Numerical Simulation of Optimized Step-Wise Depressurization for Enhanced Natural Gas Hydrate Production in the Nankai Trough of Japan

Round 1

Reviewer 1 Report

The main objective of this study is to investigate the viability of utilizing step-wise depressurization for the gas and water production behaviors in the Nankai Trough of Japan using a single vertical well. The manuscript at first glance looks well. However, there are also some flaws in this manuscript, and it is not able to recommend for publication in its current state and condition. the following items are suggested:

(1) The title is not catchy and does not reflect essential contents, may be confused between natural gas production and natural gas hydrate production.

(2) The introduction needs to be further revised to highlight the purpose of the study. What others have studied and what needs further research in the topic-numerical simulation of optimized step-wise depressurization for enhanced natural gas production from hydrate deposits.

Based on the practice of natural gas hydrate production in the Nankai Trough,  Japan and in the Shenhu Area of South China Sea, China, it has been proven that the stepwise depressurization method is an effective method for natural gas hydrate production, and all trial production has adopted the stepwise depressurization method, not the direct depressuaization method. So the method of step-wise depressurization for enhancing natural gas production and the numerical simulation results obtained in this study are not innovative enough.

(3) I am unconvinced about the appropriateness and adequacy of the dimension (thickness) of the burdens, as described by the authors. Moreover, the horizontal boundary of 200 m of 3D model is also insufficient. This is an open system, a true boundary has to be located at a depth where the same negligible pressure drop is observed after the 360 days of production or a longer time.

4. Less important issues. In my opinion, items (2) and (3) are by far the most important issues that need to be addressed. There are some other, less important issues that also need to be addressed. Fox examples,

a)      “Compared with the direct depressurization method, the step-wise depressurization method significantly increased the cumulative gas production by more than 10% and alleviated a large amount of gas and water production in a short period.”  I am unable to clarify this sentence.

b)      As authors mentioned, “The gas production rate remained at approximately 2.0×10⁴ m³/d, which was consistent with the field data from the Nankai Trough with the exception of the first day … … Given the similar results observed in other studies, it could be concluded that the model was accurate and has been successfully established.” The authors only compared the gas production without describing the water production. The effect of water production on the results is obvious and needs additional description.

No

Author Response

The main objective of this study is to investigate the viability of utilizing step-wise depressurization for the gas and water production behaviors in the Nankai Trough of Japan using a single vertical well. The manuscript at first glance looks well. However, there are also some flaws in this manuscript, and it is not able to recommend for publication in its current state and condition. the following items are suggested:

1. The title is not catchy and does not reflect essential contents, may be confused between natural gas production and natural gas hydrate production.

• Responses to comments:

Thanks for the reviewer’s comments. The revised title, "Numerical simulation of optimized step-wise depressurization for enhanced natural gas hydrate production in the Nankai Trough of Japan," has been updated to clarify the content of the study.

 

2. The introduction needs to be further revised to highlight the purpose of the study. What others have studied and what needs further research in the topic-numerical simulation of optimized step-wise depressurization for enhanced natural gas production from hydrate deposits.

Based on the practice of natural gas hydrate production in the Nankai Trough, Japan and in the Shenhu Area of South China Sea, China, it has been proven that the stepwise depressurization method is an effective method for natural gas hydrate production, and all trial production has adopted the stepwise depressurization method, not the direct depressuaization method. So the method of step-wise depressurization for enhancing natural gas production and the numerical simulation results obtained in this study are not innovative enough.

• Responses to comments:

Thanks for the reviewer’s comments. The second paragraph of the Introduction introduces the main extraction methods and several field tests for offshore hydrate production, highlighting the current issues of sand production risk and low gas production efficiency. The third paragraph describes the optimization of the depressurization method at laboratory scale. The fourth paragraph states the scarcity of the researches on step-wise depressurization at large scale. Therefore, this study aims to simulate and optimize step-wise depressurization at large scale, as well as to identify the optimal depressurization strategy. We have rewritten the purpose of this study in the Introduction.

The reviewer mentioned that the step-wise depressurization method was employed in the practice of natural gas hydrate production in the Nankai Trough, Japan and the Shenhu Area of the South China Sea, China. However, the specific values for the magnitude of each depressurization step and the maintenance time after each depressurization step are crucial aspects that require further investigation to determine the optimal depressurization strategy. The direct depressurization method is used as a reference case for comparison purposes (Please see Page 2, Line 88).

 

3. I am unconvinced about the appropriateness and adequacy of the dimension (thickness) of the burdens, as described by the authors. Moreover, the horizontal boundary of 200 m of 3D model is also insufficient. This is an open system, a true boundary has to be located at a depth where the same negligible pressure drop is observed after the 360 days of production or a longer time.

• Responses to comments:

Thanks for the reviewer’s comments. Moridis and Kowalsky [1] stated that a 30-meter-thick overburden and underburden layers may be sufficient to simulate the boundary effects caused by the heat exchange and pressure diffusion. Similarly, Yu et al. [2], Sun et al. [3], Feng et al. [4], and other researchers [5,6] have also adopted a 30-meter-thick overburden and underburden layers in their simulation studies. Furthermore, based on their long-term simulation results, a length of 200 meters in the radial direction was sufficient and not affected by the boundary effects. Due to the proven adequacy of this model size in other similar studies, this research adopts the same dimensions for conducting step-wise depressurization investigations.

 

4. Less important issues. In my opinion, items (2) and (3) are by far the most important issues that need to be addressed. There are some other, less important issues that also need to be addressed. Fox examples,
5. a) “Compared with the direct depressurization method, the step-wise depressurization method significantly increased the cumulative gas production by more than 10% and alleviated a large amount of gas and water production in a short period.” I am unable to clarify this sentence.

• Responses to comments:

Thanks for the reviewer’s comments. Compared to the direct depressurization method, the step-wise depressurization method significantly increased the cumulative gas production by over 10%. Furthermore, this method could mitigate the rapid generation of gas and water production during the depressurization process (Please see Page 1, Line 17).

 

4. b) As authors mentioned, “The gas production rate remained at approximately 2.0×10⁴ m³/d, which was consistent with the field data from the Nankai Trough with the exception of the first day … … Given the similar results observed in other studies, it could be concluded that the model was accurate and has been successfully established.” The authors only compared the gas production without describing the water production. The effect of water production on the results is obvious and needs additional description.

• Responses to comments:

Thanks for the reviewer’s comments. In similar studies, there is a common phenomenon of higher water production compared to field test. In the study of Yu et al. [2], the water production was approximately three times of the actual production rate, while in the study of Sun et al. [3], it was about four times, and in the study of Zhu et al. [6], it was about six times. This discrepancy could be attributed to the model simplifications and the presence of water-blocking devices in the actual field tests. Due to these differences, this study did not include a comparison of the water production rates.

 

References

1. Moridis, G.J.; Kowalsky, M.B. Response of Oceanic Hydrate-Bearing Sediments to Thermal Stresses. Spe J. 2007, 12, 253–268.
2. Yu, T.; Guan, G.; Abudula, A. Production Performance and Numerical Investigation of the 2017 Offshore Methane Hydrate Production Test in the Nankai Trough of Japan. Appl. Energy 2019, 251, 113338.
3. Sun, J.; Ning, F.; Zhang, L.; Liu, T.; Peng, L.; Liu, Z.; Li, C.; Jiang, G. Numerical Simulation on Gas Production from Hydrate Reservoir at the 1st Offshore Test Site in the Eastern Nankai Trough. J. Nat. Gas Sci. Eng. 2016, 30, 64–76.
4. Feng, Y.; Chen, L.; Suzuki, A.; Kogawa, T.; Okajima, J.; Komiya, A.; Maruyama, S. Numerical Analysis of Gas Production from Layered Methane Hydrate Reservoirs by Depressurization. Energy 2019, 166, 1106–1119.
5. Chen, L.; Feng, Y.; Kogawa, T.; Okajima, J.; Komiya, A.; Maruyama, S. Construction and Simulation of Reservoir Scale Layered Model for Production and Utilization of Methane Hydrate: The Case of Nankai Trough Japan. Energy 2018, 143, 128–140.
6. Zhu, H.; Xu, T.; Yuan, Y.; Xia, Y.; Xin, X. Numerical Investigation of the Natural Gas Hydrate Production Tests in the Nankai Trough by Incorporating Sand Migration. Appl. Energy 2020, 275, 115384.

Reviewer 2 Report

1.       It is suggested that the author add a brief introduction for Nankai trough, such as its location and geological condition, the existing problem for the production in this area, etc, to help the reader get familiar with the application field.

2.       Table 2: please add the meaning for the parameters used. Such as what is S*, etc.

3.       Please add the governing equation used in the simulation.

4.       Is the absolute (intrinsic) permeability not changing with the production and the change of effective permeability only affected by the change of relative permeability?

5.       Figure 3: why the field gas production dropped to 0 at day 6? Is this a maintenance? Please add brief explanations.

6.       Figure 4: It is suggested that the authors use same left axis range (0-8) for Figure 3 and Figure 4.

7.       Figure 5: there are two figures numbered (d).

8.       It is suggested that the author add a summary diagram before section 4.1, which show the cases with groups, for example, group 1 incudes cases 0-3, and the focus of this group is the impact of different depressurization gradients. Group 2 incudes cases 3,6, and 9, their focus is the impact of different maintenance times, and so on.

Author Response

1. It is suggested that the author add a brief introduction for Nankai trough, such as its location and geological condition, the existing problem for the production in this area, etc, to help the reader get familiar with the application field.

• Responses to comments:

Thanks for the reviewer’s comments. We have revised the Introduction section to emphasize the issues associated with the exploitation process in the Nankai Trough region. Since the model was built based on the actual geological parameters, we have added the introduction of the Nankai Trough's location and geological conditions in Section 2.2 for the detailed explanation (Please see Page 2, Line 48 and Line 57).

 

2. Table 2: please add the meaning for the parameters used. Such as what is S*, etc.

• Responses to comments:

Thanks for the reviewer’s comments. We have added a description of the variables in Table 2. For example, S* represents a scaled saturation, which is an intermediate variable in the capillary pressure model. The specific calculation formula for S* is given as  (Please see page 5, line 172, Table 2).

 

3. Please add the governing equation used in the simulation.

• Responses to comments:

Thanks for the reviewer’s comments. We utilized the TOUGH+HYDRATE simulator, which is governed by the equation:. We have included the main governing equation in Section 2.1 for reference (Please see Page 3, Line 111).

 

4. Is the absolute (intrinsic) permeability not changing with the production and the change of effective permeability only affected by the change of relative permeability?

• Responses to comments:

Thanks for the reviewer’s comments. In this study, the changes in the pore structure of the reservoir have not been considered in the TOUGH+HYDRATE simulator, and therefore, we assume that the absolute permeability remains constant. As a result, the effective permeability is solely influenced by the relative permeability. Considering the inherent variations in permeability across each sub-layer, we have chosen to present the effective permeability instead of the relative permeability to provide a more intuitive observation.

 

5. Figure 3: why the field gas production dropped to 0 at day 6? Is this a maintenance? Please add brief explanations.

• Responses to comments:

Thanks for the reviewer’s comments. This is due to the occurrence of the severe sand production issues during the actual production test, leading to the well shut-in and a subsequent reduction in gas production rates to zero. We have made revisions to the Introduction section to provide a clear explanation of this phenomenon (Please see Page 2, Line 48).

 

6. Figure 4: It is suggested that the authors use same left axis range (0-8) for Figure 3 and Figure 4.

• Responses to comments:

Thanks for the reviewer’s comments. Due to the rapid initial gas production rates during depressurization, we have adjusted the left axis of Figures 3 and 4 to the range from 0 to 30 (Please see Page 7, Line 198 and Line 211, Figures 3 and 4).

 

7. Figure 5: there are two figures numbered (d).

• Responses to comments:

Thanks for the reviewer’s comments. We have made corrections to the numbering of the subfigures (Please see Page 8, Line 234, Figure 5).

 

8. It is suggested that the author add a summary diagram before section 4.1, which show the cases with groups, for example, group 1 incudes cases 0-3, and the focus of this group is the impact of different depressurization gradients. Group 2 incudes cases 3,6, and 9, their focus is the impact of different maintenance times, and so on.

• Responses to comments:

Thanks for the reviewer’s comments. We categorized the cases into two main groups: Group A-C, which investigates the impact of different depressurization gradients under the same maintenance time; and Group D-F, which examines the influence of different maintenance times under the same depressurization gradient, as shown in Table 4 (Please see Page 9, Line 248 and Line 253, Table 4).

Reviewer 3 Report

The work "Numerical simulation of optimized step-wise depressurization for enhanced natural gas production in the Nankai Trough of Japan" is devoted to a theoretical study of the influence of the step-wise depressurization method on the extraction of gas from a reservoir containing natural gas hydrate, as well as to the study of changes occurring in such a reservoir during long-term storage, including the gas and water production characteristics. The introduction provides an overview of the latest research in the required field. The description of the methods is detailed and scrupulous, which makes it possible to reproduce the results. The results themselves are clear, the conclusions correspond to them.

Comments:
1. The abstract does not give an understanding of what theoretical method was used.
2. (Fig. 4) Why the gas production rate slowly lowers, whereas the water production rate is constant? The distribution of gas in a hydrate on a macroscopic scale should be more or less uniform.
3. Fig. 8 caption refers to Case 0, however the picture itself doesn't present any data about Case 0.
4. Fig. 9 doesn't need the gap between 0 and 45 values.

Due to the fact that the comments do not address the main conclusions, the text of the article requires only minor edits, after which it should be published.

Author Response

The work "Numerical simulation of optimized step-wise depressurization for enhanced natural gas production in the Nankai Trough of Japan" is devoted to a theoretical study of the influence of the step-wise depressurization method on the extraction of gas from a reservoir containing natural gas hydrate, as well as to the study of changes occurring in such a reservoir during long-term storage, including the gas and water production characteristics. The introduction provides an overview of the latest research in the required field. The description of the methods is detailed and scrupulous, which makes it possible to reproduce the results. The results themselves are clear, the conclusions correspond to them.

Comments:

1. The abstract does not give an understanding of what theoretical method was used.

• Responses to comments:

Thanks for the reviewer’s comments. Our study was conducted based on the TOUGH+HYDRATE simulator, whose governing equation is described in detail in Section 2.1. We have made revisions to some parts of the abstract to emphasize the theoretical method employed in our research (Please see Page 1, Line 12).

 

2. (Fig. 4) Why the gas production rate slowly lowers, whereas the water production rate is constant? The distribution of gas in a hydrate on a macroscopic scale should be more or less uniform.

• Responses to comments:

Thanks for the reviewer’s comments. The water production rate does indeed correlate with the gas production rate, but with a slower decline trend compared to the gas production rate. This trend is not clearly evident in the displayed graph. Therefore, we have magnified the curve for the first 0.5 days to enhance the visibility and facilitate the analysis (Please see Page 7, Line 211).

 

3. Fig. 8 caption refers to Case 0, however the picture itself doesn't present any data about Case 0.

• Responses to comments:

Thanks for the reviewer’s comments. Fig. 8 describes the incremental cumulative gas production of Cases 1-9 relative to Case 0. We have made modifications to the title of Fig. 8 to eliminate any possible misunderstanding (Please see Page 12, Line 347).

 

4. Fig. 9 doesn't need the gap between 0 and 45 values.

Responses to comments:

Thanks for the reviewer’s comments. We have made modifications to Fig. 9 by removing the gap between 0 and 45 (Please see Page 13, Line 348).

Round 2

Reviewer 1 Report

No