EP3990747A1

EP3990747A1 - Method for abandoning wellbores

Info

Publication number: EP3990747A1
Application number: EP19765548.3A
Authority: EP
Inventors: Pradyumna Thiruvenkatanathan; Tommy LANGNES
Original assignee: BP Exploration Operating Co Ltd; Lytt Ltd
Current assignee: BP Exploration Operating Co Ltd; Lytt Ltd
Priority date: 2019-06-25
Filing date: 2019-06-25
Publication date: 2022-05-04
Also published as: WO2020260928A1; CA3145162A1; US20220251943A1

Abstract

A method of abandoning a wellbore can include obtaining a first sample data set within a wellbore, wherein the first sample data set is a sample of an acoustic signal originating within the wellbore; determining a plurality of frequency domain features of the first sample data set; identifying a fluid flow location within the wellbore using the first plurality of frequency domain features; setting a barrier (130A, 131A, 130B, 131B, 130C) at or above the fluid flow location; obtaining a second sample data set above the barrier, wherein the second sample data set is a sample of an acoustic signal originating within the wellbore above the barrier; determining a second plurality of frequency domain features of the second sample data set; and identifying that a fluid flow rate or flow mechanism at the fluid flow location has been reduced or eliminated and/or identifying another fluid flow location using the second plurality of frequency domain features.

Description

METHOD FOR ABANDONING WELLBORES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

BACKGROUND

[0002] Within a hydrocarbon production well, various fluids such as hydrocarbons, water, gas, and the like can be produced from the formation into the wellbore. The production of the fluid can result in the movement of the fluids in various downhole regions, including with the subterranean formation, from the formation into the wellbore, and within the wellbore itseif. Following production, plugs are positioned in a well to be abandoned in order to prevent leaks of fluid from the well.

BRIEF SUMMARY OF THE DISCLOSURE

[0003] In an embodiment, a method of abandoning a wellbore comprises obtaining a first sample data set within a wellbore, wherein the first sample data set is a sample of an acoustic signal originating within the wellbore; determining a first plurality of frequency domain features of the first sample data set; identifying a first fluid flow location within the wellbore using the first plurality of frequency domain features; setting a first barrier at or above the first fluid flow location; obtaining a second sample data set within the wellbore above the first barrier, wherein the second sample data set is a sample of an acoustic signal originating within the wellbore above the first barrier; determining a second plurality of frequency domain features of the second sample data set; and identifying that that a fluid flow rate or fluid flow mechanism at the first fluid flow location has been reduced or eliminated and/or identifying a second fluid flow location within the wellbore using the second plurality of frequency domain features. The first sample data and the second sample data set can comprise a sample of an acoustic signal originating within the wellbore, and can be representative of the acoustic signal across a frequency spectrum.

[0004] In an embodiment a system for abandoning a wellbore, the system comprising: a receiver unit comprising a processor and a memory, wherein the receiver unit is configured to receive an acoustic signal from a sensor disposed in a wellbore, wherein a processing application is stored in the memory, and wherein the processing application, when executed on the processor, configures the processor to: receive a first baseline acoustic signal data set from the sensor, wherein the first baseline acoustic signal data set comprises an indication of the acoustic signal received over a first depth interval while the wellbore is shut in; receive a first flowing acoustic signal data set, wherein the first flowing acoustic signal data set comprises an indication of the acoustic signal received over the first depth interval while a first pressure differential is induced within the wellbore; determine a baseline fluid flow log using the first baseline acoustic signal data set; determine a flowing fluid flow log using the first flowing acoustic signal data set; subtract the baseline fluid flow log from the flowing fluid flow log to provide a first sample data set; determine a first plurality of frequency domain features of the first sample data set; identify a first fluid flow location within the wellbore using the first plurality of frequency domain features; determine a change in a flowrate or flow mechanism at the first fluid flow- location using the first sample data set; and generate an output indicative of the first fluid flow location and a change in the flow rate or flow mechanism at the first fluid flow- location. The acoustic signal can be indicative of the acoustic signal across a frequency spectrum.

[0005] In an embodiment, a method of comparing acoustic signals obtained between different acoustic sensor operations in a wellbore comprises: obtaining a first baseline sample data set over a first depth interval within a wellbore, wherein the first baseline data set is a sample of an acoustic signal originating within the wellbore; determining at least one frequency domain feature of the first baseline sample data set; inducing a first pressure differential within the wellbore; obtaining a first acoustic data set over the first depth interval within the wellbore while inducing the first pressure differential; determining at least one frequency domain feature of the first acoustic data set; subtracting the at least one frequency domain feature of the first baseline sample data set from the at least one frequency domain feature of the first acoustic data set to obtain a first sample data set over the first depth interval; obtaining a second baseline sample data set over a second depth interval within the wellbore, w-herein the second baseline sample data set is a sample of an acoustic signal originating within the wellbore, wherein the second depth interval overlaps with the first depth interval; determining at least one frequency domain feature of the second baseline sample data set; inducing a second pressure differential within the wellbore; obtaining a second acoustic data set over the second depth interval within the wellbore while inducing the second pressure differential; determining at least one frequency domain feature of the second acoustic data set; subtracting the at least one frequency domain feature of the second baseline sample data set from the at least one frequency domain feature of the second acoustic data set to obtain a second sample data set over the second depth interval; and comparing the second sample data set to the first sample data set over the second depth interval,

[0006] in an embodiment, a system for of comparing acoustic signals obtained between different acoustic sensor operations in a wellbore, the system comprising: a receiver unit comprising a processor and a memory, wherein the receiver unit is configured to receive an acoustic signal from a sensor disposed in a wellbore, wherein a processing application is stored in the memory', and wherein the processing application, when executed on the processor, configures the processor to: receive a first baseline sample data set over a first depth interval within the wellbore, wherein the first baseline data set is a sample of an acoustic signal originating within the wellbore; determine at least one frequency domain feature of the first baseline sample data set; receive a first acoustic data set over the first depth interval within the wellbore, wherein the first acoustic data sat is an acoustic signal obtained while a first pressure differential is induced within the wellbore; determine at least one frequency domain feature of the first acoustic data set; subtract the at least one frequency domain feature of the first baseline sample data set from the at least one frequency domain feature of the first acoustic data set to obtain a first sample data set over the first depth interval; receive a second baseline sample data set over a second depth interval within the wellbore, wherein the second baseline sample data set is a sample of an acoustic signal originating within the wellbore, wherein the second depth interval overlaps with the first depth interval; determine at least one frequency domain feature of the second baseline sample data set; receive a second acoustic data set over the second depth interval within the wellbore, wherein the second acoustic data sat is an acoustic signal obtained while a second pressure differential is induced within the wellbore; determine at least one frequency domain feature of the second acoustic data set; subtract the at least one frequency domain feature of the second baseline sample data set from the at least one frequency domain feature of the second acoustic data set to obtain a second sample data set over the second depth interval; compare the second sample data set to the first sample data set over the second depth interval; and generate an output indicative of the comparison between the second sample data set and the first sample data set..

[0007] In an embodiment, a method of abandoning a wellbore comprises: obtaining a first sample data set over a first depth interval within a wellbore, wherein the first sample data set comprises a first acoustic data set having a first baseline acoustic sample data set subtracted therefrom, wherein the first acoustic data set is obtained over the first depth interval while a first pressure differential is induced in the wellbore, and wherein the first baseline acoustic sample data set is obtained over the first depth interval while the wellbore is shut in; identifying a fluid flow location within the first depth interval using the first sample data set; obtaining a second sample data set over a second depth interval within a wellbore, wherein the second sample data set is obtained after a barrier is set at or above the fluid flow location, wherein the second sample data set comprises a second acoustic data set having a second baseline acoustic sample data set subtracted therefrom, wherein the second acoustic data set is obtained over the second depth interval while a second pressure differential is induced in the wellbore, wherein the second baseline acoustic sample data set is obtained over the second depth interval while the wellbore is shut in, and wherein the second depth interval is overlaps the first depth interval; comparing the first sample data set to the second sample data set; and determining whether or not fluid flow at the fluid flow location Is substantially blocked by the barrier.

[0008] In an embodiment, a system for abandoning a wellbore, the system comprising: a receiver unit comprising a processor and a memory, wherein the receiver unit is configured to receive an acoustic signal from a sensor disposed in a wellbore, wherein a processing application is stored in the memory, and wherein the processing application, when executed on the processor, configures the processor to: receive a first baseline acoustic sample data set and a first acoustic data set from the sensor, wherein the first acoustic data set is an acoustic signal obtained over a first depth interval while a first pressure differential is induced in the wellbore, and wherein the first baseline acoustic sample data set is an acoustic signal obtained over the first depth interval while the wellbore is shut in, determine a first sample data set over a first depth interval within the wellbore, wherein the first sample data set comprises the first acoustic data set having the first baseline acoustic sample data set subtracted therefrom; identify a fluid flow location within the first depth interval using the first sample data set; receive a second baseline acoustic sample data set and a second acoustic data set from the sensor,wherein the second acoustic data set is an acoustic signal obtained over a second depth interval while a second pressure differential is induced in the wellbore and after a barrier is set at or above the fluid flow location, and wherein the second baseline acoustic sample data set is an acoustic signal obtained over the second depth interval while the wellbore is shut in and after the barrier is set at or above the fluid flow location; determine a second sample data set over the second depth interval within the wellbore, wherein the second sample data set comprises the second acoustic data set having the second baseline acoustic sample data set subtracted therefrom; compare the first sample data set to the second sample data set; and determine whether or not fluid flow at the fluid flow location is substantially blocked by the barrier; and generate an output indicative the determination of whether or not the fluid flow at the fluid flow location is substantially blocked by the barrier.

[0009] These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims.

[0010] Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention in order that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:

[0012] Figure 1 is a schematic, cross-sectional illustration of a downhole wellbore environment according to embodiments of this disclosure.

[0013] Figure 2 is a schematic, cross-sectional illustration of another downhole wellbore environment according to embodiments of this disclosure,

[0014] Figure 3 A is a schematic view of a wellbore environment 100B prior to placement of well barriers.

[0015] Figure 3B is a schematic view of a wellbore environment 100C after placement of well barriers.

[0016] Figure 4A is a schematic, cross-sectional view of an embodiment of a well with a wellbore tubular having an optical fibre associated therewith.

[0017] Figure 4B is a schematic, cross-sectional view of another embodiment of a well with a wellbore tubular having an optical fibre associated therewith.

[0018] Figure 5 illustrates an embodiment of a schematic processing flow for an acoustic signal. [0019] Figures 6A and 6B illustrate exemplary acoustic depth-time block graphs.

[0020] Figures 7A, 7B, and 7C illustrate exemplary filtered acoustic depth-time graphs.

[0021] Figure 8 illustrates an exemplary fluid flow log according to embodiments of this disclosure.

[0022] Figure 9 schematically illustrates a computer that can be used to carry out various steps according to some embodiments of this disclosure.

[0023] Figure 10 is a schematic showing baseline logs for three runs of Example 1 : Run 1 prior to placement of a first well barrier element WBEI, referred to in Figure 10 as“Pre WBE1 placement; Run 2 after placement of first well barrier element WBEI, referred to in Figure 10 as“Post WBE1 placement”; and Run 3 after placement of second and third well barrier elements WBE2/3, referred to in Figure 10 as“Post WBE2/3 placement.”

[0024] Figure 11 is a schematic showing the DAS logs (e.g., the acoustic logs) for the baseline and C bleed of Run 3 (e.g., after setting of second and third well barrier elements WBE2/3) of Example 1.

[0025] Figure 12 is a schematic of the DAS logs obtained during the B bleeds of Run 2 (e.g., after placement of first well barrier element WBE1) and Run 3 (e.g., after placement of second and third well barrier elements WBE2/3) of Example 1.

[0026] Figure 13 is a schematic showing the DAS logs for the baseline, the B bleed and the C bleed for Run 3 (e.g., after placement of second and third well barrier elements WBE2/3) of Example 1.

[0027 ] Figure 14A is a schematic of the DAS logs for Run 1 (e.g., prior to placement of first well barrier element WBE1), including one hour averaged comparisons for the baseline, the B bleed, and the C bleed of Example 1.

[0028] Figure 14B is a schematic of the DAS logs for the baseline corrected C bleed (e.g., the C bleed minus the baseline) of Run 1 (e.g., prior to placement of first well barrier element WBE 1 ) and a baseline smoothed log of the C bleed of Run 1 of Example 1.

[0029] Figure 15 A is a schematic of the DAS logs for Run 3 (e.g., after placement of the second and third well barrier elements WBE2/3), including one hour averaged comparisons for the baseline, the B bleed, and the C bleed of Example 1.

[0030] Figure 15B is a schematic of the DAS logs for the baseline corrected C bleed (e.g., the C bleed minus the baseline) of Run 3 (i.e., after placement of second and third well barrier dements WBE2/3) and a baseline smoothed log of the C bleed of Run 3 of Example 1. [0031] Figure 16 is a schematic of the DAS logs of the baseline smoothed C bleeds of Run 1 (e.g., prior to placement of first well barrier element WBE1) and Run 3 (i.e., after placement of second and third well barrier elements WBE2/3) of Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Unless otherwise specified, any use of any form of the terms“connect,”“engage,” “couple,”“attach,” or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. In the following discussion and in the claims, the terms“including” and“comprising” are used in an open-ended fashion, and thus should be interpreted to mean“including, but not limited to . . . Reference to up or down will be made for purposes of description with“up,”“upper,”“upward,”“upstream,” or “above” meaning toward the surface of the wellbore and with“down,”“lower,”“downward,” “downstream,” or“below” meaning toward the terminal end of the well, regardless of the wellbore orientation. Reference to inner or outer will be made for purposes of description with“in,”“inner,” or“inward” meaning towards the central longitudinal axis of the wellbore and/or wellbore tubular, and“out,”“outer,” or“outward” meaning towards the wellbore wall. As used herein, the term“longitudinal” or“longitudinally” refers to an axis substantially aligned with the central axis of the wellbore tubular, and“radial” or“radially” refer to a direction perpendicular to the longitudinal axis. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art with the aid of this disclosure upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings.

[0033] Disclosed herein is a real time signal processing architecture that allows for the identification of the presence, location, rate, and flow mechanism of various downhole fluid flows (fluid flow refers to fluid inflow, fluid flow within the wellbore, within an annulus, or any combination thereof, which may be indicative of a“leak”), whereby one or more well barriers can be positioned at or above the one or more fluid flow locations during plugging and abandonment (P&A) operations. The signal processing architecture can be utilized to identify one or more fluid flow events including fluid flow detection, pressure source identification, flow path identification, and phase detection of an flow fluid in the wellbore (within a casing, within an annulus, etc.), the formation (e.g., overburden monitoring, etc.), or moving between the formation and wellbore. As used herein, the term “fluid flow mechanism” can refer to the fluid flow pathway, source, and/or flow type or phase of a flowing fluid, As used herein, the term“real time” refers to a time that takes into account various communication and latency delays within a system, and can include actions taken within about ten seconds, within about thirty seconds, within about a minute, within about five minutes, or within about ten minutes of the action occurring.

[0034] In general, zonal isolation and well integrity management are concerns not only from the standpoint of operational risk and requirements, but also from an environmental impact perspective. Fluid flow detection techniques can include the use of temperature sensors, pressure sensors, casing collar locators, multi-finger calipers, spinners, and sometimes, density measurement tools deployed in well using intervention technologies, as well as other non-invasive evaluation / assessment techniques for detecting flow behind casing (e.g., temperature logging, ultrasonic imaging, oxygen activation (for detection of water flow behind casing) with neutrons, and the like).

[0035] While one or a combination of these tools may help provide a qualitative, and sometimes quantitative, estimates of fluid flow locations between the production tubing and the production casing, these methods suffer from being‘point’ measurement tools (i.e., tools that can only transduce a single physical parameter at a certain discrete location / depth at any one instance in time). This means that the fluid flows/leaks may not be captured accurately or captured at all unless the tools are positioned at the right location at the right time and/or unless the fluid flow or leak is large enough to generate a transducable signal. This typically results in longer data acquisition times and limited representations, which can often impede decision making and support. None of these tools offer the capability to monitor the flow of hydrocarbons behind multiple barriers, for example, in the casing-casing annuli, and this presents a challenge in maintaining well integrity. Multi-finger calipers are also often used to investigate any diameter variations along the tubing but this process does not quantify the extent, rate, or phase of leaking fluid. This also only provides an indication of potential fluid flow location based on mechanical assessment of the tubing. Each of these methods generally only provide an indication of a fluid flow location and do not provide the means to assess changes in fluid flow rate or fluid flow mechanism (e.g,, changes in fluid flow pathways, sources, flow types, etc.).

[0036 ] As described in more detail herein, distributed fibre optic (DFO) sensors for well integrity assessment use the fibre to monitor properties along the length of a wellbore.

Similarly, distributed temperature sensing systems (DTS) can be used to measure the temperature along the wellbore. The main advantage of these DFO sensors is that the measurement can be made along the entire length of the wellbore over long periods of time as the entire deployed fibre cable is the sensor. This can avoid the need to move the too! and aid in more economical operations. The full wellbore coverage would also enable studies of fluid flow evolution through time and depth, consequently enabling precise identification of when and where fluid flows occur, rather than piecing together the picture from various steps in the logging operation. The use of DTS for leak detection however, brings a few limitations including: 1) the use of thermal profiles alone for leak identification often results in inconclusive results, and 2) it is difficult to achieve controlled shut in versus flowing conditions outside of casing to compare and determine leak locations from baseline thermal profiles.

[0037] As disclosed herein, a new approach to plugging and abandoning a well is described using Distributed Acoustic Sensors (DAS) as the primary data input. This type of system offers not only identification of leaks and fluid flow behind casing, but also enables categorization of these events in real time or near real time. A data processing architecture is also described that processes voluminous DAS data in near real time (e.g., within a second, within ten seconds, etc.) to identify and classify leaks and other“fluid flow events” indicative of well barrier performance with a single fibre optic cable deployed in well. In embodiments, the data can also be used in conjunction with surface and peripheral sensor data to enable semi -quantitative assessments of fluid flow rates.

[0038] As further disclosed herein, the DAS data can be used with additional sensor data such as pressure data as the primary sensor inputs for determining in-well and near wellbore fluid flows. The processing methodology uses a fluid flow event detection algorithm that detects and captures acoustic events that are then processed in real-time using a spectral descriptor framework for signature recognition and identification of fluid flow. In embodiments, the outputs of the fluid flow event detection algorithm can then be correlated in time with the additional sensor data (e.g., the pressure gauge measurements). The correlation of the signals can enable identification of: a pressure source, a location of a leak, a flow rate of the leak, a ieak flow path, and/or a predominant phase of a flowing fluid, and thus be utilized to determine where to set a barrier for well abandonment and/or determine whether or not well barrier placement has successfully plugged the well (e.g., that placement of one or more well barriers has reduced or eliminated fluid flow at one or more identified fluid flow locations).

[0039] The method may also allow_' for monitoring fluid flows behind multiple barriers which are usually not detected using conventional leak detection diagnostics tools. This ability enables monitoring of hydrocarbon migration up pathways adjacent to wellbores to shallower zones (cross-flow) and/or into well annuli, thereby enabling real time monitoring of fluid movements in the formation and/or annuli and evaluating how to best plug such fluid flows for well abandonment,

[0040] As described in more detail herein, the system comprises a DAS interrogator connected to the fibre optic cable deployed in the well. Various sensors (e.g., the distributed fibre optic acoustic sensors, etc.) can be used to obtain an acoustic sampling at various points aiong the wellbore. The acoustic sample can then be processed using signal processing architecture with various feature extraction techniques (e.g., spectral feature extraction techniques) to obtain a measure of one or more frequency domain features that enable selectively extracting the acoustic signals of interest from background noise and consequently aiding in improving the accuracy of the identification of the movement of fluids and/or solids (e.g., liquid ingress locations, gas influx locations, constricted fluid flow locations, etc.) in real time. As used herein, various frequency domain features can be obtained from the acoustic signal, in some contexts the frequency domain features can also be referred to as spectral features or spectral descriptors. The signal processing techniques described herein can also help to address the big-data problem through intelligent extraction of data (rather than crude decimation techniques) to considerably reduce real time data volumes at the collection and processing site (e.g., by over 100 times, over 500 times, or over 1000 times, or over 10,000 times reduction).

[0041] The acoustic signal can be obtained in a manner that allows for a signal to be obtained along the entire wellbore or a portion of interest. Fibre optic distributed acoustic sensors (DAS) capture acoustic signals resulting from downhole events such as gas influx, liquid influx, fluid flow past restrictions, and the like as well as other background acoustics as well. This mandates the need for a robust signal processing procedure that distinguishes acoustic signals resulting from events of interest from other noise sources to avoid false positives in the results. This in turn results in a need for a clearer understanding of the acoustic fingerprint of in-well event of interest (e.g., fluid flow detection, leak detection, etc.) in order to be able to segregate a noise resulting from an event of interest from other ambient acoustic background noise. As used herein, the resulting acoustic fingerprint of a particular event can also be referred to as a spectral signature. The spectral signature can be defined by a plurality of different frequency domain features and/or combination and modifications thereof, and corresponding thresholds or ranges for the plurality of different frequency domain features and/or combination and modifications thereof, as described in more detail herein. [0042] The ability to identify various fluid flow events in the wellbore may allow for appropriate actions to be taken in order to plug the leaks and prepare the well for abandonment, For example, a barrier can be positioned at or above one or more identified fluid flow locations, and the DAS system utilized to determine whether or not the barrier is successful at reducing or eliminating the fluid flow at the one or more fluid flow locations. As described herein, frequency domain features (e.g., also referred to as spectral descriptors) can be used with DAS acoustic data processing in real time to provide various downhole surveillance applications. More specifically, the data processing techniques can be applied for various downhole fluid profiling such as events including fluid flow / inflow / outflow detection, fluid phase segregation, well integrity monitoring, in-well leak detection (e.g., downhole casing and tubing leak detection, leaking fluid phase identification, etc.), annular fluid flow detection, overburden monitoring, fluid flow detection behind a casing, fluid induced hydraulic fracture detection in the overburden, and the like, and can thus be utilized to determine a degree of success in blocking fluid flow(s) at one or more identified fluid flow locations via the seting of one or more well barriers. Such events may be referred to herein as“fluid flow” events.

[0043] In addition to the use of DAS data, additional sensor data such as pressure sensors and/or flow sensors can be used to obtain data within the wellbore. As an example, a flow sensor or pressure sensor can be used to detect fluid flow within the wellbore and/or an annulus within the wellbore. The sensors can be used with controlled shut-in and/or flow conditions to correlate in time the resulting pressure and/or flow conditions with the processed DAS data. The resulting correlation can then he used to determine a presence (or absence) and/or location of fluid flow.

[0044] Referring now to Figure 1, an example of a wellbore operating environment 100 is shown. As will be described in more detail below, embodiments of completion assemblies comprising distributed acoustic sensor (DAS) system in accordance with the principles described herein can be positioned in environment 100.

[0045] As shown in Figure 1, exemplary environment 100 includes a wellbore 114 traversing a subterranean formation 102, casing 1 12 lining at least a portion of wellbore 114, and a tubular 120 extending through wellbore 114 and casing 112. A plurality of spaced screen elements or assemblies 118 are provided along tubular 120. In addition, a plurality of spaced zonal isolation devices 117 and gravel packs 122 can he provided between tubular 120 and the sidewall of wellbore 114. In some embodiments, the operating environment 100 includes a workover and/or drilling rig positioned at the surface and extending over the wellbore 114. [0046] In general, the wellbore 114 can be drilled into the subterranean formation 102 using any suitable drilling technique. The wellbore 114 can extend substantially vertically from the earths surface over a vertical wellbore portion, deviate from vertical relative to the earth's surface over a deviated wellbore portion, and/or transition to a horizontal wellbore portion. In general, all or portions of a wellbore may be vertical, deviated at any suitable angle, horizontal, and/or curved. In addition, the wellbore 114 can be a new wellbore, an existing wellbore, a straight wellbore, an extended reach wellbore, a sidetracked wellbore, a multilateral wellbore, and other types of wellbores for drilling and completing one or more production zones. As illustrated, the wellbore 114 includes a substantially vertical producing section 150, which is an open hole completion (e.g., casing 112 does not extend through producing section 150). Although section 150 is illustrated as a vertical and open hole portion of wellbore 114 in Figure 1, embodiments disclosed herein can be employed in sections of wellbores having any orientation, and in open or cased sections of wellbores. The casing 112 extends into the wellbore 114 from the surface 113 and is cemented within the wellbore 114 with cement 111.

[01147] Tubular 120 can be lowered into wellbore 114 for performing an operation such as drilling, completion, workover, treatment, and/or production processes. In the embodiment shown in Figure 1, the tubular 120 is a completion assembly string including a distributed acoustic sensor (DAS) sensor coupled thereto. However, in general, embodiments of the tubular 120 can function as a different type of structure in a wellbore including, without limitation, as a drill string, casing, liner, jointed tubing, and/or coiled tubing. Further, the tubular 120 may operate in any portion of the wellbore 1 14 (e.g., vertical, deviated, horizontal, and/or curved section of wellbore 114). Embodiments of DAS systems described herein can be coupled to the exterior of the tubular 120, as depicted in Figure 4B, or in some embodiments, disposed within an interior of the tubular 120, as shown in Figure 4A. When the DAS fibre is coupled to the exterior of the tubular 120, as depicted in Figure 4B, the DAS can be positioned within a control line, control channel, or recess in the tubular 120. In some embodiments, a sand control system can include an outer shroud to contain the tubular 120 and protect the system during installation. A control line or channel can be formed in the shroud and the DAS system can be placed in the control line or channel. In some embodiments, the tubular and/or the DAS fiber can be removed prior to or subsequent utilization of the DAS system as described herein to identify a (first) fluid flow location, followed by removal prior to setting a barrier at or above the identified (first) fluid flow location. [0048] The tubular 120 extends from the surface to the producing zones and generally provides a conduit for fluids to travel from the formation 102 to the surface, A completion assembly including the tubular 120 can include a variety of other equipment or downhole tools to facilitate the production of the formation fluids from the production zones. For example, zonal isolation devices 117 are used to isolate the various zones within the wellbore 114. In this embodiment, each zonal isolation device 1 17 can be a packer (e.g., production packer, gravel pack packer, frac-pac packer, etc.). The zonal isolation devices 1 17 can be positioned between the screen assemblies 1 18, for example, to isolate different gravel pack zones or intervals along the wellbore 1 14 from each other. In general, the space between each pair of adjacent zonal isolation devices 117 defines a production interval.

[0049] The screen assemblies 118 provide sand control capability. In particular, the sand control screen dements 1 18, or other filter media associated with wellbore tubular 120, can be designed to allow fluids to flow therethrough but restrict and/or prevent particulate matter of sufficient size from flowing therethrough. In some embodiments, gravel packs 122 can be formed in the annulus 119 between the screen elements 118 (or tubular 120) and the sidewall of the wellbore 114 in an open hole completion. In general, the gravel packs 122 comprise relatively coarse granular material placed in the annulus to form a rough screen against the ingress of sand into the wellbore while also supporting the wellbore wall. The gravel pack 122 is optional and may not be present in all completions.

[0050] The fluid flowing into the tubular 120 may comprise more than one fluid component. Typical components include natural gas, oil, water, steam, and/or carbon dioxide. The relative proportions of these components can vary over time based on conditions within the formation 102 and the wellbore 1 14. Likewise, the composition of the fluid flowing into the tubular 120 sections throughout the length of the entire production string can vary significantly from section to section at any given time.

[0051] As fluid flows into the wellbore 114 and into the completion assembly string, the flow of the various fluids into the wellbore 114 and/or through the w'ellbore 1 14 can create acoustic sounds that can be detected using the acoustic sensor such as the DAS system . Each type of fluid flow event such as the different fluid flows and fluid flow locations can produce an acoustic signature with unique frequency domain features. For example, a fluid flow or

“leak” representing fluid flow past a restriction, through an annulus, and/or through the formation can create unique sound profiles over a frequency domain such that each event may have a unique acoustic signature based on a plurality' of frequency domain features. In some embodiments, the event or acoustic signature can comprise thresholds or ranges for a plurality of different frequency domain features, combinations of frequency domain features, or modifications of a plurality of frequency domain features,

[0052] In Figure 1, the DAS comprises an optical fibre 162 based acoustic sensing system that uses the optical backscatter component of light injected into the optical fibre for detecting acoustic/vibration perturbations (e.g., dynamic strain) along the length of the fibre 162. The light can be generated by a light generator or source 166 such as a laser, which can generate light pulses. The optical fibre 162 acts as the sensor element with no addition transducers in the optical path, and measurements can be taken along the length of the entire optical fibre 162. The measurements can then be detected by an optical receiver such as sensor 164 and selectively filtered to obtain measurements from a given depth point or range, thereby providing for a distributed measurement that has selective data for a plurality of zones along the optical fibre 162 at any given time, In this manner, the optical fibre 162 effectively functions as a distributed array of acoustic sensors spread over the entire length of the optical fibre 162, which typically spans at least a portion of the production zone 150 of the wellbore 1 14, to detect downhole acoustic signals/vibration perturbations. When used in an abandonment system, the DAS system can span a portion of the weilbore between a lower zona! isolation device (e.g., a plug, etc.) and a zone desired to be isolated as part of the abandonment process

[0053] The light reflected back up the optical fibre 162 as a result of the backscatter can travel back to the source, where the signal can be collected by a sensor 164 and processed (e.g., using a processor 168), In general, the time the light takes to return to the collection point is proportional to the distance traveled along the optica! fibre 162, The resulting backseatered light arising along the length of the optical fibre 162 can be used to characterize the environment around the optical fibre 162. The use of a controlled light source 166 (e.g., having a controlled spectral width and frequency) may allow the backscatter to be collected and any disturbances along the length of the optical fibre 162 to be analyzed. In general, any acoustic or dynamic strain disturbances along the length of the optical fibre 162 can result in a change in the properties of the backscattered light, allowing for a distributed measurement of both the acoustic magnitude, frequency and in some cases of the relative phase of the disturbance

[0054] An acquisition device 160 can be coupled to one end of the optical fibre 162. As discussed herein, the light source 166 can generate the light (e.g., one or more light pulses), and the sensor 164 can collect and analyze the backseattered light returning up the optical fibre 162. In some contexts, the acquisition device 160 including the light source 166 and the sensor 164 can be referred to as an interrogator. In addition to the light source 166 and the sensor 164, the acquisition device 160 generally comprises a processor 168 in signal communication with the sensor 164 to perform various analysis steps described in more detail herein. While shown as being within the acquisition device 160, the processor can also be located outside of the acquisition device 160 including being located remotely from the acquisition device 160, The sensor 164 can be used to obtain data at various rates and may obtain data at a sufficient rate to detect the acoustic signals of interest with sufficient bandwidth in an embodiment, depth resolution ranges of between about 1 meter and about 10 meters can be achieved.

[0055] While the system 100 described herein can be used with a DAS system to acquire an acoustic signal for a location or depth range in the weilbore 1 14, in general, any suitable acoustic signal acquisition system can be used with the processing steps disclosed herein. For example, various microphones or other sensors can be used to provide an acoustic signal at a given location based on the acoustic signal processing described herein. The benefit of the use of the DAS system is that an acoustic signal can be obtained across a plurality of locations and/or across a continuous length along the wellbore 114 rather than at discrete locations,

[0056] In addition to the DAS system, a surface sensor or sensor system 152 can be used to obtain additional data for the wellbore. The surface sensor system 152 can comprise one or more sensors such as pressure sensors, flow sensors, temperature sensors, and the like. The sensors can detect the conditions within the tubular 120 and/or in one or more annuli such as annuli 119. While only a single annulus between the tubular 120 and the easing 1 12 is illustrated in Figure 1, multiple annuli can be present. For example, more than one casing string (also referred to herein as a tubular string or casing) can often be set at or near the surface of the wellbore during drilling, which can result in two or more annuli (e.g., an annulus between the tubular 120 and the easing 112, an annulus between a first casing 112 and a second easing, an annulus between a casing string and the wellbore wall, etc.).

[0057] In embodiments, the wellbore comprises one or more tubular strings and one or more annuli disposed between: (i) two adjacent tubular strings of the one or more tubular strings, (is) a tubular string of the one or more tubular strings and the formation 102, or (iii) both (i) and (ii). For example, as depicted in Figure 2, which is a schematic, cross-sectional illustration of another downhole wellbore environment 100A according to embodiments of this disclosure, weilbore environment 100A comprises wellbore 114, tubular 120, and first casing 112A, second casing 1 12B, third casing 112C, and fourth casing 1 12D. As depicted in Figure 2, a first annulus 1 19A is positioned between wellbore 114 and first casing 112A and second casing 112B. A second annulus 119B is positioned between second casing 112B and third casing 1 12C. A third annulus 119C is positioned between third casing 112C and fourth casing 112D. In embodiments, identifying a fluid flow location comprises determining an annulus of the one or more annuli and a depth at which the fluid flow location is present. The fluid flow locations identified according to this disclosure can comprise, for example, a location of fluid flow from the formation 102 into the wellbore 114, a location of flow between the formation 102 and an annulus between a tubular string or casing and the wellbore wall (e.g., between the formation 102 and first annulus 119A, second annulus 119B, or third annulus 119C), or a location of flow between annuli formed between a plurality of tubular strings in the wellbore 1 14 (e.g., between first annulus 119A and second annulus 119B or between second annulus 119B and third annulus 119C).

[0058] As used herein, reference to the term“surface” (113) can refer to a location above or at the well head (e.g., at the Kelly bushing, rig floor, etc.), near the ground level, and/or within the first 100 m, within the first 150 m, within the first 200 m, or within about the first 300 m along the wellbore as measured from ground level.

[0059] Specific spectral signatures can be determined for each fluid flow event by considering one or more frequency domain features. The resulting spectral signatures can then be used along with processed acoustic signal data to determine if a fluid flow event is occurring at a depth range of interest. The spectral signatures can be determined by considering the different types of movement and flow occurring within a wellbore and characterizing the frequency domain features for each type of movement.

[0060] For the flow of gas into the wellbore, the proximity to the optical fibre 162 can result in a high likelihood that any acoustic signals generated would be detected by the optical fibre 162. The flow of a gas into the wellbore would likely result in a turbulent flow over a broad frequency range. For example, the gas flow acoustic signals can be between about 0 Hz and about 1000 Hz, or alternatively between about 0 Hz and about 500 Hz. An increased power intensity may occur between about 300 Hz and about 500 Hz from increased turbulence in the gas flow. An example of the acoustic signal resulting from the influx of gas into the wellbore can include frequency filtered acoustic intensity in depth versus time graphs for five frequency bins. The five frequency bins represent 5 Hz to 50 Hz, 50 Hz to 100 Hz, 100 Hz to 500 Hz, 500 Hz to 2000 Hz, and 2000 Hz to 5000 Hz. The acoustic intensity in the first three bins can have frequency ranges up to about 500 Hz, with a nearly undetectable acoustic intensity in the frequency range above 500 Hz. At least a portion of the frequency domain features may not be present above 500 Hz, which can help to define the signature of the Influx of gas.

[0061] For the flow of a fluid behind a casing in the wellbore, the proximity of the fluid flow to the optical fibre 162 can result in the acoustic signal being detected. The flow behind the casing can generally be characterized by a flow of fluid through one or more restrictions based on a generally narrow or small leak path being present. The flow through such a restriction may be characterized by an increase in spectral power in a frequency range between about 0 Hz to about 300 Hz with a main energy contribution in the range of about 0 Hz to about 100 Hz, or between about 0 Hz and about 70 Hz.

[0062] Based on the expected sound characteristics from the potential acoustic signal sources, the acoustic signature of each fluid flow event can be defined relative to background noise contributions. Referring again to Figure 1, the processor 168 within the acquisition device 160 can be configured to perform various data processing to detect the presence of one or more fluid flow events along a length of the wellbore 114. The acquisition device 160 can comprise a memory 170 configured to store an application or program to perform the data analysis. While shown as being contained within the acquisition device 160, the memory 170 can comprise one or more memories, any of which can be external to the acquisition device 160. In an embodiment, the processor 168 can execute the program, which can configure the processor 168 to filter the acoustic data set spatially, determine one or more frequency- domain features of the acoustic signal, compare the resulting frequency domain feature values to the acoustic signatures, and determine whether or not a fluid flow event is occurring at the selected location based on the analysis and comparison. The analysis can be repeated across various locations along the length of the wellbore 114 to determine the occurrence of one or more fluid flow events and/or fluid flow event locations along the length of the wellbore 114,

[0063] At the same time, one or more wellbore parameters can be measured with the sensor system 152. For example, the sensors can be used to detect the pressure(s), flow rate(s), temperature(s), and the like at one or more locations at or near the surface of the wellbore and/or within the wellbore. For example, a pressure in the tubular, and one or more annuli can be monitored over time. The measurements can be stored with a time stamp and/or stored with the acquired acoustic data set so that the two data sets can be time correlated after processing the acoustic signal.

[0064] When the acoustic sensor comprises a DAS system, the optical fibre 162 can return raw optical data in real time or near real time to the acquisition unit 160. In an embodiment, the raw data can be stored in the memory 170 for various subsequent uses. The sensor 164 can be configured to convert the raw optical data into an acoustic data set. Depending on the type of DAS system employed, the optical data may or may not be phase coherent and may be pre-processed to improve the signal quality (e.g., for opto-electronic noise normalization / de-trending single point-reflection noise removal through the use of median filtering techniques or even through the use of spatial moving average computations with averaging windows set to the spatial resolution of the acquisition unit, etc,).

[0065] As shown schematically in Figure 5, an embodiment of a system for detecting various fluid flow event conditions such as a leak detection can comprise a data extraction unit 402, a processing unit 404, a peripheral sensor data correlation unit 408, and/or an output or visualization unit 406. The system comprises of a DAS interrogator 160 connected to the fibre optic cable 162 deployed in the wellbore. The data from the DAS interrogator is transmited in real time to a data processing unit 402 that receives and processes the data in real time. The data processing unit 402 can perform a variety of processing steps on the acoustic sample data. In an embodiment, the acoustic sample can be noise de-trended. The noise de-trended acoustic variant data can be subjected to an optional spatial filtering step following the pre-processing steps, if present, This is an optional step and helps focus primarily on an interval of interest in the wellbore. For example, the spatial filtering step can be used to focus on an interval where there is maximum likelihood of fluid flow when a fluid flow event is being examined. In an embodiment, the spatial filtering can narrow the focus of the analysis to a reservoir section and also allow a reduction in data typically of the order of ten times, thereby simplifying the data analysis operations. The resulting data set produced through the conversion of the raw optical data can be referred to as the acoustic sample data.

[0066] This type of filtering can provide several advantages in addition to the data set size reduction. Whether or not the acoustic data set is spatially filtered, the resulting data, for example the acoustic sample data, used for the next step of the analysis can be indicative of an acoustic sample over a defined depth (e.g., the entire length of the optical fibre, some portion thereof, or a point source in the wellbore 1 14). In some embodiments, the acoustic data set can comprise a plurality of acoustic samples resulting from the spatial filtering to provide data over a number of depth ranges. In some embodiments, the acoustic sample may contain acoustic data over a depth range sufficient to capture multiple points of interest. In some embodiments, the acoustic sample data contains information over the entire frequency range at the depth represented by the sample. This is to say that the various filtering steps, including the spatial filtering, do not remove the frequency information from the acoustic sample data.

[0067] The processing unit 402 can also be used to generate and extract acoustic descriptors (e.g., also referred to as frequency domain features herein) from the acoustic data set. In an embodiment, the data extraction unit 402 can obtain the optical data and perform the initial pre-processing steps to obtain the initial acoustic information from the signal returned from the sensor in the wellbore. Various analyses can be performed including frequency domain feature extraction, frequency band extraction, frequency analysis and/or transformation, intensity and/or energy calculations, and/or determination of one or more frequency domain features of the acoustic data. In order to obtain the frequency domain features, the data processing unit 402 can be further configured to perform Discrete Fourier transformations (DFT) or a short time Fourier transform (STFT) of the acoustic variant time domain data measured at each depth section along the fibre or a section thereof to spectrally check the conformance of the acoustic sample data to one or more acoustic signatures. The spectral conformance check can be used to determine if the expected signature of an event is present in the acoustic sample data. Spectral feature extraction through time and space can be used to determine the spectral conformance and determine if an acoustic signature (e.g., a gas influx signature, fluid flow signature, etc.) is present in the acoustic sample in order to classify the events within the acoustic signal. Within this process, various frequency domain features can be calculated for the acoustic sample data.

[0068] The frequency domain features represent specific properties or characteristics of the acoustic signals. While a number of frequency domain features can be determined for the acoustic sample data, not every frequency domain feature may be used in the characterization of each acoustic signature. In some embodiments, the frequency domain features that are calculated can be a plurality of different frequency domain features. Some frequency domain features can represent transformed or modified frequency domain features, including combinations or mathematical modifications (e.g., ratios, multiplications, formula, etc.) of a plurality of frequency domain features. The term“frequency domain features” is used here to refer to not only the frequency domain features obtained from the acoustic signal, but also any combinations or modifications thereof.

[0069] The use of the frequency domain features to identify one or more fluid flow events has a number of advantages. First, the use of the frequency domain features results in significant data reduction relative to the raw DAS data stream. Thus, a number of frequency domain features can be calculated to allow for event identification while the remaining data can be discarded or otherwise stored, while the remaining analysis can performed using the frequency domain features. Even when the raw DAS data is stored, the remaining processing power is significantly reduced through the use of the frequency domain features rather than the raw acoustic data itself. Further, the use of the frequency domain features provides a concise, quantitative measure of the spectral character or acoustic signature of specific sounds pertinent to downhole fluid surveillance and other applications that may directly be used for real-time, application-specific signal processing.

[0070] As a further consideration in selecting the frequency domain feature(s) for an event, the dimensionality of the frequency domain feature should be compact, A compact representation is desired to decrease the computational complexity of subsequent calculations. The frequency domain feature should also have discriminant power. For example, for different types of audio signals, the selected set of descriptors should provide altogether different values. A measure for the discriminant power of a feature is the variance of the resulting feature vectors for a set of relevant input signals. Given different classes of similar signals, a discriminatory descriptor should have low variance inside each class and high variance over different classes. The frequency domain feature should also be able to completely cover the range of values of the property it describes. As an example, the chosen set of frequency domain features should be able to completely and uniquely identify the signatures of each of the acoustic signals pertaining to a selected downhole surveillance application or event as described herein. Such frequency domain features can include, but are not limited to, the spectral centroid, the spectral spread, the spectral roll-off, the spectral skewness, the root mean square (RMS) band energy (or the normalized subband energies / band energy ratios), a loudness or total RMS energy, spectral flatness, , spectral scope, spectral kurtosis, a spectra! flux, spectral entropy, and a spectral autocorrelation function. In embodiments, a single frequency domain feature is utilized to determine the presence (or absence) of a fluid flow event and identify a fluid flow location of the fluid flow event, which information is subsequently utilized to locate one or more well barriers during plugging and abandonment operations and/or determine whether or not such plugging operations have been successful at reducing or eliminating the fluid flow at the identified fluid flow location. In alternative embodiments, a plurality (e.g,, at least two) of different frequency domain features are utilized to determine the presence (or absence) of the fluid flow event and identify the fluid flow location of the fluid flow event, which information is subsequently utilized to locate one or more well barriers during plugging and abandonment operations and/or determine whether or not such plugging operations have been successful at reducing or eliminating the fluid flow at the identified fluid flow location.

[0071] The spectra! centroid denotes the“brightness” of the sound captured by the optical fibre 162 and indicates the center of gravity of the frequency spectrum in the acoustic sample. The spectra! centroid can be calculated as the weighted mean of the frequencies present in the signal, where the magnitudes of the frequencies present can be used as their weights in some embodiments. I^’he value of the spectral centroid, C , of the 1^th frame of the acoustic signal captured at a spatial location on the fibre, may be written as;

Where X_i(k), is the magnitude of the short time Fourier transform of the i^th frame where 'fc' denotes the frequency coefficient or bin index, N denotes the total number of bins and /(fe) denotes the centre frequency of the bin. The computed spectral centroid may be scaled to value between 0 and 1. Higher spectral centroids typically indicate the presence of higher frequency acoustics and help provide an immediate indication of the presence of high frequency noise. The calculated spectral centroid can he compared to a spectral centroid threshold or range for a given event, and when the spectra! centroid meets or exceeds the threshold, the event of interest may be present,

[0072] The absolute magnitudes of the computed spectra! centroids can be scaled to read a value between zero and one. The turbulent noise generated by other sources such as fluid flow and flow may typically be in the lower frequencies (e.g., under about 100 Hz) and the centroid computation can produce lower values, for example, around or under 0.1 post rescaling. The introduction of fluid or fluid carrying sand can trigger broader frequencies of sounds (e.g., a broad band response) that can extend in spectral content to higher frequencies (e.g., up to and beyond 5,000 Hz), This can produce centroids of higher values (e.g., between about 0.2 and about 0.7, or between about 0,3 and about 0,5), and the magnitude of change would remain fairly independent of the overall concentration of sanding assuming there is a good signal to noise ratio in the measurement assuming a traditional electronic noise floor (e.g., white noise with imposed flicker noise at lower frequencies).

[0073] The spectral spread can also be determined for the acoustic sample. The spectral spread is a measure of the shape of the spectrum and helps measure how the spectrum is distributed around the spectral centroid. In order to compute the spectral spread, S,, one has to take the deviation of the spectrum from the computed cen troid as per the following equation (all other terms defined above);

Lower values of the spectral spread correspond to signals whose spectra are tightly concentrated around the spectral centroid. Higher values represent a wider spread of the spectral magnitudes and provide an indication of the presence of a broad band spectral response. The calculated spectral spread can be compared to a spectral spread threshold or range, and when the spectral spread meets or exceeds the threshold or falls within the range, the event of interest may be present.

[0074] The spectra! roll-off is a measure of the bandwidth of the audio signal. The Spectral roll-off of the i^lh frame, is defined as the frequency bin‘y’ below which the accumulated magnitudes of the short-time Fourier transform reach a certain percentage value (usually between 85% - 95%) of the overall sum of magnitudes of the spectrum.

Where c = 85 or 95. The result of the spectral roll-off calculation is a bln index and enables distinguishing acoustic events based on dominant energy contributions in the frequency domain, (e.g., between gas influx and fluid flow, etc.)

[0075] The spectral skewness measures the symmetry of the distribution of the spectral magnitude values around their arithmetic mean.

[0076] The RMS band energy provides a measure of the signal energy within defined frequency bins that may then be used for signal amplitude population. The selection of the handwidths can be based on the characteristics of the captured acoustic signal. In some embodiments, a sub-band energy ratio representing the ratio of the upper frequency in the selected band to the lower frequency in the selected band can range between about 1.5:1 to about 3:1. In some embodiments, the sub-band energy ratio can range from about 2.5:1 to about 1.8:1, or alternatively be about 2:1. In some embodiment, selected frequency ranges for a signal with a 5,000 Hz Nyquist acquisition bandwidth can include: a first bin with a frequency range between 0 Hz and 20 Hz, a second bin with a frequency range between 20 Hz and 40 Hz, a third bin with a frequency range between 40 Hz and 80 Hz, a fourth bin with a frequency range between 80 Hz and 160 Hz, a fifth bin with a frequency range between 160 Hz and 320 Hz, a sixth bin with a frequency range between 320 Hz and 640 Hz, a seventh bin with a frequency range between 640 Hz and 1280 Hz, an eighth bin with a frequency range between 1280 Hz and 2500 Hz, and a ninth bin with a frequency range between 2500 Hz and 5000 Hz. While certain frequency ranges for each bin are listed herein, they are used as examples only, and other values in the same or a different number of frequency range bins can also be used, In some embodiments, the RMS band energies may also be expressed as a ratiometric measure by computing the ratio of the RMS signal energy within the defined frequency bins relative to the total RMS energy across the acquisition (Nyquist) bandwidth. This may help to reduce or remove the dependencies on the noise and any momentary variations in the broadband sound.

[0077] The total RMS energy of the acoustic waveform calculated in the time domain can indicate the loudness of the acoustic signal, In some embodiments, the total RMS energy can also be extracted from the temporal domain after filing the signal for noise.

[0078] The spectral flatness is a measure of the noisiness / tonality of an acoustic spectrum, it can be computed by the ratio of the geometric mean to the arithmetic mean of the energy· spectrum value and may be used as an alternative approach to detect broadbanded signals (e.g., such as those caused by sand ingress). For tonal signals, the spectral flatness can be close to 0 and for broader band signals it can be closer to 1.

[0079] The spectral slope provides a basic approximation of the spectrum shape by a linearly regressed line. The spectral slope represents the decrease of the spectral amplitudes from low to high frequencies (e.g., a spectral tilt). The slope, the y-lntersection, and the max and media regression error may be used as features.

[0080] The spectral kurtosis provides a measure of the flatness of a distribution around the mean value.

[8081] The spectral flux is a measure of instantaneous changes in the magnitude of a spectrum. It provides a measure of the frame-to-frame squared difference of the spectral magnitude vector summed across all frequencies or a selected portion of the spectrum. Signals with slowly varying (or nearly constant) spectral properties (e.g.: noise) have a low- spectral flux, while signals with abrupt spectral changes have a high spectral flux. The spectral flux can allow for a direct measure of the local spectral rate of change and consequently serves as an event detection scheme that could be used to pick up the onset of acoustic events that may then be further analyzed using the feature set above to identify and uniquely classify the acoustic signal.

[0082] The spectral autocorrelation function provides a method in which the signal is shifted, and for each signal shift (lag) the correlation or the resemblance of the shifted signal with the original one is computed. This enables computation of the fundamental period by choosing the lag, for which the signal best resembles itself, for example, where the autocorrelation is maximized. This can be useful in exploratory signature analysis / even for event detection for well integrity monitoring across specific depths where well barrier elements to be monitored are positioned.

[0083] Any of these frequency domain features, or any combination of these frequency domain features, can be used to provide an acoustic signature for a fluid flow event. In embodiments, a selected set of characteristics can be used to provide the acoustic signature for each fluid flow event, and/or all of the frequency domain features that are calculated can be used as a group in characterizing the acoustic signature for a fluid flow event. The specific values for the frequency domain features that are calculated can vary depending on the specific attributes of the acoustic signal acquisition system, such that the absolute value of each frequency domain feature can change between systems. In some embodiments, the frequency domain features can be calculated for each event based on the system being used to capture the acoustic signal and/or the differences between systems can be taken into account in determining the frequency domain feature values for each signature between the systems used to determine the values and the systems used to capture the acoustic signal being evaluated. In embodiments, subtraction of a baseline acoustic signal, as described in Example 1 , from a flowing acoustic signal can be utilized to decouple optica! parameter variations, for example, allowing direct comparison (i.e.,“like for like” comparison) of difference logs (e.g., allowing comparison of sample data sets comprising the flowing acoustic signal from which the baseline acoustic signal has been subtracted). In this manner, a scaling can be effected without the need for an autocalibration each time the DAS sensor is removed and redeployed within the wellbore.

[0084] In order to obtain the frequency domain features, the acoustic sample data can be converted to the frequency domain. In an embodiment, the raw optical data may contain or represent acoustic data in the time domain. A frequency domain representation of the data can be obtained using a Fourier Transform. Various algorithms can be used as known in the art. In some embodiments, a Short Time Fourier Transform technique or a Discrete Time

Fourier transform can be used. The resulting data sample may then be represented by a range of frequencies relative to their power levels at which they are present. The raw optical data can be transformed into the frequency domain prior to or after the application of the spatial filter. In general, the acoustic sample will be in the frequency domain in order to determine the spectral centroid and the spectral spread. In an embodiment, the processor 168 can he configured to perform the conversion of the raw acoustic data and/or the acoustic sample data from the time domain into the frequency domain. In the process of converting the signal to the frequency domain, the power across all frequencies within the acoustic sample can he analyzed. The use of the processor 168 to perform the transformation may provide the frequency domain data in reai time or near real time.

[0085] The data processing unit 402 can then be used to analyze the acoustic sample data in the frequency domain to obtain one or more of the frequency domain features and provide an output with the determined frequency domain features for further processing. In some embodiments, the output of the frequency domain features can include features that are not used to determine the presence of every event.

[0086] The output of the processor with the frequency domain features for the acoustic sample data can then be used to determine the presence of one or more fluid flow events at one or more locations in the wellbore corresponding to depth intervals over which the acoustic data is acquired or filtered. In some embodiments, the determination of the presence of one or more fluid flow events can include comparing the frequency domain features with the frequency domain feature thresholds or ranges in each fluid flow event signature. When the frequency domain features in the acoustic sample data match one or more of the fluid flow event signatures, the event can be identified as having occurred during the sample data measurement period, which can be in real time. Various outputs can be generated to display or indicate the presence (or absence) of the one or more fluid flow events.

[0087] The processed acoustic data (i.e , the frequency domain features), which can have a significantly smaller file size (typically over 1000 X smaller) can then be written into a file (e.g., an ASCII file) in a memory at certain intervals (e.g., every second, every ten seconds, etc.), which can then be retrieved and transmitted through network using a data collection and transmission software. This process can be executed in real time or near real time for transmission of data.

[0088] The data transmitted from the DAS interrogator (which can include the frequency domain feature data) can then be further processed using a sequence of data processing steps as shown in the processing sequence 404 in Figure 5 The processing sequence 404 can comprise a series of steps including an event detection step, a signature extraction step, an event classification step, a leak or fluid flow identification step, and an output step. The descriptor data are first processed using an event-detection algorithm to determine the presence of any anomalous acoustic response(s) that may be triggered by a fluid leak/flow.

While there are several ways to implement the event detection algorithm, amplitude thresholding of the data relative to surface noise captured by the DAS on the fibre optic cable dispersed at or near the surface (e.g., within the first 100 meters) of the well head can be used. As an example of amplitude thresholding, an acoustic intensity over the entire bandwidth can be averaged over the surface or near surface measurements (e.g., in the first 300 m of acoustic data) acquisitions to provide an estimate of the average surface acoustic noise. A threshold can then he taken as a percentage of this average. For example, the amplitude threshold can be between about 90% and about 95% of the average. The presence of the signal within the wellbore can be detected when the amplitude of the acoustic event captured exceeds the threshold value. The frequency and amplitude characteristics of the surface noise may also be used to suppress and/or reduce the background noise within the selected window to identify presence of signals at the surface, if needed. This enables a zero point depth recognition, helps to reduce or eliminate surface noise contributions, helps to reduce or eliminate the DAS interrogator noise contributions, allows for the capture of acoustic events and renders the captured events in a format ready for signature recognition, and uses processed data (as compared to raw DAS data) as the primary feed to the processing sequence. While amplitude thresholding is used, other time based digital processing approaches could also be used.

[0089] Once the data is initially processed, the anomalous events can be recognized (e.g., as events having amplitudes over the thresholds), and the corresponding data from the portion of the acoustic sample can be extracted as a depth-time event block. Figure 6A illustrates an example of a depth-time event block show depth versus amplitude. Once the depth-time blocks are amplitude thresholded, the corresponding data may appear as shown in Figure 6B, with the surface noise filtered out and the anomalous events highlighted.

[0090] In the second step 412 of the processing sequence 404, the acoustic event blocks can be further analyzed by extracting the frequency domain features at the event depths and times identified by the anomalous event detection step and comparing the extracted frequency domain features to the fluid flow event signatures to match the frequency domain features for each identified event with an appropriate signature. The extraction of the frequency domain features can be performed prior to the data being sent to the processing sequence such that the extraction of the frequency domain features involves filtering the received frequency domain features for the depth and times identified by the anomalous event detection, or the extraction of the frequency domain features can be performed only after the anomalous depth-time blocks have been identified.

[0091] In either case, the resulting frequency domain features can be compared with one or more fluid flow event signatures to identify if integrity fluid flow event has occurred in the event classification step 414. In some embodiments, the fluid flow event signatures can include frequency domain signatures for a liquid leak/ flow, a gas ieak/flow, or another such event (e.g,, an unrecognized event category or other non-flow signature, which can be used for comparison),

[0092] The event classification step 414 can be executed at each depth location along the fibre and may depend on the acoustic signatures captured at the locations identified to have an anomalous event. Once classified into the appropriate category, the intensities of the events can be determined using the normalized RMS values within the appropriate frequency hands extracted on site (e.g,, which can already be one of the descriptors obtained in the extracted frequency domain features) from the raw acoustic data. The descriptor data can then be transformed and re-written as an event matrix. These steps can be executed in near real time at the data integration server, and the transformed decision ready well integrity' event data can be stored along with some or all of the acoustic descriptor data. The classified event data may also he visualized as a three dimensional depth versus time versus event type intensity plot as shown in Figure 7A and Figure 7B to illustrate fluid flow events as a function of depth and time.

[0093] The fluid flow event matrix may be further filtered to highlight and visualize certain types of fluid flow_'· events as shown in Figure 7C. These may also be aligned in depth to the well completion schematic and / or the geological maps (e.g., discrete pressure zones) to ascertain the source of the leaking fluid in case of liquid leaks. In embodiments, a fluid flow' event or location (e.g., depth) is correlated with one or more structural features within the wellbore, and a source of the fluid flow determined based on the correlating of the one or more fluid flow events or locations (e.g,, depths) with the one or more structural features.

[0094] Accordingly, in embodiments of this disclosure, subsequent to detection of a leak (e.g., a fluid flow) utilizing the feature extraction and the event signatures as described hereinabove, a flow log can be determined using a feature that is representative of the turbulent noise caused by the leak. Such a feature representative of the turbulent noise can comprise, for example, acoustic power, spectral intensity, and the like in the frequency bands identified for the fluid flow. For example, in embodiments, the leak detection and identification step 416, the event matrix may also be processed further to obtain semi- quantitative leak assessment by filtering the event matrix to extract the events correlating to gas or liquid leaks/flows and then integrating the filtered intensity data through time to provide fluid flow logs, an example of which is shown in Figure 8. [0095] In producing a visualization fluid flow log, the RMS spectral energy for depth sections that do not exhibit the spectral conformance to specific fluid flow events can be set to zero. This allows those depth points or zones having one or more frequency domain features greater than the thresholds to be easily observed. Figure 8 represents an example of an embodiment of a fluid flow log showing acoustic intensity against depth. This figure illustrates the locations having fluid flow as peaks in the acoustic intensity. The acoustic intensity and its visualization on the fluid flow log can therefore be used to identify the relative contribution of the fluid flows at different points along the wellbore. For example, it may be possible to determine which zone is contributing the greatest proportion of the fluid flows, which zone contributes the second greatest portion of the fluid flows, and so on. This may also allow for correlation of one or more zonal isolation devices, potential leak/flow locations, and/or fluid flow through the formation along the length of the wellbore.

[0096] The use of the processing sequence 404 can result in a suitable identification of the fluid flows within the wellbore to be plugged and abandoned. In an optional processing step in the peripheral sensor data correlation unit 408, the resulting processed data can be correlated with external sensor data such as that provided by a sensor system at or near the surface of the wellbore. This processing sequence may be used with the DAS system to determine the flow path for the leaks, especially in cases where there are multiple casing strings or leak paths at or near a depth determined to have a fluid flow. The process may also be used to provide a semi-quantitative estimate of the volumes of fluid associated with the fluid flow when combined with surface measurements (e.g., hieed off rate measurements, surface pressure gauge data, etc.).

[6097] The correlation process can generally comprise the use of changing surface measurement data as a comparison with the identified fluid flow event process. For example, changing pressure or flow data at the surface can be used as a correlation with the fluid flow identification data. It may he expected that as the fluid flow occurs, a shut in annulus may have a pressure rise and/or an increased flow rate (e.g., a bleed off flow rate). When multiple annuli or leak paths are present, the use of the pressure or flow data can help to identify which leak path(s) are specifically experiencing the fluid flows, while the fluid flow depth would be known from the fluid flow' event detection sequence. While described herein as a leak or fluid flow path, a number of potential paths are available for fluid flow within the wellbore. For example, a leak can occur past a restriction or barrier in one or more annuli, between a casing and the formation, and/or within the formation or a hydrocarbon zone, and potentially, into a production assembly. For example, fluid flow within a hydrocarbon zone in the formation can be monitored using any of the methods and system s described herein.

[0098] In an embodiment, a correlation process may begin by shuting in a well. This may allow a base reading to be taken of both the surface sensor data and the frequency domain features of the wellbore without fluid flow. Once the baseline readings have been obtained, a leak path can he triggered to potentially induce a fluid flow. For example, an annulus can be opened to bleed off pressure (e.g., induce a pressure differential), which can potentially induce fluid flow within that annulus if there is a leak in fluid communication with the selected annulus. This may create a pressure differential between the selected annulus and a neighboring annulus or annuli. The pressure differential can be determined to assess the fluid flow potentials. Once one leak path has been tested, it can be closed and another leak path can be triggered. This sequence can continue until all of the desired leak paths that are to be tested are triggered. The DAS monitoring system can remain active during the induced flow process to monitor for leaks and ascertain the leaking fluid phase or phases. Inducing the differential (e.g., inducing a first pressure differential prior for determination of a first fluid flow location prior to setting of a well barrier at or above the first fluid flow location and/or inducing a second pressure differential subsequent setting of the first well barrier at or above the first fluid flow location) can comprise, for example, opening a flow valve within an annulus of the one or more annuli; and inducing a fluid flow based on opening of the flow valve.

[0099] In embodiments, a sample data set (e.g., a first sample data set that is a sample of the acoustic signal originating in the wellbore) is obtained within the wellbore by obtaining the baseline acoustic signal data set while the wellbore is shut in, inducing the pressure differential within the wellbore, as described above, obtaining a flowing acoustic sample data set while inducing the pressure differential, and subtracting the baseline acoustic signal data set from the flowing acoustic signal data set to provide the sample data set from which the plurality of frequency domain features are determined and utilized as described herein to identify the fluid flow location within the wellbore.

[00100] Once the data is obtained from the sensors and the DAS system, which can include the fluid flow event data determined from the processing sequence 404 to determine the presence of absence of any fluid flow events, the data can be correlated through time to determine a fluid flow location and fluid flow path. For example, the filtered fluid flow acoustic intensities obtained from the processing sequence 404 can be integrated through time at each depth location to obtain fluid flow data (e.g., which can be visualized as fluid flow logs) for the stages of the fluid flow path triggering (e.g,, the annular pressure bleed process). This data can then be aligned in time with the pressures, pressure differentials, flow data, etc. for each trigger operation to determine the fluid flow points and flow paths. For example, it may be determined that a given fluid flow path only triggers a fluid flow at a given depth rather than over a number of depths. From this data, the fluid flow logs can be determined for each tubular, casing string, or the like.

[00101] In some embodiments, all of the surface sensor data can be used in this process. The pressure data, including the induced pressure differentials, may be used to determine the fluid flow paths and fluid flow locations. In embodiments, the bleed off rates can be used to provide a quantitative assessment of the leak rates from each fluid flow path. This data can then be stored and/or outputted and used in the future for further fluid flow identification, comparison, and/or quantification.

[00102] Once a first fluid flow event in a first depth interval (e.g., the entire length of the wellbore or a portion thereof) has been confirmed and the location of the first fluid flow event determined, one or more first well barriers can be set in an attempt to plug the well. Any barriers known to those of skill in the art and with the help of this disclosure can be utilized. By way of non-limiting examples, the one or more well barriers can comprise bridge plugs, packers, cement plugs or columns, or combinations thereof, and the like. The acoustic sensor can be removed from the wellbore 114 prior to the setting of the one or more first well barriers employed in an attempt to plug the first fluid flow at the first fluid flow location.

[00103] Subsequent to the setting of the first well barrier(s), the fluid flow detection process can be repeated. That is, the acoustic sensor can be re-deployed into the well within a second depth interval overlapping the first depth interval (e.g., generally, a depth interval comprising at least a portion of the first depth interval and above a location(s) at which the one or more first well barriers have been positioned).. When the acoustic sensor is redeployed, the well barrier can block the ability' to deploy the fiber below the well barrier.

As a result, the acoustic sensor can be deployed to extend between a point at or near the well barrier towards the surface of the wellbore. Once redeployed, a second sample data set can be obtained and utilized as described hereinabove to identify whether or not a fluid flow rate or mechanism was reduced or eliminated and/or to determine a second fluid flow location. If a second fluid flow presence and a second fluid flow location are determined, one or more second well barriers can be set in an attempt to plug the fluid flow at the second fluid flow location. Again, the acoustic sensor can be removed from the wellbore prior to the seting of the one or more second well barriers employed in the attempt to plug the second fluid flow at the second fluid flow location, The process can be repeated as necessary to prepare the well for abandonment. Accordingly, in embodiments of this disclosure, fluid flow logs can be compared (“like for like”), and the effectiveness of a flow barrier validated. That is, a barrier that is placed can be validated based on the identified reduction or elimination of the fluid flow rate and/or the fluid flow mechanism at the first fluid flow location In embodiments, an effective barrier is one that reduces the fluid inflow at the fluid inflow location such that a flow' rate of the/any remaining fluid flow at the inflow location being blocked by the barrier is less than 80, 85, 90, 95, 96, 97, 98, 99, or 100% of the original flow rate of the leak (e.g,, the original fluid flow rate), or that the fluid flow rate is zero or substantially zero after placement of the barrier.

[00104] For example, as depicted in Figures 3A and 3B, which are schematics of a wellbore environment 100B prior to placement of well barriers, and a wellbore environment 100C after placement of well barriers, respectively, tubular 120 can be removed from the wellbore 114 and one or more well barriers set, at a fluid flow location determined with the use of the DAS system as described herein, to plug the well for abandonment. In some embodiments, a baseline acoustic signal can be obtained and the first well barrier can be set on the basis of the producing zone within the wellbore such that the well barrier will generally extend through and above the producing zone. Thus, the DAS system can be utilized s described herein to determine one or more locations of fluid flow at or above which one or more well barriers can be positioned.

[00105] The DAS system can also be utilized to determine whether or not the fluid flow has been reduced or eliminated by the setting of the one or more well barriers. By way of example, in the embodiment of Figure 3B, a first barrier comprising a first cement plug

130A has been set at a first location in the wellbore, wherein the first location is within first casing 112A, a second barrier comprising a first bridge plug 131 A and a second cement plug

130B is positioned at a second location in the wellbore, wherein the second location is above the first location and within second casing 112B, and a third barrier comprising a second bridge plug 131B and a third cement plug 130 C has been set at a third location in the wellbore, wherein the third location is above the second location and within third casing

112C. The DAS system can be utilized as described herein to determine a location at or above which to set the first well barrier comprising the first cement plug 130A, the second well barrier comprising the first bridge plug 131A and the second cement plug 130B, and/or the third well barrier comprising the second bridge plug 131B and the third cement plug 130C. Alternatively or additionally, the DAS system can be utilized as described herein to determine if the setting of the first well barrier comprising the first cement plug 130A, the second well barrier comprising the first bridge plug 131 A and the second cement plug 130B, and/or the third well barrier comprising the second bridge plug 13 IB and the third cement plug 130C has reduced or eliminated fluid flow. In aspects, any number of well barriers can he positioned within the wellbore environment, with one or more of the well barriers positioned at or above a fluid flow location determined via the DAS system as described herein.

[00 106] In some embodiments, the fluid flow can be a leak path behind a casing or within an annulus (e.g., between casing strings and/or between a casing string and a wellbore wall). The identification of the location of the fluid flow may be allow for a separate procedure to be identified and performed to stop the fluid flow. For example, a fluid flow behind a casing can be addressed through the use of a repair process comprising perforating the casing and injecting cement behind the casing (e.g., a squeeze cement procedure). This may he in addition to setting a well barrier within the wellbore.

[00107] Also provided herein is a method of comparing acoustic signals obtained between different acoustic sensor operations or deployments in a wellbore. For example, the method can allow for the comparison between a first acoustic signal from a first deployment of the fiber and a second acoustic signal from a second deployment of the fiber after a well barrier has been placed in the wellbore. While described in the context of being redeployed after a well barrier has been placed In the wellbore, the method can allow for a comparison between acoustic signals obtained between any deployments of the fiber, regardless of whether or not there are changes made within the wellbore or not.

[00108] The method comprises obtaining a first baseline sample data set over a first depth interval within a wellbore, as described herein. The first baseline data set can be a sample of an acoustic signal originating within the wellbore. In some embodiments, the baseline data set can be obtained when the wellbore is shut-in and/or when a stable pressure is maintained within the wellbore. At least one frequency domain feature of the first baseline sample data set can be determined. A first pressure differential can be induced within the wellbore, as described herein, to provide for a fluid flow. A first acoustic data set can be obtained over the first depth interval within the wellbore while inducing the first pressure differential, as described herein. At least one frequency domain feature of the first acoustic- data set can then be determined. The at least one frequency domain feature of the first baseline sample data set can be subtracted from the at least one frequency domain feature of the first acoustic data set to obtain a first sample data set over the first depth interval, A second baseline sample data set can be obtained over a second depth interval within the wellbore, as described herein. The second baseline sample data set can be a sample of an acoustic signal originating within the wellbore, and the second depth interval can overlap with the first depth interval. At least one frequency domain feature of the second baseline sample data set can be determined, as described herein. A second pressure differential can be induced within the wellbore, as described herein, and a second acoustic data set can be obtained over the second depth interval within the wellbore while inducing the second pressure differential, as described herein, At least one frequency domain feature of the second acoustic data set can be determined, as described herein, and the at least one frequency domain feature of the second baseline sample data set can be subtracted from the at least one frequency domain feature of the second acoustic data set to obtain a second sample data set over the second depth interval. The second sample data set can be compared to the first sample data set over the second depth interval. In some embodiments, a fluid flow reduction can be determined at a fluid flow location based on comparing the second sample data set to the first sample data set. As noted hereinabove, the first baseline sample data set and the first acoustic data set can be obtained with an acoustic sensor disposed in the wellbore within the first depth interval, and the second baseline sample data set and the second acoustic data set can be obtained with the acoustic sensor disposed in the wellbore within the second depth interval. The method can thus further comprise removing the acoustic sensor from the wellbore between obtaining the first baseline sample data set and obtaining the second baseline sample data set (e.g., the acoustic sensor 164 can be removed from the wellbore 114 prior to setting a well barrier element (e.g., a cement plug 130A/13GB/13GC and/or a bridge plug 131 A/131 B) and/or performing a workover procedure to reduce the fluid flow in an attempt to plug fluid flow at an identified fluid flow location, and redeployed in the wellbore 114 subsequent the setting of the well barrier element),

[00109] Any of the systems and methods disclosed herein can be carried out on a computer or other device comprising a processor, such as the acquisition device 160 of Figure 1. Figure 9 illustrates a computer system 780 suitable for implementing one or more embodiments disclosed herein such as the acquisition device or any portion thereof. The computer system 780 includes a processor 782 (which may be referred to as a central processor unit or CPU) that is in communication with memory devices including secondary' storage 784, read only memory (ROM) 786, random access memory (RAM) 788, input/output (I/O) devices 790, and network connectivity devices 792. The processor 782 may he implemented as one or more CPU chips.

[00110] It is understood that by programming and/or loading executable instructions onto the computer system 780, at least one of the CPU 782, the RAM 788, and the ROM 786 are changed, transforming the computer system 780 in part into a particular machine or apparatus having the novel functionality taught by the present disclosure. It is fundamental to the electrical engineering and software engineering arts that functionality that can be implemented by loading executable software into a computer can be converted to a hardware implementation by well-known design rules. Decisions between implementing a concept in software versus hardware typically hinge on considerations of stability of the design and numbers of units to be produced rather than any issues involved in translating from the software domain to the hardware domain. Generally, a design that is still subject to frequent change may be preferred to be implemented in software, because re-spinning a hardware implementation is more expensive than re-spinning a software design. Generally, a design that is stable that will be produced in large volume may be preferred to be implemented in hardware, for example in an application specific integrated circuit (ASIC), because for large production runs the hardware implementation may be less expensive than the software implementation. Often a design may be developed and tested in a software form and later transformed, by well-known design rules, to an equivalent hardware implementation in an application specific integrated circuit that hardwires the instructions of the software. In the same manner as a machine controlled by a new ASIC is a particular machine or apparatus, likewise a computer that has been programmed and/or loaded with executable instructions may be viewed as a particular machine or apparatus.

[00111] Additionally, after the system 780 is turned on or booted, the CPU 782 may execute a computer program or application. For example, the CPU 782 may execute software or firmware stored in the ROM 786 or stored in the RAM 788. In some cases, on boot and/or when the application is initiated, the CPU 782 may copy the application or portions of the application from the secondary storage 784 to the RAM 788 or to memory space within the

CPU 782 itself, and the CPU 782 may then execute instructions that the application is comprised of. In some cases, the CPU 782 may copy the application or portions of the application from memory accessed via the network connectivity devices 792 or via the I/O devices 790 to the RAM 788 or to memory space within the CPU 782, and the CPU 782 may then execute instructions that the application is comprised of. During execution, an application may load instructions into the CPU 782, for example load some of the instructions of the application into a cache of the CPU 782, In some contexts, an application that is executed may be said to configure the CPU 782 to do something, e.g., to configure the CPU 782 to perform the function or functions promoted by the subject application. When the CPU 782 is configured in this way by the appiication, the CPU 782 becomes a specific purpose computer or a specific purpose machine.

[00112] The secondary storage 784 is typically comprised of one or more disk drives or tape drives and is used for non-volatile storage of data and as an over-flow data storage device if RAM 788 is not large enough to hold all working data. Secondary storage 784 may be used to store programs which are loaded Into RAM 788 when such programs are selected for execution. The ROM 786 is used to store instructions and perhaps data which are read during program execution. ROM 786 is a non-volatile memory device which typically has a small memory capacity relative to the larger memory capacity of secondary storage 784. The RAM 788 is used to store volatile data and perhaps to store instructions. Access to both ROM 786 and RAM 788 is typically faster than to secondary storage 784, The secondary storage 784, the RAM 788, and/or the ROM 786 may be referred to in some contexts as computer readable storage media and/or non-transitory computer readable media,

[00113] I/O devices 790 may include printers, video monitors, liquid crystal displays (LCDs), touch screen displays, keyboards, keypads, switches, dials, mice, track balls, voice recognizers, card readers, paper tape readers, or other well-known input devices,

[00114] The network connectivity devices 792 may take the form of modems, modem banks, Ethernet cards, universal serial bus (USB) interface cards, serial interfaces, token ring cards, fibre distributed data interface (FDDI) cards, wireless local area network (WLAN) cards, radio transceiver cards that promote radio communications using protocols such as code division multiple access (CDMA), global system for mobile communications (GSM), long-term evolution (LTE), worldwide interoperability for microwave access (WiMAX), near field communications (NFC), radio frequency identity (RFID), and/or other air interface protocol radio transceiver cards, and other well-known network devices. These network connectivity devices 792 may enable the processor 782 to communicate with the Internet or one or more intranets. With such a network connection, it is contemplated that the processor 782 might receive information from the network, or might output information to the network (e.g., to an event database) in the course of performing the above-described method steps. Such information, which is often represented as a sequence of instructions to be executed using processor 782, may he received from and outputted to the network, for example, in the form of a computer data signal embodied in a carrier wave. [00115] Such information, which may include data or instructions to be executed using processor 782 for example, may be received from and outputted to the network, for example, in the form of a computer data baseband signal or signal embodied in a carrier wave. The baseband signal or signal embedded in the carrier wave, or other types of signals currently used or hereafter developed, may be generated according to several methods well-known to one skilled in the art. The baseband signal and/or signal embedded in the carrier wave may be referred to in some contexts as a transitory signal.

[00116] The processor 782 executes instructions, codes, computer programs, scripts which it accesses from hard disk, floppy disk, optical disk (these various disk based systems may all be considered secondary' storage 784), flash drive, ROM 786, RAM 788, or the network connectivity devices 792. While only one processor 782 is shown, multiple processors may he present. Thus, while instructions may he discussed as executed by a processor, the instructions may be executed simultaneously, serially, or otherwise executed by one or multiple processors. Instructions, codes, computer programs, scripts, and/or data that may be accessed from the secondary storage 784, for example, hard drives, floppy disks, optical disks, and/or other device, the ROM 786, and/or the RAM 788 may he referred to in some contexts as non-transitory instructions and/or non-transitory information.

[00117] In an embodiment, the computer system 780 may comprise two or more computers in communication with each other that collaborate to perform a task. For example, but not by way of limitation, an application may be partitioned in such a way as to permit concurrent and/or parallel processing of the instructions of the application. Alternatively, the data processed by the application may be partitioned in such a way as to permit concurrent and/or parallel processing of different portions of a data set by the two or more computers. In an embodiment, virtualization software may be employed by the computer system 780 to provide the functionality of a number of servers that is not directly bound to the number of computers in the computer system 780. For example, virtualization software may provide twenty virtual servers on four physical computers. In an embodiment, the functionality disclosed above may be provided by executing the application and/or applications in a cloud computing environment. Cloud computing may comprise providing computing services via a network connection using dynamically scalable computing resources. Cloud computing may be supported, at least in part, by virtualization software. A cloud computing environment may be established by an enterprise and/or may be hired on an as-needed basis from a third party provider. Some cloud computing environments may comprise cloud computing resources owned and operated by the enterprise as well as cloud computing resources hired and/or leased from a third party provider.

[00118] In an embodiment, some or all of the functionality disclosed above may be provided as a computer program product. The computer program product may comprise one or more computer readable storage medium having computer usable program code embodied therein to implement the functionality disclosed above. The computer program product may comprise data structures, executable instructions, and other computer usable program code. The computer program product may be embodied in removable computer storage media and/or non-removable computer storage media. The removable computer readable storage medium may comprise, without limitation, a paper tape, a magnetic tape, magnetic disk, an optical disk, a solid state memory chip, for example analog magnetic tape, compact disk read only memory (CD-ROM) disks, floppy disks, jump drives, digital cards, multimedia cards, and others. The computer program product may be suitable for loading, by the computer system 780, at least portions of the contents of the computer program product to the secondary storage 784, to the ROM 786, to the MAM 788, and/or to other non-volatile memory' and volatile memory of the computer system 780. The processor 782 may process the executable instructions and/or data structures in part by directly accessing the computer program product, for example by reading from a CD-ROM disk inserted into a disk drive peripheral of the computer system 780. Alternatively, the processor 782 may process the executable instructions and/or data structures by remotely accessing the computer program product, for example by downloading the executable instructions and/or data structures from a remote server through the network connectivity devices 792. The computer program product may comprise instructions that promote the loading and/or copying of data, data structures, files, and/or executable instructions to the secondary storage 784, to the ROM 786, to the RAM 788, and/or to other non-volatile memory and volatile memory of the computer system 780.

[00119] In some contexts, the secondary storage 784, the ROM 786, and the RAM 788 may be referred to as a non-transitory computer readable medium or a computer readable storage media. A dynamic RAM embodiment of the RAM 788, likewise, may be referred to as a non-transitory computer readable medium in that while the dynamic RAM receives electrical power and is operated in accordance with its design, for example during a period of time during which the computer system 780 is turned on and operational, the dynamic RAM stores information that is written to it. Similarly, the processor 782 may comprise an internal

RAM, an internal ROM, a cache memory, and/or other internal non-transitory storage blocks, sections, or components that may he referred to in some contexts as non-transitory computer readable media or computer readable storage media,

EXAMPLES

[00120] The embodiments having been generally described, the following examples are given as particular embodiments of the disclosure and to demonstrate the practice and advantages thereof. It is understood that the examples are given by way of illustration and are not intended to limit the specification or the claims in any manner.

Example 1

[00121] in this Example, a DAS system as described herein was utilized to plug a well for abandonment. Figure 10 is a schematic showing baseline logs for three runs: Run 1 prior to placement of a first well barrier element (WBE1), referred to in Figure 10 as“Pre WBE1 placement; Run 2 after placement of WBE1 , referred to in Figure 10 as “Post WBE1 placement”; and Run 3 after placement of second and third well barrier elements (WBE2/3), referred to in Figure 10 as“Post WBE2/3 placement.” DAS logs were also obtained while inducing a first pressure by bleeding the B annulus for Run 2 after placement of WBE1 and Run 3 after placement of WBE2/3, and while inducing a second pressure differential by bleeding the C annulus for Run 2 after placement of WBE1 and for Run 3 after placement of WBE2/3.

[00122] Figure 10 is a schematic showing baseline logs for each of the three runs: Run 1 prior to placement of a first well barrier element (WBE1), referred to in Figure 10 as“Pre

WBE1 placement; Run 2 after placement of WBE1, referred to in Figure 10 as“Post WBE1 placement”; and Run 3 after placement of the second and third well barrier dements

(WBE2/3), referred to in Figure 10 as“Post WBE2/3 placement.” WBE1 was placed at a first depth of 9000 feet; WBE 2 was placed at a second depth of about 5500 feet; and WBE 3 was also placed at the second depth of about 5500 feet. As seen in Figure 10, little to no acoustic noise was captured in the baseline data in Run 3, after the placement of WBE2/3, and, as expected, similar behavior was observed in the acoustic response in Run 2 and Run 3. The logs indicate effective barrier performance. The baseline logs were obtained by obtaining the signal from the DAS sensor in the weilbore and averaging the relative acoustic amplitude over time.

Consistent behavior was observed throughout the baseline logging duration,

[00123] As noted above, subsequent the placement of the second and third well barrier elements WBE2/3, another baseline log (e.g., baseline for Run 3) was performed by rerunning the DAS sensor in the well to the second depth. A second pressure differential was induced by bleeding the C annulus (referred to in Figure 11 as“C Bleed”), and a DAS log obtained during the C bleed of Run 3. Figure 1 1 is a schematic showing the DAS logs (e.g., the acoustic logs) for the baseline and C bleed of Run 3 after setting of WBE2/3. As can be seen in Figure 1 1, little to no acoustic noise is observed in the zone above the top of the cement (TOC) of WBE2/3, and similar behavior to the baseline is observed during the C bleed.

[00124] As noted above, a first pressure differential was induced by performing a bleed of the B annulus (referred to as“B Bleed” in the Figures). Figure 12 is a schematic of the DAS logs obtained during the B bleed of Run 2 (e.g., after placement of WBE1) and during the B bleed of Run 3 (e.g., after placement of WBE2/3). As seen in Figure 12, litle to no acoustic noise is captured in the zone above the TOC of WBE2/3.

[00125] Figure 13 is a schematic showing the DAS iogs for the baseline, the B bleed and the C bleed for Run 3 (e.g., after placement of the second and third well barrier elements WBE2/3). As seen in Figure 13, the trend remained the same in the B bleed and no significant noise zones were observed,

[00126] Figure 14A is a schematic of the DAS logs for Run 1 (e.g., prior to placement of first well barrier element WBE1), including one hour averaged comparisons for the baseline, the B bleed, and the C bleed. Figure 14B is a schematic of the DAS logs for the baseline corrected C bleed (e.g., the C bleed minus the baseline) of Run 1 and a baseline smoothed log of the C bleed of Run 1, which was obtained by subtracting the C bleed from the baseline and then smoothing (e.g., running a median filter or moving average). Figure 15A is a schematic of the DAS logs for Run 3 (e.g., after placement of the second and third well barrier elements WBE2/3), including one hour averaged comparisons for the baseline, the B bleed, and the C bleed. Figure 15B is a schematic of the DAS logs for the baseline corrected C bleed (e.g., the C bleed minus the baseline) of Run 3 and a baseline smoothed log of the C bleed of Run 3.

[00127] Figure 16 is a schematic of the DAS logs of the baseline smoothed C bleeds of Run 1 (e.g., prior to placement of WBE 1) and Run 3 (e.g., after placement of WBE2/3). As seen in Figure 16, a reduction in the baseline smoothed flow noise observed in Runs 1 and 3 evidences a drop in overall flow noise at shallower depths during the bleed, indicating successful barrier placement and performance.

[00128] While various embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the disclosure. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the subject matter disclosed herein are possible and are within the scope of the disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e g., from about 1 to about 10 includes, 2, 3, 4 etc,; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, R_L and an upper limit, R_u is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=R_L+k*(R_u-R._L), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, ... 50 percent, 51 percent, 52 percent, , 95 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed. Use of the term "optionaily" with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, etc.

[00129] Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present disclosure. Thus, the claims are a further description and are an addition to the embodiments of the present disclosure. The discussion of a reference is not an admission that it is prior art to the present disclosure, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.

ADDITIONAL DESCRIPTION

[00130] The particular embodiments disclosed above are illustrative only, as the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. While compositions and methods are described in broader terms of "having”,“comprising," "containing," or "including" various components or steps, the compositions and methods can also "consist essentially of’ or "consist of" the various components and steps. Use of the term“optionally” with respect to any element of a claim means that the element is required, or alternatively, the element is not required, both alternatives being within the scope of the claim .

[00131] Numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range are specifically disclosed. In particular, every range of values (of the form, "from about a to about b," or, equivalently, "from approximately a to h," or, equivalently, "from approximately a-b") disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles "a" or "an”, as used in the claims, are defined herein to mean one or more than one of the element that it introduces, If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents, the definitions that are consistent with this specification should be adopted.

[00132] Embodiments disclosed herein include:

[00133] A: A method of abandoning a wellbore, the method comprising: obtaining a first sample data set within a wellbore, wherein the first sample data set is a sample of an acoustic signal originating within the wellbore; determining a first plurality of frequency domain features of the first sample data set; identifying a first fluid flow location within the wellbore using the first plurality of frequency domain features; seting a first barrier at or above the first fluid flow location; obtaining a second sample data set within the wellbore above the first barrier, wherein the second sample data set is a sample of an acoustic signal originating within the wellbore above the first barrier; determining a second plurality of frequency domain features of the second sample data set; and identifying that a fluid flow rate or fluid flow mechanism at the first fluid flow location has been reduced or eliminated and/or identifying a second fluid flow' location within the wellbore using the second plurality of frequency domain features.

[00134] B: A system for abandoning a wellbore, the system comprising: a receiver unit comprising a processor and a memory, wherein the receiver unit is configured to receive an acoustic signal from a sensor disposed in a wellbore, wherein a processing application is stored in the memory, and wherein the processing application, when executed on the processor, configures the processor to: receive a first baseline acoustic signal data set from the sensor, wherein the first baseline acoustic signal data set comprises an indication of the acoustic signal received over a first depth interval while the wellbore is shut in; receive a first flowing acoustic signal data set, wherein the first flowing acoustic signal data set comprises an indication of the acoustic signal received over the first depth interval while a first pressure differential is induced within the wellbore: determine a baseline fluid flow log using the first baseline acoustic signal data set; determine a flowing fluid flow log using the first flowing acoustic signal data set; subtract the baseline fluid flow log from the flowing fluid flow' log to provide a first sample data set; determine a first plurality of frequency domain features of the first sample data set: identify a first fluid flow location within the welibore using the first plurality of frequency domain features; determine a change in a flow rate or flow mechanism at the first fluid flow location using the first sample data set; and generate an output indicative of the first fluid flow location and a change in the flow rate or flow mechanism at the first fluid flow location.

[00135] C: A method of comparing acoustic signals obtained between different acoustic sensor operations in a wellbore, the method comprising: obtaining a first baseline sample data set over a first depth interval within a wellbore, wherein the first baseline data set is a sample of an acoustic signal originating within the wellbore; determining at least one frequency domain feature of the first baseline sample data set; inducing a first pressure differential within the wellbore; obtaining a first acoustic data set over the first depth interval within the wellbore while inducing the first pressure differential; determining at least one frequency domain feature of the first acoustic data set; subtracting the at least one frequency domain feature of the first baseline sample data set from the at least one frequency domain feature of the first acoustic data set to obtain a first sample data set over the first depth interval; obtaining a second baseline sample data set over a second depth interval within the wellbore, wherein the second baseline sample data set is a sample of an acoustic signal originating within the wellbore, wherein the second depth interval overlaps with the first depth interval; determining at least one frequency domain feature of the second baseline sample data set; inducing a second pressure differential within the wellbore; obtaining a second acoustic data set over the second depth interval within the wellbore while inducing the second pressure differential; determining at least one frequency domain feature of the second acoustic data set; subtracting the at least one frequency domain feature of the second baseline sample data set from the at least one frequency domain feature of the second acoustic data set to obtain a second sample data set over the second depth interval; and comparing the second sample data set to the first sample data set over the second depth interval . [00136] D: A system for of comparing acoustic signals obtained between different acoustic sensor operations in a wellbore, the system comprising: a receiver unit comprising a processor and a memory, wherein the receiver unit is configured to receive an acoustic signal from a sensor disposed in a wellbore, wherein a processing application is stored in the memory, and wherein the processing application, when executed on the processor, configures the processor to: receive a first baseline sample data set over a first depth interval within the wellbore, wherein the first baseline data set is a sample of an acoustic signal originating within the wellbore; determine at least one frequency domain feature of the first baseline sample data set; receive a first acoustic data set over the first depth interval within the wellbore, wherein the first acoustic data sat is an acoustic signal obtained while a first pressure differential is induced within the wellbore; determine at least one frequency domain feature of the first acoustic data set; subtract the at least one frequency domain feature of the first baseline sample data set from the at least one frequency domain feature of the first acoustic data set to obtain a first sample data set over the first depth interval; receive a second baseline sample data set over a second depth interval within the wellbore, wherein the second baseline sample data set is a sample of an acoustic signal originating within the wellbore, wherein the second depth interval overlaps with the first depth interval; determine at least one frequency domain feature of the second baseline sample data set; receive a second acoustic data set over the second depth interval within the wellbore, wherein the second acoustic data sat is an acoustic signal obtained while a second pressure differential is induced within the wellbore; determine at least one frequency domain feature of the second acoustic data set; subtract the at least one frequency domain feature of the second baseline sample data set from the at least one frequency domain feature of the second acoustic data set to obtain a second sample data set over the second depth interval; and compare the second sample data set to the first sample data set over the second depth interval; and generate an output indicative of the comparison between the second sample data set and the first sample data set..

[00137 E: A method of abandoning a wellbore, the method comprising: obtaining a first sample data set over a first depth interval within a wellbore, wherein the first sample data set comprises a first acoustic data set having a first baseline acoustic sample data set subtracted therefrom, wherein the first acoustic data set is obtained over the first depth interval while a first pressure differential is induced in the wellbore, and wherein the first baseline acoustic sample data set is obtained over the first depth interval while the wellbore is shut in; identifying a fluid flow location within the first depth interval using the first sample data set; obtaining a second sample data set over a second depth interval within a wellbore, wherein the second sample data set is obtained after a barrier is set at or above the fluid flow location, wherein the second sample data set comprises a second acoustic data set having a second baseline acoustic sample data set subtracted therefrom, wherein the second acoustic data set is obtained over the second depth interval while a second pressure differential is induced in the wellbore, wherein the second baseline acoustic sample data set is obtained over the second depth interval while the wellbore is shut in, and wherein the second depth interval is overlaps the first depth interval; comparing the first sample data set to the second sample data set; and determining whether or not fluid flow at the fluid flow location is substantially blocked by the barrier.

[00138] F: A system for abandoning a wellbore, the system comprising: a receiver unit comprising a processor and a memory, wherein the receiver unit is configured to receive an acoustic signal from a sensor disposed in a wellbore, wherein a processing application is stored in the memory, and wherein the processing application, when executed on the processor, configures the processor to: receive a first baseline acoustic sample data set and a first acoustic data set from the sensor, wherein the first acoustic data set is an acoustic signal obtained over a first depth interval while a first pressure differential is induced in the wellbore, and wherein the first baseline acoustic sample data set is an acoustic signal obtained over the first depth interval while the wellbore is shut in, determine a first sample data set over a first depth interval within the wellbore, wherein the first sample data set comprises the first acoustic data set having the first baseline acoustic sample data set subtracted therefrom; identify a fluid flow location within the first depth interval using the first sample data set; receive a second baseline acoustic sample data set and a second acoustic data set from the sensor, wherein the second acoustic data set is an acoustic signal obtained over a second depth interval while a second pressure differential is induced in the wellbore and after a barrier is set at or above the fluid flow location, and wherein the second baseline acoustic sample data set is an acoustic signal obtained over the second depth interval while the wellbore is shut in and after the barrier is set at or above the fluid flow location; determine a second sample data set over the second depth interval within the wellbore, wherein the second sample data set comprises the second acoustic data set having the second baseline acoustic sample data set subtracted therefrom; compare the first sample data set to the second sample data set; determine whether or not fluid flow at the fluid flow location is substantially blocked by the barrier; and generate an output indicative the determination of whether or not the fluid flow at the fluid flow location Is substantially blocked by the barrier. Each of embodiments A, B, C, D, E, and F may have one or more of the following additional elements: Element 1 : further comprising: setting a second barrier at or above the second fluid flow location; and substantially blocking fluid flow from the first fluid flow location and the second fluid flow location using the first barrier and the second barrier. Element 2: wherein at least one of the first sample data set or the second sample data set is representative of the acoustic signal across a frequency spectrum. Element 3: wherein obtaining the first sample data set comprises: obtaining a baseline acoustic signal data set while the wellbore is shut in; obtaining a baseline fluid flow log using the baseline acoustic signal data set; inducing a pressure differentia! within the wellbore; obtaining a flowing acoustic signal data set while inducing the pressure differential; obtaining a flowing fluid flow log using the flowing acoustic signal data set; and subtracting the baseline fluid flow log from the flowing fluid flow log. Element 4: wherein the wellbore comprises one or more tubular strings and one or more annuli disposed between at least one of: i) two adjacent tubular strings of the one or more tubular strings, ii) a tubular string of the one or more tubular strings and a formation, or iii) both i and ii, and wherein inducing the pressure differential comprises releasing a fluid from an annulus of the one or more annuli. El ement 5: wherein the baseline acoustic signal data set is a time averaged acoustic data set. Element 6: wherein the barrier (e.g., the first barrier, the second barrier, or both the first barrier and the second harrier) comprise a bridge plug, a packer, a cement plug, or a combination thereof. Element 7: wherein the first fluid flow location, the second fluid flow location, or both the first fluid flow location and the second fluid flow location comprise: a location of flow from a formation into the wellbore, a location of flow between the formation and an annulus between a tubular string and the wellbore wall, or a location of flow between annuli formed between a plurality of tubular strings in the wellbore. Element 8: wherein identifying the first fluid flow location comprises comparing the first plurality of frequency domain features with a fluid flow event signature, and/or wherein identifying the second fluid flow location comprises comparing the second plurality of frequency domain features with a fluid flow event signature. Element 9: further comprising: correlating the first fluid flow location with one or more structural features within the wellbore; and determining a source of the fluid flow at the first fluid flow location based on the correlating of the first fluid flow location with the one or more structural features. Element 10: wherein the wellbore comprises one or more tubular strings and one or more annuli disposed between at least one of: i) two adjacent tubular strings of the one or more tubular strings, ii) a tubular string of the one or more tubular strings and a formation, or iii) both i and ii, and wherein identifying the first fluid flow location or the second fluid flow location comprises determining an annulus of the one or more annuli and a depth at which the first fluid flow location or the second fluid flow location is present. Element 1 1 : wherein the processing application, when executed on the processor, further configures the processor to: receive a second baseline acoustic signal data set from within the wellbore, wherein the second baseline acoustic signal data set comprises an Indication of the acoustic signal received over a second depth interval of the wellbore while the wellbore is shut in, subsequent the setting of a barrier at or above the identified first fluid flow location, wherein the second depth interval overlaps the first depth interval; receive a second flowing acoustic signal data set, wherein the second flowing acoustic signal data set comprises an indication of the acoustic signal received over the second depth interval while a second pressure differential is induced within the wellbore, subsequent the setting of the barrier at or above the identified first fluid flow location; determine a second baseline fluid flow log using the second baseline acoustic signal data set; determine a second flowing fluid flow log using the second flowing acoustic signal data set; subtract the second baseline fluid flow log from the second flowing fluid flow log to provide a second sample data set; determine a second plurality of frequency domain features of the second sample data set; determine that a fluid flow rate or a fluid flow mechanism at the first fluid flow location within the wellbore has been reduced or eliminated and/or identify a second fluid flow location using the second plurality of frequency domain features; and generate an output indicative of the identified reduction or elimination of the fluid flow at the first fluid flow location and/or indicative of the second fluid flow location. Element 12: The system of claim 12 further comprising: validating the barrier based on the identified reduction or elimination of fluid flow rate or the fluid flow mechanism at the first fluid flow location. Element 13: further comprising: the sensor, wherein the sensor comprises a fibre optic cable disposed within the wellbore; and an optical generator coupled to the fibre optic cable, wherein the optical generator is configured to generate a light beam and pass the light beam into the fibre optic cable. Element 14: wherein the wellbore comprises one or more tubular strings and one or more annuli disposed between at least one of: i) two adjacent tubular strings of the one or more tubular strings, ii) a tubular string of the one or more tubular strings and a formation, or iii) both i and ii, and wherein where the first fluid flow location, the second fluid flow location, or both comprise: a location of flow from a formation into the wellbore, a location of flow between the formation and an annulus between a tubular string and the wellbore wall, or a location of flow between annuli formed between a plurality of tubular strings in the wellbore. Element

15: wherein inducing the first pressure differential and/or inducing the second pressure differential comprises: opening a flow valve within an annulus of the one or more annuli; and inducing a fluid flow based on opening of the flow valve. Element 16: wherein the first pressure differential and/or the second pressure differential is indicative of a difference in pressure between an annulus of the one or more annuli and an adjacent flow path in the wellbore. Element 17: wherein the processing application, when executed on the processor, further configures the processor to: integrate or time average an acoustic intensity within specified frequency bands for fluid flow in the wellbore, and determine a relative fluid flowrate for fluid flow based on the integrated acoustic intensity. Element 18: wherein the output comprises a fluid flow log. Element 19: further comprising: determining a fluid flow reduction at a fluid flow location based on comparing the second sample data set to the first sample data set. Element 20: wherein the first baseline sample data set and the first acoustic data set are obtained with an acoustic sensor disposed in the wellbore within the first depth interval, wherein the second baseline sample data set and the second acoustic data set are obtained with the acoustic sensor disposed in the wellbore within the second depth interval, and wherein the method further comprises: removing the acoustic sensor from the wellbore between obtaining the first baseline sample data set and obtaining the second baseline sample data set. Element 21 : wherein identifying the fluid flow location within the first depth interval using the first sample data set comprises determining a plurality of frequency domain features of the first sample data set. Element 22: wherein the plurality of frequency domain features of the first sample data set comprise at least two frequency domain features selected from the group consisting of a spectral centroid, a spectral spread, a spectral roll-off, a spectral skewness, an RMS band energy, a total RMS energy, a spectral flatness, a spectral slope, a spectral kurtosis, a spectral flux, spectral entropy, a spectral autocorrelation function, and combinations thereof.

[00139] While various embodiments in accordance with the principles disclosed herein have been shown and described above, modifications thereof may be made by one skilled in the art without departing from the spirit and the teachings of the disclosure. The embodiments described herein are representative only and are not intended to be limiting.

Many variations, combinations, and modifications are possible and are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiments) are also within the scope of the disclosure.

Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present invention(s). Furthermore, any advantages and features described above may relate to specific embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages or having any or all of the above features,

[00140] Additionally, the section headings used herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or to otherwise provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings might refer to a“Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the“Background” is not to he construed as an admission that certain technology is prior art to any invention(s) in this disclosure. Neither is the“Summary” to be considered as a limiting characterization of the invention(s) set forth in issued claims, Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the inventions), and their equivalents, that are protected thereby, in all instances, the scope of the claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.

[00141] Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Use of the term“optionally,”“may,”“might,”“possibly,” and the like with respect to any element of an embodiment means that the element is not required, or alternatively, the element is required, both alternatives being within the scope of the embodiment(s). Also, references to examples are merely provided for illustrative purposes, and are not intended to he exclusive.

[00142] While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.

[00143] Also, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.

Claims

CLAIMS We claim:

1. A method of abandoning a wellbore, the method comprising:

obtaining a first sample data set within a wellbore, wherein the first sample data set is a sample of an acoustic signal originating within the wellbore; determining a first plurality of frequency domain features of the first sample data set;

Identifying a first fluid flow location within the wellbore using the first plurality of frequency domain features;

setting a first barrier at or above the first fluid flow location;

obtaining a second sample data set within the wellbore above the first barrier, wherein the second sample data set is a sample of an acoustic signal originating within the wellbore above the first barrier;

determining a second plurality of frequency domain features of the second sample data set; and

identifying that a fluid flow rate or fluid flow mechanism at the first fluid flow

location has been reduced or eliminated and/or identifying a second fluid flow- location within the wellbore using the second plurality of frequency domain features,

2. The method of claim 1 further comprising:

seting a second barrier at or above the second fluid flow location; and

substantially blocking fluid flow from the first fluid flow location and the second fluid flow location using the first barrier and the second barrier.

3. The method of claim I, wherein at least one of the first sample data set or the second sample data set is representative of the acoustic signal across a frequency spectrum.

4. The method of any of claim 1 to claim 3, wherein obtaining the first sample data set comprises:

obtaining a baseline acoustic signal data set while the wellbore is shut in;

obtaining a baseline fluid flow log using the baseline acoustic signal data set;

inducing a pressure differential within the wellbore:

obtaining a flowing acoustic signal data set while inducing the pressure differential; obtaining a flowing fluid flow log using the flowing acoustic signal data set; and subtracting the baseline fluid flow log from the flowing fluid flow log.

5. The method of claim 4, wherein the wellbore comprises one or more tu bular strings and one or more annuli disposed between at least one of: i) two adjacent tubular strings of the one or more tubular strings, ii) a tubular string of the one or more tubular strings and a formation, or iii) both i and ii, and wherein inducing the pressure differential comprises releasing a fluid from an annulus of the one or more annuli.

6. The method of claim 4 or claim 5, wherein the baseline acoustic signal data set is a time averaged acoustic data set.

7. The method of any of claim 1 to claim 6, wherein the barrier comprises a bridge plug, a packer, a cement plug, or a combination thereof.

8. The method of claim any of claim 1 to claim 7, wherein the first fluid flow location, the second fluid flow location, or both the first fluid flow location and the second fluid flow location comprise: a location of flow from a formation into the wellbore, a location of flow between the formation and an annulus between a tubular string and the wellbore wall, or a location of flow between annuli formed between a plurality of tubular strings in the wellbore.

9. The method of claim any of claim 1 to claim 8, wherein identifying the first fluid flow location comprises comparing the first plurality of frequency domain features with a fluid flow event signature, and/or wherein identifying the second fluid flow location comprises comparing the second plurality of frequency domain features with a fluid flow event signature.

10. The method of any of claim 1 to claim 9 further comprising:

correlating the first fluid flow location with one or more structural features within the wellbore; and

determining a source of the fluid flow at the first fluid flow location based on the correlating of the first fluid flow location with the one or more structural features.

1 1. The method of claim any of claim 1 to claim 10, wherein the wellbore comprises one or more tubular strings and one or more annuli disposed between at least one of: i) two adjacent tubular strings of the one or more tubular strings, ii) a tubular string of the one or more tubular strings and a formation, or iii) both i and ii, and wherein identifying the first fluid flow location or the second fluid flow location comprises determining an annulus of the one or more annuli and a depth at which the first fluid flow location or the second fluid flow location is present.

12. A system for abandoning a wellbore, the system comprising:

a receiver unit comprising a processor and a memory, wherein the receiver unit is configured to receive an acoustic signal from a sensor disposed in a wellbore, wherein a processing application is stored in the memory, and wherein the processing application, when executed on the processor, configures the processor to:

receive a first baseline acoustic signal data set from the sensor, wherein the first baseline acoustic signal data set comprises an indication of the acoustic signal received over a first depth interval while the welibore is shut in; receive a first flowing acoustic signal data set, wherein the first flowing acoustic signal data set comprises an indication of the acoustic signal received over the first depth interval while a first pressure differential is induced within the wellbore;

determine a baseline fluid flow log using the first baseline acoustic signal data set; determine a flowing fluid flow log using the first flowing acoustic signal data set; subtract the baseline fluid flow log from the flowing fluid flow log to provide a first sample data set;

determine a first plurality of frequency domain features of the first sample data set; identify a first fluid flow location within the welibore using the first plurality of frequency domain features;

determine a change in a flow rate or flow mechanism at the first fluid flow location using the first sample data set; and

generate an output indicative of the first fluid flow location and a change in the flow rate or flow mechanism at the first fluid flow location.

13. The system of claim 12, wherein the processing application, when executed on the processor, further configures the processor to:

receive a second baseline acoustic signal data set from within the wellbore, wherein the second baseline acoustic signal data set comprises art indication of the acoustic signal received over a second depth interval of the wellbore while the wellbore is shut in, subsequent the setting of a barrier at or above the identified first fluid flow location, wherein the second depth interval overlaps the first depth interval;

receive a second flowing acoustic signal data set, wherein the second flowing acoustic signal data set comprises an indication of the acoustic signal received over the second depth interval while a second pressure differential is induced within the wellbore, subsequent the setting of the harrier at or above the identified first fluid flow location;

determining a second baseline fluid flow log using the second baseline acoustic signal data set;

determining a second flowing fluid flow log using the second flowing acoustic signal data set;

subtract the second baseline fluid flow log from the second flowing fluid flow log to provide a second sample data set;

determine a second plurality of frequency domain features of the second sample data set;

determine that a fluid flow rate or a fluid flow mechanism at the first fluid flow location within the wellbore has been reduced or eliminated and/or identify a second fluid flow location using the second plurality of frequency domain features; and

generate an output indicative of the identified reduction or elimination of the fluid flow at the first fluid flow location and/or indicative of the second fluid flow location.

14. The system of claim 12 further comprising:

validating the barrier based on the identified reduction or elimination of fluid flow rate or the fluid flow mechanism at the first fluid flow location.

15. The system of any of claim 12 to claim 14 further comprising:

the sensor, wherein the sensor comprises a fibre optic cable disposed within the wellbore; and

an optical generator coupled to the fibre optic cable, wherein the optical generator is configured to generate a light beam and pass the light beam into the fibre optic cable

16. The system of any of claim 12 to 15, wherein the wellbore comprises one or more tubular strings and one or more annuli disposed between at least one of: i) two adjacent tubular strings of the one or more tubular strings, ii) a tubular siring of the one or more tubular strings and a formation, or iii) both i and ii, and wherein where the first fluid flow location, the second fluid flow location, or both comprise: a location of flow from a formation into the wellbore, a location of flow between the formation and an annulus between a tubular string and the wellbore wall, or a location of flow between annuli formed between a plurality of tubular strings in the wellbore

17. The method of claim 16, wherein inducing the first pressure differential and/or inducing the second pressure differential comprises:

opening a flow valve within an annulus of the one or more annuli; and

inducing a fluid flow based on opening of the flow valve.

18. The system of claim 16 or claim 17, wherein the first pressure differential and/or the second pressure differential is indicative of a difference in pressure between an annulus of the one or more annuli and an adjacent flow path in the wellbore.

19. The system of any of claim 12 to claim 18, wherein the processing application, when executed on the processor further configures the processor to:

integrate or time average an acoustic intensity within specified frequency bands for fluid flow in the wellbore, and

determine a relative fluid flowrate for fluid flow based on the integrated acoustic intensity

20. The system of any of claim 12 to claim 19, wherein the output comprises a fluid flow log.

21. A method of comparing acoustic signals obtained between different acoustic sensor operations in a wellbore, the method comprising: obtaining a first baseline sample data set over a first depth interval within a wellbore, wherein the first baseline data set is a sample of an acoustic signal originating within the welibore;

determining at least one frequency domain feature of the first baseline sample data set;

inducing a first pressure differential within the wellbore;

obtaining a first acoustic data set over the first depth interval within the welibore while inducing the first pressure differential;

determining at least one frequency domain feature of the first acoustic data set; subtracting the at least one frequency domain feature of the first baseline sample data set from the at least one frequency domain feature of the first acoustic data set to obtain a first sample data set over the first depth interval;

obtaining a second baseline sample data set over a second depth interval within the wellbore, wherein the second baseline sample data set is a sample of an acoustic signal originating within the welibore, wherein the second depth interval overlaps with the first depth interval;

determining at least one frequency domain feature of the second baseline sample data set;

inducing a second pressure differential within the wellbore;

obtaining a second acoustic data set over the second depth interval within the

wellbore while inducing the second pressure differential;

determining at least one frequency domain feature of the second acoustic data set; subtracting the at least one frequency domain feature of the second baseline sample data set from the at least one frequency domain feature of the second acoustic data set to obtain a second sample data set over the second depth interval; and comparing the second sample data set to the first sample data set over the second depth interval

The method of claim 21 further comprising:

determining a fluid flow reduction at a fluid flow location based on comparing the second sample data set to the first sample data set

23. The method of claim 21 or claim 22, wherein the first baseline sample data set and the first acoustic data set are obtained with an acoustic sensor disposed in the wellbore within the first depth interval, wherein the second baseline sample data set and the second acoustic data set are obtained with the acoustic sensor disposed in the wellbore within the second depth interval, and wherein the method further comprises:

removing the acoustic sensor from the wellbore between obtaining the first baseline sample data set and obtaining the second baseline sample data set

24. A system for of comparing acoustic signals obtained between different acoustic sensor operations in a wellbore, the system comprising:

receive a first baseline sample data set over a first depth interval within the

wellbore, wherein the first baseline data set is a sample of an acoustic signal originating within the wellbore;

determine at least one frequency domain feature of the first baseline sample data set; receive a first acoustic data set over the first depth interval within the wellbore,

wherein the first acoustic data sat is an acoustic signal obtained while a first pressure differential is induced within the wellbore;

determine at least one frequency domain feature of the first acoustic data set;

subtract the at least one frequency domain feature of the first baseline sample data set from the at least one frequency domain feature of the first acoustic data set to obtain a first sample data set over the first depth interval;

receive a second baseline sample data set over a second depth interval within the wellbore, wherein the second baseline sample data set is a sample of an acoustic signal originating within the wellbore, wherein the second depth interval overlaps with the first depth interval;

determine at least one frequency domain feature of the second baseline sample data set; receive a second acoustic data set over the second depth interval within the wellbore, wherein the second acoustic data sat is an acoustic signal obtained while a second pressure differential is induced within the wellbore;

determine at least one frequency domain feature of the second acoustic data set; subtract the at least one frequency domain feature of the second baseline sample data set from the at least one frequency domain feature of the second acoustic data set to obtain a second sample data set over the second depth interval;

compare the second sample data set to the first sample data set over the second depth interval; and

generate an output indicative of the comparison between the second sample data set and the first sample data set.

25. A method of abandoning a wellbore, the method comprising:

obtaining a first sample data set over a first depth interval within a wellbore, wherein the first sample data set comprises a first acoustic data set having a first baseline acoustic sample data set subtracted therefrom, wherein the first acoustic data set is obtained over the first depth interval while a first pressure differential is induced in the wellbore, and wherein the first baseline acoustic sample data set is obtained over the first depth interval while the wellbore is shut in;

identifying a fluid flow location within the first depth interval using the first sample data set;

obtaining a second sample data set over a second depth interval within a wellbore, wherein the second sample data set is obtained after a barrier is set at or above the fluid flow location, wherein the second sample data set comprises a second acoustic data set having a second baseline acoustic sample data set subtracted therefrom, wherein the second acoustic data set is obtained over the second depth interval while a second pressure differential is induced in the wellbore, wherein the second baseline acoustic sample data set is obtained over the second depth interval while the wellbore is shut in, and wherein the second depth interval overlaps the first depth interval;

comparing the first sample data set to the second sample data set; and

determining whether or not fluid flow at the fluid flow location is substantially

blocked by the barrier.

26. The method of claim 25, wherein identifying the fluid flow location within the first depth interval using the first sample data set comprises determining a plurality of frequency domain features of the first sample data set,

27. The method of claim 26, wherein the plurality of frequency domain features of the first sample data set comprise at least two frequency domain features selected from the group consisting of a spectral centroid, a spectral spread, a spectral roll-off, a spectral skewness, an RMS band energy, a total RMS energy, a spectral flatness, a spectral slope, a spectral kurtosis, a spectral flux, spectral entropy, a spectral autocorrelation function, and

combinations thereof.

28. A system for abandoning a wellbore, the system comprising:

receive a first baseline acoustic sample data set and a first acoustic data set from the sensor, wherein the first acoustic data set is an acoustic signal obtained over a first depth interval while a first pressure differential is induced in the wellbore, and wherein the first baseline acoustic sample data set is an acoustic signal obtained over the first depth interval while the wellbore is shut in, determine a first sample data set over a first depth interval within the wellbore,

wherein the first sample data set comprises the first acoustic data set having the first baseline acoustic sample data set subtracted therefrom; identify a fluid flow location within the first depth interval using the first sample data set;

receive a second baseline acoustic sample data set and a second acoustic data set from the sensor, wherein the second acoustic data set is an acoustic signal obtained over a second depth interval while a second pressure differential is induced in the wellbore and after a barrier is set at or above the fluid flow location, and wherein the second baseline acoustic sample data set is an acoustic signal obtained over the second depth interval while the wellbore is shut is and after the barrier is set at or above the fluid flow location;

determine a second sample data set over the second depth Interval within the

wellbore, wherein the second sample data set comprises the second acoustic data set having the second baseline acoustic sample data set subtracted therefrom;

compare the first sample data set to the second sample data set; and

determine whether or not fluid flow at the fluid flow location is substantially blocked by the barrier; and

generate an output indicative the determination of whether or not the fluid flow at the fluid flow location is substantially blocked by the harrier.