Research Progress and Technical Challenges of Geothermal Energy Development from Hot Dry Rock: A Review

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the attachment. 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Language could be refined to improve clarity and smoothness. Please revise any awkward expressions to ensure the text reads naturally and fluently.

Author Response

Response to Reviewer #1

Thanks for your constructive suggestions and positive comments. We have now revised the manuscript following your comments carefully. Below are point-by-point responses to your comments. The corresponding modifications and corrections were made and highlighted in red in the marked manuscript.

 

Comment 1: Line 93: Definition of Technologies. The paragraph references “base technologies, key technologies, and game-changing technologies” but does not provide clear definitions. please clarify what each category entails and how they relate to the rest of the paper^’s content.

Response: Thank you for pointing this out. As suggested by the reviewer, we have clarified these technologies in the revised manuscript (in Page 3 Lines 97-111).

 

Comment 2: Line 100: Roadmap for the Review. Consider including a brief roadmap in the final paragraph of the Introduction. This roadmap should outline the paper^’s structure (e.g., “section 2 addresses …, section 3 focuses on …, etc.”) so that readers can easily follow the progression of ideas.

Response: We agree with the reviewer’s assessment. As suggested by the reviewer, we have added a brief roadmap in the final paragraph of the Introduction (in Page 3 Lines 114-118).

 

Comment 3: Line 176: Missing citation. There is a statement here requiring a citation. Kindly provide an appropriate reference to support the claim or data presented in this line.

Response: Thank you for pointing this out. As suggested by the reviewer, we have added the appropriate reference in the revised manuscript in Page 5 Line 193. In fact, this latest progress news primarily originates from the FORGE project website (https://utahforge.com/).

 

Comment 4: Lines 176-180: Rapid Topic shift. The paragraph shifts topics abruptly without a transitional sentence. please add a short transitional phrase or sentence to guide the reader smoothly from one topic to the next.

Response: Thank you for pointing this out. As suggested by the reviewer, we have rewritten this part in the revised manuscript (in Page 5 Lines 193-203).

 

Comment 5: Lines 232-235: Missing citation. The content in these lines appears to rely on external data or references. Please provide citations to substantiate the information presented.

Response: Thank you for pointing this out. In fact, this content was derived from internal academic exchanges within the China Geological Survey and does not have a strict literature source. Based on the reviewers' suggestions, we have removed this part to ensure the article is more rigorous.

 

Comment 6: Line 396: Global summary Needed. Before concluding the section, it would be helpful to include a concise summary of the current global status of HDR development and to briefly discuss other regions beyond China and the United States. This would give a broader perspective to the reader.

Response: We agree with the reviewer’s assessment. As suggested by the reviewer, we have added a concise summary of the current global status of HDR development and to briefly discuss other regions beyond China and the United States (in Page 11 Lines 412-421).

 

Comment 7: Line 399: Colloquial phrasing. The phrase “scientific and technological battle” may sound informal. Consider rephrasing it in more academic or neutral language, such as “focused scientific and technological efforts.”

Response: Thank you for pointing this out. According to your suggestion, we have rewritten this sentence in Page 11 Lines 423-424.

 

Comment 8: Lines 397-408: Clear problem statement. While these lines introduce the importance of technological breakthroughs, they do not mention the main barriers or risks that hinder HDR commercialization. Please include a concise problem statement to help readers understand the principal challenges.

Response: Thank you for pointing this out. According to your suggestion, we have added a concise problem statement to help readers understand the principal challenges in the revised manuscript (in Page 11 Lines 427-433).

 

Comment 9: Line 404: Overly General statement. The statement “breakthroughs in these technologies contribute to promoting the commercial development of HDR geothermal resources in the world” is quite broad. consider specifying which breakthroughs (e.g., improved drilling efficiency, cost reductions, seismic mitigation) are most critical for making HDR viable.

Response: Thank you for pointing this out. According to your suggestion, we have rewritten this part in the revised manuscript (in Page 11 Lines 427-433).

 

Comment 10: Lines 463-465: Underdeveloped Techniques. The claim that china’s HDR optimization techniques remain underdeveloped requires comparative data. If possible, include information on how these methods compare quantitatively to U.S. or European techniques (e.g., drilling rates, success rates, or cost benchmarks).

Response: Thank you for pointing this out. According to your suggestion, we have added information on how these methods compare quantitatively to U.S. techniques in the revised manuscript (in Page 14 Lines 492-519).

 

Comment 11: Lines 494-495: Quantitative Insights. Statements referencing “long-life drill bits” or “high-temperature resistant tools” would benefit from approximate temperature thresholds, rate of penetration (ROp) improvements, or even cost comparisons. Adding these data points would make the discussion more concrete.

Response: We agree with the reviewer’s assessment. As suggested by the reviewer, we have added these data points in the revised manuscript (in Page 15 Lines 549-556).

 

Comment 12: Lines 578-624: Micro-seismic control. The section thoroughly addresses mitigation measures (traffic light systems, monitoring protocols), but examples or real-time data from projects beyond the Gonghe Basin would strengthen the argument. Providing additional case studies would illustrate how these strategies effectively reduce seismic risks.

Response: Thank you for pointing this out. According to your suggestion, we have added the additional case studies would illustrate how these strategies effectively reduce seismic risks (in Page 17 Lines 661-665).

 

Comment 13: Line 791: Concluding summary. The technical level assessment of HDR in China vs. the U.S. is informative, but the paper would benefit from a short concluding summary. Summarize the key challenges identified throughout the discussion and highlight the most pressing research priorities (e.g., drilling efficiency, induced seismicity, cost competitiveness).

Response: Thank you for pointing this out. As suggested by the reviewer, we have summarized the key challenges identified throughout the discussion and highlight the most pressing research priorities (in Page 21 Lines 855-875).

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Title of the article: Research Progress and Technical Challenges of Geothermal Energy Development from Hot Dry Rock: A Review

This paper summarizes the base technologies, key technologies, and game-changing technologies to promote the commercialization of HDR geothermal resources.

The key technologies include high-resolution exploration and characterization of HDR, efficient and complex fracture network reservoir creation, effective micro-seismic control, fracture network connectivity and reservoir characterization.

 In addition, the authors conducted a comparative analysis of China's and the United States' respective advantages in some key technologies of HDR development. This will facilitate collaboration between China and the United States to jointly promote the commercial development of HDR geothermal resources.

COMMENTS

1. A summary of findings should be included in the abstract. This will make the paper's abstract concise and informative while conveying the content of the paper.
2. The research gaps have been given at the end of the introduction. In addition to this, the authors should have stated clearly the objectives.
3. Refer to 5 of 26: As part of the U.S. Department of Energy’s Energy Earthshots™, it 
   is a department-wide effort to dramatically reduce the cost of EGS by 90%, bringing it 
   down to $45 per megawatt hour by 2035. Achieving this goal will contribute significantly 
   to reach the targets of 100% carbon-pollution-free electricity by 2035 and net-zero emissions across the U.S. economy by 2050. This is accomplished through accelerating research and development in multiple aspects such as driving down costs of drilling, well casing and related materials, advancing engineering techniques for more efficient well-drilling, collecting better-quality data for subsurface understanding, and ensuring proper containment of new reservoirs and geothermal fluids.

 

Specific Comment:

This study is very important and will have a very high contribution towards global energy needs and sustainability.

Water is injected into fissures in the hot, dry rock through the injector and then produced through the producer. The power plant at the surface uses the produced heat energy to drive turbines in a generator. After the hot fluid transfers its heat to the power plant, the cooler fluid can be injected again into the hot, dry rock.

It is expected that geothermal extraction will be significant in its magnitude all over the world if extraction technology of the geothermal heat from the dry rocks becomes commercially viable. The main challenges of the extraction technology from the hot dry rocks are control of water loss, drilling costs, and better mapping techniques and fracture stimulation revealing heat corruption for hot dry rock resources.

Comments

1. Authors should have included a summary of Figure 1. Schematic of the enhanced geothermal system, (a) hydraulic fracturing and (b) heat production should have been given.

2. The risks and challenges are related to exploration, drilling and managing the resource. Is the world prepared to undertake these risks and challenges?

3. What are the capital expenses of an HDR?

4. The glossary should have been included for readers.

5.  Ecological and Environmental Considerations should have been given a sub-section. The world is facing Global warming.

Comments on the Quality of English Language

The English language needs to be attended to.

Author Response

Response to Reviewer #2

This paper summarizes the base technologies, key technologies, and game-changing technologies to promote the commercialization of HDR geothermal resources. The key technologies include high-resolution exploration and characterization of HDR, efficient and complex fracture network reservoir creation, effective micro-seismic control, fracture network connectivity and reservoir characterization. In addition, the authors conducted a comparative analysis of China's and the United States' respective advantages in some key technologies of HDR development. This will facilitate collaboration between China and the United States to jointly promote the commercial development of HDR geothermal resources.

Response: Thanks for your constructive suggestions and positive comments. We have now revised the manuscript following your comments carefully. Below are point-by-point responses to your comments. The corresponding modifications and corrections were made and highlighted in red in the marked manuscript.

 

Comment 1: A summary of findings should be included in the abstract. This will make the paper's abstract concise and informative while conveying the content of the paper.

Response: We agree with the reviewer’s assessment. As suggested by the reviewer, we have added the summary of findings in the abstract in the revised manuscript (in Page 1 Lines 18-40).

 

Comment 2: The research gaps have been given at the end of the introduction. In addition to this, the authors should have stated clearly the objectives.

Response: Thank you for pointing this out. As suggested by the reviewer, we have added the objectives of this work at the end of the introduction (in Page 3 Lines 106-113).

 

Comment 3: Refer to 5 of 26: As part of the U.S. Department of Energy’s Energy Earthshots™, it is a department-wide effort to dramatically reduce the cost of EGS by 90%, bringing it down to $45 per megawatt hour by 2035. Achieving this goal will contribute significantly to reach the targets of 100% carbon-pollution-free electricity by 2035 and net-zero emissions across the U.S. economy by 2050. This is accomplished through accelerating research and development in multiple aspects such as driving down costs of drilling, well casing and related materials, advancing engineering techniques for more efficient well-drilling, collecting better-quality data for subsurface understanding, and ensuring proper containment of new reservoirs and geothermal fluids.

Response: Thank you for pointing this out. Yes, these research plans reflect the U.S. Department of Energy's confidence and ambition in the large-scale development of deep high-temperature geothermal resources (include HDR geothermal resources).

 

Comment 4: Authors should have included a summary of Figure 1. Schematic of the enhanced geothermal system, (a) hydraulic fracturing and (b) heat production should have been given.

Response: Thank you for pointing this out. As suggested by the reviewer, we have added a summary of Figure 1 in the revised manuscript (in Page 2 Lines 70-79).

 

Comment 5: The risks and challenges are related to exploration, drilling and managing the resource. Is the world prepared to undertake these risks and challenges?

Response: Thank you for pointing this out. The world is making significant progress in addressing the risks and challenges of HDR development, but full preparedness remains a work in progress. Exploration risks are being mitigated through advanced geophysical surveys and AI-driven resource modeling. Drilling challenges, such as high temperatures and hard rock conditions, are being tackled with innovations in high-temperature-resistant tools and efficient drilling techniques. Resource management, including induced seismicity and reservoir optimization, is improving with real-time monitoring and multi-field coupling studies. While countries like the U.S. and China are leading in pilot projects, global collaboration and continued investment are essential to fully prepare for the risks and unlock HDR's potential as a sustainable energy source.

 

Comment 6: What are the capital expenses of an HDR?

Response: Thank you for pointing this out. The capital expenses for the HDR development are significant, primarily driven by high-temperature drilling, reservoir stimulation, and infrastructure costs. As suggested by the reviewer, we have already discussed these topics in detail in the revised manuscript.

 

Comment 7: The glossary should have been included for readers.

Response: We appreciate the reviewer's suggestion regarding the inclusion of a glossary. However, we believe that the technical terms used in the manuscript are well-defined within the context of the text and are commonly understood by the target audience in the field of HDR/EGS development. To ensure clarity, we have made efforts to explain specialized terms upon their first use. Adding a glossary might disrupt the flow of the manuscript and could be redundant for the intended readership. If the reviewer feels that specific terms still require further clarification, we would be happy to address those in the revised manuscript. Thank you for your valuable feedback.

 

Comment 8: Ecological and Environmental Considerations should have been given a sub-section. The world is facing Global warming.

Response: We sincerely appreciate the reviewer's suggestion to include a dedicated sub-section on ecological and environmental considerations. However, the primary focus of this manuscript is on the technical development and challenges of HDR geothermal energy, with an emphasis on advancing drilling, reservoir stimulation, and resource evaluation technologies. While we acknowledge the critical importance of addressing global warming and environmental impacts, these aspects, though briefly mentioned in the context of HDR's potential as a clean energy source, are not the central theme of this study. Additionally, given the already substantial length of the manuscript, expanding on ecological and environmental considerations would exceed the recommended scope and word limit. We believe that the current structure effectively balances technical depth with broader implications, and we hope the reviewer finds this approach acceptable. Thank you for your valuable feedback.