Review Reports

Round 1

Reviewer 1 Report (Previous Reviewer 1)

Comments and Suggestions for Authors

I believe this revised manuscript is acceptable for acceptance and publication.

Author Response

Thank you for helping to review this paper. There were no comments to answer for reviewer 1.

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

This article focuses on risk assessment in carbon sequestration and has high quality. I think we can give a decision on 'Minor Revision'. However, the author should appropriately elaborate on the following comments I have made.
1. The authors provide detailed introductions to FEP, IM, IPM, and causal analysis in Chapter 2. However, I think the logical connection and necessity explanation between different tools are somewhat insufficient, which can easily give people the impression of "tool stacking". I think the author can add some overview of the methods at the beginning of the "Methods" section, clarifying the role of each tool in the risk assessment process.
2. The research case in this article is based on a specific AoR and well configuration, and it has not been explained whether the framework is applicable in other basins or under different data integrity conditions. I think it is best for the authors to appropriately supplement the analysis of the scope and limitations of the method or framework in the discussion, in order to enhance the engineering promotion value and significance of the research conclusions in the paper.
3. In the process of risk calculation, multiple failure events are mainly regarded by the authors as independently occurring. However, in practical engineering, there may be significant coupling amplification effects between different failure modes. Authors should clearly state in the outcome discussion the direction of the impact of the independence assumption on the risk value (overestimation or underestimation), and discuss the possible changes in risk outcomes under strong coupling conditions.
4. The authors assume in the paper that increasing the number of barriers can effectively reduce overall risk. However, some multi barrier well types in the results still show high risks, and this phenomenon lacks quantitative explanation. I think the authors can appropriately state the analysis of single barrier contribution in the manuscript, explaining whether there are cases where "low-quality barriers" cause redundant barriers to fail, in order to verify the validity of the results. 

Author Response

Reviewer 2

1. The authors provide detailed introductions to FEP, IM, IPM, and causal analysis in Chapter 2. However, I think the logical connection and necessity explanation between different tools are somewhat insufficient, which can easily give people the impression of "tool stacking". I think the author can add some overview of the methods at the beginning of the "Methods" section, clarifying the role of each tool in the risk assessment process.

Thank you for the comment. We have added the following to the introduction section, and the details, along with an example, are presented in Section 2.

“The FEP approach will be utilized for the qualitative risk assessment, which will then be converted to a semi-quantitative risk assessment with the help of an interaction matrix (IM) which represent the interaction of the one element with the other which include cause and effect, incident potential matrix (IPM) used for the ranking of the risk level (probability x severity) and color coding, and cause-effect plot diagram from which the critical element in the system can be defined. This semi-quantitative scoring system will help prioritize which well components are at risk of failure and guide targeted rework/repair of those components. The risk assessment utilized in this paper will be conducted barrier by barrier, to facilitate a better understanding of the risk assessment framework.”

2. The research case in this article is based on a specific AoR and well configuration, and it has not been explained whether the framework is applicable in other basins or under different data integrity conditions. I think it is best for the authors to appropriately supplement the analysis of the scope and limitations of the method or framework in the discussion, in order to enhance the engineering promotion value and significance of the research conclusions in the paper.

Thank you for the comment

In this study, we have already given “ Assumptions that are implemented for the downhole condition, and the well component of the well is taken from the studies conducted by Abid et al. (2024). The only difference that will be taken into account is the absence of subsidence.” Hence, if any basin has a condition other than what was assumed, then the risk value will be changed. It must be noted that this risk assessment framework is highly sensitive to “1) number of barriers, 2) well completion, 3) well configuration, and 4) the depth to which the well penetrates in the subsurface,” which is also provided in the paper.

The limitations of the work have already been provided in the paper. “Nonetheless, some of the key limitations in the presented risk assessment framework include the coarse granularity of the IPM, where probability is binned as A–E and severity is categorized as 1–5. For a finer, more discriminating assessment, the axes should use more bins (e.g., A–J and 1–10) or continuous scores with uncertainty ranges, which would reduce ties and yield more precise prioritization. Secondly, when the risk score is assigned to any of the cells in the interaction matrix, it is given as a whole, whereas it would be better to assign the value to each of the sub-mechanisms within the cell (e.g., debonding, micro-annulus, cracks) and then take the average of them. Under strong coupling conditions that can exist between the failure mechanisms, this independence assumption may have the tendency to underestimate the absolute risk magnitude, especially of the last barrier; however, the relative ranking and prioritization of wells and barriers are expected to remain robust. These limitations are planned to be accommodated in our future studies, which will improve the reliability and robustness of the risk assessment framework”.

3. In the process of risk calculation, multiple failure events are mainly regarded by the authors as independently occurring. However, in practical engineering, there may be significant coupling amplification effects between different failure modes. Authors should clearly state in the outcome discussion the direction of the impact of the independence assumption on the risk value (overestimation or underestimation), and discuss the possible changes in risk outcomes under strong coupling conditions.

Thank you for the comment.

We have stated in the limitation section that “Secondly, when the risk score is assigned to any of the cells in the interaction matrix, it is given as a whole, whereas it would be better to assign the value to each of the sub-mechanisms within the cell (e.g., debonding, micro-annulus, cracks) and then take the average of them. Under strong coupling conditions that can exist between the failure mechanism, this independence assumption may have the tendency to underestimate the absolute risk magnitude, especially for the last barrier, however, the relative ranking and prioritization of wells and barriers are expected to remain robust”.

4. The authors assume in the paper that increasing the number of barriers can effectively reduce overall risk. However, some multi barrier well types in the results still show high risks, and this phenomenon lacks quantitative explanation. I think the authors can appropriately state the analysis of single barrier contribution in the manuscript, explaining whether there are cases where "low-quality barriers" cause redundant barriers to fail, in order to verify the validity of the results. 

Thank you for the comments. In the multiple barrier system, the risk value for the last barrier is higher than the well system with single barrier due to the coupling effect described by you in the previous comment. Specifically, the last barrier in a multibarrier system is subjected to the cumulative impact of degradation mechanisms and interactions originating from the barriers below it, and it represents the final line of defense against upward migration toward USDWs and the surface. The following paragraph has been added to the conclusion sections as well.

“The presence of multiple barriers does not mean that a uniform reduction in risk will take place if the individual quality of the barrier or functional role differs. In the multi-well-barrier system, a higher risk is usually associated with the last barrier, which shows the consequence amplification rather than barrier redundancy failure. These last barriers are taken as the last line of defense against migration toward USDWs and the surface.”

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

This manuscript presents a practical, structured screening framework for prioritizing CCS Area-of-Review wells, especially legacy wells, by translating qualitative FEP considerations into a semi-quantitative ranking. The workflow is intuitive, transparent in concept, and potentially useful for operators and regulators who need a defensible way to triage large well inventories and focus diagnostics or remediation on the highest-risk well types and barriers. However, I have some comments.

1. It is unclear who assigned the probability or severity ratings and under what protocol. The paper should state whether scoring is author judgment, expert elicitation, or literature data-based rules, and describe how consistency and disagreements were handled.

2. The manuscript claims decision support but does not clearly map outputs to actions. Add a concise mapping from risk tiersr rankings to recommended diagnostics, monitoring intensity, and remediation triggers.

3. The manuscript contains many typos and editorial errors that materially hurt readability and perceived rigor. Examples include misspellings such as “Storgae” (storage), “Acdification” (acidification), and “Mena risk value” (mean risk value). Also, the keywords are missing.

Author Response

Reviewer 3

1. It is unclear who assigned the probability or severity ratings and under what protocol. The paper should state whether scoring is author judgment, expert elicitation, or literature data-based rules, and describe how consistency and disagreements were handled.

Thank you for the comment. The following has been added to the manuscript draft

“The value of probability and severity values that were designated to each of the interaction matrix cells was from the structured expert judgment by the authors, acting as subject matter experts (SME). Scoring followed predefined probability and severity categories defined in the incident potential matrix to ensure consistency across all wells and barriers.”

2. The manuscript claims decision support but does not clearly map outputs to actions. Add a concise mapping from risk tiersr rankings to recommended diagnostics, monitoring intensity, and remediation triggers

Thank you for the comment. The following table has been added to the discussion section.

Ranking with respect to risk class	Diagnostic approach	Monitoring intensity	Remediation trigger
High (corresponding to high IPM value or high mean cause -effect value)	Immediate diagnostic (for example, CBL/UCIT, pressure testing, annular integrity check, etc.)	Continuous or high frequency monitoring	Immediate evaluation for remedial action (for instance, secondary cementing, plug replacement, etc.)
Medium	Diagnostics should be targeted on critical barrier	Monitoring frequency should be increased	Remediation if degradation indicators or rising risk scores are observed
Low to very low	No immediate diagnostic needed	Periodic or routine surveillance	Until the risk level is changed no remediation is needed

 

3. The manuscript contains many typos and editorial errors that materially hurt readability and perceived rigor. Examples include misspellings such as “Storage” (storage), “Acdification” (acidification), and “Mena risk value” (mean risk value). Also, the keywords are missing.

Thank you for your comment. The draft has been revised and the typos corrected.

 

 

 

 

 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1: "the components that affect the well are the water composition and gas. (Line 30)" — This phrasing is vague and incomplete; specify "water composition and presence of gas/CO₂" or rephrase to match earlier matrix elements (e.g., "water composition and gas/CO₂ interactions").  In addition,  "along with the UIC Class IV injection well"(line ~159) —Again, factual inaccuracy; should be "UIC Class VI injection well" based on EPA UIC classification for CO₂ sequestration.

2: "From the results, it was found that the risk levels of the Type 2 and 3 wells were the highest, while Types 4–6 clustered in the medium-to-low range, and Types 7–9 and the Class VI well were dominated by Low/Very-Low classes." (line ~23) — "Class VI well" is written inconsistently as "Class VI" here but later as "UIC Class IV" in the title and methodology; correct to consistent "Class VI" throughout since it refers to CO₂ geologic sequestration injection wells.

3: Figure 1 caption "Figure 1. Legacy well within the AoR chosen for the risk assessment Arbad et al. (2022)."(line ~154). Missing period or colon after "assessment"; also add "from" before "Arbad et al. (2022)" for grammatical correctness. Moreover, "converted the qualitative risk assessment on the CSC well"  "CSC" is likely a typo for "CCS" (Carbon Capture and Storage); correct to "CCS well".

4: Both carbon storage and hydrogen storage in oil reservoirs consider the structural risk of geological reservoirs. The above discussion needs to be reflected in the original text, and relevant articles should be used to assist understanding:-- Effect of Nanomaterials on Improving the Apparent Viscosity of Heavy Oil and the Environmental Evaluation of Reservoir Environment. ---From CO2 Sequestration to Hydrogen Storage: Further Utilization of Depleted Gas Reservoirs.

5: All components (casing, cement, plugs) in all areas are marked with an "×", but "drill through" and "open hole" are marked with "✓" everywhere, and "Number of barriers for plugs: 1". This means a fully open/unprotected well; however, a "1" barrier count is contradictory if there are no barriers, and the number should be clarified or corrected to 0 if there are indeed no plugs/barriers. Moreover, The statement "Many countries signed the Paris Agreement in 2015" is vague and grammatically awkward ("controlling CO2 emissions" should be changed to "controlling CO₂ emissions").

6: The statement “Number of barriers with respect to the plug: 2 plugs are present, of which one plug is placed in the open hole” conflates “number of obstacles” with “plugs”. Since plugs are obstacles, for the sake of accuracy, it should be rewritten as “Number of plugs (obstacles): 2, one in the opening”. (Table 1, AoR Type 4 Well)

Reviewer 2 Report

Comments and Suggestions for Authors

I read the manuscript with great interest; however, it lacks novelty. The topic has been extensively covered in recent literature, and the proposed approach does not introduce significant methodological advancements. Several recently published papers address similar frameworks and concepts, including:

1. Risk Assessment of Selected CCS Wells through Feature, Event, and Process Method and Comparison of the Barrier Effect
2. Risk Matrix for Legacy Wells within the Area of Review (AoR) of Carbon Capture & Storage (CCS) Projects
3. Strategic Qualitative Risk Assessment of Thousands of Legacy Wells within the Area of Review (AoR)
4. A New Methodology for Quantitative Risk Assessment of CO₂ Leakage in CCS Projects
5. Study on Quantitative Assessment of CO₂ Leakage Risk Along the Wellbore under Geological Storage
6. Advancing Well Integrity Screening for CCS Applications: WISCoS Tool
7. Blowout Probability Estimation for CCS New and Converted Wells
8. Modeling Framework Developed to Assess Integrity of Legacy Wells for CO₂ Storage
9. Well Engineering Aspects and Risk Analysis of CCS Wells
10. Quantitative Risking of CCS Projects with Legacy Wells

Given the overlap with these studies, the manuscript does not demonstrate a clear, novel contribution to the field.