A Review of the Visualization Analysis of the Pore-Scale Formation and Decomposition of CO₂ Hydrates for Carbon Capture and Storage

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. In the manuscript, the authors mentioned microfluidic chips, X-ray micro CT, and Raman spectroscopy. The reviewer suggests adding a table to appropriately compare the advantages and disadvantages of these visualization techniques in terms of resolution, applicable conditions (pressure/temperature), real-time performance, etc. in pore scale hydrate research, in order to guide readers in choosing appropriate research tools.
2. The reviewer believes that the authors should propose more targeted future research directions based on the shortcomings of current research in the conclusion or outlook section, such as how to overcome the limitations of existing visualization technologies in three-dimensional dynamic and complex fluid environments? How to elevate the micro mechanism of pore scale to the prediction model of macro reservoir scale?
3. Since the authors mentioned CO2 capture/storage in the manuscript, it is suggested that when discussing permeability changes or hydrate distribution patterns, it is appropriate to discuss how these pore scale changes affect the injection efficiency, reservoir cap rock sealing, and long-term stability during CCS processes, in order to highlight the engineering application value of the review.
4. Hydrates can indeed play a crucial role in areas such as carbon sequestration and energy development. Therefore, the statements in lines 33-35 of the manuscript require support from the following references: Risk prediction of gas hydrate formation in the wellbore and subsea gathering system of deep-water turbidite reservoirs: case analysis from the south china sea;  -Effects of crosslinking agents and reservoir conditions on the propagation of fractures in coal reservoirs during hydraulic fracturing;  -From CO2 sequestration to hydrogen storage: further utilization of depleted gas reservoirs.
5. When introducing intrinsic dynamic models such as Englezos or Kim Bishnoi models, it is recommended to add appropriate explanations for the physical meanings of key parameters K * and kd, and point out the range and limitations of these empirical parameters under different experimental systems (such as salinity and different pore media).
6. Authors need to carefully proofread to ensure consistency in the translation of professional terms such as/Dissection or decomposition. At the same time, verify that all variable symbols in formulas and charts (such as permeability k, porosity) are consistent with standard conventions in the field to avoid confusion.

Author Response

Reviewer 1:

Comment 1. In the manuscript, the authors mentioned microfluidic chips, X-ray micro CT, and Raman spectroscopy. The reviewer suggests adding a table to appropriately compare the advantages and disadvantages of these visualization techniques in terms of resolution, applicable conditions (pressure/temperature), real-time performance, etc. in pore scale hydrate research, in order to guide readers in choosing appropriate research tools.

Response:

(1) Thank you for your careful review.

(2) The original intention of mentioning CT and Raman technology is to illustrate that technological innovation significantly promotes the development of experiments in the field of microfluidic technology. We deeply apologize for the language issue that caused you to misunderstand that the three visualization techniques can only be used separately. We have made corresponding changes in the text. In the experimental system using microchannel chips discussed in this article, microfluidic chips, CT, and Raman technologies each have their own advantages and strong compatibility. Microfluidic technology precisely controls flow and temperature pressure conditions. Microfluidic technology, as the core support of the experimental platform, can regulate key experimental conditions such as flow state, temperature, and pressure. Raman spectroscopy technology, with its in-situ and real-time measurement advantages, can accurately capture the molecular level dynamic changes during hydrate synthesis in the microchannel chip environment, while CT technology (especially micro CT) Focusing on the spatial structure dimension, non-destructive scanning of the porous medium hydrate system inside microchannel chips can obtain three-dimensional images with micrometer level resolution, clearly presenting the occurrence form, spatial distribution pattern, and saturation degree changes of hydrates in pores at different synthesis stages, providing key basic data support for constructing more accurate numerical models.

(3) The details are as follows：

The relevant literature on the use of CT technology is indicated in the text.

The manuscript has been improved by separately listing the application of Raman spectroscopy technology in section 2.2. The specific additions are as follows: Innovations in experimental techniques have significantly advanced this field. Within microfluidic experiments, analytical methods such as micro-Raman spectroscopy serve to assess CO₂ hydrate crystallization dynamics. For example, Ouyang et al. [83]combined Raman spectroscopy with microfluidic technology to achieve in-situ chemical analysis of the hydrate generation process, revealing the correlation between the different permeability of porous media and nucleation patterns: in low-permeability stochastic and physical pore networks, CO₂ hydrate undergoes homogeneous nucleation (stochastic multi-point nucleation) due to limited CO₂-water mixing; conversely, high-permeability homogeneous pore networks enable non-uniform nucleation (predictable interfacial growth) by promoting continuous gas-liquid interface contact. Wells et al. [90] use Raman technology to quantified the propagation rate of hydrates inside high-pressure microfluidic device and found that under low supercooling (2 K) and moderate pressure, growth was primarily mass-transfer-limited. However, when pressure exceeded a critical threshold, reaction kinetics became the dominant factor restricting growth.

Comment 2. The reviewer believes that the authors should propose more targeted future research directions based on the shortcomings of current research in the conclusion or outlook section, such as how to overcome the limitations of existing visualization technologies in three-dimensional dynamic and complex fluid environments? How to elevate the micro mechanism of pore scale to the prediction model of macro reservoir scale?

Response:

• Thank you for your precise comment, which are of great significance for improving the depth and extensibility of this study.
• We fully agree with your suggestion. Given the limitations of our current work, we have added a new section titled "Future Research Directions" in Section 4.1, with the aim of proposing more targeted directions for future research.
• This manuscript has been revised in section 4.1 based on your feedback in this regard. The specific additions are as follows：

Despite significant progress, critical gaps remain between current research and CCS engineering demands. Three targeted future directions are proposed:

First, overcoming limitations of visualization technologies in 3D dynamic and complex fluid environments. Current studies rely mainly on 2D microfluidic chips [61,79,83] or static 3D characterization [17,18]; future efforts should develop time-lapse micro-CT [52,78], holographic microscopy for real-time 3D tracking, optimize microfluidic chips to simulate complex reservoir fluids [94], and integrate in-situ Raman spectroscopy [83,90,91] with 3D visualization to capture morphological and compositional changes simultaneously.

Second, elevating pore-scale mechanisms to macro reservoir-scale prediction models. Existing simulations excel at pore-scale mechanisms [22,80], but upscaling re-mains challenging. Future work should establish a multi-scale coupling framework (pore network-core-reservoir) [52,89,109], incorporate geological constraints, and vali-date models with field test data to improve reservoir-scale prediction reliability.

Third, strengthening synergy between experimental visualization and numerical simulation. Current research often separates the two [60,114]; future efforts should use high-precision visualization data to calibrate simulation parameters, and leverage simulation results to guide targeted experiments, enhancing mechanism interpretation accuracy and model reliability.

Comment 3. Since the authors mentioned CO₂ capture/storage in the manuscript, it is suggested that when discussing permeability changes or hydrate distribution patterns, it is appropriate to discuss how these pore scale changes affect the injection efficiency, reservoir cap rock sealing, and long-term stability during CCS processes, in order to highlight the engineering application value of the review.

Response:

• Thank you for your valuable and constructive comment. Your suggestion to link pore-scale changes to core engineering issues in CCS effectively bridges the gap between fundamental pore-scale research and practical industrial applications. This insight is crucial for highlighting the engineering value of our review, and we fully agree with and highly appreciate your guidance.
• In the revised manuscript, we will supplement and expand the discussion section Given the limitations of our current work, we have added a new section, "Engineering Implications for CCS," in Section 4.1, aiming to elucidate the engineering significance of pore-scale research on carbon dioxide hydrate for CCS.
• This manuscript has been revised in section 4.1 based on your feedback in this regard. The specific additions are as follows：

Notably, these pore-scale insights underpin CCS engineering optimization. The pore-filling hydrate growth mode effectively reduces sandstone permeability, demonstrating the potential of CO₂ hydrate as a storage mechanism in the CO₂ storage process[18]. Pore structure evolution regulates CO₂ injection efficiency: expanded pores and enhanced connectivity reduce injection pressure, while excessive hydrate formation or mineral precipitation may cause blockage and fracturing. In long-term storage, hydrate-induced pore filling enhances permanence, while unexpected dissociation threatens reservoir stability. These mechanisms clarify the engineering significance of pore-scale research for CCS.

The relevant achievements deepen understanding of CO₂ hydrate storage and dynamics, providing a robust theoretical foundation and experimental support for advancing Carbon dioxide geo-sequestration (CGS) and energy storage technologies.

Comment 4. Hydrates can indeed play a crucial role in areas such as carbon sequestration and energy development. Therefore, the statements in lines 33-35 of the manuscript require support from the following references: Risk prediction of gas hydrate formation in the wellbore and subsea gathering system of deep-water turbidite reservoirs: case analysis from the south china sea;  -Effects of crosslinking agents and reservoir conditions on the propagation of fractures in coal reservoirs during hydraulic fracturing;  -From CO₂ sequestration to hydrogen storage: further utilization of depleted gas reservoirs.

Response:

• Thank you for your valuable recommendation of these high-quality references, which effectively strengthen the academic support for the application prospects of hydrate-based technologies mentioned in lines 33–35 of the manuscript. We fully agree with your suggestion and have carefully integrated the references into the corresponding section, supplementing and refining the statement to enhance the rigor and persuasiveness of the argument.
• We have revised the original sentences in lines 33–35 to explicitly link the application prospects of hydrate technologies to the core themes of the recommended references, with clear citation correspondence. The modified content is as follows: In past few decades, hydrate-based technologies have been recognized for their significant and promising application prospects in the energy sectors [1,2]. Carbon di-oxide hydrates find application in diverse fields such as carbon dioxide sequestration [3–5], natural gas hydrate exploitation. To advance hydrate technology, a funda-mental understanding of their growth behavior and morphological influences is essen-tial. Microfluidic chips are uniquely suited for this purpose, as they offer precise con-trol over temperature, pressure, and flow conditions, enabling the real-time, visual monitoring of hydrate crystal formation.

Comment 5. When introducing intrinsic dynamic models such as Englezos or Kim Bishnoi models, it is recommended to add appropriate explanations for the physical meanings of key parameters K * and kd, and point out the range and limitations of these empirical parameters under different experimental systems (such as salinity and different pore media).

Response:

• Thank you for your meticulous comment. Your suggestion to clarify the physical meanings of key parameters (K* and kd) in intrinsic dynamic models and elaborate on their applicable ranges and limitations under different experimental systems (is crucial for enhancing the accuracy and clarity of the manuscript. This revision helps readers better understand the inherent characteristics of these empirical parameters and avoids potential misunderstandings in practical application, which we highly appreciate.
• In response to your comment, we have supplemented and expanded Section 1.2.2 of the manuscript. The core modification focuses on two aspects: First, explicitly explain the physical meanings of the key parameters K*and kd in the models; Second, systematically elaborate on the applicable ranges, limitations, and parameter correction requirements of K* and kd when experimental conditions change, combining relevant literature and experimental data to support the discussion.
• The specific additions are as follows：

Due to the difficulty of completely eliminating mass- and heat-transfer resistances under laboratory conditions, an accurate measurement of the intrinsic formation rate constant has not been achieved and no universally accepted standard value exists. Consequently, all currently reported CO₂-hydrate formation rate parameters are only apparent K* values that are strongly dependent on the experimental setup[45–47]. Their calculation is essentially based on the transient gas-consumption or mole-number profile, and they must be re-fitted whenever salinity, ion type, or porous-medium characteristics are changed.

The constant k_d is usually written in an Arrhenius-type equation as: , in which k₀ is the intrinsic reaction rate constant, ΔEa represents the activation energy. Clarke and Bishnoi [48] measured the intrinsic rate constant of CO₂ hydrate decomposition, kd = 1.83×10⁸ mol·m⁻²·Pa⁻¹·s⁻¹ (ΔEa = 102.9 kJ mol⁻¹), in a pure water stirred tank with fully eliminated mass and heat transfer resistance, it was used as an intrinsic benchmark for subsequent studies. Decomposing the intrinsic rate constant has significant limitations: its determination is based on ideal conditions of "no salt, non-porous, and mass and heat transfer without resistance". When salinity or porous media are introduced into the experimental system, corrections need to be made based on the system's characteristics[49,50].

 

Comment 6. Authors need to carefully proofread to ensure consistency in the translation of professional terms such as/Dissection or decomposition. At the same time, verify that all variable symbols in formulas and charts (such as permeability k, porosity) are consistent with standard conventions in the field to avoid confusion.

Response:

• Thank you for providing detailed and reliable suggestions. We have thoroughly proofread the manuscript to address the consistency of professional terms and the standardization of variable symbols, with the following specific revisions completed:
• To address your concerns, we have implemented a comprehensive review and revision of the entire manuscript, focusing on two core aspects: First, conduct a systematic proofread of professional terminology (especially synonymous terms such as "dissociation" and "decomposition") to confirm consistency in their use while adhering to academic conventions in the hydrate research field; Second, verify all variable symbols in formulas, charts, and text descriptions (e.g., permeability, porosity) against widely accepted standard conventions in the field, ensuring uniform expression across the entire manuscript to avoid reader confusion.
• The specific details of the modifications are as follows.

In the field of gas hydrate research, "dissociation" and "decomposition" are widely recognized as synonymous terms, both referring to the process of hydrate lattice breakdown and release of encapsulated gas. We used these two terms interchangeably throughout the manuscript solely to avoid repetitive expression and enhance the readability of the text, rather than indicating different physical or chemical processes. All uses of the two terms are consistent in core meaning, complying with the academic consensus and standard definitions in the field.

For the core technical terms and variable symbols in the formula, we have ensured that they are consistent with the widely accepted symbols in authoritative literature. We have strictly adopted the official and widely accepted expressions in hydrate research.

 

Reviewer 2 Report

Comments and Suggestions for Authors

I reviewed the manuscript entitled “A Review on the Visualization Analysis of Porous Scale Formation and Decomposition of CO₂ Hydrates as New Energy Sources.” The manuscript summarizes pore-scale visualization and numerical simulation studies related to the formation and dissociation of CO₂ hydrates in porous media, with particular focus on microfluidic methods and multiscale modelling. The references are reviewed and analyzed with the focuses on hydrate nucleation, growth, distribution and decomposition kinetics. The findings in numerical simulation reveal micro-mechanisms and predict macroscopic behaviors. Overall, the manuscript is scientifically valuable. The following questions should be addressed before the manuscript is considered for publication:

1. Line 60-61, references are also needed for porous media and others.
2. Line 225-237, the description of the microfluidic experimental setup is too detailed but lacks unique advantages and relative analysis.
3. Some figures (e.g., those adapted from previous works) need clearer captions describing their relevance to the text.
4. Terms such as “driving force,” “memory effect,” “dissolution-diffusion-crystallization mechanism,” etc. should be consistently defined.
5. More explicit experimental conclusions are expected for nucleation mechanisms (homogeneous vs heterogeneous) of CO₂
6. The section on numerical methods should analyze the strengths and weaknesses of MD, CFD, and LBM.

Author Response

Reviewer 2:

I reviewed the manuscript entitled “A Review on the Visualization Analysis of Porous Scale Formation and Decomposition of CO₂ Hydrates as New Energy Sources.” The manuscript summarizes pore-scale visualization and numerical simulation studies related to the formation and dissociation of CO₂ hydrates in porous media, with particular focus on microfluidic methods and multiscale modelling. The references are reviewed and analyzed with the focuses on hydrate nucleation, growth, distribution and decomposition kinetics. The findings in numerical simulation reveal micro-mechanisms and predict macroscopic behaviors. Overall, the manuscript is scientifically valuable. The following questions should be addressed before the manuscript is considered for publication:

Comment 1. Line 60-61, references are also needed for porous media and others.

Response:

• Thank you for your comment concerning our manuscript.
• Since the following section specifically addresses the case in porous media, no citation was provided in lines 60–61. We recognize that this was inappropriate and have added the necessary citation in the revised version.
• The specific additions are as follows: The equilibrium conditions and growth morphology of carbon dioxide hydrates have been investigated in four environments: pure water [8–10], different gas compositions [11–13], additives [14–16], porous media[17–20].

 

Comment 2: Line 225-237, the description of the microfluidic experimental setup is too detailed but lacks unique advantages and relative analysis.

Response:

• Thank you for your insightful comment. We fully agree that the original description of the microfluidic experimental setup (Lines 225–237) overly focused on technical details while lacking a clear articulation of its unique advantages and comparative analysis with traditional experimental methods. Your feedback helps us refine the content to highlight the core value of the experimental design, which is crucial for readers to understand the rationality and innovation of the study. We highly appreciate your targeted guidance.
• In response to your comment, we have strengthened comparative analysis to clarify the superiority of the microfluidic setup in pore-scale hydrate research, linking the experimental design to the study’s core objectives (revealing pore-scale mechanisms).
• This manuscript has been revised in section2.1.1 based on your feedback in this regard. The specific additions are as follows：

Compared with traditional high-pressure reactors, the unique advantages of this microfluidic system lie in three aspects: (1) Real-time visualization of pore-scale dynamics: it enables direct observation of hydrate nucleation, growth, and decomposition at the microscale (μm level), which is inaccessible with bulk reactors that only provide macroscopic average data; (2) Precise control of key parameters: temperature and pressure can be regulated with high accuracy, and fluid flow velocity is adjustable to simulate different reservoir flow conditions; (3) Mimicry of realistic porous media: microfluidic chips can replicate heterogeneous pore-throat structures (e.g., natural sandstone pores or regular geometric models, bridging the gap between idealized laboratory conditions and actual reservoir environments. These advantages make microfluidic technology an indispensable tool for revealing pore-scale hydrate mechanisms.

 

Comment 3: Some figures (e.g., those adapted from previous works) need clearer captions describing their relevance to the text.

Response:

• Thank you for your constructive and valuable comment. We fully agree that clear figure captions—especially for figures adapted from previous works—are crucial to establishing a strong connection between the visual content and the core arguments of the manuscript. Your reminder helps us enhance the readability and logical coherence of the study, ensuring readers can quickly grasp the relevance of each figure to the text. We highly appreciate your meticulous review and guidance.
• To address your concern, we have implemented a systematic revision of all figures adapted from previous work. We retain the original key information (such as data sources and experimental conditions) in the description; And we have added clear descriptions related to the current research, including how charts and graphs support the text's arguments, supplement the proposed mechanisms, or provide basic background for the research.
• Based on your feedback, modifications have been made to this aspect. Each referenced image has been described in the text and revised to a more appropriate title. And here we did not list the changes but marked in red in the revised paper.

 

Comment 4: Terms such as “driving force,” “memory effect,” “dissolution-diffusion-crystallization mechanism,” etc. should be consistently defined.

Response:

• Thank you for your valuable suggestions. Defining core technical terms is critical to maintaining the academic rigor, readability, and clarity of the manuscript. It helps avoid potential ambiguities in concept understanding and ensures that readers can accurately grasp the core connotations of these terms. Your reminder effectively improves the standardization of the manuscript, and we highly appreciate your careful review and guidance.
• To address your concern, we have added clear and standardized definitions for each term when it first appears in the manuscript, and changed them to a uniform expression when mentioned subsequently. The specific revisions as follows:

"Dissolution-Nucleation-Growth " (Section 2.2): Defined as a three-step pathway governing hydrate phase transition;

"Driving force" (Section 1.2.1): Defined as a generic term referring to the thermodynamic imbalance or kinetic gradient that promotes hydrate phase transition (formation or decomposition);

"Memory effect" (Section 2.3): Defined as a unique phenomenon where the aqueous phase retains certain structural features after hydrate decomposition.

 

Comment 5: More explicit experimental conclusions are expected for nucleation mechanisms (homogeneous vs heterogeneous) of CO₂

Response:

• Thank you for your insightful comments. We will provide clearer experimental conclusions on the nucleation mechanism of CO₂. Thank you for your insightful and targeted comment. We fully agree that explicitly clarifying the nucleation mechanisms (homogeneous vs. heterogeneous) of CO₂ hydrates based on experimental results is crucial to enhancing the depth and clarity of the study’s conclusions. Your suggestion helps us focus on the core scientific question of hydrate nucleation, and we highly appreciate your guidance in strengthening the manuscript’s experimental contribution.
• Based on microfluidic observations of CO₂ hydrate formation in porous media with controlled wettability and water saturation, the dominant nucleation mechanism of CO₂ hydrates at the pore scale is heterogeneous nucleation, while homogeneous nucleation is negligible under the experimental conditions Heterogeneous nucleation is strongly regulated by the medium’s wettability and water saturation, which determine the spatial distribution of the CO₂-water interface and thus the nucleation sites.
• To address your request, we have supplemented and refined Section 2.2, Nucleation Mode and Growth Morphology by：

The heterogeneous distribution of medium partial water saturation and wettability induces spatial variations in hydrate formation, and moderate wettability and partial water saturation conditions may provide the optimal environment for hydrate formation [78]. If the medium is hydrophilic [79](such as quartz sandstone), water covers the particle surface to form a continuous water film, with CO₂ distributed in the central region of the pores, nucleation occurring at the water-CO₂ interface or in the water, and growing along the water film towards the pore wall to form pore-filling hydrates; if the medium is hydrophobic (some carbonates), CO₂ preferentially contacts the solid surface, water existing as discrete droplets suspended in CO₂, hydrates growing wrapped around the surface of the medium particles to form particle-wrapping hydrates.

 

Comment 6: The section on numerical methods should analyze the strengths and weaknesses of MD, CFD, and LBM.

Response:

• Thank you for your suggestion, which has provided relevant missing details in the new revised manuscript. Analyzing the strengths and weaknesses of numerical methods in the context of pore-scale CO₂ hydrate research is crucial to enhancing the comprehensiveness and guidance of the numerical methods section. We highly appreciate your meticulous review and valuable guidance.
• We have supplemented the advantages and disadvantages of these three methods. First, for each method (MD, CFD, LBM), we explicitly elaborate on its core strengths in pore-scale CO₂ hydrate research; Second, we systematically analyze its inherent weaknesses;
• We have added targeted analysis of the strengths and weaknesses of MD, CFD, and LBM in Section 3.1, with specific supplements as follows:

MD excels at uncovering molecular-level mechanisms of hydrate formation and dissociation—such as hydrogen-bond network reorganization, guest-molecule encapsulation, and interfacial water structure—yet its nanometre–nanosecond reach precludes direct coupling to flow or field-scale forecasts.

Compared to MD simulations, CFD is suitable for more complex geometries and is particularly suitable for studying the hydrates kinetics of in pore networks [58,106–110] and their impact on permeability and fluid distribution, but it must rely on empirical constitutive relations for nucleation and other micro-events and carries a heavy grid-generation and interface-tracking burden.

Since LBM's algorithm is based on grid points, this makes it very convenient to handle complex pore structures directly converted from CT scan images without generating complex computational meshes, its downside is weakened numerical stability under high density/viscosity contrasts or strong non-equilibrium conditions, and its parameters require repeated experimental calibration.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript number energies- 4013810 reviews “the visualization analysis of porous scale formation and decomposition of CO₂ hydrates as new energy sources”. Although the manuscript worth consideration, there are some major issues that need to be fixed before recommending this manuscript for publication in the journal of Energies.

1. The title of the manuscripts states “visualization analysis of porous scale..” as the theme of the review. However, the manuscript consists of “1. Introduction”, “2. Mechanism of Hydrate Formation and Decomposition”, and “3. Numerical Modeling for Hydrate Research”. There are no sections related to the critical analysis of the literature related to “visualization”. This should be the main focus of the manuscript. In its current form, the manuscript is mainly focusing on fundamentals, which have been already covered in many reviews and books. The manuscript should provide a novel contribution that distinguishes it from prior review articles.
2. The title of the manuscripts states “……CO₂ hydrates as new energy sources”. CO₂ hydrates have a potential application in energy storage, but they themselves are not new energy sources. The authors need to reformulate their review article accordingly.
3. The Introduction and Mechanism of Hydrate Formation and Decomposition sections frame hydrates in “energy sectors” and talk about “energy extraction” and “gas productivity,” which is appropriate for CH₄ hydrates, not for CO₂. Thus, these sections need to be aligned with CO₂ hydrates. Remove the contents related to CH₄ hydrates and keep the focus only on CO₂ hydrates.
4. What is “Porous scale”? Pores vary widely in scale? Which scale is targeted here?  
5. There are some repetitions and inaccuracies in using the English language. Please correct.
6. Overall, the manuscript needs reformulation and re-writing to sharpen the focus on the theme mentioned in the title.

Author Response

Reviewer 3:

The manuscript number energies- 4013810 reviews “the visualization analysis of porous scale formation and decomposition of CO₂ hydrates as new energy sources”. Although the manuscript worth consideration, there are some major issues that need to be fixed before recommending this manuscript for publication in the journal of Energies.

Comment 1: The title of the manuscripts states “visualization analysis of porous scale..” as the theme of the review. However, the manuscript consists of “1. Introduction”, “2. Mechanism of Hydrate Formation and Decomposition”, and “3. Numerical Modeling for Hydrate Research”. There are no sections related to the critical analysis of the literature related to “visualization”. This should be the main focus of the manuscript. In its current form, the manuscript is mainly focusing on fundamentals, which have been already covered in many reviews and books. The manuscript should provide a novel contribution that distinguishes it from prior review articles.

Response:

• Thank you for your insightful comments. We apologize for the unclear expression that led to misunderstanding—this review’s core focus is pore-scale visualization of CO₂ hydrate formation/decomposition in microfluidic channels.
• We focused on introducing the relevant content of microchips after the basic theory of carbon dioxide hydrates, and analyzed the experimental and simulation results in this case in the second and third parts, and added the advantages of this method. And all subsequent analyses were conducted under these conditions.
• The specific additions are as follows:

Compared with traditional high-pressure reactors, the unique advantages of this microfluidic system lie in three aspects: (1) Real-time visualization of pore-scale dynamics: it enables direct observation of hydrate nucleation, growth, and decomposition at the microscale (μm level), which is inaccessible with bulk reactors that only provide macroscopic average data; (2) Precise control of key parameters: temperature and pressure can be regulated with high accuracy, and fluid flow velocity is adjustable to simulate different reservoir flow conditions; (3) Mimicry of realistic porous media: microfluidic chips can replicate heterogeneous pore-throat structures (e.g., natural sandstone pores or regular geometric models, bridging the gap between idealized laboratory conditions and actual reservoir environments. These advantages make microfluidic technology an indispensable tool for revealing pore-scale hydrate mechanisms.

 

Comment 2: The title of the manuscripts states “……CO₂ hydrates as new energy sources”. CO₂ hydrates have a potential application in energy storage, but they themselves are not new energy sources. The authors need to reformulate their review article accordingly.

Response:

• Thank you for your rigorous and accurate comment. We fully acknowledge the inaccuracy in the original title’s description of "CO₂ hydrates as new energy sources"—CO₂ hydrates are not new energy sources themselves but serve as a stable storage medium for Carbon Capture and Storage (CCS), which aligns with the core focus of our review. Your correction is critical to ensuring the academic accuracy and rigor of the manuscript, and we highly appreciate your meticulous review and professional guidance.
• We have revised the title to "A Review on the Visualization Analysis of Pore Scale Formation and Decomposition of CO₂ Hydrates for Carbon Capture and Storage", which not only corrects the inaccurate expression of "new energy sources" but also highlights the core application scenario of CCS. At the same time, we adjusted the relevant descriptions in the abstract and introduction, emphasizing that CO₂ hydrate is a stable storage medium for CCS, and added Engineering Implications for CCS and Future Research Directions in the conclusion to ensure consistency throughout the entire text.

 

Comment 3: The Introduction and Mechanism of Hydrate Formation and Decomposition sections frame hydrates in “energy sectors” and talk about “energy extraction” and “gas productivity,” which is appropriate for CH₄ hydrates, not for CO₂. Thus, these sections need to be aligned with CO₂ hydrates. Remove the contents related to CH₄ hydrates and keep the focus only on CO₂ hydrates.

Response:

• Thank you for your precise comment. We fully recognize that the original manuscript inappropriately mixed content related to methane hydrates with the core focus on carbon dioxide hydrates, leading to thematic ambiguity.
• Currently, the vast majority of hydrate-related studies focus on CH₄ hydrates, while dedicated research on CO₂ hydrates remains relatively scarce. And the kinetic mechanisms of hydrate formation and decomposition (e.g., nucleation kinetics, growth pathway, decomposition thermodynamics) are consistent between CH₄ and CO₂
• We have thoroughly revised the manuscript to center exclusively on CO₂ hydrates:

1. Delete all content unique to CH₄ hydrates;
2. Retain and refine the analysis of common hydrate kinetic characteristics (applicable to CO₂ hydrates);
3. Supplement CO₂-specific research content and application orientation (e.g., links to Carbon Capture and Storage (CCS)) to enhance the manuscript’s relevance and depth.

 

Comment 4: What is “Porous scale”? Pores vary widely in scale? Which scale is targeted here?

Response:

• Thank you for your meticulous and valuable comment.
• We fully acknowledge that the term "Porous scale" used in the original manuscript is inaccurate and ambiguous, failing to clearly define the targeted research scale. We have corrected this inappropriate expression and revised the title to "A Review on the Visualization Analysis of Pore Scale Formation and Decomposition of CO₂ Hydrates for Carbon Capture and Storage".
• "Porous scale" (referred to as "pore scale" in the revised manuscript) refers to the microscopic to mesoscopic scale focusing on the internal pores of porous media. Pores in natural and artificial porous media range from nanometers to centimeters. This is verified by the pore/throat width of microfluidic chips (50–700 μm, Table 2) and the matching of numerical simulation scales with experimental dimensions.

 

Comment 5: There are some repetitions and inaccuracies in using the English language. Please correct.

Response:

• Thank you for your detailed and reliable suggestions. We sincerely thank you for your professional reminder. This was indeed an inaccuracy in the wording of our manuscript, and your suggestion is crucial for enhancing the precision and academic rigor of this work.
• We tried our best to improve the manuscript and made some changes to the manuscript. These changes will not influence the content and framework of the paper. And here we did not list the changes but marked in red in the revised paper.

 

Comment 6: Overall, the manuscript needs reformulation and re-writing to sharpen the focus on the theme mentioned in the title.

Response:

• Thank you for your insightful comment. We fully agree that the manuscript’s focus on the theme specified in the title needs to be sharpened through targeted reformulation and rewriting.
• The specific details of the modification are as follows:

1. We have re-emphasized the central focus -formation/decomposition mechanisms of CO₂ hydrates process in microfluidic models- in the abstract, introduction, and conclusion.
2. A dedicated subsection on "Engineering Implications and Future Research Directions" has been added in Section 4. This section links pore-scale insights to practical CCS engineering optimization and proposes three targeted future research directions, strengthening the manuscript’s practical value and academic extension.
3. We have systematically proofread the main text, rectifying imprecise technical descriptions, inconsistent terminology usage, and logical flaws. All revisions ensure the accuracy of content while maintaining consistency with the core theme.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Accept.