Overview of Cement Bond Evaluation Methods in Carbon Capture, Utilisation, and Storage (CCUS) Projects—A Review

Abstract

Cement bond evaluation helps check wellbore integrity and zonal isolation in carbon capture, utilisation, and storage (CCUS) projects. This overview describes various cement bond evaluation methods, focusing on acoustic logging and ultrasonic imaging tools supplemented by emerging data-driven interpretation techniques. Their advantages, limitations, and recent advancements are described with illustrative example on ultrasonic-image-based machine learning classifier that detect microannulus. Key research gaps remain in field-scale validation of long-term cement behaviour and in establishing comprehensive 3-D bond-strength benchmarks. To address these gaps, this review recommends (i) creating an open, standardised ML dataset for CCUS well logs, (ii) adopting best-practice pressure-monitoring protocols during and after injection, and (iii) integrating ML analytics with advanced modelling while exploring alternative binder systems. The next step is to test these ML models on real CO₂-storage well data, paving the way toward more reliable cement-bond integrity assessments in future CCUS projects.

1. Introduction

1.1. Brief Overview of Carbon Capture, Utilisation, and Storage (CCUS)

Carbon dioxide (CO₂) is generally regarded as the primary cause of global temperature increase due to its heat-trapping properties as a greenhouse gas [1,2,3]. Carbon capture, utilisation, and storage (CCUS) projects have gained considerable attention as a vital approach for mitigating greenhouse gas emissions in response to escalating concerns about climate change. CCUS involves the capture of CO₂ emissions from various industrial processes and power plants, its transportation, its utilisation in products and processes (e.g., enhanced oil recovery, construction materials, chemicals), and their injection into geological formations for long-term storage [4,5,6,7,8]. Therefore, CCUS technology is crucial for achieving carbon neutrality goals and reducing greenhouse gas emissions [9,10].

CCUS projects are rapidly expanding worldwide, with 41 in operation, 26 under construction, and 325 in the advanced and early development phases as of the end of 2023 [11]. Current trends include integrating circular CO₂ utilisation with partial geological sequestration and scaling multi-user hubs, particularly in Asia-Pacific [10,12]. To ensure public confidence, a concise priority is the integrity of injection and storage wells—principally via robust cementing and completion—to minimise leakage risk over project life.

In CCUS projects, brownfield and greenfield sites present distinct risks to the long-term containment of injected CO₂. Brownfield projects must evaluate legacy well integrity (materials not designed for CO₂ injection, potential microannulus, channelling, casing corrosion), whereas greenfield projects must optimise completion design for long-term containment rather than injectivity alone [13,14,15]. Pressure-management wells also warrant targeted surveillance because they can evolve late-life leakage pathways distinct from injectors [16,17].

Maintaining well integrity—preventing uncontrolled fluid release—requires durable barriers, sound cementing/completions, and materials compatible with CO₂-rich environments. A central element is the evaluation of cement bond quality to verify zonal isolation and overall wellbore integrity [18,19]. Evaluating the strength of the cement bond to the formation is vital for maintaining structural integrity and preventing CO₂ migration to the surface, thus ensuring the success of CCUS projects [20].

It is important to note that the evaluation techniques commonly used in the petroleum industry are not always transferable to CCUS projects because project objectives and operating envelopes differ (production maximisation vs. safe and permanent CO₂ storage) [21,22]. CO₂ properties and interactions with well materials mandate CCUS-specific approaches for cement bond evaluation and broader well integrity assessment.

1.2. Importance of Cement Bond Evaluation in CCUS Projects

Carbon capture and storage wells require a specialised design and construction to ensure long-term integrity and prevent CO₂ leakage. The critical components of CCUS wells include the casing, tubing, cement, and packers [21,23]. The cement placed around the casing provides zonal isolation and prevents CO₂ migration. However, potential leakage pathways exist, such as through poor cementation, microannulus, and flow through the cap rock [20,24]. Geochemical reactions between cement and CO₂-rich brine can cause deterioration, leading to changes in porosity, permeability, and mechanical properties. Mechanisms that could cause cement failure include the dissolution of primary minerals, the precipitation of carbonates, and the re-dissolution of carbonates [25]. Poor cement jobs, geochemical reactions, pressure, and temperature contribute to potential leakage [20]. To mitigate risks, CCUS well designs should consider dual containment, material selection for corrosive environments, and periodic inspections. Maintaining proper cement integrity is crucial for preventing undetected CO₂ migration from storage zones [26].

Maintaining a sealed outer annulus, the space between the outermost casing and the formation, is vital for ensuring the long-term integrity of the storage complex and preventing the potential leakage of injected CO₂ in CCUS projects [22,23]. This requirement is particularly critical for well components that penetrate the primary caprock and extend into the storage reservoir. These components represent potential pathways for CO₂ migration from the intended storage zone. A competent cement seal in the outer annulus prevents vertical fluid migration, unintended fluid communication between the geological layers, and CO₂ leakage through the annular space. Failure to robustly seal the outer annulus, particularly in areas where the wellbore passes through permeable formations or faults, could compromise the effectiveness of the storage project and lead to environmental and safety hazards [21,24].

Figure 1 illustrates the possible leakage pathways of CO₂ gas through several mechanisms, such as small gaps near the casing–cement or cement–formation boundary; channelling along the cement; and the quality of the cement itself, which can deteriorate over time.

The length of the cement zone plays a vital role in ensuring the long-term integrity and containment of injected CO₂ in CCUS sites. There is no universally prescribed minimum length for the cement zone. Instead, the required length is determined by a combination of site-specific factors and regulatory requirements. Geologic characteristics such as the depth of the injection zone, the presence of overlying cap rock, and the potential for fluid migration through faults or fractures all influence the cement zone length required to isolate the storage reservoir effectively. Moreover, the anticipated pressure and temperature conditions during injection and the chemical compatibility between the cement and formation fluids must be considered when designing an adequate cement barrier [28,29]. Regulatory frameworks, which vary by jurisdiction, typically stipulate minimum cement zone requirements based on these site-specific factors and industry best practices. For instance, NORSOK D-010, used in Norway, emphasises the importance of proper cement coverage to ensure well integrity without specifying a fixed minimum length [30]. ISO 16530 [31] and ISO 27914 [32] also provide recommendations for cement zone design and verification without prescribing specific minimum length. Similarly, the regulatory framework for CCUS in Australia, outlined in documents such as the “Regulatory Guiding Principles for Carbon Dioxide Capture and Geological Storage” and the “Offshore Petroleum and Greenhouse Gas Storage Act 2006”, requires detailed assessments of the proposed reservoir and risk mitigation strategies rather than adherence to a fixed minimum length across all projects [33,34].

CCUS projects must ensure well integrity to guarantee the safe and secure long-term storage of CO₂. After construction, combinations of pressure tests, cement bond logs, acoustic logging techniques, fibre optic distributed acoustic sensing (DAS), and temperature sensing (DTS) are employed to assess the well’s mechanical strength and sealing capacity and to identify potential leakage pathways [35,36]. Throughout the operational life of the well, continuous monitoring methods, such as annulus pressure monitoring, downhole sensors, and gas detectors, are utilised to detect any signs of integrity issues. When repurposing old wells for CCUS, a rigorous evaluation is conducted by reviewing the original well design, construction materials, and completion methods to determine their compatibility with CO₂ injection requirements [14]. This may involve using corrosion-resistant alloys and CO₂-resistant cement and conducting comprehensive logging and testing to identify weaknesses in the existing well structure. The geomechanical modelling and analysis of in situ stress conditions are also crucial to determine the maximum safe injection pressures and avoid fracturing the formation [37]. Based on risk assessment, old wells may be categorised as suitable for reuse, requiring remedial work, or unsuitable for CCUS. The decision to reuse an old well depends on a thorough evaluation of its integrity, the costs of any necessary remedial work compared with those of drilling a new well, and the overall risk profile of the CCUS project [38].

In contrast to new wells, legacy wells initially designed for oil and gas production often lack comprehensive records of their construction and cement integrity. This necessitates new logging runs to assess their suitability for CCUS. Legacy wells may not have been designed to withstand the corrosive nature of CO₂-rich fluids or ensure a long-term sealing capacity. Evaluating these wells involves reviewing the available records, followed by targeted logging and testing to identify potential leakage pathways or weaknesses, which can be complicated by multiple casing layers [39]. However, new wells explicitly drilled for CCUS incorporate CO₂-resistant materials and specialised components such as CO₂ injectors and monitoring systems [40]. They undergo initial post-construction testing and ongoing monitoring using methods such as distributed acoustic sensing (DAS), temperature sensing (DTS), and periodic wireline logging with advanced tools such as pulsed-neutron logs [41]. The inclusion of CO₂ injectors and water producers in new wells adds complexity, as the long-term effects of CO₂ exposure on cement and well materials must be carefully considered, often through geochemical modelling and simulation. The mechanisms of cement degradation in CO₂ injection wells involve three stages: (1) dissolution of primary minerals, (2) precipitation of carbonates, and (3) re-dissolution of carbonates. The first two stages lead to decreased rock porosity and mechanical property degradation, while the transition to stage three results in increased porosity and mechanical stiffness [25]. Challenges in evaluating legacy wells include the lack of a unified data structure, the need to estimate cement bond and barrier permeabilities, and the potential for undetected leakage pathways owing to outdated well designs or materials. To address these challenges, a risk-based approach is often used, involving the development of methodologies to estimate potential CO₂ leakage rates and support decision-making in site screening and selection. One of the approaches is by utilising sustained casing pressure (SCP) data which can be used to back-calculate the effective permeability of cement. This permeability data can then be further used to estimate CO₂ leakage rates for that specific well [38].

1.3. Objectives of This Review

This review synthesises contemporary methods for cement bond evaluation in CCUS—emphasising acoustic/ultrasonic logs (CBL/VDL, ultrasonic imaging/AABT) and complementary surveillance (DAS/DTS, temperature/noise)—to (i) delineate CO₂-related failure mechanisms and leakage pathways, (ii) assess tool capabilities/limitations and interpretation practices, (iii) compile recent field/lab evidence, and (iv) propose concise recommendations and research priorities for reliable, fit-for-purpose well integrity evaluation.

2. Cement Bond Evaluation Techniques

2.1. Description of Cement Bond Logging Tools

Cement bond evaluation is crucial for ensuring wellbore integrity, particularly in the context of CO₂ injection for carbon storage. Various logging tools have been used for cement bond evaluation, each with distinct technologies and applications. The primary logging tools used for cement bond evaluation are acoustic and ultrasonic tools and other technologies, such as temperature logging, noise logging, oxygen activation logs, neutron–neutron logging, and fibre optic measurements [42,43]. Acoustic tools, such as pulse-echo and flexural measurements, are the leading techniques for cement job evaluation. Acoustic logging is the primary method used to assess cement bond quality, allowing an analysis of casing and formation waves [42,44]. In recent years, ultrasonic tools have gained traction in the industry and have been developed to provide a high-quality assessment of cement bond quality [45].

However, these tools differ in their operating principles and applications. The cement bond log (CBL) is one of the most commonly used tools for evaluating cement bond quality. Acoustic waves measure the amplitude and travel time of sound waves propagating through the cement, casing, and formation. The CBL provides a quantitative indication of cement bond quality by comparing the amplitude and travel time of the reflected waves from different interfaces. This tool assesses casing annular conditions and provides measurements related to the cement quantity, compressive strength, and bonding to the pipe and formation. CBL tools are essential for determining the isolation of cement and ensuring its zonal integrity [46,47]. Despite its wide use, the CBL has its limitations. One limitation is that the CBL measures the average travel time and amplitude around the borehole. If there is any issue with the quality of the cement bond, then one cannot determine on which side of the borehole the issue lies because the CBL does not provide azimuth information. In CCUS wells, this azimuthal averaging can mask localised channels or microannulus responsible for CO₂ migration. With advances in technology, new tools have been developed.

One example is the segmented bond tool (SBT) [48]. The SBT measures attenuation similar to the CBL attenuation, including azimuthal information (60-degree coverage per segment) to determine the location of any potential cement bond issues around the borehole. Figure 2 provides a comparison of CBL and SBT data. In CBL data, the attenuation is an average reading from around the wellbore. Therefore, there is no way to tell which part of the wellbore requires more attention.

Meanwhile, with the additional azimuthal information from the SBT, the area with possible poor cement bonding can be located more accurately. Further improvements in the acoustic logging tool led to a new tool called the azimuthally acoustic bond tool (AABT 2.0), which offers an enhanced azimuth resolution capability that can be useful for determining the azimuth and angle range of cement channelling [44]. This tool uses phased-arc array transmitters to provide detailed evaluations of cement bond quality in different directions around the wellbore. In current CCUS workflows, SBT/AABT commonly replace or augment legacy CBL-only evaluations to pinpoint localised channelling and microannulus.

In recent years, there has been an increased focus on developing and utilising ultrasonic tools for cement bond evaluation in the industry. These tools utilise ultrasonic waves to assess the quality of the cement bond. Ultrasonic logging tools effectively provide indirect measurements of cement quality. They are commonly used to detect the presence of cement behind pipes and evaluate the bond strength between cement and formations [43,49]. Ultrasonic logging tools typically use a broadband pulse (200–700 kHz), which is much higher than the pulse used by the conventional CBL. These higher frequencies allow for more detailed and accurate evaluations of the cement bond because they are better able to detect small air voids or microannulus that may exist in the cement sheath. However, compared with acoustic logging tools, ultrasonic logging tools are more sensitive to irregularities around the pipe, such as pipe rugosity or the presence of corrosion or deposits, which can affect the accuracy of the measurements. Moreover, using high frequencies in ultrasonic logging tools limits their use in heavy mud such as oil-based mud (OBM). For CCUS, the details and accuracy of ultrasonic logging tool are particularly useful for diagnosing microannulus and small-scale channelling.

For well integrity evaluation, acoustic logging tools are standard for indirectly measuring cement quality and finding the top of cement (TOC). However, they have limitations in measurement and interpretation accuracy [43]. Ultrasonic tools are preferred for their high-resolution imaging capabilities, and they are extensively used for cement bond evaluation [45]. Other technologies such as logging-while-drilling (LWD) sonic tools offer alternatives for cement bond evaluations, especially in scenarios where wireline operation is not feasible [50,51]. Additionally, new technologies are being developed to evaluate the cement quality behind multiple casing layers in a single logging run, which can be cost-effective for well abandonment [39]. In practice, CCUS well integrity assessments benefit from combining azimuthal acoustics (SBT/AABT) with ultrasonics and selective temperature/noise surveys.

2.2. Principles of Acoustic Logging and Ultrasonic Imaging Tools

Acoustic logging tools are essential for evaluating cement bond integrity, particularly in the context of carbon sequestration or CCUS projects. These tools utilise sound waves to examine the cement bond quality between the casing and formation. By assessing the travel time and amplitude of the reflected waves, acoustic logging tools offer detailed insights into the bonding strength and integrity of the cement sheath [52,53,54]. The process of generating and analysing acoustic waves with logging tools is initiated by emitting sound pulses from a transmitter into the wellbore. These sound waves propagate through the casing, cement sheath, and formation, encountering interfaces that reflect some energy to the receiver. The received signals are then processed to extract information about the acoustic properties of the materials, including the cement bond quality. Several parameters measured during acoustic logging include the following:

• Transit Time: This parameter reflects the acoustic impedance of the encountered materials, with variations indicating changes in material properties, such as bonding strength. In CCUS wells, lower acoustic impedance can accompany debonding or channelling created by thermal/pressure cycling.
• Amplitude: The strength of the received acoustic signal correlates with material density and integrity. Lower amplitudes suggest poor bonding or the presence of a microannulus in the cement sheath. Azimuthal low-amplitude sectors are consistent with localised CO₂ migration risks.
• Attenuation: This parameter provides information about the material’s acoustic properties and cement bond quality based on the reduction in the strength of the signal as it propagates through the wellbore, casing, and cement. Elevated attenuation may reflect scattering from carbonation-induced microcracks.
• Oscilloscope Pictures: Visual representations of acoustic signals help to interpret cement bond quality and identify issues such as fluid annulus thickness or poor bonding.

By analysing the amplitude, travel time, and attenuation of reflected waves, operators can evaluate the integrity of the cement bond and identify potential issues, such as microannulus or poor bonding [55,56,57].

Figure 3 shows a schematic of acoustic waveforms in a cement bond log. Different cement bond conditions will give different amplitude characteristics at different travel times. In this figure, the 57 µs/ft line represents a signal from the casing (casing–cement boundary). The significantly high amplitude right after the casing represents the possibility of a free pipe, resulting from an acoustic signal resonating along the casing.

A well-bonded cement sheath shows consistent transit times, high amplitudes, and low attenuation levels, indicating effective zonal isolation and wellbore integrity. Conversely, poor bonding, microannulus, or fluid channels behind the casing may lead to longer transit times, reduced amplitudes, and increased attenuation, signalling potential cement bond issues [58,59]. The analysis of acoustic waves during logging offers detailed information on material acoustic properties, enabling operators to evaluate the cement bond quality and identify anomalies that could compromise wellbore integrity especially in CCUS operations.

Ultrasonic imaging tools employ a rotating transducer to emit and receive ultrasonic waves, generate cross-sectional images of the annular space, and identify areas with poor cement bonds. The high-frequency broadband pulses used by ultrasonic imaging tools cause a part of the casing to vibrate. This vibration can die off rapidly or slowly, depending on the material behind the casing. Some energy is reflected to the tool, and some continues propagating through the casing wall until it is completely attenuated. As a result, the reflected waves recorded by the transducers contain resonances. Some measurements taken by ultrasonic imaging tools include the following:

• Echo amplitude: This is an indicator of casing conditions. Reduced echo amplitude can flag debonding or fluid behind pipe in CO₂ injectors.
• Internal radius of the casing: This is calculated from the transit time of the main echo.
• Casing thickness: This value is calculated from the resonant frequency.
• Acoustic impedance of the material behind the casing: This is calculated from the resonance form. Azimuthal maps of impedance enable targeted remediation where channelling is suspected.

The resonance is strong when fluid is behind the casing, and it is weaker if there is solid cement behind the casing. The different resonance characteristics determine the acoustic impedance value of the cement. The cement impedance value can be displayed in a 3D view, which provides full spatial coverage around the wellbore. In practice, by applying a threshold value to the cement impedance, areas with relatively low impedance values could be flagged as poor bond intervals with liquid or gas filling.

Zemanek et al. [60] could be considered the first to introduce early-generation ultrasonic technology with a tool called the borehole televiewer (BHTV). Subsequently, other advancements in ultrasonic imaging tools were made to keep up with the demand from the industry. One such advancement is the cement evaluation tool [61], which uses an ultrasonic pulse with an array of eight transducers, each covering an azimuthal area of 45°. Hayman et al. [62] introduced the second generation of ultrasonic tools with a tool called the ultrasonic imager (USI). This tool uses a single rotating transducer to achieve greater accuracy in covering the pipe circumference. It also incorporates improved model-based signal processing, which is less sensitive to environmental effects. Graham et al. later developed a similar tool called circumferential acoustic scanning tool-visualisation (CAST-V) [63]. Advancements in ultrasonic imaging tools have recently been made by several companies [64,65,66] aiming to overcome the challenges related to cement integrity evaluation in more extreme environments. In current CCUS practice, these newer imagers complement azimuthal acoustic tools to localise CO₂-related debonding and to confirm casing thickness/corrosion trends before remedial work.

2.3. Advantages and Limitations of Acoustic Logging and Ultrasonic Imaging Tools

Both acoustic logging and ultrasonic imaging have advantages and limitations, influencing their selection and use in cement bond evaluations for CCUS projects. As an early-generation tool for cement bond evaluation, acoustic logging tools have been developed with a high degree of sophistication, offering reliable assessments essential for confirming well integrity [51]. Acoustic logging tools offer real-time assessment capabilities, allowing operators to promptly identify bonding issues, such as microannulus or poor bonding. Providing immediate feedback on cement bond conditions enables quick decision-making and intervention to ensure zonal isolation and wellbore integrity. Additionally, acoustic logging tools offer extensive wellbore coverage, allowing operators to visualise the cement bond conditions surrounding the well casing and comprehensively evaluate cement integrity [58,67]. These advantages make acoustic logging tools valuable for assessing cement bond quality in CCUS projects. However, these tools have some limitations. Conventional acoustic logging tools can struggle with azimuth and angle range resolution when evaluating channelling, which can mask localised CO₂-related microannulus. This limitation has led to the development of improved tools, such as the azimuthally acoustic bond tool (AABT 2.0), for enhanced resolution [44]. Therefore, CCUS workflows commonly augment or replace CBL-only passes with SBT/AABT for azimuthal diagnosis.

However, ultrasonic imaging techniques provide detailed visualisations of the cement bond conditions, offering valuable insights into zonal isolation and well integrity. The high-resolution imaging capability of ultrasonic tools allows for the identification of small-scale defects that may not be easily detectable with an acoustic logging tool. Figure 4 displays an example of data from the ultrasonic logging tool. The data in track 3 on the right show an acoustic waveform from the CBL compared with other tracks from ultrasonic logging. In CCUS cases, low impedance patches can indicate liquid-filled microannulus or carbonation-weakened annuli. This detailed visualisation enables precise assessments of the cement bond integrity, which is crucial for the success and safety of wellbore operations, as in CCUS projects. However, ultrasonic imaging techniques face limitations and challenges, particularly in high-pressure and high-temperature wells. Signal distortion and attenuation in these environments can affect the accuracy of imaging results, thereby affecting the reliability of the assessment. Additionally, interpreting ultrasonic imaging data in complex well environments requires specialised expertise and calibration to evaluate cement bond integrity accurately. The complexity of well environments, such as multiple casings or intricate geometries, can present challenges for ultrasonic imaging techniques, affecting the comprehensive evaluation of cement bond conditions [68].

Table 1. Summary of main advantages and disadvantages of different acoustic/ultrasonic logging tools used for evaluating cement sheath integrity.

Tool	Key Advantages	Key Disadvantages
Cement Bond Log (CBL)	Widely used standard sonic log; simple amplitude and travel-time measurements give an intuitive overall bond assessment.	No azimuthal coverage (only provides an average bond measurement), so it can miss narrow channels. Results are sensitive to tool decentralisation and microannulus (small casing-cement gaps), which can lead to misleading poor-bond indications.
Segmented Bond Tool (SBT)	Measures bond quality in multiple circumferential sectors (e.g., six 60° segments), enabling detection of non-uniform cement and channels around the casing. Particularly useful in eccentric or irregular wells, since each sector is logged independently to pinpoint localised bonding defects.	Discrete sector coverage means lower spatial resolution than fully azimuthal imaging (defects smaller than a sector can be overlooked). The tool’s design is more complex than a standard CBL, and like other acoustic logs it requires continuous fluid coupling to transmit signals.
Azimuthally Acoustic Bond Tool (AABT)	Employs phased-array acoustic transmitters for full 360° coverage, greatly improving azimuthal resolution to locate channels and quantify their orientation/extent. Offers a detailed circumferential bond map, so small channels or partial bond issues can be identified more reliably than with older tools.	Higher complexity and cost—AABT’s advanced phased-array hardware and data processing make it less common and require specialised expertise. As a newer technology, it may rely on proprietary equipment and has limited public field validation data compared to conventional tools.
Ultrasonic Imaging Tool (USIT)	High-frequency pulse-echo imaging provides much finer vertical and radial resolution than sonic tools, delivering a detailed 360° map of casing cement coverage. Measures acoustic impedance of the annulus, which allows direct identification of liquid vs. solid behind casing (less dependent on cement properties and more effective at detecting channels or voids).	Requires liquid in the wellbore for acoustic coupling (cannot transmit through gas). Logging is slower with dense data and the equipment is more expensive than a basic CBL, increasing operational cost. Like other acoustic methods, its signals can be distorted by heavy mud or multiple casing strings, complicating interpretation in such conditions.

summarises the main advantages and disadvantages of different tools used to evaluate the cement sheath integrity.

In CO₂ injector wells, the advantages of acoustic logging tools, such as real-time assessment and extensive wellbore coverage, are particularly beneficial for quickly identifying and addressing bonding issues. The limitations of ultrasonic imaging techniques, such as signal distortion and challenges in complex well environments, may pose challenges in accurately assessing cement bond integrity under these conditions. Therefore, a combined workflow is recommended—azimuthal acoustics (SBT/AABT) to localise channelling/microannulus, ultrasonics to characterise impedance/thickness, and selective temperature/noise surveys for confirmation—when selecting the most suitable method for cement bond evaluation in high-pressure and high-temperature CO₂ injector wells in CCUS projects.

3. Challenges in Cement Bond Evaluation for CCUS

3.1. Presence of CO₂ and Other Reactive Fluids

The interaction between CO₂ and cementitious materials in carbon capture, utilisation, and storage (CCUS) wells complicates cement bond evaluation due to its role in accelerating cement degradation. Portland cement, the most commonly used material in oil and gas wells, is valued for its sealing properties and ability to prevent fluid migration, making it suitable for drilling operations. However, its performance in CCUS wells is compromised by its vulnerability to degradation when exposed to CO₂-rich environments and reactive fluids. This degradation often involves coupled chemical carbonation and mechanically driven debonding at the steel-cement and cement-rock interfaces, which significantly weakens the mechanical properties of the cement. From a logging perspective, such damage typically presents as increased CBL/VDL amplitude and travel-time changes (poorer shear coupling), azimuthally localised channelling on segmented bond tools, and reduced ultrasonic impedance. As a result, Portland cement’s suitability for such applications is increasingly being questioned [69].

Alternatives such as geopolymers and non-Portland cement are being actively explored due to their superior chemical resistance and durability in CO₂-rich conditions [70,71]. In practice, geopolymer families (e.g., low-Ca fly ash-based, slag-rich, and metakaolin systems) often exhibit lower permeability evolution and higher interfacial bond stability under CO₂-rich brines than Portland analogues, making them promising candidates for mitigating the risk of structural degradation and CO₂ leakage. Additionally, Portland cement’s resilience can be enhanced with additives such as nanomaterials, smectite clay waste, or pozzolanic substances, latex/epoxy modifiers, silica flour or nano-silica, and expansive agents, which improve resistance to carbonation and aggressive fluids [72,73,74]. These interface-oriented modifications aim to densify the interfacial transition zone and reduce microannulus formation; diagnostically, improved adhesion is expected to reduce CBL amplitudes, suppress azimuthal channelling on segmented tools, and increase ultrasonic acoustic impedance. However, the long-term effectiveness of these enhanced formulations in preventing CO₂ leakage remains under investigation.

When exposed to wet supercritical CO₂ under reservoir conditions, Portland cement experiences carbonation, altering its microstructure and diminishing its mechanical properties [75,76]. Mechanically induced microannulus from thermal/pressure cycling can then interact with chemical alteration, accelerating loss of bond strength and permeability. This degradation is compounded by the presence of aggressive fluids in real-world applications such as CO₂-enhanced oil recovery (CO₂-EOR), CO₂/H2S co-sequestration, and matrix acidification [77]. The altered cement structure weakens the annular seal, increasing the risk of CO₂ leakage [78]. Studies have emphasised the need to better understand these degradation mechanisms, mainly how compressive stress influences cement integrity during CO₂ attacks [25,79]. In logs, these coupled effects frequently manifest as higher CBL amplitudes with azimuthal asymmetry and diminished ultrasonic impedance, aiding early detection.

Cement degradation significantly threatens the long-term success of CCUS projects by compromising wellbore integrity and increasing the likelihood of CO₂ leakage. Prolonged exposure to CO₂ alters both the transport and mechanical properties of the cement, leading to the formation of preferential pathways for fluid migration [78]. Where preferential pathways form, segmented bond tools can localise channelling azimuthally, while ultrasonic cased-hole imagers can differentiate poor bond from fluid-filled microannulus, informing targeted remediation.

The changes in the hydraulic properties of wellbores due to cement degradation can also lead to the migration of subsurface greenhouse gases and brines [80]. Moreover, reductions in compressive and shear bond strengths heighten the risk of wellbore failure over time [81]. Mitigation options include compositionally tailored binders (e.g., geopolymers) and interface-focused additives (latex/epoxy modifiers, silica flour/nano-silica, expansive agents) to limit microannulus formation and carbonation-driven weakening; their success should be validated by combined CBL/SBT/ultrasonic workflows. These challenges underscore the urgent need for robust cementing strategies and the development of CO₂-resistant cement formulations to ensure the long-term safety and effectiveness of carbon sequestration [82,83,84].

Accurately assessing cement bond quality in the presence of reactive fluids such as CO_2-rich brine necessitates the adoption of advanced evaluation techniques. One promising approach is using novel ultrasonic cased-hole imagers designed explicitly for enhanced cement evaluation [69]. In CCUS settings, pairing ultrasonic imaging with azimuthally segmented bond tools improves sensitivity to localised debonding and channels associated with CO2-induced degradation and interface failure. These tools provide a detailed visualisation and assessment of the cement bond quality behind the casing, offering critical insights into the integrity of cement sheaths in CCUS wells.

Advancements in cement bond evaluation using well logs also present valuable opportunities. By leveraging well log data and interpretation techniques, operators can effectively assess the cement bond with the casing and formation, even under challenging reservoir conditions, providing a comprehensive understanding of cement integrity [85]. Furthermore, applying enhanced ultrasonic measurements, combining traditional pulse-echo techniques with innovative flexural wave concepts, allows for a more comprehensive evaluation of cement properties and bond integrity [86]. Critically, materials choices influence log response: geopolymers often show higher acoustic impedance and lower attenuation than Portland systems, while adhesion-enhancing additives tend to restore shear coupling—signals that can be tracked over time to verify durability in CO₂-rich environment. These innovative approaches provide operators with the tools and materials necessary to accurately evaluate and ensure long-term cement integrity in CCUS environments, contributing to the overall success and safety of carbon sequestration projects.

3.2. Impact of Cement Sheath Deterioration over Time

The cement sheath can deteriorate through various mechanisms over time, impacting zonal isolation and well integrity. One significant mechanism is the formation of a microannulus at the cement–sheath interface, leading to issues such as annular pressures, crossflows between reservoirs, and flow behind the casing [87]. Changes in wellbore temperature and pressure during fracturing and flow back can induce stress variations in the cement sheath, potentially causing compression, tensile failure, or micro-clearance between the casing and the cement sheath [88]. Additionally, sudden thermomechanical fatigue failure can lead to microannulus and fractures in the cement sheaths [89]. From a diagnostics perspective, these mechanically driven features typically manifest as increased CBL/VDL amplitudes with azimuthal asymmetry on segmented bond tools and reduced ultrasonic impedance behind casing, enabling early localisation of debonded intervals.

Mechanical and chemical degradation frequently act in concert in CCUS environments: exposure to CO₂ drives carbonation and decalcification that weaken the interfacial transition zone, while thermal/pressure cycling promotes microannulus opening and fracture propagation [89]. This coupled pathway accelerates permeability growth and loss of shear coupling; diagnostically, operators observe higher CBL amplitudes, azimuthally localised channels on segmented tools, and lower ultrasonic impedance consistent with fluid-filled gaps. Therefore, understanding and mitigating mechanical and chemical degradation mechanisms are crucial for ensuring the long-term sustainability of cement sheaths in CCUS projects.

Effectively managing the long-term risks associated with cement sheath integrity in CCUS wells necessitates a multifaceted approach. Advanced monitoring technologies, such as downhole sensors, provide real-time data on temperature, pressure, and stress variations within the wellbore [90]. These real-time data enable the proactive identification and mitigation of potential cement degradation issues. Furthermore, developing and utilising predictive models based on computational simulations are essential. These models, incorporating factors such as material properties and wellbore dynamics, and the evolving bond quality inferred from CBL/VDL, azimuthal segmented bond tools, and ultrasonic images, can forecast the long-term behaviour of the cement sheath under varying reservoir conditions, supporting surveillance plans that target intervals with deteriorating bond signatures for remediation [91,92].

In addition to monitoring and prediction, utilising innovative materials is vital to enhancing cement sheath resilience. Incorporating engineered additives with expansive properties or enhanced durability into cement formulations can significantly improve long-term performance and reduce the likelihood of degradation [93]. Interface-focused additives (e.g., latex/epoxy modifiers, silica flour or nano-silica, and expansive agents) and alternative binders such as geopolymers can densify the interfacial transition zone and limit microannulus formation under CO₂-rich environment; improvements should be verified by reduced CBL amplitudes, diminished azimuthal channelling on segmented tools, and increased ultrasonic impedance over time. By integrating these advanced monitoring strategies, predictive modelling techniques, and novel material solutions, operators can effectively assess and mitigate the long-term risks associated with cement deterioration, ensuring the integrity and effectiveness of CCUS operations while minimising environmental impact.

4. Advances in Cement Bond Evaluation Techniques

Applying machine learning (ML) and artificial intelligence (AI) in cement bond evaluation presents a significant advancement for CCUS projects. ML algorithms, trained on diverse datasets, including ultrasonic and sonic logs, offer the potential for a more accurate and efficient assessment of cement bond integrity. AI-driven predictive models can identify patterns and predict bond quality, leading to improved decision-making in well construction and monitoring [18,94,95,96]. The benefits include enhanced evaluation accuracy and efficiency, real-time insights into cement quality for proactive maintenance, and the identification of potential issues such as crossflow, preventing costly well failures [97]. Additionally, AI facilitates the development of predictive maintenance strategies to optimise the performance and longevity of CCUS projects. Figure 5 compares the results of ML-assisted interpretations with human expert interpretations. In this example, ML-assisted interpretations show very similar results to human expert interpretations in many intervals, proving that machine learning can significantly contribute to accurately evaluating cement bond integrity.

However, challenges remain in fully leveraging AI for cement bond evaluation. The reliance on high-quality data for training AI algorithms can be limiting, as such data may not always be readily available or may require extensive pre-processing [96]. Furthermore, the interpretability of AI models is crucial, as stakeholders need to understand the rationale behind AI-driven decisions. Equally important is explicit uncertainty quantification and calibration of ML outputs against independent bond-strength proxies to support operational decisions. Additionally, the initial investment required for implementing AI technologies can be a barrier for some organisations [97].

Several research gaps need to be addressed to enhance the accuracy and reliability of AI-based cement bond evaluation. Firstly, advanced methods for accurately interpreting ultrasonic data for casing annulus characterisation are needed [99]. Secondly, current research primarily focuses on single-slice image analysis, highlighting the need for comprehensive 3D analysis techniques to evaluate cement-to-formation bond strength [18]. Lastly, while ML has been explored for cement quality risk assessment [100], there is a lack of research integrating experimental analysis with ML to assess bond performance and corrosion severity. In addition, standardised datasets and reporting protocols—covering tool settings, well geometry, and ground-truth tests—are needed to enable reproducible benchmarking and external validation of models.

Several strategies can be implemented to address these gaps: Developing autonomous interpretation methods leveraging ML can allow for the accurate characterisation of borehole properties and casing annulus features [99]. Additionally, advancing imaging techniques to enable a comprehensive 3D analysis of bond strength is crucial [18]. Finally, integrating experimental analysis with ML can provide a more holistic understanding of bond performance and corrosion. Actionable roadmap: (1) calibrate acoustic/ultrasonic log features to lab-measured bond-strength proxies and report uncertainty; (2) fuse multi-physics data (CBL/SBT/AABT + ultrasonic, optionally DAS/DTS) with joint inversion; (3) publish anonymised benchmark datasets with predefined train/validation splits; (4) define performance metrics (e.g., agreement with expert labels, blind-well validation) and error budgets; (5) develop guidelines for non-ideal geometries (eccentric/multi-string) and HPHT conditions. These advancements will significantly enhance the accuracy and reliability of cement bond evaluation, improving well integrity and zonal isolation for the long-term success and safety of CCUS projects.

5. Conclusions and Recommendations

This review underscores the critical role of accurate cement bond evaluation in ensuring the success and safety of CCUS projects. While advancements have been made in evaluation techniques, significant research gaps remain, particularly regarding the long-term behaviour of cement in the CO₂-rich environments typical of CCUS reservoirs. Further investigation is needed to fully understand the complex interactions between cement and reactive fluids such as CO₂ and to establish calibrated lab-field links (time-lapse logging paired with mechanical testing) for degradation mechanisms and mitigation strategies during field lifetimes.

Several recommendations are proposed to enhance the reliability and effectiveness of cement bond evaluation in CCUS. (1) Align future work with the specific challenges identified: data quality, azimuthal/geometry limits of acoustic tools, ultrasonic performance in HPHT, and model interpretability—each paired with targeted tasks (calibration to strength proxies, multi-tool fusion, standardised datasets, uncertainty reporting). (2) Standardise shared protocols and benchmark datasets to enable reproducible comparisons across tools and wells. (3) Deploy time-lapse field pilots linking logging outputs to remediation outcomes to validate decision thresholds. (4) Authors’ opinion on non-conventional cements: geopolymers appear most suitable for CO₂-rich, corrosive environments due to low permeability and chemical resistance; calcium-aluminate systems are promising for HPHT and sulphate exposure; MgO-based and polymer-modified cements may offer niche benefits (expansion/sealing or flexibility) but demand careful placement design and compatibility checks with casing/formation. Where CO₂ exposure is moderate and operations favour proven supply chains, Class G Portland cement with tailored additives remains appropriate. Selection should be context-specific, balancing durability, pumpability, and verification via CBL/ultrasonic logs. Through continued collaboration and innovation among researchers, industry stakeholders, and policymakers, we can overcome existing challenges and advance cement bond evaluation techniques to ensure the long-term success of CCUS initiatives.