WO2007003445A1

WO2007003445A1 - Sensor system for gas lift wells

Info

Publication number: WO2007003445A1
Application number: PCT/EP2006/050660
Authority: WO
Inventors: Philip Head
Original assignee: Philip Head
Priority date: 2005-02-03
Filing date: 2006-02-03
Publication date: 2007-01-11

Abstract

A meter system for measuring parameters of conditions within production tubing (1), comprising a plurality of meters being linked by a single fibre-optic cable (10). Some of the meters comprise a sensing element employing a Bragg grating sensor and ideally the meters are situated on a tool (7) disposable down hole or engagable with a side pocket in the production tube. The tool may be a pump.

Description

Sensor system for gas lift wells

Real-time monitoring of artificial lift systems

The invention relates to the real-time monitoring of artificial lift systems.

Gas lift is a well understood and very commonly used method of helping to lift hydrocarbons from reservoir to surface.

This is achieved by injecting under pressure from surface gas into the production tubing annulus. Down the length of the production tubing are located gas lift valves. Each are set to a pre defined cracking pressure, so that they meter gas into the production tubing, which in turn helps to lift the oil to surface.

If a valve is not working correctly or is not allowing sufficient gas to enter the production tubing, then production is not optimised and the net flow rate is not maximised.

Sensors are utilised to monitor conditions in the production tubing, and such sensors may measure various physical parameters such as pressure and temperature and often rely on the transmission of strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a sensing element. In a pressure sensor, the sensing element may be bonded to the elastic structure with a suitable adhesive. An industrial process sensor is typically a transducer that responds to a measure and with a sensing element and converts the variable to a standardized transmission signal, e.g., an electrical or optical signal, that is a function of the measure and. Industrial process sensors utilize transducers that include pressure measurements of an industrial process such as that derived from slurries, liquids, vapors and gasses in refinery, chemical, pulp, petroleum, gas, pharmaceutical, food, and other fluid processing plants. Industrial process sensors are often placed in or near the process fluids, or in field applications. Often, these field applications are subject to harsh and varying environmental conditions that provide challenges for designers of such sensors. Typical electronic, or other, transducers of the prior art often cannot be placed in industrial process environments due to sensitivity to electromagnetic interference, radiation, heat, corrosion, fire, explosion or other environmental factors. It is also known that the attachment of the sensing element to the elastic structure can be a large source of error if the attachment is not highly stable. In the case of sensors that measure static or very slowly changing parameters, the long term stability of the attachment to the structure is extremely important. A major source of such long term sensor instability is a phenomenon known as "creep", i.e., change in strain on the sensing element with no change in applied load on the elastic structure, which results in a DC shift or drift error in the sensor signal.

Certain types of fiber optic sensors for measuring static and/or quasi-static parameters require a highly stable, very low creep attachment of the optical fiber to the elastic structure. Various techniques exist for attaching the fiber to the structure to minimize creep, such as adhesives, bonds, epoxy, cements and/or solders. However, such attachment techniques may exhibit creep and/or hysteresis over time and/or high temperatures. One example of a fiber optic based sensor is that described in U.S. Pat. No. 6,016,702 entitled "High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments" to Robert J. Maron, which is incorporated herein by reference in its entirety. In that case, an optical fiber is attached to a compressible bellows at one location along the fiber and to a rigid structure at a second location along the fiber with a Bragg grating embedded within the fiber between these two fiber attachment locations and with the grating being in tension. As the bellows is compressed due to an external pressure change, the tension on the fiber grating is reduced, which changes the wavelength of light reflected by the grating. If the attachment of the fiber to the structure is not stable, the fiber may move (or creep) relative to the structure it is attached to, and the aforementioned measurement inaccuracies occur. In another example, a optical fiber Bragg grating pressure sensor where the fiber is secured in tension to a glass bubble by a UV cement is discussed in Xu, M. G., Beiger, H., Dakein, J. P,; "Fibre Grating Pressure Sensor With Enhanced Sensitivity Using A Glass-Bubble Housing", Electronics Letters, 1996, VOl 32, pp. 128-129.However, as discussed hereinbefore, such attachment techniques may exhibit creep and/or hysteresis over time and/or high temperatures, or may be difficult or costly to manufacture.

It is an objective of this invention to improve the monitoring of conditions in an artificial lift systems, and particularly to monitor valves deployed in such systems.

According to the present invention, there is provided a fibre optic based flow meter measuring the gas flow in each valve into the production tubing. The present invention may employ fibre optics and sensing elements integral with the fibre such as pressure and accelerometers to determine such parameters such as the distributed temperature along the power cable, the pump inlet pressure, discharge pressure, pressure across a venturi to determine actual flow rate, temperature in the motor windings, vibration of the system, and or the type of noise generated indicting the current condition of bearings etc, contamination of the motor oil through optical sensors etc.

According to another aspect of the present invention, Bragg grating are bonded to inconel diaphragms, when the diaphragm surface is subjected to deformation by the pressure applied against it the bragg grating measures the strain from which when corrected for temperature can correlated to the actual pressure.

According to another aspect of the present invention, the bragg grating can be sandwiched between two thin inconel or other low creep metal discs and pressure can be applied on either side of the disc and the nett differential pressure will be determined.

The use of fibre optics for measuring temperature of the well bore production fluids is well understood. The present invention fibre optics to monitor an electrical submersible pump power system i.e. the cable, the motor, and in addition the well bore fluid conditions.

The advantage of the fibre optic means of sensing is that there are no electronics, as a result the housing is very compact and can be hermetically sealed from the tough gas environment. Finally, because the sensor is based on bragg grating technology, a whole series of sensors can be daisy chained together for no additional cable costs.

It is a further aspect of this invention that all the required parameters may be determined through a single fibre. The following embodiment will illustrate the invention by way of example

Figure 1 is a side view of an electrical submersible pump (ESP) in a well.

Figure 2 is a similar view to figure 1 with an indication of the sensor types and positions piggy backed on the common fibre link

Figure 3a shows a side view with a conventional ESP system

Figure 3b shows a detailed side view of the pressure sensor cartridge

Figure 3c shows a detailed cross section of a power cable

Figure 3d shows a detailed cross section of an electrical motor

Figure 4a shows a side view of a bottom intake ESP

Figure 4b shows a detailed side view of the pressure measuring cartridges

Figure 5c shows a self supporting ESP power cable.

Figure 6. is a side cross section of a well at a gas lift side pocket mandrel

Figure 7 is a plan cross section of a well at a gas lift side pocket mandrel

Figure 8 is a blown up detail of figure 7

Figure 9 is a plan view of the sensor housing Figure 10 is a side section YY of figure 9

Figure 11 is a side section XX of figure 9

Figure 12 is a similar view to figure 11 with end seal removed

Figure 13 is a similar view to figure 12 with one half of the pressure diaphragm removed

Figure 14. is a section side view of one embodiment of the invention

Figure 15. is a section side view of a further embodiment of the invention

Figure 16 is a exploded section side view of assemble shown in Figure 2

Figure 17 is a side view of the assembly prior to connection to a metal clad fibre optic cable

Figure 18 is a side view of the assembly after connection to a metal clad fibre optic cable

Figure 19 is a section side view of a further embodiment of this invention

Figure 20 is a section side view of a further embodiment of this invention

Figure 21 is an exploded section side view of the invention and the parts required to mount it in a carrier tube Figure 22 is a section side view of the invention assembled in the carrier shown in Figure 21.

Figure 23 is a section side of a differential sensor used in conjunction with a venture flow meter

Figure 24 is a section side of a differential sensor used in conjunction with a venture flow meter, with the venturi throat removed for well access

Figure 25 is a section side as shown in Figure 24, with a running tool setting the venturi throat in place.

Figure 26 is a section side as shown in Figure 24, with a running tool being removed from the well.

Figure 27 is a section end view of one embodiment of the invention

Figure 28 is a plan view of the embodiment shown in figure 1

Figure 29 is a section side view of the embodiment shown in figure 1

Figure 30 is a section side view of a further embodiment of this invention

Figure 31 is a plan view of the embodiment shown in figure 30

Figure 32 is a similar view to figure 31 showing the fibre routing.

Figure 33 is a section XX side view of figure 32 Figure 34 is a section YY side view of figure 33

Referring to figure 1 there is shown a well bore 1 with perforations 2 through which fluid flows 3. A power cable 4 is strapped to production tubing 5 on the end of which is attached a pump 6 driven by a electrical motor 7. This is a conventional electrical submersible pump arrangement and well understood by those skilled in the art.

Referring to figure 2, there is shown schematically where a fibre would be positioned and what sensors would be employed. The fibre 10 could be independently strapped to the tubing from surface, however, it would be better if it was embedded within the power cable so that the temperature along the cable could be continuously monitored. At the termination of the cable in the motor, the fibre would be run separately and through three separate pressure chambers. In each chamber a pressure sensor integral with the fibre would measure pressure in the chamber. Each chamber would measure pressure from the following positions, pump inlet pressure, Pl pump discharge pressure P2 and pressure at the throat of a venturi P3, the fibre could also be extended to pass through the motor windings providing a full heat profile of the motor and other sensors such as optical and acoustic sensors could determine the motor oil quality or the status of the bearings. Finally the fibre could be spliced onto a tail hanging from the motor which would extend more inline sensors across the reservoir providing real time pressure P4,...Pn and matching temperature measurements.

Referring to figure 3a to 3d there is shown some further embodiments. Figure 3c shows a cross section of a typical power cable.19 In the centre there is a small space left by the conductors in this space a fibre 20 would be laid, this would provide temperature information about all three conductors. Inside the motor the three conductors would be connected to each phase of the motor. In addition the fibre from the cable would be spliced 22 to a fibre 21 already threaded into the motor wire laminations 23. The fibre could have numerous sensors on it, or splices such as a tailpipe splice 24, or external pressure sensor assembly 25. The arrangement shown consists of three integral fibre optic pressure sensors 26 separated by a pressure bulkhead 27 and installed in a tube 28. The tube 28 has seals 29 which are in a corresponding position to the seals 27. The tube 28 is installed in a bore 30 of the flowmeter 31, and when correctly positioned, chamber 32 measures pump discharge pressure. Chamber 33 measures pressure in the ventrui throat and chamber 34 measures pump inlet pressure.

Figures 3e and 3f show a further embodiment of a pump having sensors linked by a fibre optic cable. Pressure is measured at 60, 62 and 64, temperature is measured along the cable 65 and along the stator 66. The pump performance is also monitored by an XY accelerometer at 63. The temperature in the stator 66 can be monitored in many positions 67.

Referring to figure 4a to c there is shown an inverted pump arrangement; this is relatively new and becoming a more common method for pump installation. In this instance the pump is suspended on a self supporting power cable 40. In detail it consist of a hydraulic steel tube for the core, around this is helically laid a number of electrical conductors 42 to transmit electrical power. This is encapsulated in a jacket 43 Fibre optic cables 44 are embedded in the jacket to provide temperature feedback. Finally an external stainless steel sheave 45 is wrapped, seam welded and swaged down onto the jacket to complete the assembly. At the termination of this cable would be a sub to mount the pressure sensors 50. These sensors (shown in detail in figure 4b) would be like those described above and would measure the same pump pressures (shown by a representation of hydraulic tubes from each sensor linked to the relevant pump section). In this case the pressure drop through the bore of the motor would be used to determine the flow rate.

Referring to figures 6-13, the production tubing 1 is located inside a casing 2. Gas is injected into the annulus area 3 under pressure and enters the gas lift valve via port 4. the gas lift mandrel 5 contains regulator and springs for fine adjustment. The sensor assembly 6 is attached to the port 4. The gas flows through an orifice 7 in the sensor and the pressure is sensed via porting 8 and 9 and the difference applied across a inconel membrane 10. The membrane 10 consists of two parts 11 and 12, which when assembled sandwich a bragg grating fibre 13. Each layer when put together is laser welded to hermetically seal it to its mating part. So laser welding would be performed at interface 14, 15 and 16. Two bragg gratings are required, the first measures both strain caused by pressure and temperature, and the second measures strain caused only by temperature, differential equations allow the pressure value to be determined.

The nature of bragg grating fibre optic sensors is that the a large number of grating can be interrogated on a single fibre, so for a typical gas lift well installation, only a single fibre would be required to convey all the data back to surface.

Referring to figures 14 to 18 there is shown an absolute pressure and temperature sensor utilising bragg grating fibre optics. A housing 1. has a hermetically sealed tube 2 attached to it by welding or other permanent bonding means. A fibre optic cable incorporating two bragg grating sensing elements is fed into the tube and one bragg brating element is bonded to the outer surface of the housing 3 and the other is bonded 5to the pressure sensitive dish 4, the fibre is allowed to follow a helical path 6, which when assembled 7. does not create any unnecessary stress in the fibre. The pressure sensitive disc sits in a bore 8, which has concentric undercuts machined into it. A process interface sleeve 10 has an interferance fit at the interface 11. This swages the pressure sensitive disc against the concentric undercuts 8 and forms many metal to metal seals. The process interface sleeve 10 is located in a similar bore 12 to the pressure sensitive disc. And this is swaged into the concentric undercuts 13 using a ring 14. on the outer surface of the process connection is a conventional autoclave type metal to metal process fitting 15, which also allows the process fluid into the metal to metal sealed pressure sensing chamber 16. In the atmospheric chamber 20, one bragg grating sensor 3 provides a reference strain for ambient conditions, and the bragg grating sensor 7 attached to the pressure sensing disc 4 has strain for ambient conditions together with the applied pressure. If the reference stain is deducted from the combined strains measured by the bragg grating sensor 7, then a true measure of absolute pressure can be determined. It maybe necessary to connect a single pressure sensor module 30 to a metal clad fibre 21. First two sides of the fibre need to be aligned and fused together using a fibre optic splicing tool (not shown) but well understood in the industry. Once fused together, minimum stress should be applied to this joint 22. Over the outside of the metal clad cable is a sleeve

23. This can be slide down the outside of the tube and threaded into a bore

24. On its tip is an autoclave type profile 25 which forms a metal to metal seal when fully installed in the bore 26, Once secure in this bore the outer nut is made tight against a metal to metal seal olive 28. Thus the whole bragg grating pressure sensor is sealed in a metal to metal assembly. Referring to figures 19 and 20 there is shown two embodiments of a differential pressure sensor. In this design, the bragg grating sensor 40 is bonded to one pressure sensing disc 41, the fibre continues to the circumference of the disc where it is wrapped in a small annular void 42, and in this void a further bragg grating sensor element 43 is also located. The space between the two discs 44 and 45 is filled under vacuum with a suitable elastic potting / bonding material. The discs 44 and 45 are retained using the same process described above.

Referring to figures 21 and 22 there is shown how the pressure sensor is mounted in one embodiment of this invention. An absolute pressure sensor 50 already connected to a metal clad fibre optic cable 21. is aligned with a bore 51 in a "pup" joint 52 of a tubing string to be deployed in a well. At the centre of the bore 51 is a autoclave profile 15' which matches that of the pressure sensor 50. A retaining plate 53 is placed over the pressure sensor and holds the two side of the autoclave surfaces 15 and 15' tightly together, when the cap head screws 54 are made tight.

Referring to figures 23 to 26 there is shown a further embodiment of this invention, with the pressure sensors mounted to measure the pressure across the throat of a venturi type flowmeter. The flowmeter body 60 is a short section of tubing which is conveyed into the well. Inside the tube 60 is a bore 61 in which is installed a retrievable venturi profile 62. The external surface of the profile has three sets of seals 63 which are energised by a ratchet mechanism 64 and lock down collet 65. Ports in the venturi profile 66 and 67 align with ports 68 and 69 of the bragg grating differential pressure sensor, this can be designed for very small differences in pressure, even though the whole assembly may be subjected to significant absolute pressure. A further absolute pressure sensor 70 could be installed to measure the external pressure, which if installed in a well with an electrical submersible pump would provide the pump inlet pressure.

A running tool 71 could be used to retrieve the venturi profile, either to resize the profile of to provide full bore access to the well for well servicing etc.

Referring to figures 27 to 29 there is shown an absolute pressure and temperature sensor utilising bragg grating fibre optics. A housing 1 has a chamber 2. machined in it. The chamber is closed by a 2nd disc 3. A bragg grating fibre is bonded to the inside surface 4 of the housing 1, this measures the external pressure. A bragg grating fibre is bonded to theinternal surface of disc 3 on face 5and this measures the stain on face 5 which is a measure of the internal pressure inside the tube 6. A further bragg grating fibre is bonded to a non strained surface of the housing, and this just measures the strain caused by the change in temperature. This is then used to correct the pressure strained bragg gratings and surfaces 4 and 5.

The housing is attached to the tube using a set of cap head screws 7, which incorporate an anti vibration and anti back off locking washer 8. The entire assembly has metal to metal seals, 10 is a ring seal and 11 is a tapered ring which expands the recess it seats in and creates a multiude of metal to metal seals along the face 12.

The fibre optic cable is conveyed to surface via a metal clad tube 13. To join the fibre in the sensor and the fibre in the tube, a splicing system is employed. Once the fibres are spliced together 14 and tested the outer tube 15 is slide over the metal clad tube 13 and made tight to both the housing at 16 and the metal clad tube AT 17, auto clave type sealing surfaces are used wherever possible.

Referring now to figures 31 to 34 there is shown a combined absolute and differential pressure sensor. The absolute sensor has the same construction as described above. The differential sensor will now be described. It measures the difference in pressure between Pl and P2. Pl is transmitted into a chamber 20, via porting 21 to a inner chamber 22. The inner chamber has been formed by a support disc 23, and the pressure sensing disc 24. The pressure sensing disc is retained by a tapers ring 25, which generates metal to metal seals along the surface 27. Pl is applied over the whole surface 30, however, the supporting disc 23, will allow a small deflection of the disc before supporting it and preventing catastrophic failure. So if for any reason Pl ports 21 become plugged, P2 cannot cause a mechanical failure of the disc 30. Between the two disc surfaces 30 and 31 is bonded a bragg rating fibre, and as the fibre is strained it only measures the differential pressure between pi and p2. The path of the fibre is shown by, as it traverses the centre of the discs while generous radius of bends 41 are used so that the fibre ends in the differential chamber 42

Claims

1. A meter system for measuring parameters of conditions within production tubing, comprising a plurality of meters being linked by a single fibre-optic cable.

2. A meter system according to claim 1 wherein at least some of the meters comprise a sensing element employing a Bragg grating sensor,

3. A meter system according to claim 2 wherein at least some of the meters are situated on a tool disposable down hole

4. A meter system according to claim 2 wherein the tool may engage with a side pocket in the production tube.

5. A meter system according to claim 2 wherein the tool is a pump.

6. A meter system according to any previous claim wherein the meters measure at least one of the following parameter; pressure, acceleration, distributed temperature along the power cable, pressure across a venturi to determine actual flow rate, vibration of the tool,

7. A meter system according to any previous claim wherein a meter is include sdd iinn aa ppuummpp ttoo mmeeaassuurree inlet pressure of a pump and/or discharge pressure of a pump

8. A meter system according to any previous claim wherein a meter is included in a motor to measure the temperature in motor windings and/or the type of noise generated indicting the current condition of bearings etc, contamination of a motor oil through optical sensors etc.

9. A meter system according to any previous claim wherein there is included a venturi, at least two of the meters measuring fluid pressure at different points in the venturi.