Method and system for barrier verification

Abstract

Acoustic evaluation of a barrier in a wellbore (1). The acoustic evaluation is performed from a top side of the barrier (3) by an acoustic sensor (6) arranged in the wellbore. The acoustic evaluation is performed by an acoustic sensor adapted for downward acoustic evaluation of the barrier.

Description

[0001] METHOD AND SYSTEM FOR BARRIER VERIFICATION

[0002] INTRODUCTION

[0003] The disclosure relates to barrier integrity verification in wellbores. In particular, the disclosure relates to methods, systems and downhole tools for acoustic evaluation of a barrier in wellbores. The wellbores may e.g. be hydrocarbon wellbores, CCUS wellbores (Carbon capture utilization and storage) or geothermal wellbores.

[0004] BACKGROUND

[0005] Physical barriers (e.g. cement plugs) may be placed in the wellbore for temporary or permanent plugging of hydrocarbon wells e.g. in P&A, slot recovery or intervention applications. The barrier may be established in e.g. a Perforate, Wash and Cement operation for sealing off the wellbore and the annuli. The wellbore barrier is typically placed over a mechanical barrier base plug. The barrier length is typically about 30m (single barrier) or about 60m (dual barrier). The barrier must be verified to ensure barrier integrity for establishing a safe barrier in the hydrocarbon wellbore. In current industry practice pressure tests are typically performed from above the barrier. These pressure tests may overlook small leaks that develop into bigger leaks over time. Acoustic techniques may be used to evaluate the pressure tested barriers by looking “sideways” (radially / laterally)) into the barrier looking for leak paths for fluids. In some cases, the barrier (e.g. a cement plug) is being drilled out and section logged in a time consuming and costly procedure, typically done only on the first three wells in a certain oil and gas field.

[0006] SUMMARY OF THE INVENTION

[0007] The invention provides in a first aspect a method for evaluation of a barrier in a wellbore. The method comprising performing acoustic evaluation of the barrier from a top side of the barrier. The method comprising downward acoustic evaluation of the barrier.

[0008] The wellbore may be vertical, horizontal or a deviated wellbore. By “downwards” is meant along the wellbore independent of the orientation of the wellbore. In a high deviation or horizontal wellbore the term "downwards" means "along the hole". In a horizontal or deviated wellbore, the measured depth of the wellbore would be the length along the wellbore instead of the true vertical depth. The true vertical depth would be the length of a vertical wellbore. The barrier includes the barrier in the main bore and the barrier in the annulus.

[0009] The method may further comprise performing acoustic evaluation of the barrier using a multi-finger tool comprising an acoustic sensor arranged on at least one finger. The least one finger of the multi-finger tool may be contacting a casing in the wellbore during acoustic evaluation of the barrier.

[0010] The method may further comprise operating an acoustic sensor in a passive mode receiving acoustic signals. The acoustic sensor may comprise a single sensor or a sensor array. An acoustic transmitter may be arranged below the barrier.

[0011] An acoustic reflector may be arranged below the barrier. The acoustic reflector may be arranged on top of a cement base for the barrier. The method may further comprise varying the pressure in the wellbore.

[0012] The method may comprise transmitting an acoustic signal into the barrier and / or casing and receiving acoustic signals from the barrier and / or casing.

[0013] The acoustic signals may be sonic or ultrasound signals.

[0014] The invention provides in a further aspect an acoustic system for evaluation of a barrier in a wellbore. The system comprising an acoustic sensor arranged in the wellbore. The acoustic sensor is adapted for acoustic evaluation of the barrier from a top side of the barrier. The acoustic sensor may be adapted for downward acoustic evaluation of the barrier. The acoustic sensor may be a single sensor or a sensor array.

[0015] The acoustic sensor system may comprise a multi-finger tool comprising at least one acoustic sensor arranged on one or more fingers. The at least one of the acoustic sensors arranged on one or more finger may be adapted for contacting a casing of the wellbore during acoustic evaluation of the barrier. The acoustic system may comprise at least one transmitter and / or transmitter array arranged below the barrier for transmitting at least one acoustic signal into the barrier. The acoustic system may further comprise a reflector for reflecting acoustic signals into the barrier. The reflector may be arranged on top of a base for the barrier. The reflector may be arranged below the barrier. The acoustic system may further comprise a transmitter or transmitter array arranged above the barrier for transmitting at least one acoustic signal into the barrier. The acoustic signal may be transmitted into the barrier in a substantially longitudinal direction of the barrier.

[0016] The invention provides a further aspect a downhole tool adapted for evaluation of a barrier in a wellbore. The downhole tool comprising an acoustic sensor. The acoustic sensor is adapted for acoustic evaluation of the barrier from a top side of the barrier. The acoustic sensor may be adapted for downward acoustic evaluation of the barrier. The acoustic sensor may be a single acoustic receiver or a receiver array.

[0017] The acoustic sensor may be a multi-finger tool comprising at least one acoustic receiver arranged on at least one of the fingers. The at least one of the acoustic receivers may be adapted for contacting a casing of the wellbore during acoustic evaluation of the barrier. At least one of the acoustic receivers may contact the inside of the casing. At least one of the fingers may be adapted for contacting a casing of the wellbore during acoustic evaluation of the barrier. At least one of the fingers may contact the inside of the casing.

[0018] The downhole tool further may further comprise at least one transmitter and / or transmitter array arranged below the barrier for transmitting at least one acoustic signal into the barrier. The downhole tool may further comprise at least one transmitter and / or transmitter array arranged above the barrier for transmitting at least one acoustic signal into the barrier. The at least one acoustic signal may be transmitted downwards into the barrier.

[0019] The downhole tool may be an imaging tool. In a further aspect it is disclosed a method for evaluation of a barrier in a wellbore, wherein the method comprising performing downward acoustic evaluation of a barrier.

[0020] In a further aspect it is disclosed an acoustic system for evaluation of a barrier in a wellbore, the system comprising at least one acoustic sensor arranged in the wellbore, wherein the acoustic sensor is adapted for downward acoustic evaluation of the barrier.

[0021] The method, system or downhole tool as disclosed may be used for evaluating an in-situ barrier in a wellbore. The method, system or downhole tool as disclosed may be used for acoustic imaging of a barrier.

[0022] The barrier is a physical barrier and may be e.g. a barrier of a fluidized plugging material, cement plug, a bismuth based plug, a bentonite based plug, a mechanical barrier or a fixed barrier (e.g. a bridge plug).

[0023] The method, system or downhole tool as disclosed may be used in well integrity applications within PP&A, temporary P&A, slot recovery, well intervention or drilling.

[0024] The invention provides acoustic evaluation of a barrier from above the barrier. The acoustic sensor(s) listen to the barrier below the tool. The invention provides a downward looking acoustic sensor system. The acoustic sensors are facing downwards towards the physical barrier. The acoustic sensors are arranged in the vicinity of the top surface of the barrier. The acoustic sensors detect acoustic signal(s) from the physical barrier main bore and annulus. The performance of the barrier as a seal is evaluated. Any leaks or potential leaks in the barrier is detected. In a passive mode the acoustic sensor detects acoustic signal(s) from potential leak(s) across the barrier.

[0025] The acoustic sensor system may include a multi-finger tool with acoustic receivers on the fingers. One or more of these acoustic receivers may be touching the casing / tubular. Acoustic signals propagating in the main bore and annulus may hit the casing and travel upwards through the casing. The receivers touching the casing gather pressure waves off the casing. Receivers touching the casing increases the signal to noise ratio providing increased sensitivity and thus higher quality and accuracy in the verification of the barrier. Increased sensitivity enables detecting even minor leaks or potential failure in the barrier integrity. When the barrier integrity has been approved by use of the invention this ensures that the barrier meets the highest standard for safety and integrity. The acoustic signals may travel upwards through the casing from the barrier below both from the barrier in the main bore and the barrier in the annulus.

[0026] The disclosed downward acoustic solution is adapted for evaluation of a physical barrier along its length (axial) instead of the current industry standard to look “sideways” (radially). The in-situ physical barrier is evaluated in a reliable and costefficient way. The physical barrier is evaluated from a top side. An acoustic sensor(s) system may be lowered down to the physical barrier e.g. on a wireline or arranged on a downhole tool. The acoustic sensors are positioned above the physical barrier and detect acoustic signals from the physical barrier without any intervention into the physical barrier. The acoustic sensors are arranged in the vicinity of the barrier. Acoustic transmitter(s) arranged below the barrier in the main bore does not interfere with the barrier. The acoustic transmitter(s) may be placed on or top of a base for the barrier, e.g. a cement base or bridge plug. The solution enables detection of small leaks and leak paths in the barrier. The solution enables the possibility for barrier verification of all barriers in a well. The solution may be used for barrier verification of all types of cement. Azimuthal barrier (e.g. cement) quality may be evaluated. The solution may e.g. be used in hydrocarbon, CCUS or geothermal wells. The solution also enables acoustic «imaging» of the barrier.

[0027] BRIEF DESCRIPTION OF DRAWINGS

[0028] Example embodiments are described with reference to the following drawings. The drawings are only illustrations and are not to scale. The example embodiments are only examples and not limiting for the invention.

[0029] Figure 1 illustrates an exemplary embodiment of acoustic sensing of a barrier in a wellbore in a passive acoustic mode using an acoustic sensor with a single sensor (receiver Rx). Figure 2 illustrates an exemplary embodiment of acoustic sensing of a barrier in a wellbore in a passive acoustic mode using an acoustic sensor with a sensor array (receiver Rx array).

[0030] Figure 3 illustrates an exemplary embodiment of acoustic sensing of a barrier in a wellbore with a sensor array (Rx) located on finger touching casing with single transmitter Tx or array of transmitters Tx above the cement plug. An acoustic reflector is arranged on a cement base for the cement plug. The cement plug may be evaluated in passive or active mode.

[0031] Figure 4 illustrates an exemplary embodiment of acoustic sensing of a barrier in a wellbore with a sensor array (Rx) located on a finger, where the finger touching a casing of the wellbore. A single transmitter Tx or array of transmitters Tx are placed below the cement plug on top of the cement base for the plug. The cement plug may be evaluated in passive or active mode.

[0032] DETAILED DESCRIPTION

[0033] Example embodiments are described with reference to the drawings. The example embodiments are not limiting for the disclosure. The drawings are illustrations for understanding of the principles of the disclosed solution. The same reference numerals are used for the same or similar features in all the drawings and throughout the description.

[0034] The example embodiments use a cement plug as a physical barrier for illustration purposes. This is however not limiting for this disclosure. The disclosure covers physical barriers in wellbores. The physical barrier may e.g. be in the form of a fluidized plugging material, cement plug, bismuth barrier, bentonite barrier or a fixed barrier as e.g. a bridge plug. The cement plug may be any type of cement usable as a permanent or temporary plug in a wellbore.

[0035] The method, system and tool provide evaluation of a barrier in a wellbore. Both the barrier in the main bore and annulus are evaluated. The evaluation of the barrier is performed from a top side of the barrier. Downward acoustic evaluation of the barrier is performed. The barrier is evaluated also in a longitudinal axial direction and not only looking radially. The acoustic receivers are facing downwards towards the physical barrier to detect acoustic signals from the barrier. The downward acoustic evaluation of the barrier is performed along the length of the casing. The wellbore may be vertical, horizontal or a deviated wellbore. By “downwards” is meant along the wellbore independent of the orientation of the wellbore. In a high deviation or horizontal wellbore the term "downwards" means "along the hole". In a horizontal or deviated wellbore, the measured depth of the wellbore would be the length along the wellbore instead of the true vertical depth. The true vertical depth is the length of a vertical wellbore.

[0036] Figures 1-4 illustrates exemplary non-limiting embodiments of acoustic evaluation of cement plugs in a vertical wellbore for illustration purposes only, but this is not to be considered limiting for the invention as explained above. Both the barrier in the main bore and the barrier in the annulus are evaluated for barrier integrity. For a barrier in the form of a cement plug, both the cement plug main bore and cement plug annulus may be evaluated for barrier integrity including e.g. cement quality and cement bonding. The downward acoustics is evaluation from a top side of the barrier. The acoustic waveforms travel downwards and upwards with receivers placed above the barrier. There are two general modes. An active mode transmitting acoustic signals into the barrier and or casing and receiving acoustic signals from the barrier and or casing, and a passive mode with just receiving acoustic signals from the barrier and or casing. The acoustic evaluation may be performed by use of one or more acoustic sensors. The acoustic sensor(s) may be arranged on a downhole tool. An acoustic transmitter(s) may be arranged on the downhole tool. The downhole tool may be an imaging tool. An acoustic transmitter(s) may be arranged below the barrier. Evaluating may be performed of the in-situ barrier. Typical barrier placed: 30m (single barrier) to 60m (dual barrier).

[0037] Examples of acoustic techniques that may be used are:

[0038] Sonic Logging as e.g. Cement Bond Log (CBL), Variable Density Log (VDL); Ultrasonic Imaging Tools as e.g. Ultrasonic Cement Evaluation Tool (UCET), Ultrasonic Pulse-Echo Tools, Acoustic Impedance Logging, Flexural Wave Logging, Cross-Dipole Sonic Logging, Acoustic Emission Monitoring. These acoustic techniques may be used alone or in combination to provide a comprehensive evaluation of the cement plug. The choice of technique depends on factors such as the wellbore conditions, the type of barrier, and the specific objectives of the evaluation.

[0039] Any of the acoustic techniques above may be used together with any of the pressure testing methods below.

[0040] Examples of pressure testing methods that may used for evaluating the physical barrier in combination with acoustic techniques are Positive Pressure Test, Negative Pressure Test, Leak-Off Test (LOT), Formation Integrity Test (FIT), Pressure Build-Up Test, Casing Pressure Test, Inflatable Packer Test, Pressure Transient Analysis, Hydraulic Fracture Test. These pressure test methods are well known in the technical field of barrier integrity testing.

[0041] Figures 1 and 2 illustrate a physical barrier in the form of a cement plug in a wellbore. The cement plug is arranged in a main bore of the wellbore inside a casing and arranged outside the casing in the annulus of the wellbore. In Figures 3 and 4 the cement plug in the main bore is arranged on top of a mechanical cement base plug.

[0042] Figure 1 illustrates an exemplary embodiment of a downhole tool 7 for evaluation of a cement plug in a wellbore 1 . The cement plug comprising a cement plug main bore 3 and a cement plug annulus 4. Figure 1 illustrates operation of an acoustic sensor in a passive acoustic mode. An acoustic sensor in the form of a single sensor 6 (receiver Rx) is used. The acoustic sensor is arranged inside the casing 20. The acoustic sensor may be arranged on a downhole tool 7. The acoustic sensor performs acoustic evaluation of the cement plug 3, 4 from a top side of the cement plug. The acoustic sensor 6 performs downward acoustic evaluation of the cement plug.

[0043] Figure 1 illustrates an acoustic signal 5 from leaks inside the cement travelling upwards towards the sensor. The illustrated acoustic signal 5 may result from leaks in the interface between the cement and the casing 20. The acoustic signals may result from leaks in the cement in the main bore and in the annulus. The acoustic signals may result from leaks from the interface between the cement plug in the main bore and the casing and the interface between the cement plug in the annulus and the casing, he single sensor (Rx) performs passive listening to the acoustic signals. The single acoustic sensor is arranged on a top side of the cement plug. In Figure 1 , the single acoustic sensor is arranged on a downhole tool above and at a distance from the cement surface. «Passive listening» is performed by the single acoustic sensor for acoustic signal(s) from potential leak(s) across the cement plug. The passive evaluation of the cement plug may or may not include pressure variation / manipulation in the wellbore. Although not shown in Figure 1 , a mechanical cement base plug may be arranged under the cement plug main bore. The mechanical cement base plug may be provided with either transmitter(s) or acoustic reflector(s). The mechanical base plug may e.g. be a bridge plug. The acoustic sensor Rx may be used for obtaining an acoustic image of the cement plug with potential leak.

[0044] Figure 2 illustrates an exemplary embodiment of a downhole tool 7 for evaluation of a cement plug in a wellbore 1 . The method in Figure 2 illustrates operation of an acoustic sensor 9 in a passive acoustic mode. An acoustic sensor in the form of a sensor array (receiver Rx array) is used. The receiver array Rx includes a single acoustic receiver at the end of the downhole tool and a receiver array with sensors arranged around the circumference of the downhole tool. The single acoustic receiver at the end of the tool is closest to the cement surface. The receiver array Rx enables downward detection of acoustic signals from the cement plug with increased sensitivity as they cover a larger area of the cement plug from directly above. The acoustic sensor is arranged inside the casing 20. The acoustic sensor is arranged in the main bore. The acoustic sensor array may be arranged on a downhole tool 7. The acoustic sensor array in Figure 2 is arranged on the downhole tool above and at a distance from the cement surface. The acoustic sensor performs acoustic evaluation of the cement plug from a top side of the cement plug. The acoustic sensor performs downward acoustic evaluation of the cement plug. Figure 2 illustrates an acoustic signal 8 from leaks inside the inside cement travelling upwards towards the sensor array. The sensor array performs passive listening to the acoustic signals. The acoustic sensor array is placed on top of the cement plug. «Passive listening» is performed by the acoustic sensor array for acoustic signal(s) from potential leak(s) across the cement plug. The passive evaluation of the cement plug may or may not include pressure variation / manipulation in the wellbore. Although not shown in Figure 2, a mechanical cement base plug may be arranged under the cement plug main bore. The mechanical cement base plug may be provided with either transmitter(s) or acoustic reflector(s). The mechanical cement base plug may e.g. be a bridge plug. The acoustic receiver array may be linear or spatial.

[0045] For the embodiments in Figures 1 and 2 a combination of pressure testing with high bandwidth acoustic sensor (20 Hz [audible sound frequency range] - 250kHz [Ultra sound frequency range] and up to 656kHz (high frequency ultrasound)) or acoustic sensor array placed above cement plug may be performed. Typical applications may be in barrier integrity yes / no (leak or no leak).

[0046] Figure 3 illustrates an exemplary embodiment for evaluation of a cement plug in a wellbore 1 . A mechanical cement base plug 15 is arranged below the cement plug in the main bore. A reflector 14 for acoustic signals is arranged on top of the cement base plug below the cement plug in the main bore. The reflector is arranged between the top of the cement base plug and the cement plug. The method in Figure 3 illustrates operation of an acoustic sensor in an active acoustic mode. The acoustic sensor may however also be operated in a passive acoustic mode. The acoustic sensor modes may be active or passive sonic and / or ultrasonic modes. The frequency range may be from 20 Hz [audible sound frequency range] - 250KHz [ultra sound frequency range] and up to 700kHz (high frequency ultrasound))). The acoustic sensor is arranged inside the casing 20. The acoustic sensor is arranged in the main bore. The acoustic sensor array may be arranged on a downhole tool. The acoustic sensor performs acoustic evaluation of the cement plug from a top side of the cement plug. The acoustic sensor performs downward acoustic evaluation of the cement plug. In Fig.3, the acoustic sensor has a sensor array 11 of receivers (Rxarray). The acoustic sensor array of receivers may have a variable bandwidth. The bandwidth may vary between (20 Hz [audible sound frequency range] - 250KHz [ultra sound frequency range] and up to 700kHz (high frequency ultrasound). The sensors in the array may also have different bandwidths. The acoustic array of transmitters 12 may be combination of monopole and dipole transmitters. In Figure 3, the receiver array is arranged as multiple finger mounted sensors (i.e. multi-finger tool). The receiver array is located circumferentially touching the casing 20 above cement plug 3. There may be at least two sensors or more. In Figure 3, the sensor array of receivers is located on fingers. In Figure 3, there is a sensor on each finger. The fingers may be attached to a tool. In Figure 3, the receivers Rxon the fingers touch the inside of the casing. In alternative embodiments, one or more of the fingers may touch the casing. One or more of the fingers may be provided with a receiver.

[0047] The exemplary embodiment illustrated in Figure 3 has an array of transmitters 12 (Txarray). The transmitter may however also be a single transmitter or a combination of a single transmitter and a transmitter array. The transmitter array is arranged on the downhole tool. As illustrated in Figure 3, the transmitter array is located on the front part of the downhole tool. This location of the transmitter array allows the transmitter to be arranged directly above the top of the cement plug and transmitting acoustic signal(s) 21 downwards into the cement plug. The emitted acoustic signal(s) 21 travel downwards into the cement plug and are reflected by the acoustic reflector 14 on top of the cement base. The reflected acoustic signal(s) 13 travel upwards through the cement plug and are received by the sensors Rxon the fingers that touch the inside of the casing. The exemplary embodiment in Figure 3 may also be operated in a passive mode by not sending an acoustic signal into the cement plug and only listening with the acoustic receivers Rx.

[0048] The exemplary embodiment with a transmitter Tx array above cement plug and reflector below cement plug on top of cement base is more complicated as waveforms have to pass twice through the cement. This sensor system may be used to obtain acoustic image of plug with potential leak, plug length, strength, CBL-VDL (Cement Bond Log)-(Variable Density Log).

[0049] Figure 4 illustrates an exemplary embodiment of a downhole tool and method for evaluation of a cement plug in a wellbore 20. A mechanical cement base (CB) 16 is arranged below the cement plug 3 in the main bore. A transmitter 17 Txor transmitter Txarray for transmitting acoustic signals is arranged on top of the cement base below the cement plug 3 in the main bore. The transmitter Txor transmitter Txarray is arranged between the top of the cement base plug and the cement plug. The acoustic transmitter array may be combination of monopole and dipole transmitters. The transmitter may also be a combination of a single transmitter and a transmitter array. The transmitter or transmitter array may be arranged on a mechanical cement base plug e.g. be a bridge plug. The method in Figure 4 illustrates operation of an acoustic system in an active acoustic mode. The acoustic system may however also be operated in a passive acoustic mode. The modes may be active or passive sonic and / or ultrasonic modes. The frequency range may be from 20 Hz [audible sound frequency range] - 250KHz [ultra sound frequency range]) and up to 700kHz (high frequency ultrasound). The acoustic sensor is arranged inside the casing. The acoustic sensor is arranged in the main bore. The acoustic sensor array (Rxarray) may be arranged on a downhole tool. The acoustic sensor performs acoustic evaluation of the cement plug from a top side of the cement plug. The acoustic sensor performs downward acoustic evaluation of the cement plug. The acoustic sensor has a sensor array of receivers 11 (Rxarray). The acoustic sensor array of receivers may have a variable bandwidth. In Figure 4, the receiver array has multiple finger mounted sensors (i.e. multi-finger tool). The receiver array is located circumferentially touching the casing 20 above cement plug. There may be at least two sensors or more. In Figure 4, the sensor array of receivers is located on fingers. In Figure 4, there is a receiver on each finger. The fingers are attached to a tool. The receivers Rxon the fingers touch the inside of the casing. The acoustic signal(s) from the transmitter or transmitter array 17 travel upwards through the cement plug. The acoustic signal(s) are received by the sensors 11 Rxon the fingers that touch the inside of the casing 20. The exemplary embodiment in Figure 4 may also be operated in a passive mode by not sending an acoustic signal and only listening with the acoustic receivers Rx.

[0050] This sensor system of a transmitter below plug on top of cement base and receiver with multiple arms clamped onto or touching casing above the cement plug, may be used to measure flexwave / 3rd echo interface and CBL-VDL ((Cement Bond Log)-(Variable Density Log). This may create a sector image of the cement plug. The multi-finger tool with acoustic receivers arranged on each finger illustrated in Figures 3 and 4 are non-limiting examples only. The number of fingers can vary. The number of fingers touching the casing during acoustic evaluation of the cement plug can vary. One or more fingers may contact the casing. The exemplary sensor tool in Figures 3 and 4 may also be operated for acoustic sensing without touching the casing. The tool may be a multi-finger tool. The tool may have one or more deployable arms 10. Different sensor elements may be arranged on the arms. Fingers are arranged on the arms. The arms may be adapted to different internal diameters of the casing. The arms may be operated during acoustic sensing. Each arm may be individually moved to different positions where one or more fingers are touching the casing and to other positions where no fingers are contacting the casing. The arms with the fingers may be individually adjusted in length and in the azimuthal position. The tool may be adapted during use depending on the measurement to be made including e.g. type of measurement technique, type of barrier, based on previous measurement(s). A transmitter(s) and / or transmitter array(s) may be arranged below the barrier for transmitting at least one acoustic signal into the barrier. Alternatively, a reflector(s) may be arranged below the barrier for reflecting acoustic signals into the barrier. The acoustic sensor, the finger mounted acoustic receivers and the transmitter(s) or reflector(s) are arranged in the main bore of the well.

[0051] One or more fingers with acoustic receivers may touch the inside of the casing. Use of acoustic receivers touching the casing increases the sensitivity of the acoustic measurement and also the ability of detecting acoustic signals originating from leaks in the annulus barrier part of the barrier. Acoustic signals from leaks in the barrier in the annulus may travel upwards via the casing. Also, acoustic signals from the barrier in the main bore may couple into the casing and are detected by the acoustic receivers in the fingers. The number of fingers with acoustic receivers may vary depending on a number of parameters including e.g. the type of acoustic measurement, which characteristics of the barrier are desired to be measured, the barrier used, the length of the barrier or the diameter of the barrier. The method and sensor system may be used to measure and confirm plug length, plug strength / density, potential leak and may also provide an acoustic image of the plug with plug length, plug strength / density, potential leak.

[0052] Active acoustic verification downwards of placed cement barriers (in-situ). Typical applications may be well integrity applications within PP&A, temporary P&A, well intervention, slot recovery, drilling.

[0053] Passive acoustic verification downwards of placed cement barriers (in-situ). Typical applications may be well integrity applications within PP&A, temporary P&A, well intervention, slot recovery, drilling.

[0054] A combination of pressure testing with variable bandwidth acoustic transmitter array placed above or below cement plug (combination of monopole and dipole transmitters) and receiver array located circumferentially touching the casing above cement plug may be performed. Receiver array may include multiple finger mounted sensors (i.e. multi-finger tool) with at least two sensors or more. This may e.g. be used for barrier integrity measuring acoustic properties of at least but not limited to compressional, shear and flexural wave forms (CBL type amplitude, cement casing bonding, cement outside casing bonding, barrier geometry - length internal and annulus, compressional strength of cement).

[0055] Having described example embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.

Claims

CLAIMS1 . Method for evaluation of a barrier in a wellbore, the method comprising performing acoustic evaluation of the barrier from a top side of the barrier.

2. Method according to claim 1 , wherein the method comprising downward acoustic evaluation of the barrier.

3. Method according to claim 1 or claim 2, further comprising performing acoustic evaluation of the barrier using a multi-finger tool comprising an acoustic sensor arranged on at least one finger.

4. Method according to claim 3, wherein at least one finger of the multi-finger tool is contacting a casing in the wellbore during acoustic evaluation of the barrier.

5. Method according to one of claims 1 -4, further comprising emitting acoustic signals by an acoustic transmitter arranged below the barrier.

6. Method according to one of claims 1 -4, wherein an acoustic reflector is arranged below the barrier.

7. Method according to claim 6, wherein the acoustic reflector is arranged on top of a base for the barrier.

8. Method according to one of claims 1 -7, wherein the method further comprising operating an acoustic sensor in a passive mode receiving acoustic signals.

9. Method according to one of claims 1 -8, wherein the acoustic sensor comprising at least one single sensor and / or at least one sensor array.

10. Method according to one of claims 1 -9, the method further comprising varying the pressure in the wellbore.11 . Method according to one of claims 1 -10, wherein the method further comprising transmitting an acoustic signal into the barrier and / or casing and receiving acoustic signals from the barrier and / or casing.

12. Method according to one of claims 1-11 , wherein the acoustic signals are sonic or ultrasound signals.

13. An acoustic system for evaluation of a barrier in a wellbore, the system comprising an acoustic sensor arranged in the wellbore, wherein the acoustic sensor is adapted for acoustic evaluation of the barrier from a top side of the barrier.

14. Acoustic system according to claim 13, wherein the acoustic sensor is adapted for downward acoustic evaluation of the barrier.

15. Acoustic system according to one of claims 13-14, wherein the acoustic sensor system comprising a multi-finger tool comprising at least one acoustic sensor arranged on one or more fingers.

16. Acoustic system according to claim 16, wherein at least one of the acoustic sensors arranged on one or more finger is adapted for contacting a casing of the wellbore during acoustic evaluation of the barrier.

17. Acoustic system according to one of claims 13-16, wherein the acoustic sensor comprising at least one single sensor and / or at least one sensor array.

18. Acoustic system according to one of claims 13-18, the system further comprising at least one transmitter and / or transmitter array arranged below the barrier for transmitting at least one acoustic signal into the barrier.

19. Acoustic system according to one of claims 13-18, the system further comprising at least one reflector arranged below the barrier for reflecting acoustic signals into the barrier.

20. Acoustic system according to claim 20, wherein the reflector is arranged on top of a base for the barrier.21 . Acoustic system according to one of claims 13-20, the system further comprising at least one transmitter and / or transmitter array arranged above the barrier for transmitting at least one acoustic signal into the barrier substantially along a longitudinal direction of the barrier.

22. Acoustic system according to one of claims 13-21 , wherein at least one transmitter and / or transmitter array is arranged below the barrier for transmitting at least one acoustic signal into the barrier, wherein the at least one transmitter and / or transmitter array are preferably arranged on a mechanical base plug for the barrier e.g. a bridge plug.

23. Downhole tool adapted for evaluation of a barrier in a wellbore, the downhole tool comprising at least one acoustic sensor, wherein the acoustic sensor is adapted for acoustic evaluation of the barrier from a top side of the barrier.

24. Downhole tool adapted for evaluation of a barrier in a wellbore, the downhole tool comprising at least one acoustic sensor, wherein the acoustic sensor is adapted for downward acoustic evaluation of the barrier.

25. Downhole tool according to one of claims 23-24, wherein the acoustic sensor comprising a multi-finger tool comprising at least one acoustic receiver arranged on at least one of the fingers.

26. Downhole tool according to claim 25, wherein at least one of the acoustic receivers is adapted for contacting a casing of the wellbore during acoustic evaluation of the barrier.

27. Downhole tool according to claim 25, wherein at least one of the fingers is adapted for contacting a casing of the wellbore during acoustic evaluation of the barrier.

28. Downhole tool according to one of claims 23-27, wherein the downhole tool further comprising at least one transmitter and / or transmitter array arranged below the barrier for transmitting at least one acoustic signal into the barrier.

29. Downhole tool according to one of claims 23-28, wherein the acoustic sensor is a single acoustic receiver and / or a receiver array.

30. Downhole tool according to one of claims 23-29, wherein the downhole tool further comprising at least one transmitter and / or transmitter array arranged above the barrier for transmitting acoustic signals downwards into the barrier.31 . Downhole tool according to one of claims 23-30, wherein the downhole tool is an imaging tool.

32. Method for evaluation of a barrier in a wellbore, wherein the method comprising performing downward acoustic evaluation of a barrier.

33. Method according to claim 31 , wherein the method further comprising at least one of claims 3-12.

34. An acoustic system for evaluation of a barrier in a wellbore, the system comprising at least one acoustic sensor arranged in the wellbore, wherein the acoustic sensor is adapted for downward acoustic evaluation of the barrier.

35. Acoustic system according to claim 34, the system further comprising at least one of claims 13-22.

36. Use of the method according to one of claims 1 -13, 32-33 or system according to one of claims 13-22, 34-35 or downhole tool according to one of claims 24-30, for evaluating an in-situ barrier.

37. Use of the method according to one of claims 1 -13, 32-33 or system according to one of claims 13-22, 34-35 or downhole tool according to one of claims 24-30, for acoustic imaging of a barrier.

38. Use of the method according to one of claims 1 -13, 32-33 or system according to one of claims 13-22, 34-35, or downhole tool according to one of claims 24-30, in well integrity applications within PP&A, temporary P&A, slot recovery, well intervention or drilling.

39. Use according to at least one of claims 36-38, wherein the barrier is a physical barrier e.g. a fluidized plugging material, a cement plug, a bismuth plug, a bentonite plug or a fixed barrier e.g. a bridge plug.

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