Geothermal Characteristics and Productivity Potential of a Super-Thick Shallow Granite-Type Enhanced Geothermal System: A Case Study in Wendeng Geothermal Field, China

Round 1

Reviewer 1 Report

The paper is well written and well organised. Actually, I enjoyed from reading this work. There are some misunderstandings which coming from the thermo-hydraulic nature of your simulations. I have the following suggestions to help to address these issues:

*Your well placement is not considering the fracture density. Previous studies such as "Impact of Well Placement in the Fractured Geothermal Reservoirs Based on Available Discrete Fractured System" showed that well placement is a strong function of the fracture map and has a great impact on the heat productivity through this. It is better to mention and discuss this issue.

*You mentioned that high in-situ stress is a favourable condition for the heat generation. However, if you run THM models, you may get other conclusions. As this stress is mainly compressive one, it may close the fractures and reduce the hydraulic conductivity. As an example, "Simulations and global sensitivity analysis of the thermo-hydraulic-mechanical processes in a fractured geothermal reservoir"shows the importance of the thermo-poro-elastic parameters on the heat extraction which emphasis the stress field impact.

*Line 200: No flow boundary condition seems to be unrealistic. It is better to assume a constant hydraulic heat. Also, open flow boundary condition for the heat transfer.

*Previous studies such as "Hydro-Thermal Modeling for Geothermal Energy Extraction from Soultz-sous-Forêts, France" shows that the large portion of the heat loss happens inside the wellbore and it is more important than the reservoir response. This heat loss is a strong function of the injection/production temperature and flow rate. Please mention this limitation in the current study.

*Symbols are not clear in the figures. Please enlarge them.

*It is better to discuss more about the figures. For example in figure 14, temperature decreases during time resulting to the viscosity increase and this requires more injection pressure to push the fluid toward the production wellbores.

Author Response

1. Your well placement is not considering the fracture density. Previous studies such as "Impact of Well Placement in the Fractured Geothermal Reservoirs Based on Available Discrete Fractured System" showed that well placement is a strong function of the fracture map and has a great impact on the heat productivity through this. It is better to mention and discuss this issue.

Answer: We have read the recommended paper and agreed with the reviewer's opinion. According to the reviewer’s comments, we mentioned and discussed this issue in Section 4.3 (line 368-374, track change version). It should be noted that MShale processes fractures into an orthogonal distribution. Thus, injection and production wells are placed on the main fracture profile to obtain the longest flow path as shown in Figure 6b. However, natural fracture distribution have the characteristics of spatial heterogeneity. Well placement is a strong function of the fracture map and has a great impact on the heat productivity [34]. When more detailed reservoir information is available in the future, the effect of fracture density should be considered in the well placement pattern.

2. You mentioned that high in-situ stress is a favourable condition for the heat generation. However, if you run THM models, you may get other conclusions. As this stress is mainly compressive one, it may close the fractures and reduce the hydraulic conductivity. As an example, "Simulations and global sensitivity analysis of the thermo-hydraulic-mechanical processes in a fractured geothermal reservoir" shows the importance of the thermo-poro-elastic parameters on the heat extraction which emphasis the stress field impact.

Answer: Thanks for recommending article. We agree with the opinion of as this stress is mainly compressive one, it may close the fractures and reduce the hydraulic conductivity. In general, EGS prefer to create longer reservoirs rather than higher ones to increase the flow path between the injection and production wells, which is more favorable for heat transfer. In the vertical direction, the in-situ stress would vary with depth. Longer fractures are formed when the reservoir is shielded by stress in the upper boundary. So, stress shielding layer is suggested when selecting target reservoir. Actually, our conclusion is consistent with the reviewer's opinion. We emphasize the stress shielding layer, which has a lower in-situ stress not high in-situ stress, as shown in the following figure (example). In order to avoid ambiguity, we give detailed expression in Section 3.2 (line 206-213, track change version).

Besides, we also mentioned the limitation without considering the mechanical filed in the last part of Conclusions (line 610-614, track change version). In this study, considering the complexity of the actual project, we simplified the numerical model. Only the coupling of the hydraulic-thermal effect was considered. The distribution characteristics of the facture spacing, aperture, and orientation should be collected to establish a more realistic hydraulic–thermal–mechanical–chemical model in the future.

3. Line 200: No flow boundary condition seems to be unrealistic. It is better to assume a constant hydraulic heat. Also, open flow boundary condition for the heat transfer.

Answer: We have corrected the boundary conditions as shown in Section 3.3.1 (line 245-248, track change version).

4. Previous studies such as "Hydro-Thermal Modeling for Geothermal Energy Extraction from Soultz-sous-Forêts, France" shows that the large portion of the heat loss happens inside the wellbore and it is more important than the reservoir response. This heat loss is a strong function of the injection/production temperature and flow rate. Please mention this limitation in the current study.

Answer: We have mentioned this limitation in the last part of Conclusions (line 607-610, track change version). Additionally, previous study shows that the large portion of the heat loss happens inside the wellbore and it is more important than the reservoir response. The wellbore model should be taken into account when more accurate heat transfer simulation is carried out later.

5. Symbols are not clear in the figures. Please enlarge them.

Answer: We have refined the figures and provided a Figure file (figure.doc) with all clear figures.

6. It is better to discuss more about the figures. For example in figure 14, temperature decreases during time resulting to the viscosity increase and this requires more injection pressure to push the fluid toward the production wellbores.

Answer: According to the reviewer’s comments, we further explored and discussed the figures in the paper. For example, figure 14 (line 443-446, track change version) and figure 10 (line 410-416, track change version).

Thank you!

Author Response File: Author Response.pdf

Reviewer 2 Report

General comments

Line 64: There needs to be more discussion on past EGS projects almost all of which have failed. An extra paragraph should be added in summarising the reasons why past EGS projects have failed and what lessons from these failures are being applied in the current study. Why is an EGS project at Wengdeng likely to work better than past projects?

Detailed comments

Line 28: This sentence is unclear. Perhaps delete it.

Line 35: The following is too vague. It should be deleted or explained: “but the overall economic benefits should be taken into con-sideration

Line 68: The following sentence is too vague. More explanation is required. “This depends on fracturing and heat trans-fer effects”

Line 103: Please explain why “which is different from sedimentary-type geothermal field”

Line: 176: Please give more information about what constitutes a stress shielding layer

Line 198: Please give more details of the grid sensitivity analysis

Line 243: What is meant by: “water internal friction”?

Line 250: The deep temeperatures indicate possible convection. Please comment.

Fig. 6: The grid layout for the model is not totally clear at this time. Is it the elliptical region in Fig. 6b? Please make this clear.

Line 348: Where are the boundaries of the model? It looks like they may be affecting the shape of the isotherms in Fig 10.

Figure 16: Are the boundary conditions affecting these results?

Line 452 etc: It would be good to reference back some of these conclusions to earlier EGS projects

Line 471: Please explain the economic trade-offs.

Line 485: The deep convective zone should have been discussed earlier

Line 487: The following point is not well explained: “the selection of a high temperature HDR reservoir in this super-thick shal-low granite is mainly influenced by the distribution of the fault zones.”

Extensive editing of English is required. I have made suggestions (in blue) on the attached pdf but probably more editing is required

Comments for author File: Comments.pdf

Author Response

Point to point responses to Reviewer 2:

1. General comments.

Line 64: There needs to be more discussion on past EGS projects almost all of which have failed. An extra paragraph should be added in summarising the reasons why past EGS projects have failed and what lessons from these failures are being applied in the current study. Why is an EGS project at Wengdeng likely to work better than past projects?

Answer: In Introduction part, we have added an extra paragraph to review and describe challenges experienced in EGS projects, based on observations from 64 EGS sites (line 60-69, track change version). Lessons from these failures are partly described in line 70-88, track change version. About why we chose Wendeng as the study area, the explanation is described in line 123-132, track change version. Wendeng geothermal field has been developed for its hydrothermal geothermal re-sources for many years. It indicates that there exists abundant thermal resources in the underground. Super-thick shallow granites are widely distributed in Wendeng geother-mal field. They are largely exposed to the surface without cap layer, the thermal structure of which is different from sedimentary-type geothermal fields. In order to evaluate wheth-er this area has the potential to develop HDR geothermal resources, Shandong provincial government has carried out geothermal geological survey, drilling and logging in Wendeng geothermal field area. The project is still in the feasibility study stage. The next plan is to select the appropriate reservoir for reservoir stimulation.

2. Line 28: This sentence is unclear. Perhaps delete it.

Answer: We deleted this sentence.

3. Line 35: The following is too vague. It should be deleted or explained: “but the overall economic benefits should be taken into con-sideration.

Answer: We deleted this sentence in this part. The explanation lies in line 458-459, Section 5.1, track change version. Drilling extra deep wells will increase the initial cost of the project.

4. Line 68: The following sentence is too vague. More explanation is required. “This depends on fracturing and heat trans-fer effects”.

Answer: We have given more explanation for this sentence in line 83-86, track change version. The effect of reservoir stimulation effect determines the amount of thermal energy that can then be extracted. During the heat extraction stage, the well layout pattern and operational parameters affect the final productivity significantly.

5. Line 103: Please explain why “which is different from sedimentary-type geothermal field”.

Answer: We have added the explanation in line 126-127 and line 567-571, track change version.

6. Line: 176: Please give more information about what constitutes a stress shielding layer.

Answer: We have explained why stress shielding layer is suggested in line 206-212, section 3.2, track change version. In general, EGS prefer to create longer reservoirs rather than higher ones to increase the flow path between the injection and production wells, which is more favorable for heat transfer. In the vertical direction, the in-situ stress would vary with depth. Longer fractures are formed when the reservoir is shielded by stress in the upper boundary. So, stress shielding layer is suggested when selecting target reservoir. We emphasize the stress shielding layer, which has a lower in-situ stress not high in-situ stress, as shown in the following figure (example).

7. Line 198: Please give more details of the grid sensitivity analysis.

Answer: We have added the results with new Table 2 and descriptions of the grid sensitivity (Section 3.3.1, line 227-242, track change version).

8. Line 243: What is meant by: “water internal friction”?

Answer: Water internal friction denotes frictional impedance due to the viscous force.

9. Line 250: The deep temeperatures indicate possible convection. Please comment.

Answer: We have added the description about the deep convection type geothermal system according to the measured temperature data of ZK1 well (Section 4.1, line 303-309; Section 5, line 571-576, track change version).

10. Fig. 6: The grid layout for the model is not totally clear at this time. Is it the elliptical region in Fig. 6b? Please make this clear.

Answer: We have added explanation for EGS reservoir region based on the stimulation results (Section 4.3, line 363-366; Fig6a and Fig6b, line376, track change version).

11. Line 348: Where are the boundaries of the model? It looks like they may be affecting the shape of the isotherms in Fig 10.

Answer: The geometric size of the model is 1000 m (x) × 700 m (y) × 200 m (z). The geometric size of EGS reservoir zone is 700 m (x) × 400 m (y) × 35 m (z) (line 230-231). The boundaries of EGS reservoir zone are open for mass and heat transfer with surrounding rock. The model size is large enough to avoid heat transfer impact from the boundary (line 245-248). We did several tests before modeling to ensure that the cold halo did not spread to the model boundary.

12. Figure 16: Are the boundary conditions affecting these results?

Answer: As described in above answer, the model size is large enough to avoid heat transfer impact from the boundary (line 245-248). We did several tests before modeling to ensure that the cold halo did not spread to the model boundary.

13. Line 452 etc: It would be good to reference back some of these conclusions to earlier EGS projects.

Answer: According to the reviewer’s comments, we referenced these conclusions to earlier EGS projects (line 533-540).

14. Line 471: Please explain the economic trade-offs.

Answer: We deleted this sentence in this part. The explanation lies in line 458-459, Section 5.1, track

15. Line 485: The deep convective zone should have been discussed earlier.

Answer: We have discussed the deep convection type geothermal system according to the measured temperature data of ZK1 well in Section 4.1, line 303-309.

16. Line 487: The following point is not well explained: “the selection of a high temperature HDR reservoir in this super-thick shal-low granite is mainly influenced by the distribution of the fault zones.”

Answer: We have added the explanation in line 303-309. Below 1100 m, the temperature drops because the well has penetrated along the water-conducting fault, which indicated possible convection type geothermal system. This is good for traditional hydrothermal geothermal development, but the temperature is still too low for HDR reservoirs, which require further drilling. Generally, geothermal gradients increase linearly with depth in this super-thick shallow granite area, as long as water faults are not drilled. Therefore, when choosing shallow high-temperature HDR reservoirs in such super-thick granite areas, attention should be paid to avoiding deep water fault zones.

17. Extensive editing of English is required. I have made suggestions (in blue) on the attached pdf but probably more editing is required

Answer: We sincerely thank the reviewer for the careful revision of our paper. We have made careful revisions according to the suggestions in the attached pdf. In addition, we hired native English speakers to review the editing and grammar of the full article.

Thank you!

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper by Haiyang Jiang et al have carried out field test, laboratory test and numerical simulation in succession to evaluate EGS productivity potential of Wendeng geothermal field. The geothermal characteristics and deep rock mechanical properties are identified based on real geological and core data of the borehole ZK1. Then, a numerical model of reservoir hydraulic fracturing based on a discrete fracture network is established. Thermal extraction simulations were conducted to assess the long-term dynamic productivity of EGS based on the fracturing results. It is recommended to be published after the following issues are resolved.

1.In line 64, the whole paragraph tries to focus on the numerical method used, but the first sentence does not lead the whole paragraph;

2.In line 160, it is still too arbitrary to form a complex seam network. Can we combine experiment and numerical simulation;

3.In line 193, the grid sensitivity was verified without further description. For example, several groups of different grid experiments were conducted, or the error range was not given, after all, corresponding experimental results were not given;

4.Is it wrong to introduce formula 7 in line 222 and 245, because the corresponding formula does not go to number 7;

5.When figure10 was analyzed in line 343, the characteristics of temperature distribution over time could be properly analyzed and the thermal diffusion efficiency of EGS reservoir could be quantitatively described;

6.Is there any error in the order of the value ranges when line 341,510 quantitatively describes the power generation and efficiency for 20 years.

Author Response

Point to point responses to Reviewer 3:

1. In line 64, the whole paragraph tries to focus on the numerical method used, but the first sentence does not lead the whole paragraph.

Answer: According to the reviewer’s comments, we divided the original paragraph into two paragraphs. The new paragraph is focused on the numerical method used, seen in Section 1 (line 78-98, track change version).

2. In line 160, it is still too arbitrary to form a complex seam network. Can we combine experiment and numerical simulation.

Answer: Brittleness index is an indicator of whether the reservoir is easy to form fracture network through volume fracturing. It is difficult to determine the suture network by brittleness index alone. Therefore, we have removed this original expression in original line 160. Besides, we stated hydraulic shear fracturing in naturally fractured formation has been proven effective in EGS projects, thus it was adopted for simulation this time (Section 3.2, line 191-196, track change version). And, the fracture network is formed through the self-support of staggered natural fractures after water shear fracturing. The formation with developed fractures is generally selected as the target reservoir (line 201-204, track change version).

3. In line 193, the grid sensitivity was verified without further description. For example, several groups of different grid experiments were conducted, or the error range was not given, after all, corresponding experimental results were not given.

Answer: We have added the results with new Table 2 and descriptions of the grid sensitivity (Section 3.3.1, line 227-242, track change version).

4. Is it wrong to introduce formula 7 in line 222 and 245, because the corresponding formula does not go to number 7.

Answer: We have checked and corrected similar issues throughout the article to avoid misunderstanding.

5. When figure10 was analyzed in line 343, the characteristics of temperature distribution over time could be properly analyzed and the thermal diffusion efficiency of EGS reservoir could be quantitatively described.

Answer: We have further analyzed the figure 10 and described the change rule of temperature distribution and thermal diffusion with time (line 410-416, track change version).

6. Is there any error in the order of the value ranges when line 341,510 quantitatively describes the power generation and efficiency for 20 years.

Answer: The order of values is chronological. Both power generation and efficiency decrease over time.

Thank you!

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

More suggested edits to the English are shown in blue. Technically the paper is now OK

Comments for author File: Comments.pdf

Author Response

Thank you very much for your careful reading and revision for our paper.

The detailed changes can be seen in track change version. We also provide a clear version.

Best regards,

Haiyang Jiang