Review Reports

Round 1

Reviewer 1 Report

This is an interesting and useful concept that requires further investigations. The paper makes a good effort to present the issues and describe the situation used as the example. My main concern is that the paper does not satisfactorily describe the limitations, caveats and restrictions of the proposed storage of water in the disused underground coal mine workings. Essentially, this is a variant on Managed Aquifer Recharge, or Aquifer Storage and Recovery and is used in the hard rock mining industry on a regular basis. Use in a coal mine setting is more unusual as there are inherent difficulties in sub-surface management and usually significant water quality issues to be overcome, including heavy metal and hydrocarbon dissolution and storage collapse resulting in high risk to any existing mining activities. Hence this technology is not used in active coal mines where other sources of water are available. 

Your closing paragraph of section 3 which outlines the peculiar conditions of the mine and stratigraphy is a fundamental statement for this work and should be stated up-front to emphasise that mine conditions need to be considered first, prior to conceptualisation and subsequent numerical estimations of storage volumes. Indeed, parameters such as the Terzaghi effective stress are only applicable here because of the specific mine conditions and would not be universally correct.

Ultimately, only 2 bores are calculated to be in "good" locations and presumably indicate conditions that should be considered for goaf storage, yet These locations show opposing characteristics on many maps (e.g. coal seam floor sand ratio of 0.6 for H70 and only 0.3 for H63), despite the final results being good for both. This should be elaborated to demonstrate how opposing values in combinations can result in a good location.

An underground storage is apparently already in use (above the 12301 mining face) but I could not determine how this relates spatially to any of the results or to the analysed boreholes' locations. This is a critical omission. As should be a full verification of the methodology, but perhaps I missed something?

There is also no coherent summary of the integrated map results. How would these spatial distributions be used to guide future reservoir constructions. Indeed, the engineering aspects of what is meant by "built" are not covered.

The figures in general are adequate, but not great! Figure 2 needs description, legend and explanation of the different colours, layers and symbols. The contour maps should be enlarged so the numbers can easily read, or modified to enhance the borehole locations, at least.

The goafing figure seems to imply homogeneous fracturing which is not accurate but might be considered for bulk storage parameterisation as long as flow is not critical and we are interested in total storage capability.

The empirical formula #2 requires a reference to the original documentation and explanation why this can be used in this mine's conditions. A generalised formula of this nature is likely highly site-dependent and should not be relied upon unless there is adequate evidence to show the relationship holds across multiple sites. This is not clear in the paper.

Overall I think this is a methodology that should be further investigated at other mines and under differing conditions. The potential to provide a quantitative assessment of potential risk from re-injection of water to disused workings t an active mine is something that should be a focus of research.

The paper’s English is passable but should be reviewed and improved prior to publication.

The paper can be understood but requires further English editing and some re-organisation. The paper's logic does not flow, which makes reading and evaluation difficult. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper authors investigated the geological factor that influence the underground water storage capacity. The paper presents a numerical methodology to rank the reservoir. However, paper presentation needs to be improved, it is not clear what is the original work of this paper.

1. In introduction, many papers studying the effect of geological factors on storage have been reported, so what is the novelty of this paper, is the mathematical model and algorithm the novelty?

2. Figure 1, it would be helpful to have the color scale and legend.

3. Is HJT has a full form or is it well know mine field?

4. What is the relation between h1 and R, h2 and F?

5. Eq 1-23 are the correlations based on previous studies or based on the experiments considered? 

The paper is mostly well written with few grammatical errors.

1. On page 2, line 66, "A mathematical model", a should be in lower case.

2. Equations need to be referred as Equation 1, 2 and so on in text instead of formula 1, 2 (Page 3)

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

(1) In the Abstract, add some quantitative key data appropriately to support the relevant conclusions. In Figure1,  add relevant textual explanations as appropriate. Currently, there are arrows in Figure 1 but no explanations.

(2) The title of the second part (2. Materials and Methods) should be revised because the manuscript is not an experimental study, but a computational simulation study. You know, there are already some relevant analyses available regarding your research content. What is the difference (progressiveness) between the model built in your research (Part 2) and the model in previous research?

(3) In Section 3.1, it is best to include a map that introduces the regional geological conditions. In this way, the regional structure can be better and more vividly displayed. Correspondingly, the length of the main text of this section can be appropriately reduced.

(4) The statement in Lines 40-42 needs to be supportted by some references. ① https://doi.org/10.1016/j.oceaneng.2023.114949; ② https://doi.org/10.1007/s11053-023-10202-7; ③ https://doi.org/10.1016/j.jcis.2022.12.160

(5) Chemical reactions occur between water and rock, which in turn affect water storage capacity. For this reason, water compatibility and water-rock reaction are also important aspects in the study of water storage capacity. Did the authors consider the effect of this factor?

(6) Can the research in the manuscript roughly estimate the water storage capacity (storage capacity) of the study area? 

The language of the manuscript requires moderate polishing and revision. Academic manuscripts should avoid using long and difficult sentences as much as possible, and use academic language in a standardized manner.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for responding to my comments. You have demonstrated that you appreciate all issues that I considered and have adequately considered them in your paper.

 

There are a few places I would phrase slightly differently, but I think the text is adequate as is, so I do not suggest any changes.

Reviewer 3 Report

The paper can be accepted for publication now, and it was fine now.