Experimental and Numerical Methods for Hydraulic Fracturing at Laboratory Scale: A Review
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript provides a comprehensive review of experimental and numerical methods used in hydraulic fracturing at the laboratory scale. It covers a wide range of topics, including design considerations, scaling factors, and various numerical simulation techniques. However, there are several areas where the manuscript could be improved to enhance its clarity, depth, and overall quality. With these revisions, the manuscript has the potential to make a significant contribution to the literature on hydraulic fracturing.
- The review is largely descriptive and lacks critical analysis of the methods and findings discussed. While the authors have done a good job of summarizing existing literature, they do not provide sufficient insight into the limitations, strengths, and weaknesses of the different experimental and numerical approaches. The authors should include a more critical evaluation of the methods reviewed, discussing the trade-offs between different approaches and identifying gaps in the current research. This would help readers understand the relative merits of each method and guide future research directions.
- The section on scaling factors is brief and does not delve deeply into the challenges and limitations of scaling laboratory results to field conditions. The authors mention the importance of scaling but do not provide a detailed discussion on how scaling factors are determined or validated. The authors should expand this section to include a more detailed discussion on the methodologies used to determine scaling factors, the challenges associated with scaling, and examples of successful or unsuccessful scaling attempts in the literature.
- While the manuscript covers a broad range of topics, it lacks discussion on some of the more recent advances in hydraulic fracturing research, particularly in the areas of machine learning and advanced numerical simulations.
- Some sections, such as the discussion on numerical simulation methods (Section 5), are quite detailed, while others, such as the section on design considerations (Section 2), are relatively superficial. This inconsistency in depth can make the review feel unbalanced. The design considerations section could be expanded to include more discussion on the impact of sample preparation, stress regimes, and fluid properties on experimental outcomes.
- The manuscript would benefit from more figures and diagrams to help illustrate key concepts, particularly in the sections discussing numerical methods and experimental setups. Visual aids can greatly enhance the reader's understanding of complex processes.
- The manuscript focuses primarily on laboratory-scale experiments and numerical simulations but does not sufficiently discuss how these methods translate to practical applications in the field.
- In addition, the authors should expand the conclusion to include a discussion of the limitations of current methods, and specific recommendations for future research directions.
- Some references are outdated, and more recent studies should be included to ensure the review is up-to-date.
Author Response
Response to Reviewer-1
Comment-1: The manuscript provides a comprehensive review of experimental and numerical methods used in hydraulic fracturing at the laboratory scale. It covers a wide range of topics, including design considerations, scaling factors, and various numerical simulation techniques. However, there are several areas where the manuscript could be improved to enhance its clarity, depth, and overall quality. With these revisions, the manuscript has the potential to make a significant contribution to the literature on hydraulic fracturing.
The review is largely descriptive and lacks critical analysis of the methods and findings discussed. While the authors have done a good job of summarizing existing literature, they do not provide sufficient insight into the limitations, strengths, and weaknesses of the different experimental and numerical approaches. The authors should include a more critical evaluation of the methods reviewed, discussing the trade-offs between different approaches and identifying gaps in the current research. This would help readers understand the relative merits of each method and guide future research directions.
Response: We sincerely appreciate the reviewer’s positive feedback on the comprehensiveness of our review and the relevance of our manuscript. We acknowledge the need for further improvements in clarity and depth and will carefully address these areas in our revisions to enhance the overall quality of the manuscript. Thank you for your valuable insights. A dedicated discussion has been included to explain the limitations, strengths, and weaknesses of the different experimental and numerical approaches on page 36 (lines 1093-1147).
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Comment-2: The section on scaling factors is brief and does not delve deeply into the challenges and limitations of scaling laboratory results to field conditions. The authors mention the importance of scaling but do not provide a detailed discussion on how scaling factors are determined or validated. The authors should expand this section to include a more detailed discussion on the methodologies used to determine scaling factors, the challenges associated with scaling, and examples of successful or unsuccessful scaling attempts in the literature.
Response: Thank you for your comment. Additional information related to scaling factor development, challenges, and comparisons with field data has been added in Section 3, Scaling Factors in Hydraulic Fracturing Experimentation. See page 8 (lines 294-368).
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Comment-3: While the manuscript covers a broad range of topics, it lacks discussion on some of the more recent advances in hydraulic fracturing research, particularly in the areas of machine learning and advanced numerical simulations.
Response: The authors appreciate this observation. A new section demonstrating the application of machine learning and its integration with numerical simulations has been added to the manuscript on page 10 (lines 858-888).
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Comment-4: Some sections, such as the discussion on numerical simulation methods (Section 5), are quite detailed, while others, such as the section on design considerations (Section 2), are relatively superficial. This inconsistency in depth can make the review feel unbalanced. The design considerations section could be expanded to include more discussion on the impact of sample preparation, stress regimes, and fluid properties on experimental outcomes.
Response: The authors agree with the reviewer’s observation. More details have been added to the Design Considerations section to enhance discussions on sample specifications (Page 3; lines 108-112, 102-124, 139-145), stress regimes (Page 6; lines 188-199), saturation conditions (Page 7; lines 221-228), and fracturing fluids (Page 7; lines 243-287).
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Comment-5: The manuscript would benefit from more figures and diagrams to help illustrate key concepts, particularly in the sections discussing numerical methods and experimental setups. Visual aids can greatly enhance the reader's understanding of complex processes.
Response: Thank you for your valuable feedback. Additional figures have been included in the sections on experimental setups and numerical methods. See Figures 6, 7, 8, 15, and 17.
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Comment-6: The manuscript focuses primarily on laboratory-scale experiments and numerical simulations but does not sufficiently discuss how these methods translate to practical applications in the field.
Response: Thank you for your comment. The discussion has been expanded to address the extension of laboratory and numerical methods to practical applications in the field. See page 35 (lines 1093-1113, 1127-1147).
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Comment-7: In addition, the authors should expand the conclusion to include a discussion of the limitations of current methods, and specific recommendations for future research directions.
Response: Thank you for your comment. The summary has been revised to include a discussion of the limitations of current methods and specific recommendations for future research directions. See page 37 (lines 1204-1215, 1227-1236).
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Comment-8: Some references are outdated, and more recent studies should be included to ensure the review is up-to-date.
Response: Thank you for your observation. Recent references have been added to the draft, and some outdated references have been removed.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe research experiment discussed provides valuable insights into hydraulic fracturing experimentation and modelling, but such studies have various limitations which must be acknowledged:
- This experimental work lacks detailed methodologies or scaling factors to effectively bridge the gap between laboratory and field-scale applications, limiting generalizability.
- What about the variability of geological conditions across different reservoirs, which could lead to inaccuracies in field scenarios. Clarification from authors is needed.
- Limited discussion on the uncertainties and limitations of numerical simulations, such as model calibration and geological data integration, affects predictive reliability. Clarification from authors is needed.
- In manuscript influence of fracturing fluid properties is mentioned but not thoroughly explored in terms of fluid types, viscosities, and additives across geological settings. Authors should address this issue.
- There is little emphasis on validating numerical models with real-world field data, reducing confidence in their predictive capabilities. Clarification from authors is needed.
- Figures 5 and 6 depict schematic diagrams of the true triaxial testing apparatus used in hydraulic fracturing experiments. The authors should briefly explain the significance of these figures, emphasizing their relevance to the study. It's also important to highlight the differences between the two figures, as they may represent distinct experimental configurations or stages. This clarification will help future researchers understand the methodology and replicate the experiments accurately.
- In Figure 17, b and c illustrate the same effect of confining pressure on both permeability and hydraulic conductivity. However, this is somewhat unclear, as permeability is a material property, whereas hydraulic conductivity depends on both the material and the fluid's properties. A clearer distinction should be made between the two, as the influence of confining pressure on these properties may differ due to the additional dependence of hydraulic conductivity on fluid characteristics.
Author Response
Response to Reviewer-2
Comment-1: The research experiment discussed provides valuable insights into hydraulic fracturing experimentation and modeling, but such studies have various limitations that must be acknowledged: This experimental work lacks detailed methodologies or scaling factors to effectively bridge the gap between laboratory and field-scale applications, limiting generalizability.
Response: We sincerely appreciate the reviewer’s acknowledgment of the insights provided by our research. More information related to scaling factor development, challenges, and comparison with field data has been added in Section 3: Scaling Factors in Hydraulic Fracturing Experimentation. See page 8 (lines 294-368).
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Comment-2: What about the variability of geological conditions across different reservoirs, which could lead to inaccuracies in field scenarios? Clarification from the authors is needed.
Response: Thank you for your comment. We acknowledge the variability of geological conditions across different reservoirs and its impact on the accuracy of field-scale scenarios. In our manuscript, we discuss in-situ stress conditions, geological heterogeneity, temperature, and pressure variations to highlight the limitations of laboratory testing. Specifically, in the section on geological conditions, we address how reservoir heterogeneity varies from one field to another and how limited consideration of these properties may lead to inaccuracies in field-scale conclusions. Please refer to page [lines 966–1016] for further details.
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Comment-3: There is limited discussion on the uncertainties and limitations of numerical simulations, such as model calibration and geological data integration, which affects predictive reliability. Clarification from the authors is needed.
Response: Thank you for your comment. The discussion on the uncertainties and limitations of numerical simulation, including aspects such as model calibration and geological data integration, has been expanded in the discussion section. See page 36 (lines 114-147).
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Comment-4: The manuscript mentions the influence of fracturing fluid properties but does not thoroughly explore fluid types, viscosities, and additives across geological settings. The authors should address this issue.
Response: Thank you for your comment. The fracturing fluid section has been improved to include the effects of rheological properties and the nature of the fracturing fluid on hydraulic fracturing. See page 7 (lines 243-287).
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Comment-5: There is little emphasis on validating numerical models with real-world field data, reducing confidence in their predictive capabilities. Clarification from the authors is needed.
Response: Thank you for your comment. To emphasize the importance of validating numerical models with real-world field data, a discussion has been added on this topic. See page 36 (lines 1128-1147).
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Comment-6: Figures 5 and 6 depict schematic diagrams of the true triaxial testing apparatus used in hydraulic fracturing experiments. The authors should briefly explain the significance of these figures, emphasizing their relevance to the study. It's also important to highlight the differences between the two figures, as they may represent distinct experimental configurations or stages. This clarification will help future researchers understand the methodology and replicate the experiments accurately.
Response: The importance of these figures and their relevance to the objectives of this work have been added to the manuscript. Additionally, the differences between the two figures have been clarified. See page 16 (lines 545-557), page 17 (lines 578-581, 584-586).
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Comment-7: In Figure 17, parts (b) and (c) illustrate the same effect of confining pressure on both permeability and hydraulic conductivity. However, this is somewhat unclear, as permeability is a material property, whereas hydraulic conductivity depends on both the material and the fluid's properties. A clearer distinction should be made between the two, as the influence of confining pressure on these properties may differ due to the additional dependence of hydraulic conductivity on fluid characteristics.
Response: Thank you for your comment. The distinction between permeability and hydraulic conductivity, along with an explanation of the reasons behind their variation with confining pressure, has been added to the manuscript. See page 34 (lines 1061-1064, 1067-1070).
Reviewer 3 Report
Comments and Suggestions for AuthorsThe paper "Experimental and Numerical Methods for Hydraulic Fracturing at Laboratory Scale: A Review" provides a detailed review of experimental and numerical methods for hydraulic fracturing. The authors present on the combination of experimentation and numerical modelling to better understand the mechanics of hydraulic fracturing. Hydraulic fracturing is widely used in increasing oil and natural gas production capacity in oil and gas reservoirs, mostly in unconventional formations such as shale. The research on the hydraulic fracturing experimental and numerical methods arose from the necessity to reduce field testing costs and to improve process understanding under controlled conditions. The paper is clearly structured, comprising the introduction, an overview of experimental methods, numerical approaches, discussion, and conclusions. Tables and diagrams assist to elaborate on a variety of experimental setups and numerical simulations. The writing is in academically precise language and heavily referenced from previous works. Descriptions of methods are complete enough that the work can be easily followed, aside from the occasional technicality that makes understanding difficult for one outside this field. The principal merits of the text remain in the wide-ranging treatment of methods and their drawbacks; its chief failings are the lack of parameters obtained from practical field data and the scant appreciation of the environmental aspects. The environmental impact of hydraulic fracturing and the costs involved in experimental and numerical methods need to be addressed in order for the paper to present a holistic view. It would also be illustrative to include examples from the oil and gas industry as the methods proposed in the paper will show how they get implemented in the field. In spite of the ample discussion on numerical methods, the paper could provide an even deeper investigation into their limitations and possible improvement.
Author Response
Response to Reviewer-3
Comment-1: Comment-1 : The paper "Experimental and Numerical Methods for Hydraulic Fracturing at Laboratory Scale: A Review" provides a detailed review of experimental and numerical methods for hydraulic fracturing. The authors present on the combination of experimentation and numerical modelling to better understand the mechanics of hydraulic fracturing. Hydraulic fracturing is widely used in increasing oil and natural gas production capacity in oil and gas reservoirs, mostly in unconventional formations such as shale. The research on the hydraulic fracturing experimental and numerical methods arose from the necessity to reduce field testing costs and to improve process understanding under controlled conditions. The paper is clearly structured, comprising the introduction, an overview of experimental methods, numerical approaches, discussion, and conclusions. Tables and diagrams assist to elaborate on a variety of experimental setups and numerical simulations. The writing is in academically precise language and heavily referenced from previous works.
Descriptions of methods are complete enough that the work can be easily followed, aside from the occasional technicality that makes understanding difficult for one outside this field.
Response: We sincerely appreciate the reviewer’s positive feedback and meticulous review. The introduction to hydraulic fracturing is provided at the beginning of the paper, followed by a discussion of its necessity. The importance of experimentation and numerical simulations is outlined, and the limitations of both methods are discussed in more detail in the discussion section to ensure accurate interpretations and provide guidelines for future research. Furthermore, the logical sequence of sections ensures that a non-technical reader can follow the article without difficulty.
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Comment-2: The principal merits of the text remain in the wide-ranging treatment of methods and their drawbacks; its chief failings are the lack of parameters obtained from practical field data and the scant appreciation of the environmental aspects.
Response: Thank you for your comment. The importance of field data and its role in experimental and numerical methods for hydraulic fracturing have been discussed in more detail. See page 35 (lines 1093–1047). Additionally, environmental aspects of hydraulic fracturing have been addressed at the beginning of the discussion section. See page 28 (lines 893–900).
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Comment-3: The environmental impact of hydraulic fracturing and the costs involved in experimental and numerical methods need to be addressed in order for the paper to present a holistic view.
Response: Thank you for your valuable suggestion. The major environmental concerns related to hydraulic fracturing have been added in the discussion section. See page 28 (lines 893–901). Additionally, a discussion on the costs associated with experimental and numerical methods in hydraulic fracturing has been incorporated into the draft. See page 36 (lines 1148–1160).
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Comment-4: It would also be illustrative to include examples from the oil and gas industry as the methods proposed in the paper will show how they get implemented in the field.
Response: We appreciate the reviewer’s suggestion to include real-world examples from the oil and gas industry. However, due to the limited availability of openly published field data, we were unable to incorporate specific case studies. Nonetheless, we have included an example of swarm fracture scaling in Section 3, demonstrating how this approach successfully predicts hydraulic fracturing properties that align with observed field data. This serves as a representative case to illustrate the practical applicability of our methods.
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Comment-5: In spite of the ample discussion on numerical methods, the paper could provide an even deeper investigation into their limitations and possible improvement.
Response: Thank you for your comment. The limitations of numerical methods, along with possible improvements, have been expanded upon in the discussion section. See page 35 (lines 1114–1147).
