How to Minimize the Environmental Contamination Caused by Hydrocarbon Releases by Onshore Pipelines: The Key Role of a Three-Dimensional Three-Phase Fluid Flow Numerical Model
Round 1
Reviewer 1 Report
Interesting results are presented in the manuscript, which may be of interest to readers. However, the presentation form has some minor drawbacks.
1. The most basic remark is that the figs 4, 6-17, 21 are of poor quality and look the same. It is difficult to compare them with each other. I understand that it is very difficult to demonstrate the results of 3D flow modeling, but it is necessary to change the form of presentation of the results. It is possible to reduce the number of drawings and improve the averaged results by analogy with figs 18-20.
2. You need to check the text again. For example, two red lines in the caption of Figure 19.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper deals with the environmental contamination by means of a three-dimensional numerical solver. The paper drives very interesting conclusions supported by detailed numerical simulations and sound physical interpretation of said results. In my opinion, the paper deserves publication. I however have a few comments that the authors may want to consider in a revised version of their paper:
- Page 5: They use an explicit integration scheme in time: Which order in time? Can they motivate their choice, especially in regard to the dispersion error and the numerical dissipation introduced by the numerical time integration scheme?
- Page 6: "The rock compressibility [...] all over the grid k_x = k_y = k_z": Can the authors further substantiate their hypothesis of constant hydraulic conductivity and isotropic assumption of k_x, k_y, and k_z?
- I would suggest wrapping up the equations of the model and the boundary conditions in a clearer way, rather than embedding them in the text.
- Some of the axis and colorbar labels are missing: see Figs. 6 and 7
- A more detailed description of the model's limits is expected in the conclusions
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript titled "How to minimize the environmental contamination caused by hydrocarbon releases by onshore pipelines: The key role of a three-dimensional three-phase fluid flow numerical model" by Feo et al reports a numerical study of the contamination impact and the migration of the contaminant into the surrounding environment due to the presence of a spilled oil pipeline. A three-dimensional model based on a high-resolution shock-capturing conservative method (the CactusHydro code, which was developed previously) was employed. The effects of various parameters, such as oil type, density, the unsaturated zone depth, its saturation, the hydraulic gradient, and the pressure of the oil pipeline were investigated. The results suggest that the pipeline pressure, water saturation and the unsaturated zone depth significantly influences the contaminant migration, whereas the oil density and the hydraulic gradient have limited effects.
Overall, the topic under study is a very important and interesting one. The manuscript is easy to understand, simulation seems to be well executed, and conclusions are well supported by the presented evidence. That said, the manuscript could use some more work on English writing. The literature review is not very well done, and most importantly, although the work provides a lot of food for thoughts, the novelty of the work is not clearly spelled out. Major revision is recommended. My detailed comments are provided below.
More work could be used on the writing. I found quite a few typos and grammatical errors here and there. For instance, Line 16: "on" ought to be "of". Line 23: missing "in" before "unsaturated zone". Line 2216, typo. Line 235, typo. Line 243, I guess "superficial" should be "interfacial"??
I find the literature review of this manuscript is quite insufficient. Although many previous papers were cited, they are dismissed quickly with one or two sentences. For instance, Line 82, 19 papers were simply dismissed by one sentence, without any discussion. The only one paper that the authors discussed is Ref. 39, which according to the authors presented a very similar study as the current one. So what is the new contribution of the current paper compared to the previous one? It is expected that a literature review should at least answer the following questions: what is already known in the literature? What is missing (knowledge gap)? Why is it important to fill the knowledge gap (potential significance of the work)? What is the limitation of the previous methods? What will be done differently in the current work to overcome the previous limitations? I suggest the authors significantly rewrite the introduction section with a focus on addressing the questions above.
Related to the comment on literature review, clear statements of the novelty of the work are missing. Specifically, what is the novelty of the work and where does it go beyond the previous ones in literature? The authors should clearly spell that out.
Line 199, 2MPa and 3600 were picked as the initial conditions of the potential leak. How representative of this choice? Can the authors provide some reference or basis that the choice was based on?
The structure of the results section needs to be designed. For example, 3.2 and 3.3 are on the same level of structures, but they are certainly not parallel sections. Also the authors decided to list the results of each case one by one, which is okay, but in general not a good practice. I suggest the authors structure the results part by the parameters of interest, One way to do this is to present results of a baseline case, and then do Effect of saturation, effect of density, etc. And by the way, the subsections are quite confusing, 3.3 and 3.3.1 should be more like 3.3.1 and 3.3.2 and same to 3.4 and 3.4.1. Also in this sense, it might be a good idea to perform analysis based on nondimensional parameters, such as capillary number, bond number, and viscosity ratio, which may help to make the discussion more efficient.
Section 3.3, the use of "unsaturated zone" is very misleading here. In general, the unsaturated zone is the portion of the subsurface above the groundwater table. The soil and rock in this zone contain air as well as water in its pores, so it is not equivalent to a dry zone. I would use dry zone in section 3.3 and use unsaturated zone in 3.3.1. And "partially unsaturated zone" is rarely used.
For almost all the figures (Figures 3-17) I suggest the authors label all the subfigures and describe each subfigure in the caption. And to avoid confusion, it would be better to use a different symbol other than sigma for saturation contour, as sigma has been used to denote interfacial tension.
Section 3.8 belongs to the Materials and Method section.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Round 2
Reviewer 3 Report
The authors have amended their manuscript and have added some new information, which is very helpful. Specifically, the restructuring of the sections and addition of the novelty statement of the current work have helped to strengthen the paper. Overall, I think the authors have done a very good job in answering my questions and addressing my comments. That being said, I think the introduction part, particularly the literature review, could still use some more work. For instance, Line 84 - 101, if I read this correctly, the 18 lines of text is only one sentence. This is probably the longest sentence I have ever seen. It is extremely hard to follow, and I don't really see the points that the sentence is trying to make. This needs to be rewritten. In addition, I still spotted a few text editing errors here and there (e.g., inconsistent font, inappropriate change of lines, etc.) Publication of this paper is recommended after those minor issues are properly addressed. I don’t have any more questions or comments.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
