Control Mechanism of Microbial Degradation on the Physical Properties of a Coal Reservoir

Round 1

Reviewer 1 Report

In this paper, the influence of microbial methanogenesis on the physical properties of coal reservoirs was studied. The effects of microbial gas production on the physical properties of coal were analyzed by means of low temperature liquid nitrogen adsorption test, isothermal adsorption and diffusion coefficient test on two coal samples before and after anaerobic degradation. But on the whole, the research content is shallow, there is no discussion of some reasons, only a statement of the results.

The following are specific review comments :

1. 3.1 Biological methane output characteristics of methane production unit μmol / g is how to convert, please explain in detail.

2. The third chapter discusses the language is not refined enough, too cumbersome.

3. The specific difference between the two coal samples used in the experiment, and the results of the comparison of the two coals in the conclusion are not explained, please explain in detail in the conclusion.

4. The conclusion is too simple, not clear enough, the proposal should be more specific.

5. Figure 6 is not complete. Please check the manuscript.

Author Response

1. 3.1 Biological methane output characteristics of methane production unit μmol / g is how to convert, please explain in detail.

Response:

The amount of gas in the experiment is generally represented by ml. Under standard conditions, 1 mol gas = 22.4L, that is, 1ml is about 0.0446 μmol. μmol / g is the total amount of gas produced in a group of experiments divided by the total mass of coal samples used in this group, representing the average amount of methane produced by 1g coal samples after microbial degradation after the end of the experimental period.

2. The third chapter discusses the language is not refined enough, too cumbersome.

Response:

The contents of 3.2.1 and 3.4 are deleted, and the deleted context has been marked red in the paper.

 

3. The specific difference between the two coal samples used in the experiment, and the results of the comparison of the two coals in the conclusion are not explained, please explain in detail in the conclusion.

Response:

HM is lignite is low rank coal and JM is bituminous coal is medium rank coal. The comparison results of two kinds of coal have been explained in the conclusion.

 

4. The conclusion is too simple, not clear enough, the proposal should be more specific.

Response:

The comparison results of HM and JM are increased.

(3) Because the metamorphic degree of bituminous coal is higher than that of lignite, the structure of bituminous coal is more difficult to degrade than that of lignite, resulting in a longer hydrolysis stage and the overall gas production lags behind that of lignite. After microbial degradation, the pore content of each pore size of lignite samples decreased, while bituminous coal only had obvious micropore effect, and the number of mesopores increased slightly, because the metamorphic degree of lignite was low and microorganisms were easy to transform it. Compared with lignite, the diffusion coefficient of bituminous coal after microbial degradation increases less, but compared with that before degradation, the diffusion coefficient increases greatly, which can make methane gas escape in a large amount in the coal seam after microbial transformation, so as to achieve the effect of increasing coalbed methane production.

 

5. Figure 6 is not complete. Please check the manuscript.

Response:

Figure 6 (a) has been deleted before the analysis, so there is no figure a, and the remaining part of the figure has been modified.

Reviewer 2 Report

Dear Authors,

The present study aims to observe the effect of microbial methane producers on coal reservoirs. Samples were obtained from coal reservoirs, and coal's physical properties were evaluated before and after microbial activity. Authors claim that bacterial enzymes decompose macromolecules in coal, changing coal properties and increasing the coalbed methane production. The study is interesting. However, important information is missing. How many samples were collected? Where are the negative controls of the study? How many replicates were tested? Statistical analysis to support the results is essential. What bacteria were used? How were the bacteria defined as methanogenic? Many surprises appeared along the results because were not mentioned or explained in the material and methods, such as the association of lignite to HM, and bituminous to JM coal samples; or the time points planned for the experiment; or pore structures (micropores, mesopores, and macropores. The figures have incomplete legends and need to be improved.

All these major points need to be addressed. Other minor specific points I have identified are the following.

 

Introduction –

 

L56-57 – substitute “some researchers have come to the opposite conclusions, saying” for “some studies have shown”.

 

Material and Methods –

 

L104-106 – Why is the sentence in between parentheses? Please, remove them and include weighed specifications, including brand, model, and manufacturer.

 

L115 – substitute “is not detected, it needs to be” for “was not detected, it was”.

 

L119-120 – Please, rewrite this sentence. It looks unclear. Is it the 1:10 range? What does the bacterial liquid mean?  Why is the sentence in between parentheses?

 

Results and discussion – 

 

L203 – Has been mentioned that “In high-pressure region, the adsorption of both coal samples increased significantly”. Do you have any statistical data to support this statement?

 

Figures and tables – 

 

Tab 1- Please, include the definition of all acronyms present in the table, such as “Mad”, “Aad”, “Vdaf”, “Cdaf”, “Hdaf”, “(O+S)daf”, “Ndaf”, and “Romax”.

 

Fig 5 – What is the information in a table inside the figure? It is not readable.

 

Fig. 6 – I see figure b. If there is figure a, where is it? 

Author Response

1. How many samples were collected?

Response:

Each sample is about 5 kg.

 

2. Where are the negative controls of the study?

Response:

HM and JM contrast each other.

 

3. How many replicates were tested? Statistical analysis to support the results is essential. What bacteria were used?

Response:

At the beginning of the experiment, there were multiple sets of identical parallel samples, and the bacteria used were mainly methanogens enriched from mine water.

 

4. How were the bacteria defined as methanogenic? Many surprises appeared along the results because were not mentioned or explained in the material and methods, such as the association of lignite to HM, and bituminous to JM coal samples; or the time points planned for the experiment; or pore structures (micropores, mesopores, and macropores.

Response:

The methanogen culture medium was used to enrich the bacteria from the mine water, and the enriched bacteria were sent to gene sequencing. The results of gene sequencing showed that the bacterial solution contained several methanogens.

The correlation between lignite and HM, bituminous coal and JM coal samples and the time points of the experimental plan have been added in 2.1.1 and 2.1.3.

The coal samples are low rank lignite ( HM ) from Baiyinhua Coal Mine in Inner Mongolia Autonomous Region and medium rank bituminous coal ( JM ) from Shaqu Coal Mine in Liulin County, Shanxi Province.

Gas samples are tested every three days.

 

5. The figures have incomplete legends and need to be improved.

Response:

Has been modified.

 

6. Introduction –

L56-57 – substitute “some researchers have come to the opposite conclusions, saying” for “some studies have shown”.

Response:

Has been modified.

However, some studies have shown that the microbial action of the coal increases the surface of coal fissures

 

7. Material and Methods –

L104-106 – Why is the sentence in between parentheses? Please, remove them and include weighed specifications, including brand, model, and manufacturer.

Response:

The sentence in brackets is a supplementary explanation of the precision of weighing drugs. It has been revised.

Using GL224-1SCN electronic balance, precision 0.0001g

 

L115 – substitute “is not detected, it needs to be” for “was not detected, it was”.

Response:

Has been modified.

If methane was not detected, it was re-enriched until methane is generated.

L119-120 – Please, rewrite this sentence. It looks unclear. Is it the 1:10 range? What does the bacterial liquid mean? Why is the sentence in between parentheses?

Response:

Has been modified.

After successful enrichment, the enriched bacterial liquid was expanded according to the ratio of enriched bacterial liquid to medium 1:10.

8. Results and discussion –

L203 – Has been mentioned that “In high-pressure region, the adsorption of both coal samples increased significantly”. Do you have any statistical data to support this statement?

Response:

It can be concluded from Fig.2.

 

9. Figures and tables –

Tab 1- Please, include the definition of all acronyms present in the table, such as “Mad”, “Aad”, “Vdaf”, “Cdaf”, “Hdaf”, “(O+S)daf”, “Ndaf”, and “Romax”.

Response:

Mad - moisture of coal sample, % ;

Aad-ash of coal sample, % ;

Vdaf - dry ash-free volatile matter of coal samples, %;

Cdaf, Hdaf, (O+S) daf and Ndaf represent the percentage content of C, H, ( O + S ) and N components in dry ashless base of coal samples.

Romax represents the average maximum reflectance of vitrinite of coal samples.

 

Fig 5 – What is the information in a table inside the figure? It is not readable.

Response:

This is the fitting curve equation and related parameters.

 

Fig. 6 – I see figure b. If there is figure a, where is it?

Response:

Figure 6 (a) has been deleted before, no figure (a), has been modified.

 

Round 2

Reviewer 2 Report

Dear authors,

Thank you for the answers. I have reviewed the new version and appreciated the changes made in the manuscript. Unfortunately, with the information in the manuscript, I am not convinced that the bacteria are responsible for the observed differences of methane production. Important points were not covered by your answers and I must reinforced the importance of those.

1- I think is fine that every collected sample has an amount of 5Kg. However, there are no information about how many samples were used in every comparative approach. It is essential that any comparison has enough number of independent samples to guaranty reproducibility of results. Please, include the number of independent samples used in the study.

2- I understand that HM and JM are different coal samples. None of them are comparable to a negative control. Also, each of them has different characteristics but they are not contrast samples. Whether the aim of the study is to observe the effect of the microbial methane production, the comparative analysis must be samples with or without microbial methane producers. Please, if no negative controls are used include the information on material and methods and downgrade your conclusion about the microbial effect on methane production.

3- Every set of identical parallel sample should be described on material and methods. Did your experiments demonstrate that the methanogens are exclusively bacteria? If a sequencing data has this information it must be shown in the manuscript.

4- I did not see any improvement in the legend of figures. The figure must be self explanatory. The acronyms meaning should be on legend. Please. improve the legends with information. Also, figure 5 has words in a non-english language. Please, fix it.

Best regards.

 

 

 

 

Author Response

Thank the editors and reviewers again for their constructive opinions on our revised manuscript ' Control mechanism of microbial degradation on physical properties of coal reservoirs. All the comments have greatly promoted the improvement of our paper and provided important guidance for our future research. We have carefully read all the comments and made corrections accordingly. We show the responses marked in blue in the revised version, and also outline each ( point-by-point ) change proposed in the reviewer's comments. The main amendments and responses to the editorial opinions of the paper are as follows. If there is something wrong with reply or modification, please give us an opportunity to explain and modify.

Reviewer 2

Thank you for the answers. I have reviewed the new version and appreciated the changes made in the manuscript. Unfortunately, with the information in the manuscript, I am not convinced that the bacteria are responsible for the observed differences of methane production. Important points were not covered by your answers and I must reinforced the importance of those.

 

1- I think is fine that every collected sample has an amount of 5Kg. However, there are no information about how many samples were used in every comparative approach. It is essential that any comparison has enough number of independent samples to guaranty reproducibility of results. Please, include the number of independent samples used in the study.

Response:

The number of coal samples required for the experiment is 80 ~ 100 mesh, and there will be more consumption in the process of grinding into 80 ~ 100 mesh pulverized coal. Before the experiment, two core columns with a diameter of about 25 mm were drilled from each of the two coal rock samples, and the core column was about 25 g. In this experiment, a group of blank groups and two experimental groups were set up. The experimental group was 500ml bacterial solution + 50g 80-100 mesh HM or JM pulverized coal, plus two parallel samples. The parallel sample was also set to 500ml bacterial solution + 50g 80-100 mesh HM or JM pulverized coal, and one was 500ml bacterial solution + core column. That is, a coal sample in this batch of experiments requires about 50g of coal pillars and 150g of 80-100 mesh pulverized coal, about 200g ( coal grinding loss is not counted ).

 

2- I understand that HM and JM are different coal samples. None of them are comparable to a negative control. Also, each of them has different characteristics but they are not contrast samples. Whether the aim of the study is to observe the effect of the microbial methane production, the comparative analysis must be samples with or without microbial methane producers. Please, if no negative controls are used include the information on material and methods and downgrade your conclusion about the microbial effect on methane production.

Response:

This batch of experiments was set to three groups. The first group was a blank control group with three bottles of 500ml bacterial solution without pulverized coal. The second group was HM group, three bottles of 500ml bacterial solution + 50g 80-100 mesh HM ( two bottles were parallel ), one bottle of 500ml bacterial solution + HM core column. The third group was JM, three bottles of 500ml bacterial solution + 50g 80-100 mesh JM ( two bottles were parallel ), and one bottle was 500ml bacterial solution + JM core column. The first group was the blank control of the second and third groups, and the second and third groups were compared with each other.

This paper focuses on the changes of raw coal without microbial action and coal samples degraded by microorganisms ( mainly methanogens ) in bacterial solution, so the negative control you said is raw coal without microbial action in our opinion.

 

3- Every set of identical parallel sample should be described on material and methods. Did your experiments demonstrate that the methanogens are exclusively bacteria? If a sequencing data has this information it must be shown in the manuscript.

Response:

The materials and methods of parallel samples were explained in the first two questions.

Methanogens are not the only bacterial genus in the enriched bacterial solution. The bacterial solution also contains other bacteria, and methanogens account for the majority. However, other genera have little effect on methane production and coal samples, so they are often ignored.

 

4- I did not see any improvement in the legend of figures. The figure must be self explanatory. The acronyms meaning should be on legend. Please. improve the legends with information. Also, figure 5 has words in a non-English language. Please, fix it.

Response:

The meaning of the abbreviations in the picture is explained in the original text, so I misunderstood it when I first revised it. I am sorry about this, but I have revised it in the picture now. Because in many places in the article, lignite and bituminous coal have been written as HM and JM. In order to facilitate reading and comparing the legend, HM and JM in the legend have not been modified to lignite and bituminous coal. Please forgive me. And others have been modified.

Author Response File: Author Response.docx