Numerical Simulation of a Class I Gas Hydrate Reservoir Depressurized by a Fishbone Well

Round 1

Reviewer 1 Report

(1) Hydrate deposits properties and initial conditions in Shenhu area displayed in Table 1 is necessary for simulation. Where did these data come from? Based on cognitive hypothesis or from relevant literature? In addition, the research object in this manuscript is only limited to the research area (Shenhu Sea Area), and has not mentioned the site or well. 

(2) Why the phase equilibrium relationship shown as Eqs.7 was used in the investigation in manuscript? We know that there are many hydrate phase equilibrium models. What are the reasons and reasons for choosing this model?

(3) As we all know, the mathematical model can only be applied to specific case analysis after being verified. However, the numerical model in this manuscript has not been verified. Its applicability needs to be explained and supplemented by the author.

(4) In fact, the main reason why fractures can significantly improve gas production is that they can make more reservoir fluid flow to the bottom of the well and be pumped. Distribution of gas saturation should be added in Section 3.3.3. 

(5) It is mentioned in relevant literature that the decomposition of hydrate will improve the permeability of sediment and further promote the decomposition of hydrate. However, the permeability model in the model mentioned in this manuscript (Eqs.8) is considered to be independent of hydrate saturation. I think the permeability model (Eqs.8) should be changed.

(6) In INTRODUCTION, some development strategies (such as depressurization, thermal stimulation, inhibitor injection) were mentioned. Some references should be added in REFERENCE section to improve the preciseness of these statements in this manuscript.. The following papers can be added: https://doi.org/10.1007/s40948-022-00396-0, https://doi.org/10.3390/en15165951, https://doi.org/10.1016/j.molliq.2023.121394. These papers can be found by input the above website in browser.  

(7) The language of this manuscript also needs to be polished by the polishing agency or native English speakers before it can be considered for acceptance and publication. 

Author Response

Dear Editor and Reviewers,

Thank you for your letter and for the reviewers’ comments. We appreciate the efforts the editor made to supervise the reviewing process of our manuscript. We also highly appreciate the reviewer’s carefulness, conscientiousness, and the broad knowledge on the relevant research fields. Those comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made correction accordingly. The main corrections in the paper and the responses to the editor’s reviewer’s comments are as follows:

Reviewer 1

Comments and Suggestions for Authors

1. Hydrate deposits properties and initial conditions in Shenhu area displayed in Table 1 is necessary for simulation. Where did these data come from? Based on cognitive hypothesis or from relevant literature? In addition, the research object in this manuscript is only limited to the research area (Shenhu Sea Area), and has not mentioned the site or well.

Response: Thanks to the reviewer’s comment. The target hydrate reservoir is the reservoir for the second trial test. The site and related reference have been added in the manuscript

2. Why the phase equilibrium relationship shown as Eqs.7 was used in the investigation in manuscript? We know that there are many hydrate phase equilibrium models. What are the reasons and reasons for choosing this model?

Response: Yes, there are many hydrate phase equilibrium models. Moridis developed a new model because it can matches the experimental data well. Moreover, the model is continuous and derivable, and therefore, it is suitable for numerical simulation. Thanks to the reviewer, the above explanations have been added in the revised manuscript.

3. As we all know, the mathematical model can only be applied to specific case analysis after being verified. However, the numerical model in this manuscript has not been verified. Its applicability needs to be explained and supplemented by the author.

Response: Thanks to the reviewer’s comment. A validation part has been added in section 2.2 and the results of Konno and the model in this paper is compared. The detailed contents are as follows:

Konno conducted a simulation study of a hydrate reservoir using the MH21-HYDRES simulator, and the detailed parameters are comprehensively described. In order to validate the effectiveness of the model, the simulation results of Konno and the model in this paper are compared. The target hydrate reservoir is a class III reservoir. Radial grid is used and the depth of the hydrate layer is 50m. The thickness of the single grid is 1m and therefore there are 50 grids in the vertical direction. In order to simulate the influence of overburden and underburden on hydrate dissociation, the depths of the overburden and underburden are both 50m. A vertical well is used for depressurization development and the bottom hole pressure is kept at 4MPa. The reservoir permeability is 500mD and the comparison of the results is shown in Figure 1. As shown in this figure, the peak gas production of the model in this paper is slightly higher than that of Konno. However, the shapes are quite similar and a good matching result is obtained. Therefore, the model in this paper is reliable and can be used for the subsequent simulation.

Figure 1. Comparison between Konno’s model and the model in this paper.

4. In fact, the main reason why fractures can significantly improve gas production is that they can make more reservoir fluid flow to the bottom of the well and be pumped. Distribution of gas saturation should be added in Section 3.3.3.

Response: Thanks to the reviewer’s comment. The dissociated gas mainly locates at the dissociation area of hydrates. Meanwhile, Tough+Hydrate can not distinguish the dissociated gas and the initial gas at present. Therefore, in order to avoid confusion to the readers, we just show the NGH saturation distribution in the paper.

5. It is mentioned in relevant literature that the decomposition of hydrate will improve the permeability of sediment and further promote the decomposition of hydrate. However, the permeability model in the model mentioned in this manuscript (Eqs.8) is considered to be independent of hydrate saturation. I think the permeability model (Eqs.8) should be changed.

Response: There are two kinds of models used in Tough+Hydrate simulator, that is, the phase equilibrium model and kinetic model. In the phase equilibrium model, the hydrates occupy the some of the pore space. Once the hydrates dissociate, the saturation of water and gas increase. Otherwise, the saturation of water and gas decrease. By this way, the relative permeability of gas and water is influenced by hydrate dissociation and formation. In the kinetic model, the permeability is directly related with hydrate saturation. Kowalsky and Moridis compared the simulation results of the two models and nearly same results were obtained. However, the phase equilibrium model is much faster than the kinetic model, and therefore, they suggest to use the phase equilibrium model.

Thanks to the reviewer’s comment. The above explanations are added in the revised manuscript.

6. In INTRODUCTION, some development strategies (such as depressurization, thermal stimulation, inhibitor injection) were mentioned. Some references should be added in REFERENCE section to improve the preciseness of these statements in this manuscript.. The following papers can be added: https://doi.org/10.1007/s40948-022-00396-0, https://doi.org/10.3390/en15165951, https://doi.org/10.1016/j.molliq.2023.121394. These papers can be found by input the above website in browser.

Response: Thanks to the reviewers comment. However, the first website can not open now and the we have cited the second reference in the revised manuscript.

7. The language of this manuscript also needs to be polished by the polishing agency or native English speakers before it can be considered for acceptance and publication.

Response: Thanks to the reviewer’s comment. The English has been revised and we hope it can meet the standard of the journal.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors presented a numerical Study on Class I Gas Hydrate Reservoir Depressurized by Fishbone Well.

The introduction is short and should be extended.

The mathematical formulation is well presented, but the numerical method and used software are to be presented with more details.

The boundary conditions are to be presented in Fig. 2

The boundary conditions are to be expressed mathematically.

What is the convergence criterion?

A grid sensitivity test is to be performed.

What is the used time step?

What are the characteristics of the computed used to solve the governing equations?

What is the time needed for the convergence?

A reference is to be added to Table 1.

A verification/validation of the numerical model is to be performed.

The resolution of Fig 3 is very low; legends are unreadable.

Some results related to the flow structure are to be added.

Discussion are to be improved by adding physical interpretations

The paper is to be checked against misprints and grammatical mistakes.

 

 

Author Response

Reviewer 2

The authors presented a numerical Study on Class I Gas Hydrate Reservoir Depressurized by Fishbone Well.

1. The introduction is short and should be extended.

Response: Thanks to the reviewer’s suggestion. The introduction part has been extended and research on the horizontal well is reviewed as follows:” Moreover, many other researchers also found that horizontal well can greatly increase the contact area between the wellbore and reservoir. Thus, the gas production can be enhanced. Sasaki et al and Li conducted physical experiments and found that the SAGD type well configuration can greatly accelerate the hydrate dissociation. Feng et al. compared the performance of a vertical well and a horizontal well in depressurization and thermal stimulation development. The results showed that the gas production rate, the heat transfer rate and the accumulative dissociation ratio of the horizontal well are much higher than those of the vertical well. Choudhary and Phirani built a numerical simulation model based on the geological parameters of KG Basin NGHP-02-09 site and the performances of vertical wells and horizontal wells are compared. The results showed that the horizontal well leads to the early gas production. Moridis also compared the performance of a vertical well and a horizontal well and found that production using horizontal wells is about two orders of magnitude larger than that from vertical wells accessing the same section of the HBL.”

2. The mathematical formulation is well presented, but the numerical method and used software are to be presented with more details.

Response: Thanks to the reviewer. The following descriptions are added:” The current simulators on the development of hydrate reservoirs include Tough+Hydrate, MH21-HYDRES, STOMP-HYD, etc. Among these simulators, Tough+Hydrate considers comprehensive mechanisms and becomes the most widely used simulator at present. Therefore, the Tough+Hydrate simulator was used for the subsequent research.”

3. The boundary conditions are to be presented in Fig. 2. The boundary conditions are to be expressed mathematically.

Response: There are two boundary conditions used in this paper. As described in section 3.2, the fishbone well was produced with a constant bottomhole pressure of 4MPa. Therefore, the inner boundary condition is Dirichlet boundary condition. The outer boundary condition is a closed boundary that belongs to the Neumann boundary condition. The following descriptions are added in the revised manuscript:”

The closed boundary condition that belongs to the Neumann boundary condition is used in this paper. The formula can be expressed as:

                                      (9)

Where  and  are the pressure of water phase and gas phase at the boundary , respectively, Pa.

4. What is the convergence criterion?

Response: Thanks to the reviewer’s comment. The convergence criteria is that the differences of the primary variables between two Newton iterations are all smaller than 1×10^-5. The description has been added.

5. A grid sensitivity test is to be performed.

Response: Thanks to the reviewer’s comment. Considering that grid sensitivity test is the common step before the simulation, we just described the results of the grid sensitivity in the revised manuscript briefly.

6. What is the used time step?

Response: Thanks to the reviewer’s comment. The maximum timestep used in the simulation is 10 days and the information has been added in the revised manuscript.

7. What are the characteristics of the computed used to solve the governing equations?

Response: The simulator used in the manuscript is Tough+Hydrate. The following descriptions are added in the revised manuscript:” The current simulators on the development of hydrate reservoirs include Tough+Hydrate, MH21-HYDRES, STOMP-HYD, etc. Among these simulators, Tough+Hydrate considers comprehensive mechanisms and becomes the most widely used simulator at present. Therefore, the Tough+Hydrate simulator was used for the subsequent research.”

8. What is the time needed for the convergence?

Response: The time needed for the simulation is determined by the performance of the computer. Taking my computer (CPU: INTEL i5-12600, Dram: 16G) for example, the time for a simulation is about 70 minutes.

9: A reference is to be added to Table 1.

Response: Thanks to the reviewer’s comment. The reference has been added in the revised manuscript.

10. A verification/validation of the numerical model is to be performed.

Response: Thanks to the reviewer’s comment. A validation part has been added in section 2.2.

11. The resolution of Fig 3 is very low; legends are unreadable.

Response: We are sorry that the software reduced the resolution of Fig. 3. A high resolution figure has been inserted.

12. Some results related to the flow structure are to be added. Discussion are to be improved by adding physical interpretations

Response: Thanks to the reviewer’s suggestion. We are sorry that there are many grammar mistakes in the paper which leads to some explanations unclear. The English has been revised and we hope that the flow structure and discussion are clear after modification.

13. The paper is to be checked against misprints and grammatical mistakes.

Response: Thanks to the reviewer’s comment. The English has been revised and we hope it can meet the standard of the journal.

 

We tried our best to improve the manuscript and made many changes in the manuscript. We hope that all these changes fulfil the requirements to make the manuscript acceptable for publication in Energy. Once again, we appreciate for Editors/Reviewers’ warm work earnestly.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

(1) As we all know, the hydrate reservoirs in the sea area are usually shallowly buried, with poor cementation and diagenesis. The well type construction mentioned in the manuscript will be extremely complex. Is fracturing operation more appropriate than the scheme in the manuscript (regardless of construction difficulty or cost)? Please explain appropriately!

(2) The author mentioned in his reply that, at the same time, Tough+Hydrate can not distinguish free gas from initial gas at present. However, we can see from Table 1 that there is no initial gas in the initial hydrate-bearing sediment. Therefore, I still insist that gas saturation still needs to be added in the manuscript.

(3) The study area conditions and reservoir characteristics need to be simply mentioned in the manuscript. In this way, follow-up researchers will get some basic information and reference from this study.

(4) The first reference mentioned in Comment 6 is worthy of reference, so it should be cited in manuscript. Its title is Factors affecting the lower limit of the safe mud weight window for drilling operation in hydrate-bearing sediments in the Northern South China Sea. And, the website is: https://link.springer.com/article/10.1007/s40948-022-00396-0

Author Response

Dear Editor and Reviewers,

Thank you for your letter and for the reviewers’ comments. We appreciate the efforts the editor made to supervise the reviewing process of our manuscript. We also highly appreciate the reviewer’s carefulness, conscientiousness, and the broad knowledge on the relevant research fields. Those comments are all valuable and very helpful for revising and improving our paper. We have studied comments carefully and have made correction accordingly. The main corrections in the paper and the responses to the editor’s reviewer’s comments are as follows:

Reviewer 1

Comments and Suggestions for Authors

1. As we all know, the hydrate reservoirs in the sea area are usually shallowly buried, with poor cementation and diagenesis. The well type construction mentioned in the manuscript will be extremely complex. Is fracturing operation more appropriate than the scheme in the manuscript (regardless of construction difficulty or cost)? Please explain appropriately!

Response: Thanks to the reviewer’s comment. We agree with the reviewer’s comment that the drilling of a fishbone well is quite complex in hydrate reservoirs due to the poor cementation and shallowly buried depth. However, the fishbone well is a special kind of horizontal well and the difficulty in drilling this kind of well is how to build up deflection quickly in a shallowly buried reservoir. The second trial test of China has proved that drilling a horizontal well in shallowly buried hydrate reservoir is feasible. The branches of the fishbone can be created by many methods (cavitation jet drilling, etc.) and the feasibility of drilling branches has been studied by many researchers (Zhang et al. [1]). While referring to fracturing, it is a hot topic in development of hydrate reservoirs. However, the hydrate reservoir is poorly cemented. Whether fractures can formed and exist in a long time if they are formed are not clear at present. Moreover, hydraulic fracturing will greatly damage the reservoir and whether geological disaster will be aroused is not fully studied.

Thanks to the reviewer’s comment. We have explained this in the revised manuscript briefly.

References:

[1] Zhang Y, Hu X, Wu X, Li G, Li J, Tian S, et al. A study of swirling jet drilling and its specific energy for hydrate-bearing sediments. SPE Journal, 2022; doi: https://doi.org/10.2118/212287-PA.

2. The author mentioned in his reply that, at the same time, Tough+Hydrate can not distinguish free gas from initial gas at present. However, we can see from Table 1 that there is no initial gas in the initial hydrate-bearing sediment. Therefore, I still insist that gas saturation still needs to be added in the manuscript.

Response: Really thanks to the reviewer’s valuable comment. I think there are some misunderstandings in my descriptions. There is no initial gas in the hydrate layer. However, the mixed layer which located under the hydrate layer has initial gas saturation of 0.132. Once simulation begins, the free gas in the mixed layer will flow into the hydrate layer and we can not distinguish the free gas and the dissociated gas. So we just analyzed the gas generation from the hydrate saturation distribution (the decrease of hydrate saturation means generation of dissociated gas).

3. The study area conditions and reservoir characteristics need to be simply mentioned in the manuscript. In this way, follow-up researchers will get some basic information and reference from this study.

Response: Thanks to the reviewer, in section 3.2, the study area conditions and reservoir characteristics are described as follows: “The geological parameters of the gas hydrate reservoir for the second trial test in the Shenhu area of South China Sea were investigated, and the values of main parameters were shown in Table 1 [19]. The gas hydrate reservoir in the test production area contains three main horizons, namely, hydrate layer, mixed layer and gas layer. As shown in Figure 3a, the hydrate layer is saturated with water and hydrate, the mixed layer is saturated with water, hydrate and gas, and the gas layer mainly contains gas and water.” The reference also is added and the readers could know more information about it.

References:

[19] Ye J, Qin X, Xie W, Lu H, Ma B, Qiu H, et al. The second natural gas hydrate production test in the South China Sea. China Geology, 2020; 3(2):197-209.

4. The first reference mentioned in Comment 6 is worthy of reference, so it should be cited in manuscript. Its title is Factors affecting the lower limit of the safe mud weight window for drilling operation in hydrate-bearing sediments in the Northern South China Sea. And, the website is: https://link.springer.com/article/10.1007/s40948-022-00396-0

Response: Thanks to the reviewer’s comment and we have cited this paper in the revised manuscript.

 

We tried our best to improve the manuscript and made many changes in the manuscript. We hope that all these changes fulfil the requirements to make the manuscript acceptable for publication in Energy. Once again, we appreciate for Editors/Reviewers’ warm work earnestly.

Author Response File: Author Response.docx

Reviewer 2 Report

Accept as it is

Author Response

Thanks for your effort to improve the quality of this manuscript. 

Round 3

Reviewer 1 Report

So far, the manuscript can be considered to be accepted and published.