Comparative Analysis on the Evolution of Seepage Parameters in Methane Hydrate Production under Depressurization of Clayey Silt Reservoir and Sandy Reservoir

Round 1

Reviewer 1 Report

Parameter S_H is not defined: is it ratio of pore volume share occupied by hydrates to total pore volume or ratio of hydrates volume to total rock sample volume?

Page 2, Lines 54-84: Many repetitions!  

Table 2.2 –Please, check Bottom temperatures for CHR and  SHR (seems too high), taking into account, that in the text lower temperature at the bottom of HSZ for CHR indicated as 15.15^oC.

No data about heat conductivity and heat of phase transition in hydrate-containing sediments, although these parameters strongly affects intensity of hydrate decomposition. Sometimes even more strongly than porosity and permeability , especially in high-permeable sediments.

Line 332 - Where is “… Vp first time declined…” in Figure 3.1?

The nature of second peak in figures 3.1a, 3.2a, 3.3a for Vp is not clear. Explanations are needed.

Authors have to check their text carefully: there are repetitions and gaps.

 

Author Response

Q1. Parameter S_H is not defined: is it ratio of pore volume share occupied by hydrates to total pore volume or ratio of hydrates volume to total rock sample volume?

A: Thanks very much for this valuable suggestion. The S_H was precisely defined in Page 13, Lines 394-397, and it is the ratio of the volume of gas hydrates in the reservoir pores to the total pore volume. This position is too far back, so the definitions of parameter Φ, k, and S_H were supplemented in Page 6, Lines 219-223.

Q2. Page 2, Lines 54-84: Many repetitions!

A: Thank you for pointing out this detail. Small mistakes occurred in typesetting, and the repetitions has been deleted in Page 2, Lines 72-87.

Q3. Table 2.2 –Please, check Bottom temperatures for CHR and SHR (seems too high), taking into account, that in the text lower temperature at the bottom of HSZ for CHR indicated as 15.15oC.

A: The question was been carefully considered. Taking CHR as an example，the initialization of the temperature was calculated according to the reported seafloor temperature (4.82°C) and the corresponding geothermal gradient (54.9°C/km), these values are supported by sufficient data. Table 2.2 shows the temperature of the CHR model bottom，and the underburden thickness is 20m，T_{bottom of HSZ}=T_CHR bottom - geothermal gradient * the underburden thickness = 16.25-5.49*0.2=15.152℃.

Q4. No data about heat conductivity and heat of phase transition in hydrate-containing sediments, although these parameters strongly affects intensity of hydrate decomposition. Sometimes even more strongly than porosity and permeability , especially in high-permeable sediments.

 A: Thermodynamic parameters strongly affects intensity of hydrate decomposition and need to be fully consider. The data about heat conductivity and heat of phase change was listed in Table 2.2 . The dry heat conductivity is 1.0 W/m/K in both CHR and SHR. The wet heat conductivity of CHR is 2.917 W/m/K , and the wet heat conductivity of SHR is 3.10 W/m/K. The phase change heat of gas hydrate is 53.5 kJ/mol in both CHR and SHR.

Q5. Line 332 - Where is “… Vp first time declined…” in Figure 3.1?

A: Sorry, this should be “… Qp first time declined…” in Page 12, Lines 351-355. The cooling period in Figure 3.1 is a period of 0.001days-0.1 days. A large amount of hydrate decomposition led to a sharp drop in the temperature around the well, which is not conducive to the subsequent hydrate exploitation, so the volumetric rate of CH₄ decreased rapidly.

Q6. The nature of second peak in figures 3.1a, 3.2a, 3.3a for Vp is not clear. Explanations are needed.

A: We appreciate your kind suggestions and relevant supplements are made in Page 11, lines 328-334. After the cooling period, as the temperature in the cooling area recovers, there is enough heat to supply hydrate decomposition, and the Qp rises to the second peak. Because of the pressure-drop transmission and the decomposition of hydrate around the wellbore, the Qp decreased sharply to reach a steady value.

Q7. Authors have to check their text carefully: there are repetitions and gaps.

A: Thank you for your careful reading of our manuscript. We apologize for any confusion caused and appreciate the valuable suggestions. And the paper has been carefully examined, such as the repetitions in Page 2, Lines 72-87, and the gaps between Figure 3.2 and Table 3.4.

Author Response File: Author Response.docx

Reviewer 2 Report

Report on the Manuscript Number: jmse-1673104

The submitted entitled “Comparative analysis on the evolution of seepage parameters in methane hydrate production under depressurization of Clayey Silt Reservoir and Sandy Reservoir” research article in ‘JMSE-MDPI, manuscript at first glance looks well. The writing article efforts have been made by Y. Li et al. is looking interesting although this article has been conducted on various laboratory performed experiments. 

Submitted article is a full-length article therefore all section must be improved during the addressing the comment followed by given comments / questions / suggestion, then only manuscript should be considered for publication or further action.

            I believe some of my suggestions make it improve meets journal standard, because 

This journal is one of the best journals of their own field. Moreover, each and every manuscript must have clear background of research and ongoing interests of this subject.

I am writing some suggestions / comment and the manuscript must undergo ‘major revision’ criteria.

Comments are as follows;

• In introduction need to be updated about gas hydrates, structures and modification happened due to various seafloor condition including mud, sand etc. However recently published works must be updated as documented there are numbers of works have been done.
• Moreover, Shenhu Sea area of the South China is important location for gas hydrates and numbers of models and efforts have been made to demonstrate the condition and mining of gas hydrates from this location.
• Table 1, it would be nice presentation if author could demonstrate this data graphically.
• Table1, I believe there more site in around the globe can be included like Europe etc?
• Figure 1 is good to have in this manuscript, but I believe author can enhance the quality of these two figures, I can see fonts are not much clear.
• Kindly double check the numbering style of the figure and table.
• Validation of models?
• Is there any other model can works for this study?
• In figure 3.1 Vp (CHR) trends following curve, whether Vp (SHR) following straight, Explanation?
• Similar trend of variation has been found in figure 3.2, and 3.3
• Figure 3.2, Additionally E8 -red line (below) shows irregular trends around day 50?
• I believe there is a room in discussion, in light of chemistry.
• How this attempt / approach attract scientific society, stakeholders.

Author Response

Q1. In introduction need to be updated about gas hydrates, structures and modification happened due to various seafloor condition including mud, sand etc. However recently published works must be updated as documented there are numbers of works have been done.

A: Thanks very much for this valuable suggestion. The introduction has been updated in lines 41-47, such as the research on hydrate occurrence conditions by Ning et al. , and the research on energy potential of gas hydrate by Ma et al.

Q2. Table1, I believe there more site in around the globe can be included like Europe etc?

A: This suggestion is worth thinking about. We know that natural gas hydrate is distributed in many countries in the world, including Europe. However, in order to highlight the different hydrate decomposition behaviors and gas production characteristics in the hydrate reservoir with different lithology, Several global gas hydrate production tests were summarized in Table1.1. At present, there is no hydrate production tests in Europe.

Q3. Figure 1 is good to have in this manuscript, but I believe author can enhance the quality of these two figures, I can see fonts are not much clear.

A:Thank you for your careful reading of our manuscript. The high-res image has been updated in Figure 2.1 and Figure 2.2.

Q4. Kindly double check the numbering style of the figure and table.

A: Thank you for your careful reading of our manuscript, and the paper has been carefully examined according to the journal template.

Q5. Validation of models? Is there any other model can works for this study?

A: Thanks very much for this valuable suggestion，and the validation of models is very critical and necessary. We have an excellent research team, which has conducted in-depth research on relevant models. As shown in following references, Yuan, Zhu et al. had uesd T+H to simulate gas hydrates production in China and Japan, and fitted the simulation data and the measured data in gas hydrate production tests to prove the availability of the models. In this paper, the models refers to those work, and the value of main parameters is same. Therefore, the models can be directly used in our research.

References:

Yuan Y, Xu T, Xin X, et al. Multiphase flow behavior of layered methane hydrate reservoir induced by gas production[J]. Geofluids, (2017) 7851031.

 Zhu H, Xu T, Yuan Y, et al. Numerical investigation of natural gas hydrate production tests in the Nankai Trough by incorporating sand migration[J]. Applied Energy,275 (2020) 115384.

Yuan Y. Numerical simulation on gas production potential and the geo-mechanical stability from marine natural gas hydrate through depressurization[D]. Changchun: Jilin University, 2019.

Zhu H. Numerical study on sand production processes during natural gas hydrate recovery and its impact on gas production[D]. Changchun: Jilin University, 2021.

Q6. In figure 3.1 Vp (CHR) trends following curve, whether Vp (SHR) following straight, Explanation? Similar trend of variation has been found in figure 3.2, and 3.3.

A: We appreciate your kind suggestions and relevant supplements are made in Page 11, lines 336-341。The trend of Vp depends on the decomposition process of hydrate. The SHR has large the sensible heat and the heat conduction velocity, and the hydrate decomposition is less limited by heat supply, so the Vp of SHR following straight. The sensible heat and the heat conduction velocity of CHR is weaker, and the heat supply becomes more and more difficult, the growth of the hydrate exploitation slows down, so the Vp of CHR following convex curve.

Q7. Figure 3.2, Additionally E8 -red line (below) shows irregular trends around day 50?

A: This problem is an oversight of our work, and thank you for pointing it out carefully. The data is serial in the drawing process, and the Figure 3.2 (b) has been redrawn in Page 13, lines 397. The Qp and Vp of scheme with higher permeability is always greater.

 

Figure 3.2 (b) The effect of k on Q_P and V_P in the SHR model.

Q8. I believe there is a room in discussion, in light of chemistry.

A: This suggestion is worth thinking about, and it is a great angle to discuss the hydrate decomposition from the chemical aspect. The paper mainly discusses the impact of seepage parameters on gas production with different lithology, less chemical analysis is involved. We have decided to refer to your important opinions and conduct in-depth research on them in the following work.

Q9. How this attempt / approach attract scientific society, stakeholders.

A: Thanks very much for this valuable suggestion. It is necessary to clarify the research significance, and we elaborated on significance in Page 3, lines 120-129。The paper establish two models of depressurized production of horizontal wells in layered heterogeneous reservoir which includes both CHR of W11 drilling site in the Shenhu sea area of the South China Sea and SHR of AT1 drilling site in the Eastern Nankai Trough of Japan, and the sensitivity analysis of the lithological parameters affecting the productivity of different lithological reservoir is carried out. Based on the research of this paper, the mechanism of hydrate decomposition is described in more detail, and it can provide reference for the scheme optimization and site selection of hydrate exploitation in the Shenhu sea area of the South China Sea in the following work.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Much better. Some conclusions were known before this study.

Author Response

Thanks very much for this valuable and necessary suggestion and those suggestions make the manuscript improve meets journal standard.

Reviewer 2 Report

Q8. I believe there is a room in discussion, in light of chemistry. For this question I can suggest you an article that can help authors to improve this part. Article entitled "An experimental study on doubly salt effect for methane hydrate inhibition".

Author Response

Thanks very much for this valuable and necessary suggestion and we have supplemented the light of chemistry. The manuscript have been revised.

Author Response File: Author Response.docx