Acoustic beacon

Abstract

A system and method for determining a fluid interface in a wellbore or annulus 211 comprising an acoustic beacon 213 and signal detector. The beacon has a housing (Fig 3A) and floats at a fluid interface using a buoyancy module and is connected to an acoustic signal generator (33, Fig 3), power supply and controller. The acoustic wave is detected by an optical fibre or distributed acoustic sensor (DAS) 212 that may be within the tubing or annulus. In operation, the beacon or signal emitter floats at the fluid interface and may detect a change in liquid level, typically caused by a leak or fault, by identifying a change in the time delays of receiving the signal. The system is preferably used to determine a liquid level at a liquid-gas or liquid-liquid interface. Optionally, several beacons are configured to be distinguishable from each other by using different frequencies or time delays for liquid level detection in multiple annuli.

Description

Technical field The invention relates to diagnostics of hydrocarbon producing wells, and in particular to determining fluid levels in annuli of a well. Background In a hydrocarbon producing well, there are various annuli or spaces between tubulars of the well structure, usually arranged concentrically. The annuli ensure a safe and efficient production of oil and gas. The A-annulus is a space between the production tubing and a casing string surrounding the production tubing, while the B-annulus is located between different casing strings. Subsequent annuli and indicated by letters C, D, etc. The barriers provided by the tubing or strings prevent leakage of hydrocarbons into the environment, and also prevent inflow of contaminants into the production fluids. The annuli are typically filled with a fluid, such as water, which is stationary during production. Monitoring fluid levels in the annuli is vital for ensuring the safety and integrity of the well structure. Any sudden or abnormal changes in fluid levels could indicate a potential issue, such as casing leaks, casing failure, or uncontrolled fluid flow. By monitoring fluid levels, engineers can promptly identify and address such issues, preventing any accidents or environmental hazards. Monitoring fluid levels in annuli also helps in maintaining weil control. If the fluid levels deviate significantly from the expected levels, it could indicate the influx of unwanted fluids, such as formation water or gas, into the wellbore. Detecting and addressing such influxes promptly is crucial to prevent well control issues like kicks or blowouts. Changes in fluid levels can also indicate reservoir pressure changes, fluid migration, or other reservoir characteristics. Monitoring fluid levels therefore also assists in reservoir management and optimizing production strategies. Monitoring fluid levels in the annuli can also help with monitoring the integrity of the casing and cementing. If there are significant changes in fluid levels, it may suggest casing leaks or poor cementing jobs. Monitoring fluid levels in the different annuli is therefore important for safety, well control, reservoir management, and equipment integrity. However, monitoring fluid levels is challenging because there is no direct access from the wellbore to the annuli at the fluid levels. Statement of invention According to a first aspect of the invention, there is provided a method of determining at least one fluid interface in at least one annulus surrounding a tubing of a wellbore, the method comprising: providing a buoyancy module, floating the buoyancy module at the at least one fluid interface; generating an acoustic signal with an acoustic signal generator connected to the buoyancy module; detecting the acoustic signal with an optical signal carried within an optical fibre arranged in the longitudinal direction of the tubing. The optical fibre may be configured as a distributed acoustic sensor, DAS. The at least one fluid interface may be an interface between a liquid and a gas, wherein determining the at least one fluid interface comprises determining the liquid level. Alternatively, the at least one fluid interface may be an interface between a first liquid and a second liquid, wherein the buoyancy module floats at the interface between the first and second liquids. A plurality of annuli may be arranged around the tubing, wherein the method comprises measuring a plurality of acoustic signals with said optical signal, wherein the plurality of acoustic signals are generated by a corresponding plurality of acoustic signal generators. The plurality of acoustic signals are distinguishable from each other by one or more of: different frequencies, different modulation of frequencies, and different time delays between acoustic signals. The method may further comprise detecting a change in the detected fluid level, and raising an alarm to alert an operator. The method may further comprise determining the height of a fluid column in the annulus from said determining of the fluid interface. The method may further comprise arranging the optical fibre within the tubing, or within an annulus. According to a second aspect of the invention, there is provided an acoustic beacon for use in a wellbore annulus, the acoustic beacon comprising: an acoustic signal generator; a housing; a power supply and a controller; a buoyancy module. The housing may comprise an acoustic resonator for amplifying an acoustic signal generated by said acoustic signal generator. The housing and the buoyancy module may be the same or different components. The acoustic beacon may further comprise an energy harvester for charging said power supply. The housing may be disc-shaped, oval shaped, or round shaped. The acoustic signal generator may comprise a member and a sound board, and may be arranged to generate an acoustic burst by the member impinging on the sound board. The acoustic signal generator may be arranged to generate a predetermined frequency. According to a third aspect of the invention, there is provided a system comprising the acoustic beacon according to according to the second aspect, and further comprising a distributed acoustic sensor arranged to detect an acoustic signal generated by the acoustic beacon. Figures Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 illustrates schematically a section of a wellbore including multiple annuli; Fig. 2 illustrates a radial cross section through the annuli including multiple acoustic beacons; Fig. 3 illustrates acoustic beacons; and Fig. 4 is a flow diagram. Specific description The inventors have appreciated that a fibre optic sensor, in particular a distributed acoustic sensor (DAS), can be used to remotely determine a fluid level in an annulus. The DAS comprises an optical fibre arranged to carry an optical signal. The optical signal is coupled into the fibre using an optoelectronic device, typically including a laser source. The DAS is arranged within a tubing of a wellbore, and is configured to detect an acoustic signal emitted by an acoustic beacon in the annulus outside the tubing. The acoustic signal causes strain on the fiber and modulation of the refractive index, which in turn causes a change in Rayleigh backscattering of the light the propagates through the fibre. Although Rayleigh backscattering is the dominant contribution, there are also other effects which could be used to detect acoustic signals, such as Brillouin scattering or Raman scattering. A DAS relies on Rayleigh backscattering, but more generally a fibre optic sensor can be used to detect an acoustic signal. A Fiber Bragg Grating (FBG) is another option for a fibre-based detection method. In an FBG, a periodic variation of the refractive index acts like a grating, which reflects a particular spectral portion. An acoustic or heat signal causes a variation in the grating constant, thereby changing the reflected spectrum. The acoustic signal is generated with an acoustic signal generator connected to a buoyancy module within the annulus, together acting like a beacon to indicate the fluid level within the annulus. The fluid level may be an interface between a liquid and a gas, or between two liquids with different densities such as water and liquid hydrocarbon. The beacon floats at the interface. At an interface between liquid and gas, the weight of the liquid volume displaced by the acoustic beacon is larger than the weight of the acoustic beacon. At an interface between two liquids, the weight of the acoustic beacon is larger than the weight of the displaced volume of the top liquid, but is smaller than the weight of the displaced volume of the bottom liquid. The fibre optic sensor can be used to detect an acoustic signal emitted by one acoustic beacon, but it can also be used to detect a plurality of acoustic signals emitted by a plurality of acoustic beacons. It is not uncommon to have a plurality of annuli arranged around a tubular, often referred to as the ‘A-annulus’, ‘B-annulus’, etc. Each annulus can have an acoustic beacon, preferably each emitting a unique signal such that the signals can be distinguished from one another. Fig. 1 illustrates schematically a tubing 11, surrounded by the A-annulus 12, the B-annulus 13, the C-annulus 14 and the D-annulus 15. The tubing and the four annuli all have a longitudinal axis, and arranged co-axially with respect to each other. The fluid level in each of the annuli is different, and indicated by arrows 16, 17, 18 and 19. As set out in the background, the movement of the fluid level may be an indication of a leak, or other issues. During operation of the well, the fluid levels in the annuli are typically constant in the absence of a leak. Besides monitoring for adverse effects, the measurement of the fluid level in the A-annulus can also be used to determine the pressure at the casing shoe, which is the lowest part of the fluid column in the A-annulus. The pressure at the lowest part of any of the annuli can be determined on the basis of the determined fluid level. The pressure can be calculated by adding the pressure at the top of the annulus to the weight of the gas column and the weight of the fluid column. The pressure can be monitored and an alarm can be raised when pressure exceeds a desired or safe operating window. Fig. 2 illustrates a radial cross section through the tubular and surrounding annuli, starting at the centre and extending into the surrounding sediments and seawater in the direction of arrow ‘r’ in Fig. 2A and 2B. Fig 2 B illustrates the vertical cross section in direction ‘r’, starting with the centre of tubular 21 filled with hydrocarbons. The four annuli of fig. 1 defined by concentric steel pipes are arranged around the tubular. The tubular is surrounded in order by the following annuli: the A-annulus 22, the B-annulus 23, the C-annulus 24 and the D-annulus 25. Seawater 26 surrounds the D-annulus 25. The D-annulus extends above the seabed, while other annuli extend into the formation and are surrounded by sediment 27. The arrangement of the annuli is a result of the drilling process, whereby a drill is extended into the formation starting from within the outer tubular 28. After the drill has progressed some distance, the next tubular 29 is inserted to stabilise the newly drilled section of wellbore. A new section of wellbore is drilled from tubular 29, and so on until the reservoir is reached. The annuli contain water 210, typically at different levels, and gas 211 above the water. A DAS 212 is arranged within tubular 21, preferably against the wall to improve acoustic coupling between the DAS and the tubular wall. Acoustic beacons 213 are floating at the water surface and emit a signal which can be detected by the DAS. The DAS or other fibre optic based acoustic sensor is preferably arranged in the longitudinal direction of the tubular 21, such that the fibre is at least arranged along the depth of the one or more fluid levels within the annuli. The detection may still be possible if the fibre is not arranged at the same depth as the fluid levels, but it will be more challenging to detect. The fibre can be arranged within the tubular. The fibre can be arranged within the tubular by wireline at a later stage than well completion. The fibre can be attached to the internal tubular wall, at least in the region of the fluid levels, to improve the signal quality. The fibre can alternatively be cemented into one of the annuli, or otherwise be provided within an annulus. A disadvantage of arranging the fibre within an annulus is that the annulus is more challenging to access, but an advantage is that the fibre is less disrupted by the flow of fluids from the well through the tubular 21. The location of each beacon can be determined by the DAS by analysing the propagation of the acoustic signal through the optical fibre. The acoustic signal will enter the fibre at the position closest to the beacon, and a component of the signal will then propagate both upwards and downwards through the fibre from the point of entry. The point of entry therefore corresponds to the fluid height. The optical fibre enables detection along a continuous section of the wellbore, rather than only at discrete locations. The maximum length of a fibre optical cable is up to 120km, beyond which the damping will be too large to effectively use the detector. The complete length of a typical well can be covered by a DAS. The wavelength of the optical signal may be in the region of 1500-1600nm, or the light may more specifically have a central wavelength of 1550 with a FWHM of the spectrum depending on the length of the pulse. The light is generated by a pulsed laser source, for example 100ns. The spatial resolution depends on the length of the pulse, and the example of a 100ns pulse would correspond to a 10m resolution. A close contact between the DAS and the wall of the inner tubular would increase the signal strength because the signal would not need to cross the additional fluid interfaces between the wall and the DAS. However, the DAS may not be attached to the tubular wall for the entire length. The DAS may be temporarily deployed by wireline intervention, for example, and not be pre-installed, and attaching the DAS along the entire length would be impractical. The DAS may therefore be attached to the wall only in the region where the fluid levels are expected to occur, or not be attached at all to the wall. The DAS may also be disposed after temporary use. Acoustic beacons are provided into some or all of the annuli where fluid levels need to be monitored by the DAS. Fig. 3 illustrates an embodiment of an acoustic beacon and shows a vertical cross section of a schematically represented beacon. A battery unit 31 is connected to a controller 32 and an acoustic signal generator 33. The beacon is preferably used for a long period of time, such as months or years, and the energy consumption should therefore be minimised. The controller may only instruct the signal generator to emit a signal periodically, whereby the controller and signal generator are in a sleep mode in between the emission of the acoustic signals to minimise power consumption. Optionally, a power generator 35 is provided and attached to battery unit 31 to charge the battery. An example of a power generator is a mechanical energy harvester that converts movement into electrical energy, or a thermal energy harvester that converts heat into electrical energy. The amount of electrical energy harvested is typically small, but the generated additional energy will make a significant contribution to the battery lifetime if the controller and signal generator are in sleep mode for most of the time. The acoustic signal generator may also be mechanically coupled to the housing 34 such that the housing acts as a resonator to propagate the signal to the surroundings of the beacon. The housing may have a spherical or elliptical outer shape, with the advantage of minimising the risk of getting stuck within an annulus. The housing may comprise a top and a bottom part, connected by way of a fitting arrangement 36. The fitting arrangement may be a snap fit, a screw fit, or other suitable arrangement known to the skilled person. The housing 34 defines an interior volume, which may contain air or another gas to create an effective density of the acoustic beacon that is lower than the fluid on which the beacon is intended to float. The interior volume may also contain a lightweight material such as Styrofoam or similar plastics material, to avoid the beacon sinking when there is a leak in the housing. The housing therefore carries out a function of protecting the electronic components as well as providing buoyancy. The housing is one example of a buoyancy module. Another example of a buoyancy module is a piece of low-density material, such as a Styrofoam sphere or block, which is mechanically connected to the electronic components, which are separately contained within a sealed housing. Fig. 3A illustrates a further example of a housing arranged to contain the electronic components like in Fig. 3, whereby the shape of the housing is different. The housing has the shape of a disc, with the advantage of providing a better acoustic resonator than the spherical shape of Fig. 3. A better acoustic resonator reduces the strength of the acoustic signal that is required for the DAS to distinguish over the noise level. The housing defines an acoustic cavity, which may be a non-Helmholtz resonator. The dimensions of the housing may be chosen depending on the frequency of the acoustic signal, such that the resonant frequency of the housing, or one or more of the higher harmonics of the resonant frequency, matches the frequency of the signal generator. The acoustic signal does not need to be in the audible spectrum because the DAS is not restricted to the audible spectrum, which means that a high frequency with a small wavelength can be chosen to allow for a small housing to fit in an annulus. The diameter of production tubing ranges approximately between 2 and 4 inches, or 5 and 10 cm, the liner between 9 and 10 inches, and casing sizes of 13, 20 and 39 inch diameters. A size of housing in the range of 1-10cm, more specifically in the range of 3-5cm could therefore be suitable depending on the annulus. A 5cm acoustic wavelength would in air correspond to a frequency of 6.9kHz. The acoustic signal comprises a base frequency, and it is preferable to select different signals for different annuli to be able to distinguish between the annuli. One example is a different frequency for each beacon, another example is the same frequency for each beacon but a different modulation of the frequency for each beacon. The different signals may also be generated with a different periodicity between signals. In a different embodiment, the acoustic signal comprises a short acoustic burst like a ‘click’. The click can be generated by a member impacting on a resonant sound board such as the wall of the housing. The member may be driven by an electromagnetic force or a mechanical spring. The clicks can be generated periodically. A different resonant sound board for different beacons, for example achieved by a different size of housing, will produce distinguishable clicks. The acoustic signal is detected by the modulation of the optical signal, as described above. The optical signal is processed to distinguish the signal over the noise and to distinguish the signals from different beacons. The fluid levels are derived from the time delays of the reflected signals. The signal processing may take place at the surface, and may also take place remotely from the production facility. The details of the signal processing are not described as that will be part of the knowledge of the skilled person. The data can be interpreted by an operator, or the interpretation of data can be fully automated. In an automated system, an alarm may be raised if one or more of the fluid levels change, or are outside the range of normal fluctuations. An operator can then act on the alarm and analyse the data to see whether the data indicate a leak, or a pressure change, or other unexpected change in the operating conditions. The method of determining a fluid interface is described above, and the steps are illustrated in Fig. 4: S1: providing a buoyancy module, S2: generating an acoustic signal, and S3: detecting the acoustic signal with an optical fibre. A fibre optical detection method can also be used to detect the fluid level by way of a heat signal. The beacon can be arranged to emit a heat signal instead of, or in addition to, the acoustic signal. The heat signal can be generated by temporarily sending an electric current through a heating wire contained within a floating capsule similar to the 5 capsule described before. The heat signal can also be generated by a chemical reaction. The fibre optical detection method is sufficiently sensitive to measure the heat signal. However, a signalling method based on a heat signal would require more energy due to the absorption and dissipation of heat in fluids when compared to the method based on an acoustic signal. 10 Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are 15 contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

5CLAIMS:

1. A method of determining at least one fluid interface in at least one annulus surrounding a tubing of a wellbore, the method comprising:5 providing a buoyancy module, the buoyancy module comprising an acousticsignal generator; a housing; a power supply and a controller; and a buoyancy module;floating the buoyancy module at the at least one fluid interface of the annulus;generating an acoustic signal with an acoustic signal generator connected to the 10 buoyancy module;detecting the acoustic signal with an optical signal carried within an optical fibre arranged in the longitudinal direction of the tubing.

2. The method of claim 1, wherein the optical fibre is configured as a distributed 15 acoustic sensor, DAS.

3. The method of claim 1, wherein the at least one fluid interface is an interface between a liquid and a gas, and wherein determining the at least one fluid interface comprises determining the liquid level.

204. The method of claim 1, wherein the at least one fluid interface is an interface between a first liquid and a second liquid, and wherein the buoyancy module floats at the interface between the first and second liquids.25 5. The method of any one of the preceding claims, wherein a plurality of annuli arearranged around the tubing, and wherein the method comprises measuring a plurality of acoustic signals with said optical signal, wherein the plurality of acoustic signals are generated by a corresponding plurality of acoustic signal generators.30 6. The method of claim 5, wherein the plurality of acoustic signals aredistinguishable from each other by one or more of: different frequencies, different modulation of frequencies, and different time delays between acoustic signals.

7. The method of any one of the preceding claims, further comprising detecting a 35 change in the detected fluid level, and raising an alarm to alert an operator.30 01 258. The method of any one of the preceding claims, further comprising determining the height of a fluid column in the annulus from said determining of the fluid interface.5 9. The method of any one of the preceding claims, further comprising arranging theoptical fibre within the tubing, or within an annulus.

10. An acoustic beacon for use in a method according to any one of the preceding claims, the acoustic beacon comprising:10 an acoustic signal generator;a housing;a power supply and a controller;a buoyancy module.15 11. The acoustic beacon of claim 10, wherein the housing comprises an acousticresonator for amplifying an acoustic signal generated by said acoustic signal generator.

12. The acoustic beacon of claim 10 or 11, wherein the housing and the buoyancy module are the same or different components.2013. The acoustic beacon of any one of claims 10 to 12, further comprising an energy harvester for charging said power supply.

14. The acoustic beacon of any one of claims 10 to 13, wherein the housing is disc-25 shaped, oval shaped, or round shaped.

15. The acoustic beacon of any one of claims 10 to 14, wherein the acoustic signal generator comprises a member and a sound board, and is arranged to generate an acoustic burst by the member impinging on the sound board.3016. The acoustic beacon of any one of claims 10 to 14, wherein the acoustic signal generator is arranged to generate a predetermined frequency.

17. A system, comprising the acoustic beacon according to any one of claims 10 to 16, and further comprising a distributed acoustic sensor arranged to detect an acoustic signal generated by the acoustic beacon.30 01 25

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