Numerical Simulation of Gas Hydrate Production Using the Cyclic Depressurization Method in the Ulleung Basin of the Korea East Sea
Round 1
Reviewer 1 Report
Manuscript: applsci-1405371
Title: Numerical simulation of gas hydrate production using the cyclic depressurization method in the Ulleung Basin of the Korea East Sea
Remarks:
The manuscript entitled “Numerical simulation of gas hydrate production using the cyclic depressurization method in the Ulleung Basin of the Korea East Sea”. The authors have done innovative study of gas hydrate production simulation. This article is quite comprehensive based on the topic of discussion. I would recommend this article to be published after some proper revision. There are several specific comments of mine that need to be considered, with the following:
Point 1. Abstract section needs to be revised. The abstract section lacks of specific result. Research results were not included. It should be included wisely.
Point 2. On the page 3 for the intoduction section, the author need to elaborate further explanation regarding the cyclic depressurization method and its connection to geomechanical stability according to the background of the topic since the last paragraph mentioned between sentences is quite jumpy (line 78).
Point 3. On the page 4, It would be better if the author provide an additional brief explanation regarding Figures 5 and 6, or add more detailed information about the symbols in the captions of Figures 5 and 6.
Point 4. For the chapter 3 on the page 8, in order to avoid misunderstanding about the case study in the beginning, it could be better to briefly mention about the parameters (bottomhole pressure and production time) not only for the base case but also for the other cases. While for Table 4, it is better if the writing of No.1-4 can be replaced with case study 1-4.
Point 5. The explanation in sub-chapter 3.1 on the page 9 will be more complete to understand by mentioning Figure 8 per part (a) and (b). It also applies to sub-chapter 3.3 on the page 11 for Figure 12 (a) and (b).
Point 6. It is recommended for authors to have their manuscript checked for some grammatical structures. I found many common mistakes of grammatical structures.
For example: in Line 54, the word “so” should be revised to use better “adverb”.
Line 54, “other” should be revised to “another”
Line 58, “This phenomenon lead” should be revised to “This phenomenon leads”
Line 71, “was” should be revised to “were” because “studies” are plural
Line 78, “have” should be revised to “has” because of singular word
In last paragraph of introduction, words “First, Second, Third” should be omitted. It is inappropriate to use those words in scientific paper.
Checking the common grammatical mistakes of the entire manuscript are required.
Point 7. Following reference could be referred in introduction part. “Numerical Simulation on the Dissociation, Formation, and Recovery of Gas Hydrates on Microscale Approach”
Author Response
We would like to thank the reviewer for a very thorough reading of the manuscript and helpful comments. We have made changes to the manuscript in response, with attached word file.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Dear authors,
Please find my comments below
line 13: should it rather be? :
Maintenance of Geomechanical stability is necessary for sustainable production of gas from gas hydrate deposits.
line 27: should it rather be? :
Gas that potentially can be produced from gas hydrates gas burn more cleanly producing less CO2 than petroleum and coal.
Because commonly people do not intend to lift hydrates on the surface to burn them but prefer to decompose them downhole.
There were field studies on the technology of “formation fluid extraction” which includes partial lifting of the hydrates from reservoir but this way was considered as non-cost-effective.
Liu L. et al. Progress in Global Gas Hydrate Development and Production as a New Energy Resource Acta Geologica Sinica (English Edition), 2019, 93(3): 731–755
line 43: China performed some field trials as well
Liu L. et al. Progress in Global Gas Hydrate Development and Production as a New Energy Resource Acta Geologica Sinica (English Edition), 2019, 93(3): 731–755
lines 46-49: it would be better to rephrase these sentences. It is clear what you mean, but it would be better if you asked native speaker how to write these sentences better.
line 53 : increase temperature of HBS?
line 54: lines 46-49: it would be better to rephrase these sentences. It is clear what you mean, but it would be better if you asked a native speaker or interpreter how to write these sentences better.
lines 55-58: could you please provide reference related to studies of permeability change during CH4-CO2 swapping.?
line 64: there is a typo in the Figure 2 “Hydaret”. “New hydrate phase diagram as gas swapping”. Please rephrase. Does dash line correspond to CO2 hydrate phase diagram?
line 71: simulation studies were
line 75: “because the sensible heat was readily exhausted” not clear what you mean here
line 111: “validated in previous study” probably, you missed a link here
line 111: “Hydrate formation and dissociation reactions are specified by equilibrium kinetics in STARS” are the rates specified? Also not clear what is equilibrium kinetics. I believe that there is no reaction on macroscopic level at equilibrium point from thermodynamical standpoint.
line 114: “The former does not update pore compressibility and absolute permeability after calculating displacement, stress and strain.”
I believe that this is the main drawback of this simulator and you need to say clearly if it may have influence the conclusion made after the simulations. If the conclusions can be affected dramatically you need to say clearly as well what the value of this research is. I think that you also need to mention here if the simulator takes into account the changes of these parameters due to hydrates dissociation.
It would be beneficial as well if you could place the schematic of workflow of the simulator here or if you could put the reference where this workflow can be found.
line 153: “were also differ” should it be “were also different”?
Overall comment of the results:
- Could you also please show in supplementary materials the charts on the dependences of vertical displacement on total amount of produced gas?
- In the figures 10-14 I see in “a” charts the increase on production rates till commonly 80 days, however in “b” charts there are almost linear curves on cumulative production since ~10 day. Could you please comment on it?
Author Response
We would like to thank the reviewer for a very thorough reading of the manuscript and helpful comments. We have made changes to the manuscript in response, with attached word file.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Dear Authors
I believe that there are still some typos in the text:
lines 70-72: Something is missed in this sentence
line 79: still not clear what do you mean as “heat depletion” during depressurization method”.
Also I truly believe that it would be of significant advantage for this paper is you showed comparison chart "vertical subsidence vs cumulative gas production" for different production schemes or may be bar chart total amount of gas produced till vertical subcidience (for example for 1m, for 2m, for 3m) because I believe this is one of the key point how much of gas you can produce till time when the well may collapse. Moreover you discuss it both in abstract and in conclusions.
Other than that I do not have any other comments for this paper
Thank you
Author Response
We would like to thank the reviewer for helpful second comments. We have made minor changes to the manuscript in response, as attached file.
Author Response File: Author Response.docx
