The Enhanced Oil Recovery Effect of Nitrogen-Assisted Gravity Drainage in Karst Reservoirs with Different Genesis: A Case Study of the Tahe Oilfield

Round 1

Reviewer 1 Report

The paper titled “The EOR effect of nitrogen-assisted gravity drainage in karst reservoirs with different genesis: A case study of the Tahe Oilfield” presents a lab experimental study on nitrogen-assisted gravity drainage on remaining oil in reservoirs with different karst genesis. I have the following comments on the study to adequate reply to which a recommendation can be made.

1.   Avoid using acronyms in the title, which may confuse the general readers.

2.  The novelty of the study is not established well. The author should emphasize how this study improves the understanding of the phenomenon of nitrogen-assisted gravity drainage of the mentioned oilfield over and above the others cited here.

3.  There are some typos and grammatical errors in the manuscript. A thorough check is needed.

4.  The quality of all the figures is very poor and needs much improvement. Extensive improvement is required.

5.    Much more details are to be added to the figures presenting experimental visuals.

 

6.  The conclusions section needs to be revisited and revised totally. This section should represent the salient findings of the study, which improves the knowledge base of the field over the existing understanding. The present conclusions look very generic, a repetition of what has been done in the study. 

Overall fine. Some typos and grammatical errors are there. Needs a thorough check.

Author Response

Reviewer 1:

The paper titled “The EOR effect of nitrogen-assisted gravity drainage in karst reservoirs with different genesis: A case study of the Tahe Oilfield” presents a lab experimental study on nitrogen-assisted gravity drainage on remaining oil in reservoirs with different karst genesis. I have the following comments on the study to adequate reply to which a recommendation can be made.

1. Avoid using acronyms in the title, which may confuse the general readers.

Re: Thank you very much for your comments.

According to the reviewer’s comments, we have rewritten the title in the revised manuscript as following: 

The enhanced oil recovery effect of nitrogen-assisted gravity drainage in karst reservoirs with different genesis: A case study of the Tahe Oilfield

2. The novelty of the study is not established well. The author should emphasize how this study improves the understanding of the phenomenon of nitrogen-assisted gravity drainage of the mentioned oilfield over and above the others cited here.

Re: Thank you very much for your comments. We are very sorry for making you confused.

According to the reviewer’s comments, we have emphasized how this study improves the understanding of the phenomenon of nitrogen-assisted gravity drainage of the mentioned oilfield in the Introduction section of revised manuscript as follows:

The comprehensive review of the above-mentioned research shows that there are three typical reservoirs in fractured-vuggy carbonate reservoirs according to different karst genesis, which are the epikarst zone, underground river, and fault-karst reservoirs, the type of karst reservoir formed by different factors has an important effect on the distribution of remaining oil, especially on EOR effects under varying oil recovery methods. In these works, the model was made according to the distribution probability of fractures and caves in the reservoir, not based on the geological model. And there is a certain gap between its structure and the actual reservoir, therefore, the results of the study can not directly reflect the actual situation in the oilfield.

In this work, physical models of karst reservoirs of different karst genesis based on geological models were established. The experiments of natural bottom water flooding, artificial water flooding, and N₂ flooding were carried out to obtain the distribution characteristics of the remaining oil in the various karst reservoirs. Then, the NAGD experiment was carried out. The research results are in favor of understanding the feasibility of NAGD and the effect of different reservoir types on quantitative productivity analysis and providing theoretical and technical support for the development of fracture-vuggy reservoirs in the Tahe Oilfield.

3. There are some typos and grammatical errors in the manuscript. A thorough check is needed.

Re: Thank you very much for your comments.

According to the reviewer’s comments, the manuscript has been fully revised to improve the level of language.

4. The quality of all the figures is very poor and needs much improvement. Extensive improvement is required.

Re: Thank you very much for your comments. We are very sorry for making you confused.

According to the reviewer’s comments, we have carefully checked and improved the quality of all the figures to in accordance with that adopted by the journal.

5. Much more details are to be added to the figures presenting experimental visuals.

Re: Thank you very much for your comments.

According to the reviewer’s comments, much more details have been added to the figures presenting experimental visuals in the revised manuscript.

6. The conclusions section needs to be revisited and revised totally. This section should represent the salient findings of the study, which improves the knowledge base of the field over the existing understanding. The present conclusions look very generic, a repetition of what has been done in the study.

Re: Thank you very much for your comments. We are very sorry for making you confused.

According to the reviewer’s comments, we have carefully checked and improved the Conclusions section to represent the salient findings of the study in the revised manuscript as follows:

(1) For the epikarst zone reservoir, it is easier to realize uniform displacement and greatly improve sweep efficiency in the NAGD stage, to effectively enhance the degree of remaining oil recovery in the reservoir. NAGD flooding can improve the recovery by 8.85% compared with low injection-high production N₂ flooding.

(2) For the underground river reservoirs, NAGD flooding can effectively start the inter-well residual oil in underground river reservoirs, and the final oil recovery factor is lower than that of N₂ flooding of low-injection and high-injection by 6.28%.

(3) The injection gas tends to gather at the top of the production well, the injection fluid is mainly longitudinal flow, the transversal displacement is less, and the production well is in a low-pressure area, therefore, gas channeling occurs easily. For the fault-karst reservoir, NAGD technique could increase recovery by 11.7% compared with gas flooding of low injection and high production.

The experiment result shows that nitrogen-assisted gravity drainage technology can enhance the recovery of fault-karst reservoirs to the greatest.

7. Overall fine. Some typos and grammatical errors are there. Needs a thorough check.

Re: Thank you very much for your comments.

According to the reviewer’s comments, the manuscript has been fully revised to check typos and grammatical errors and improve the level of language.

Special thanks to you for your good comments.

Reviewer 2 Report

The manuscript entitled “The EOR effect of nitrogen-assisted gravity drainage in karst reservoirs with different genesis: A case study of the Tahe Oilfield” was researching on applying nitrogen-assisted gravity drainage method in Tahe oilfield reservoirs. Both numerical and physical modelling tools were utilized. This paper is generally well-written and well-organized, and the results and analysis are presented in an appealing, informative and coherent manner. The paper is recommended to be published; however, some minor revisions are strongly recommended. For example:

1.  There are some minor grammatical errors in the writing, please make changes to improve.

2.  Basically, all the figures, especially the plots, in this paper are blurry. The author is responsible for presenting the results in the clearest form.

There are some minor grammatical errors in the writing, please make changes to improve.

Author Response

Reviewer 2:

The manuscript entitled “The EOR effect of nitrogen-assisted gravity drainage in karst reservoirs with different genesis: A case study of the Tahe Oilfield” was researching on applying nitrogen-assisted gravity drainage method in Tahe oilfield reservoirs. Both numerical and physical modelling tools were utilized. This paper is generally well-written and well-organized, and the results and analysis are presented in an appealing, informative and coherent manner. The paper is recommended to be published; however, some minor revisions are strongly recommended. For example:

1. There are some minor grammatical errors in the writing, please make changes to improve.

Re: Thank you very much for your comments. We are very sorry for making you confused.

According to the reviewer’s comments, the manuscript has been fully revised to check grammatical errors and improve the level of language.

2. Basically, all the figures, especially the plots, in this paper are blurry. The author is responsible for presenting the results in the clearest form.

Re: Thank you very much for your comments.

According to the reviewer’s comments, all the figures, especially the plots, in the revised manuscript have been carefully checked and improved.

3. There are some minor grammatical errors in the writing, please make changes to improve.

Re: Thank you very much for your comments. We are very sorry for making you confused.

The manuscript has been fully revised to correct the grammatical errors and improve the level of language.

Special thanks to you for your good comments.

Round 2

Reviewer 1 Report

The authors have addressed all the comments and queries adequately. I have no further queries and suggest accepting the manuscript for publication in its present form.