Promoting Sustainable Coal Gas Development: Microscopic Seepage Mechanism of Natural Fractured Coal Based on 3D-CT Reconstruction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The present manuscript studies the microscopic seepage mechanism of primary fractured coal based on three-dimensional reconstruction by experimental analysis, and furtherly discussed the permeability evolution characteristics combining the pore-fracture distribution. Their results provide valuable insights for the analysis of fracture seepage channels and the advancement of sustainable coal gas development, which was inspiring to the researchers in the journal. The manuscript needs accept after minor revisions.

The detailed comments are as below:

1.       In section of 2.1, how to ensure the specimens always remain vertical during rotation in the working turntable devices (Figure 3) in the process of CT scanning.

2.       The primary figures and text format of the manuscript should be referred to the latest published paper and the journal’s author guidelines. Several details need to been taken care. For example, the text size in the Figure 4 and Figure 6 is too large; the sentence of “1# specimen” in page 10 should be written as REV-1. It is recommended to be modified.

3.       In the section of Discussion, there are many descriptions about the average porosity and absolute permeability of two types tests, but the correlation between them is not obvious. It is suggested to increase the comparative analysis of them.

4.       In the section of Discussion. which factors are relevant to the results of the fracture seepage characteristics, and the results can guide for the engineering practice. Please explain and supplement it.

5.       Some place lacks the reference on the rock dynamic disaster and rock engineering, it is suggested to add the latest relevant literatures.

6.The following reference format should be corrected: “Zhang, Y.; Wang, Y.; Cui, B.; Feng, G.; Zhang, S.; Zhang, C.; Zhang, Z. A Disturbed Voussoir Beam Structure Mechanical Model and Its Application in Feasibility Determination of Upward Mining. Energies 2023, 16, 7190. https://doi.org/10.3390/en16207190”

Comments on the Quality of English Language

GOOG

Author Response

Dear Editor and Reviewers:
    Thank you very much for reviewing and commenting on our manuscript, entitled “Promoting Sustainable Coal Gas Development: Microscopic Seepage Mechanism of Primary Fractured Coal Based on Three-Dimensional Reconstruction” (Manuscript ID: sustainability-2988080). Your comments were valuable and helped us revise and improve this paper.

We have carefully revised our manuscript and made the corresponding corrections to address your concerns. The further revised portions of our manuscript are highlighted in red. The main corrections to the paper and our responses to reviewers’ comments are as follow:

Responds to the reviewers’ comments:

Reviewer #1: The present manuscript studies the microscopic seepage mechanism of primary fractured coal based on three-dimensional reconstruction by experimental analysis, and furtherly discussed the permeability evolution characteristics combining the pore-fracture distribution. Their results provide valuable insights for the analysis of fracture seepage channels and the advancement of sustainable coal gas development, which was inspiring to the researchers in the journal. The manuscript needs accept after minor revisions.

1. 
   In section of 2.1, how to ensure the specimens always remain vertical during rotation in the working turntable devices (Figure 3) in the process of CT scanning.

Response to comment 1:

Thank you very much for your professional comments. In order to ensure the specimens always remain vertical during rotation in the working turntable devices. Firstly, we fixed the base and the specimens using mastic and tape, then positioned the specimen on the work table to ensure it was perpendicular to the horizontal. Secondly, a low rotation speed was maintained throughout the scanning process to ensure stability of the test piece.

2. 
   The primary figures and text format of the manuscript should be referred to the latest published paper and the journal’s author guidelines. Several details need to been taken care. For example, the text size in the Figure 4 and Figure 6 is too large; the sentence of “1# specimen” in page 10 should be written as REV-1. It is recommended to be modified.

Response to comment 2:

We appreciated for your precious suggestion. As you suggested, the text size in Figure 4 and Figure 6 has been reduced, and the Figures and symbols in the revised manuscript were modified and carefully checked to ensure that they can meet the requirements of the journal. Thank you!

3. 
   In the section of Discussion, there are many descriptions about the average porosity and absolute permeability of two types tests, but the correlation between them is not obvious. It is suggested to increase the comparative analysis of them.

Response to comment 3:

We are very grateful to your professional suggestion. As you suggested, we have supplemented some content to analyze the correlation between the average porosity and absolute permeability. The detailed contents are as follows:

By analyzing the test results, it can be seen that there was an obvious positive correlation between the average porosity and absolute permeability of the specimen. The storage space was mainly composed of connected cracks and pores, and cracks are the main seepage channels. As the number of different layers increases, there are an increasing number of connected cracks and seepage streamlines inside the sample, and the degree of internal connectivity is immensely increased, thus forming a satisfactory seepage channel. It is apparent that the sample is mainly along the crack surface, and that it forms the migration fracture channels. Based on the average porosity distribution of each unit body, and comparing it with the calculated absolute permeability, the average porosity and ab-solute permeability change curve is obtained as illustrated in Figure 9. The permeability and porosity change curves of fractured coal samples at different positions in Figure 9(a) indicate that the changes in porosity and permeability are generally consistent. As the interception locations continue to increase, the porosity and absolute permeability of the sample increase. The rate generally exhibits a trend of first increasing and subsequently decreasing, which is consistent with the change rules affecting the aforementioned distribution of pore cracks and seepage streamlines. Since the initial state of the coal sample contains cracks in the middle, the permeability exhibits an increasing trend as the cracks expand during the seepage process. In addition, the change curve of absolute permeability with porosity in Figure 9(b) reveals that the absolute permeability exhibits a gradually in-creasing trend as the average porosity increases.

4. 
   In the section of Discussion. which factors are relevant to the results of the fracture seepage characteristics, and the results can guide for the engineering practice. Please explain and supplement it.

Response to comment 4:

Thank you for your professional comment, as we know, it is common knowledge that the influence of pore-cracks distribution on fracture seepage characteristics. Following your suggestion, we have added the content in the revised manuscript. In the section of Discussion, we have supplemented the description on the the results can guide for the engineering practice. The detailed contents are as follows:

Porosity and permeability are important parameters that reflect the characteristics of coal seam reservoirs. During coal seam excavation, coal seam gas is discharged due to stress release and further development of pore fissures under the action of unloading. The research results of this article can not only effectively extract coal seam gas, reduce coal mine gas dynamic disasters, and ensure safe and efficient production of mines, but also achieve multiple effects such as clean energy supply and atmospheric environmental protection, accelerating the promotion of "carbon peaking and carbon neutrality" realized.

5. 
   Some place lacks the reference on the rock dynamic disaster and rock engineering, it is suggested to add the latest relevant literatures.

Response to comment 5:

Thank you for your comment. Following your suggestion, we have supplemented the reference about rock dynamic disaster and rock engineering in the revised manuscript ( in the section of Introduction and Discussion ). The detailed are as follows:

[1] Si X, Luo Y, Luo S. Influence of lithology and bedding orientation on failure behavior of “D” shaped tunnel [J]. Theoretical and Applied Fracture Mechanics, 2024, 129: 104219.  https://doi.org/10.1016/j.tafmec.2023.104219.

[2] Ma Q, Liu XL, Tan YL, et al. Numerical study of mechanical properties and microcrack evolution of double-layer composite rock specimens with fissures under uniaxial compression [J]. Engineering Fracture Mechanics, 2023, 289(2): 109403. 10.1016/j.engfracmech.2023.109403.

[3] Wang X, Li J, Zhao X. et al. Propagation characteristics and prediction of blast-induced vibration on closely spaced rock tunnels[J]. Tunnelling and Underground Space Technology, 2022, 123: 104416.

[4] Teng T, Xue Y, Zhang C. Modeling and simulation on heat-injection enhanced coal seam gas recovery with experimentally validated non-Darcy gas flow [J]. Journal of Petroleum Science and Engineering, 2019, 177, 734-744.

[34] Wang X, Zhang X, Li W. et al. Analytical theoretical study on ultra-low friction characteristics of coal rock interfaces under tress wave action [J]. Journal of China coal society, 2024. https://doi.org/10.13225/j.cnki.jccs.2023.1197.

[35] ] Du Z, Sheng S, Guo J, Effect of composite activators on mechanical properties, hydration activity and microstructure of red mud-based geopolymer [J]. Journal of Materials Research and Technology, 2023, 24, 8077-8085.

[36] Liu J, He X, Huang H. et al. Predicting gas flow rate in fractured shale reservoirs using discrete fracture model and Ga-Bp neural network method [J]. Engineering Analysis With Boundary Elements. 2024, 159: 315-30. doi: 10.1016/j.enganabound.2023.12.011.

[37] Zhang Y, Cui B, Wang Y. et al. Evolution law of shallow water in multi-face mining based on partition characteristics of catastrophe theory [J]. Fractal Fract. 2023, 7: 779. https://doi.org/10.3390/fractalfract7110779

[38] Zhao C, Liu J, Dai H. et al. Frictional evolution process and stability properties of Longmaxi shale under fluid injection [J]. Energy, 2024, 294: 130910.

[39] Tu W, Li L, Zhou Z. et al. Thickness calculation of accumulative damaged zone by rock mass blasting based on Hoek-Brown failure criterion [J]. International Journal of Geomechanics, 2022, 22(2): 04021273. DOI: 10.1061/(ASCE)GM.1943-5622.0002257.

6. 
   .The following reference format should be corrected: “Zhang, Y.; Wang, Y.; Cui, B.; Feng, G.; Zhang, S.; Zhang, C.; Zhang, Z. A Disturbed Voussoir Beam Structure Mechanical Model and Its Application in Feasibility Determination of Upward Mining. Energies 2023, 16, 7190. https://doi.org/10.3390/en16207190

Response to comment 6:

Thank you for your comment. We have read the journal's guidelines carefully and revised the full reference in strict accordance with the journal's reference format requirements. The corrected reference as follows:

[8] Zhang Y, Wang Y, Cui B, et al. A disturbed voussoir beam structure mechanical model and its application in feasibility determination of upward ,ining [J]. Energies, 2023, 16: 7190. https://doi.org/10.3390/en16207190.

Thank you again for your careful work.

Best regards,

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript investigates the Promoting sustainable coal gas development: microscopic seepage mechanism of primary fractured coal based on three-dimensional reconstruction. On the whole, it is meaningful research, but there are some problems need to be made clear and further improved, as:

1. The section of Title and Abstract does not completely refine the essence of the manuscript, a further condensed elaboration of research content is needed to illustrate, which is not concise in the current form. It is suggested to further refining and improving. For example, the “Three-Dimensional Reconstruction” in the title can be rewritten to “Digital Core” or “3D-CT Reconstruction” to ensure more concise and professional.

2. In the second and third paragraphs of Introduction section, there are two places where the sentences too long and meaning is not clearly expressed, “Zhao et al. [9] established a three-dimensional digital sandstone core based on CT scanning images and performed a detailed visualization characterization of the grain size distribution and the rock sample's pore structure and noted that there are significant differences in the pore connectivity of different physical reservoirs” and “ Wang et al.’s [21] study, which was based on the three dimensional reconstruction computer tomography imaging technology, established a structural model of different pore sizes and connectivity; utilized the N-S equation to simulate the dynamic seepage process; and noted that the change in pore size not only affects the seepage characteristics in the pores, but also affects the stable seepage speed in the fractures effect of theoretical fracture trace morphology on the fissure aperture degree ” it is advice to check and modify these two sentences.

3. There are two terms should be rectified to be more professional. The “representative unite body” should be written as “representative elementary volume” the “CT images” should be written as “CT slices”.

4. In section 2.1, what is the type of the coal specimens? Different coals may have different mechanical properties. The type of the coal adopted in this study should be specified.

5. The layout should be modified with the guidelines of journal, and some details needs further improved. For example, in Figure 2, the denoted lowercase letters (a) and (b) should be centered below the figure parts; All numbers referring to “Figure X” in the text should be highlighted in blue.

Comments on the Quality of English Language

 Minor editing of English language required.

Author Response

Dear Editor and Reviewers:
Thank you very much for reviewing and commenting on our manuscript, entitled “Promoting Sustainable Coal Gas Development: Microscopic Seepage Mechanism of Primary Fractured Coal Based on Three-Dimensional Reconstruction” (Manuscript ID: sustainability-2988080 ). Your comments were valuable and helped us revise and improve this paper.

We have carefully revised our manuscript and made the corresponding corrections to address your concerns. The further revised portions of our manuscript are highlighted in red. The main corrections to the paper and our responses to reviewers’ comments are as follow:

Reviewer #2: The manuscript investigates the Promoting sustainable coal gas development: microscopic seepage mechanism of primary fractured coal based on three-dimensional reconstruction. On the whole, it is meaningful research, but there are some problems need to be made clear and further improved, as:

1. 
   The section of Title and Abstract does not completely refine the essence of the manuscript, a further condensed elaboration of research content is needed to illustrate, which is not concise in the current form. It is suggested to further refining and improving. For example, the “Three-Dimensional Reconstruction” in the title can be rewritten to “Digital Core” or “3D-CT Reconstruction” to ensure more concise and professional.

 Response to comment 1:

Thank you very much for your careful review, which is of great help for us to improve the paper quality. We have made a lot of efforts to improve the Title and Abstract. As you suggested, we have rewritten the title to “3D-CT Reconstruction” for more concise and professional. The refined abstract as follows:

Preventing coal and gas outburst dynamic disasters are essential for ensuring sustainable and safety mining. The numerous microscopic pores within the coal serve as the primary storage space for gas, making it essential to explore the structural distribution and seepage characteristics for reveal the disaster mechanism. Under the mining stress, gas within the micropores of the coal migrates outward through cracks, with these cracks exerting a significant control effect on gas migration. Therefore, the study focuses on utilizing natural fractured coal bodies as research ob-jects, employing a micro-CT imaging system to conduct scanning tests and digital core technology to reconstruct sample pore and fracture structures in three dimensions, and the pores, cracks, skeleton structure, and connectivity were characterized. A representative elementary volume containing macro cracks was selected to establish an equivalent model of the pore network, and a seepage simulation analysis was performed using the visualization software. Revealing the seepage characteristics of fractured coal mass from a microscopic perspective. The research results can guide gas drainage and early warning of dynamic disasters in deep coal mining.

2. 
   In the second and third paragraphs of Introduction section, there are two places where the sentences too long and meaning is not clearly expressed, “Zhao et al. [9] established a three-dimensional digital sandstone core based on CT scanning images and performed a detailed visualization characterization of the grain size distribution and the rock sample's pore structure and noted that there are significant differences in the pore connectivity of different physical reservoirs” and “ Wang et al.’s [21] study, which was based on the three dimensional reconstruction computer tomography imaging technology, established a structural model of different pore sizes and connectivity; utilized the N-S equation to simulate the dynamic seepage process; and noted that the change in pore size not only affects the seepage characteristics in the pores, but also affects the stable seepage speed in the fractures effect of theoretical fracture trace morphology on the fissure aperture degree ” it is advice to check and modify these two sentences.

Response to comment 2:

Thank you very much for your comment. We have modified this mistake. The modified content as follows:

Zhao et al. [16] performed a detailed visualization of the grain size distribution and pore structures, and noted that there are significant differences in the pore connectivity of different physical reservoirs

Wang et al. [28] established a structural model of different pore sizes and connectivity, and noted that the change in pore size not only affects the seepage characteristics in the pores, but also affects the stable seepage speed in the fractures

3. 
   There are two terms should be rectified to be more professional. The “representative unite body” should be written as “representative elementary volume” the “CT images” should be written as “CT slices”.

Response to comment 3:

We are very grateful to your professional suggestion. As you suggested, we have modified the two technical terms in the revised manuscript.

4. 
   In section 2.1, what is the type of the coal specimens? Different coals may have different mechanical properties. The type of the coal adopted in this study should be specified.

Response to comment 4:

Thank you very much for your professional suggestion, as we know, Different coals may have different mechanical and deformation properties. Following your suggestion, we have added the description on the coal type. In the section of 2.1, we have added some content to describe the source and type of the natural fractured coal. The detailed contents are as follows:

It is worth mentioning that the specimens utilized in the test were obtained from the 8119 working face and of No. 3-5 Coal seam in Tashan Coal Mine in Datong City, Shanxi Province. Which belongs to Carboniferous coal seam.

5. 
   The layout should be modified with the guidelines of journal, and some details needs further improved. For example, in Figure 2, the denoted lowercase letters (a) and (b) should be centered below the figure parts; All numbers referring to “Figure X” in the text should be highlighted in blue.

Response to comment 5:

Thank you for your comment. We have read the journal's guidelines carefully and revised the full Figures in strict accordance with the journal's format requirements.

 

Thank you again for your careful work.

Best regards,

 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The current study deals with the exploration of digital core technology and it’s use into assessment of the pore and fissure structure of fractured coal bodies in reservoirs, where factors such as pore size distribution, pore throat characteristics, and connectivity influence fluid migration. The current target with respect to  prior is that such studies have examined the pore and fracture characteristics of reservoirs but considering a different size of the sample, also most of those studies focused on intact coal and rock bodies of small sizes, and there was a shortage of analysis on primary fractured coal bodies with large fractures. This approach could lead to a future development in the pore and fissure structure considering seepage properties of macro-cracked coal masses and the influence into the gas flow dynamics, useful in the safety measure of gas exploration.

The paper uses clear and well-expressed technical language, presenting ideas in an organized and coherent manner. The narrative is structured effectively, offering a precise context for understanding the current state of the art and the relevance of this study prior to performing an intrusive cracking in the unaltered rock.

However,  the following aspects have been identified for more argumentation or potential improvement:

 

·       A thorough review might help eliminate repetitive phrases such as "representative elementary volume that can represent," which would improve clarity and avoid redundancies in the narrative (see Section 2.2, page 6). Additionally, there are a few instances of repeated words in the text that should be corrected.

While the study's relevance stems from the importance and uniqueness of the samples used, which provide fresh insights due to their unaltered state, all the test subjects are from the same specimen. This raises concerns about potential redundancies and the risk of insufficient data for conclusive results. Could previous studies in rock science and geology offer solutions to address these questions and provide more consistent arguments?

 

With a  clarification of the specific comments, the intended scope of the investigations is, in the concept of the reviewer, successfully achieved. Perhaps, in the discussion the observations regarding the multidisciplinary background could be helpful.

Comments on the Quality of English Language

Minor editing of the english language are required

Author Response

Dear Editor and Reviewers:
   Thank you very much for reviewing and commenting on our manuscript, entitled “Promoting Sustainable Coal Gas Development: Microscopic Seepage Mechanism of Primary Fractured Coal Based on Three-Dimensional Reconstruction” (Manuscript ID: sustainability-2988080 ). Your comments were valuable and helped us revise and improve this paper.

We have carefully revised our manuscript and made the corresponding corrections to address your concerns. The further revised portions of our manuscript are highlighted in red. The main corrections to the paper and our responses to reviewers’ comments are as follow:

Responds to the reviewers’ comments:

Reviewer #3:

The current study deals with the exploration of digital core technology and it’s use into assessment of the pore and fissure structure of fractured coal bodies in reservoirs, where factors such as pore size distribution, pore throat characteristics, and connectivity influence fluid migration. The current target with respect to prior is that such studies have examined the pore and fracture characteristics of reservoirs but considering a different size of the sample, also most of those studies focused on intact coal and rock bodies of small sizes, and there was a shortage of analysis on primary fractured coal bodies with large fractures. This approach could lead to a future development in the pore and fissure structure considering seepage properties of macro-cracked coal masses and the influence into the gas flow dynamics, useful in the safety measure of gas exploration.

The paper uses clear and well-expressed technical language, presenting ideas in an organized and coherent manner. The narrative is structured effectively, offering a precise context for understanding the current state of the art and the relevance of this study prior to performing an intrusive cracking in the unaltered rock.

However, the following aspects have been identified for more argumentation or potential improvement:

1、A thorough review might help eliminate repetitive phrases such as "representative elementary volume that can represent," which would improve clarity and avoid redundancies in the narrative (see Section 2.2, page 6). Additionally, there are a few instances of repeated words in the text that should be corrected.

Response to comment 1:

After we checked the manuscript carefully, we think your suggestion is very valuable and this part indeed requires further refinement. As you suggested, the repetitive phrases have been modified. Thanks.

 

2、While the study's relevance stems from the importance and uniqueness of the samples used, which provide fresh insights due to their unaltered state, all the test subjects are from the same specimen. This raises concerns about potential redundancies and the risk of insufficient data for conclusive results. Could previous studies in rock science and geology offer solutions to address these questions and provide more consistent arguments?

Response to comment 2:

Thank you for your professional comment, as we all know, there are many studies on the microporous structure and seepage characteristics of fractured coal / rock. In terms of research methods, which was consistent with our manuscript. For the microscopic seepage mechanism studied through digital cores, the method of selecting several unit bodies at different positions of the same specimen is generally used for research. This study selected 6 REV containing cracks at different locations of the same specimen to conduct research, which was common in the field of rock geomechanics or oil and gas production. Thanks again.

 

3、With a clarification of the specific comments, the intended scope of the investigations is, in the concept of the reviewer, successfully achieved. Perhaps, in the discussion the observations regarding the multidisciplinary background could be helpful.

 

Response to comment 3:

Thank you for your comment. Following your suggestion, we have added the content in the revised manuscript. In the section of Discussion, we have supplemented the description on the multidisciplinary background. The detailed contents are as follows:

Deep coal seams contain abundant coalbed methane resources, as shallow resources are depleted, coal mining has shifted to deeper levels [34-36]. However, the low porosity and permeability of deep coal rock make it susceptible to coal and gas outburst accidents. As an associated product in the coal-forming process, gas was the main component in coal seams and coal-measure strata [37-39]. Its properties are different from traditional sandstone and shale petroleum reservoirs. It exists in pores, joints and fissures in the form of free, adsorption and solid solution state respectively under different formation pressure. The pore and fracture system not only affects the macroscopic physical and mechanical parameters of the coal body (density and strength), but also influence the physical proper-ties of the coal body (porosity and permeability). Therefore, it is necessary to conduct an in-depth discussion on the pore structure and seepage characteristics of natural fractured coal bodies.

 Porosity and permeability are important parameters that reflect the characteristics of coal seam reservoirs. During coal seam excavation, coal seam gas is discharged due to stress release and further development of pore fissures under the action of unloading. The research results of this article can not only effectively extract coal seam gas, reduce coal mine gas dynamic disasters, and ensure safe and efficient production of mines, but also achieve multiple effects such as clean energy supply and atmospheric environmental protection, accelerating the promotion of "carbon peaking and carbon neutrality" realized.

 

Thank you again for your careful work.

Best regards,

 

Author Response File: Author Response.docx