails, voltage selector 42 changes the power supply voltage to a different level. This is detected by voltage detector switch 52 which switches off the failed sensor and switches on the redundant sensor. The two sensors may be identical. High voltage testing can also be performed without damage to the sensors because voltage detector switch 52 only switches on the sensors in response to the low voltages produced by supply 40.

Description

IMPROVEMENTS IN OR RELATING TO REDUNDANT MEASUREMENT SENSORS

The present invention relates to redundant measurement sensors and in particular, though not exclusively, to a monitoring system which uses a voltage controlled selector to switch power between a pair of sensors or gauges, for use in oil and gas wells.

Measurement and monitoring systems for many industrial applications provide redundant measurement sensors and gauges.

Where mission critical measurements are needed multiple identicai sensors are used with the additional sensors being redundant unless the main sensor fails. Redundant sensors are also used in places where access is difficult or expensive.

The installation of such multiple sensors with redundancy allows operations to be maintained while minimising repair and maintenance costs.

In the oil and gas industry, monitoring systems are reguired deep within the wellbores. Accordingly, redundancy has become common practice in order to increase the operational life of the monitoring systems. These redundant systems have taken many forms over the years, but typically fall into two categories. The first has dual redundant electronics arranged internally to a sensor module located downhole with a single cable to the surface of the well. The second, more common, arrangement has dual redundant sensors located downhole which are connected to surface by either separate cables (fully redundant system) or a single cable (multi-drop system) One area where monitoring is required is for ESP (Electric submersible pump) completions. Typically a single sensor is located below the ESP and the output signal is superimposed onto the 3-phase ESP power oable to the well surfaoe. The traditional conneotion scheme for an ESP monitoring sensor is shown in Figure 1. On the left of the Figure is the surface system connected via a downhole cable 7, to the downhole system on the right of the Figure. The ESP motor power circuit typically consists of a 400 volts 3-phase supply 1, controlled by a variable speed drive 3, which then feed a step up transformer 4. This high voltage AC supply 21 is connected to the downhole motor 8 by the cable 7. The typical ESP sensor circuit is also shown in Figure 1. A simple DC power suppiy 5 powered from AC or DC at 2, is converted in unit 5 into a DC voltage. This DC power is fed into the 3-phase circuit via a coupling choke 6. The sensor or gauge 9 connects between the downhole motor 8 on its Y-point and the local ground downhole, normally through the tubing and casing steel.

Due to the complexity of using the 3-phase power cable, dual redundant gauges in this area are of the first category as shown in Figure 1 where there is a single point connection (at the Y point) and two sets of electronics internally arranged within the sensor 9, or two separate instrument cables are run downhole to two independent gauges.

As detailed above running two cables is not a viable option in this environment. This is because cabled systems add considerable complexity and cost to DSP completions, and the introduction of a new cable, or cables, introduces a potential failure mechanism as cable damage is more likely to cause problems than the failure of the electronics which the redundant system is designed to alleviate. Such a system is therefore less reliable and consequently is not generally used.

The alternative has a single sensor with dual internal electronics. However, both sets of electronics must be running all of the time. Conseguently, the very electronics that you trying to extend the life of are in use, generally, all of the time and so the redundant electronics system is as likely to fail as the primary system.

There is further a need with ESP systems to verify the condition of the cable and motor power system using high voltage testers often applying test voltages of thousands of volts (2500V is quite common) . These testers generally will cause damage to downhole sensors and it is normal practice to use a blocking diode in the downhole system to allow the high voltage testing to be carried out as long as the test voltage is negative. This method has the drawback that if the voltage applied is inadvertently applied as positive, this may cause damage to the downhole sensor.

It is therefore an object of at least one embodiment of the present invention to provide a redundant sensor system which allows two completely identical downhole sensors to be installed with only one of them powered at any given time, so that in the event one fails the back-up can be powered up and used.

It is a further object of at least one embodiment of the present invention to provide a redundant sensor system capable of being high voltage tested in either direction without causing damage to the downhole sensors.

According to a first aspect of the present invention there is provided a redundant sensor system for use in monitoring an ESP in a wellbore, the system comprising a plurality of sensors located within the wellbore, a single power cable to a surface of the well and a voltage controlled selector for switching power between the sensors.

In this way, two completely identical downhole sensors can be installed with only one of them powered at any given time, so that in the event one fails the back-up can be powered up and used.

Preferably the plurality of sensors, are separate identical DC powered sensors.

In this way, the selected sensor can be powered while data is transmitted up the power cable.

Preferably, the system includes a data receiver, the receiver sending a signal to the voltage controlled selector to switch in the event that data is no longer received from a sensor.

In this way, the system automatically switches to a redundant sensor to prevent loss of data.

Preferably, the system includes a DC power supply having a plurality of voltage outputs and switching between the sensors is achieved by applying a different supply voltage.

Advantageously, the DC power supply is located at the surface of the well.

In this way, the power supply is kept at the surface of the well and switching is controlled at the surface of the well.

Preferably, the system includes a voltage detection switch located downhole.

In this way, a change in the supply voltage is detected downhole and the voltage is switched to a different sensor.

Preferably, the system includes a voltage converter so that the supply voltage can be returned to it's original setting for use by a redundant sensor.

In this way, an increase in the supply voltage signals a switch to a redundant sensor, but the redundant sensor will be operated from the same supply voltage as the sensor operating before the switch. All the sensors may therefore be identical.

Preferably, the voltage detection switch is arranged to switch power only after a pre-determined event has occurred to the supply voltage.

In this way, the voltage detection switch can be tolerant to residual voltage variations caused by changes in the environmental conditions. Additionally, the switch will not respond to AC signals.

In an embodiment, the pre-determined event is for a supply voltage to be applied within a voltage window and until such an event occurs, no supply voltage is switched to any sensor.

In this way, a high voltage test such as a Megger test can be applied to the line as long as the high voltage signal is higher than the voltage level of the window and has a short duration in the voltage window.

According to a second aspect of the present invention there is provided a method for continuous monitoring of an ESP in a well, comprising the steps: (a) providing first and second DC powered ESP monitoring sensors connected through a voltage detection switch to a Y-point on the ESP; (b) providing a variable DC voltage supply at a surface of the well with DC current fed into a 3-phase power cable connected to the ESP; (c) selecting a first DC voltage and using the voltage detection switch to feed the voltage to the first sensor; (d) transmitting data from the first sensor to the surface by modulating the DC current; (e) selecting a second DC voltage and using the voltage detection switch to switch the voltage to the second sensor; and (f) transmitting data fron the second sensor to the surface.

In this way, monitoring can be continued in the event that the first sensor fails.

Preferably, the method includes the step of detecting an absence of transmitted data from the first sensor and selecting the second DC voltage at the surface.

In this way, an automatic switch between the sensors can be made to ensure continuous monitoring is maintained.

Preferably, the method includes the step of programming the voltage detection switch to switch only on a pre-determined event.

In this way, voltage will only switch on a desired command and the entire system can be automated.

Preferably, the method includes the step of applying a gradual voltage at the surface in order to determine the voltage switching threshold of the voltage detection switch.

In this way, the system can be calibrated from the surface prior to use.

Preferably, the method includes the step of applying a high voltage on the 3-phase power cable to test the cable while the voltage detection switch prevents any voltage being applied to the first or second sensor.

In this way, a Megger test can be done without risk of damaging the sensors.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a schematic illustration of an ESP monitoring

system according to the prior art;

Figure 2 is a schematic illustration of ESP monitoring system located in a wellbore including a redundant sensor system according to an embodiment of the present invention; Figure 3 is a schematic illustration of an ESP monitoring system including a redundant sensor system according to an embodiment of the present invention; Figure 4 is a schematic illustration of a DC power supply and associated voltage switching components, as located at a surface cf a well, for use in the redundant sensor system of Figure 3; Figure 5 is a schematic illustration of a voltage detection switch together with sensors, as located downhole, for use in the redundant sensor system of Figure 3; Figures 6(a) and 6(b) illustrate two embodiments of a voltage sensing circuit located within the voltage detection switch of a redundant sensor system according to a further embodiment of the present invention; Figure 7 is a graph of applied voltage against current drawn to determine threshold switching voltages according to an embodiment of the present invention; and Figure 8 is a graph of voltage against time to illustrate the method of conducting a high voltage test through the redundant sensor system according to an embodiment of the present invention.

Reference is initially made to Figure 2 of the drawings which illustrates a typical ESP completion in a wellbore. An ESP motor 10 is coupled through a seal 12 to a centrifugal pump 14 and used to lift the fluids through a tubing 16 to a surface 18 of the well 20 in a manner known to those skilled in the art. In order to monitor the operation, sensors or gauges 22 are located below the ESP 10. Typically, the motor 10 is a three phase Y configuration. The motor is driven by a variable speed drive system 24 and is connected via a three phase power cable 26. The ESP monitoring system 32 can be considered to comprise two distinct parts, a surface system, generally indicated by reference numeral 28, and a downhole system, generally indicated by reference numeral 30. These two parts 28,30 communicate using the ESP power cable 26. A prior art surface system 1-6 and a downhole system 8,9 connected via a power cable 7, according to the prior art, is shown in Figure 1.

Reference is now made to Figure 3 of the drawings which illustrates an ESP monitoring system 32, comprising a surface system 28 and a downhole system 30, according to an embodiment of the present invention. Like parts to those of the prior art system have been given the same reference numerals, for reference purposes.

As described hereinbefore with reference to Figure 1, the surface system 28 comprises typically a 400 volts 3-phase supply 1, controlled by a variable speed drive 3, which then feeds a step up transformer 4. This high voltage AC supply 21 powers the downhole motor 8 via the cable 7. Power for the sensors 9a,9b is now from a DC power supply 12 powered from AC or DC at 2. DC power supply 40 has two or more output voltages as compared to the simple DC power supply 5 of the prior art.

Additionally, between the power supply 40 and the choke 6, there is located a surface voltage selector switch 42 used to switch the voltage from the power supply 40 before transmission down the cable 26.

The complete power supply system for the sensors 9a,b is shown in Figure 4. This system consists of the surface DC power supply 40, a data decoder 44, a processor based controller 46 and the voltage switch 42 allowing the processor controller to set the power supply to two or more different supply voltages.

The system is coupled to the 3-phase power cable 26 with a three phase choke 6.

Also shown in Figure 3 is the downhole system 30. The ESP motor 8 includes a Y-connection providing a star point 50 through which power to the sensors 9a,b is obtained and data is transmitted back to the surface. In this embodiment of the present invention a voltage detection switch 52 is connected to the star point 50 and two standard ESP monitoring sensors or gauges 9a,b are connected to the switch 52. The voltage detection switch 52 and the sensors 9a,b are shown in more detail in Figure 5.

The voltage detection switch 52 consists of a voltage sensing circuit 54, some selection logic 56, two power switches 58a,b and a voltage converter 60. The sensors 9a,b are those as would be used downhole currently and are both the same so that the same data will be transmitted regardless of which of the two sensors is operating.

In use, the surface system 28 and the downhole system 30 are installed in a wellbore 20 as is known in the art. The voltage switch 42 selects an output voltage from the Dc power supply 12. This first voltage is transmitted through the three phase cable 26 to the star point 50 of the ESP motor.

At the same time a high voltage AC supply is transmitted through the cable 26 to operate the FSP motor 8. The voltage sensing circuit will detect the applied DC voltage and using the logic 56 close one of the power switches 58. All the applied DC voltage is transmitted to a sensor 9b, say. Sensor 9b will thus be activated to carry out monitoring and it's output data modulated on the current and transmitted back to the surface via the star point 50 and the cable 26. This data transmission can be as presently carried out in the prior art system shown in Figure 1. While the sensor 9b is monitoring, sensor 9a is inactive and plays no role in the downhole system as there is no voltage applied to it. Sensor 9a can therefore be considered as a redundant sensor.

In the event that something occurs to the sensor 9b and the data transmission is disturbed, this disturbance will be detected at the surface by the data decoder 44. A signal is then sent to the processor 46 to operate the voltage selector switch 42. Once switched, a second voltage, in place of the first, will be transmitted down the cable 26 to the star point 50. The second voltage will be higher than the first voltage but remain as a DC voltage low in comparison to the high AC voltage for the motor 8. The voltage sensing circuit 54, in the voltage detection switch 52, will operate on the increased voltage detected sending a signal to the logic 56. If the voltage increase is sufficient to meet a pre-determined logic operation, the unit 56 will close the power switch 58b and open the power switch 58a. Voltage to the sensor 9b is thus stopped and the sensor 9b becomes inactive and redundant. The second voltage now passes to a voltage converter 60 where it is stepped down to be the same as the first voltage. This voltage is applied to the sensor 9a making it become active, start monitoring and sending data to the surface by the identical method to that of sensor Sb. As both sensors are the same, the data transmission will appear continuous at the surface. Additionally, since the sensor data is transmitted by the current drawn the increase in operating voltage does not affect the transmitted data.

Reference is now made to Figures 6(a) and 6W) which illustrate example embodiments of the voltage sensing circuit 54 in the downhole system 30. Figure 6(a) shows a simple analogue comparator 62 used to sense when the applied voltage is above a level via the sense resistors 64 and compared to a reference voltage set by reference diode 66. Figure 6(b) shows an alternative where the voltage is sensed by sense resistors 64 and this is read using an analogue to digital converter 68 and stored in a microprocessor 70.

It will be apparent to those skilled in the art that the he down hole sensing circuit 54 can be a simple analogue circuit as described in Figure 6(a) or a digital processor based system as described in 6(b) or any combination of analogue and digital measurement methods. The logic for switching the two sensors can be a) simple hardware with fixed switching thresholds or fully or partly processor controlled allowing more complex switching methods to be implemented. It is implicit in this invention that there can be many methods of activating the sensor switches 58.

In one embodiment, the sensor switches 58 are switched by a method of using a voltage window which requires the voltage to be present for a length of time. This is only one possible implementation. Other possible implementations would be to a voltage being present in long pulses, for example, where the power is present for 30 seconds and then removed for 30 seconds and then applied again. It will be apparent to those skilled in the art that it is possible to control the down hole switch behaviour by applying and removing power in any predetermined sequence of voltage and/or time to provide a crude method of control over the downhole switching system.

Using a voltage window as a switching method can be effective, but this can also be limiting, if the threshold of the voltage switching is not known. While the threshold can be theoretically determined during installation it is known that environmental faotors will oause variation in the expeoted results. The present invention provides a method for determining the threshold voltages whioh oan be determined when the system is in place.

Referring now to Figure 7, there is a graph which shows how the system can adapt to different cable lengths and differing motor systems. The motor system can cause a variable and unknown voltage loss between the surface and downhole systems.

In this method, the voltage at surface is increased gradually, plotted on the x-axis 70 in the graph 72, and by monitoring the current drawn by the downhole system 30 shown on y-axis 74 in the graph 72, it is possible to easily determine the voltage switching windows by the current drawn by the downhole sensors when switched on shown at 76 in the graph, or indeed the absence of current when the sensors are switched off 78.

Using this method the actual voltage window in any installation can easily be determined at installation, or indeed at any time after installation.

The setting of the actual voltage windows can be a) fixed at manufacture b) set up in the field manually or c) fully automatic using a microprocessor to perform voltage changes measuring the gauge current and thus automatically determining the voltage selection window.

In the same way, the surface supply 40 can be a range of fixed voltages controlled by simple switches, like relays or a manually adjustable power supply using switched mode regulation or a more basic form of control such as an AC variance. The surface supply can also be a microprocessor controlled voltage supply to allow programmatic control of the supply voltage.

In any ESP completion it is common to verify the condition of the cable 26 and the downhole motor power system. High voltage testers can be used e.g. Megger tester, which apply a high voltage down the cable. This voltage can be around 2500 V. As the redundant sensor system of the present invention can be programmed to switch only in response to a voltage window, these high voltage tests can be carried out in either direction without causing damage to the downhole sensors 9.

An illustration of a high voltage test according to an embodiment of the present invention is shown in Figure 8. The graph 80 shows applied voltage to the downhole sensors on the y-axis 82 plotted against time on the x-axis. A high voltage tester will apply high voltages 86 relatively quickly to the system, typically within a few seconds 88. The voltage selector 54 will not enable the power switch 58 for any sensor until a valid voltage, for example one of traces 92 or 94 (or more), has been present for a longer period of time 90, typically 30 or more seconds. In this way high voltage testing can be safely carried out on this system.

Thus two identical DC powered ESP monitoring sensors operate in parallel on a single ESP system. The method described here uses two such DC current gauges but with a selector circuit which selects one of the two sensors, so that only one sensor has power applied to it at any given time, controlled from surface. The selected sensor then is connected to the motor and transmits data to the surface. The selector circuit uses different DC supply voltages to select between the two gauges, and a current sensing scheme to allow the surface equipment to adapt to different installations. The voltage selector downhole further has time delay property which allow high voltages to be applied for test while both the down hole sensors are kept isolated from the motor system.

The principle advantage of the present invention is that it provides a redundant sensor system which allows two completely identical downhole sensors to be installed with only one of them powered at any given time, so that in the event one fails the back-up can be powered up and used.

A further advantage of at least one embodiment of the present invention is that it provides a redundant sensor system capable of being high voltage tested in either direction without causing damage to the downhole sensors.

It will be apparent to those skilled in the art that modifications may be made to the invention herein described without departing from the scope thereof. For example, while we have primarily referred to downhole sensors, any DC powered sensor, gauge or monitoring eguipment could be used.

Additionally, any number of identical sensors could be mounted downhole and the present invention used to switch between these for increased failure safety. It will also be apparaent that the present invention could be used for two or more different sensors, where switching occurs when a different measurement is reguired. In this way, the individual sensors can be operated in series from the surface.

Claims

CLAIMS1. A redundant sensor system for use in monitoring an ES? in a wellbore, the system comprising a plurality of sensors located within the wellbore, a single power cable to a surface of the well and a voltage controlled selector for switching power between the sensors.
2. A redundant sensor system according to claim 1 wherein the plurality of sensors, are separate identical DC powered sensors.
3. A redundant sensor system according to claim 1 or 2 wherein the system includes a data receiver, the receiver sending a signal to the voltage controlled selector to switch in the event that data is no longer received from a sensor.
4. A redundant sensor system according to any preceding claim wherein the system includes a DC power supply having a plurality of voltage outputs and switching between the sensors is achieved by applying a different supply voltage.
5. A redundant sensor system according to claim 4 wherein the DC power supply is located at the surface of the well.
6. A redundant sensor system according to any preceding claim wherein the system includes a voltage detection switch located downhole.
7. A redundant sensor system aooording to any one of olaims 4 to 6 wherein the system includes a voltage oonverter so that the supply voltage oan be returned to it's original setting for use by a redundant sensor.
8. A redundant sensor system aooording to olaim 6 or 7 wherein the voltage deteotion switoh is arranged to switoh power only after a pre-determined event has ooourred to the supply voltage.
9. A redundant sensor system aooording to olaim 8 wherein the pre-determined event is for a suppiy voitage to be applied within a voltage window and until suoh an event ooours, no supply voltage is switched to any sensor.
10. A method for continuous monitoring of an ESP in a well, comprising the steps: (a) providing first and second DC powered ESP monitoring sensors connected through a voltage detection switch to a Y-point on the ESP; (b) providing a variable DC voltage supply at a surface of the well with DC current fed into a 3-phase power cable connected to the ESP; (c) selecting a first DC voltage and using the voltage detection switch to feed the voltage to the first sensor; (d) transmitting data from the first sensor to the surface by modulating the DC current; (e) selecting a second DC voltage and using the voltage detection switch to switch the voltage to the second sensor; and (f) transmitting data from the second sensor to the surface.
11. A method aocording to claim 10 wherein the method includes the step of detecting an absence of transmitted data from the first sensor and selecting the second DC voltage at the surface.
12. A method according to claim 10 or 11 wherein the method includes the step of programming the voltage detection switch to switch only on a pre-determined event.
13. A method according to any one of claims 10 to 12 wherein the method includes the step of applying a gradual voltage at the surface in order to determine the voltage switching threshold of the voltage detection switch.
14. A method according to any one of claims 10 to 13 wherein the method includes the step of applying a high voltage on the 3-phase power cable to test the cable while the voltage detection switch prevents any voltage being applied to the first or second sensor.