Analysis of the Characteristics of Pore Pressure Coefficient for Two Different Hydrate-Bearing Sediments under Triaxial Shear

Round 1

Reviewer 1 Report

The authors worked hard in the revision of their manuscript. All my previous comments have been included on it, thus I suggest to consider the article for publication in this Journal. I only suggest to sherten the introduction and to make it more concise; in this revision it has been widely extended and now it seems to be slightly dispersive.

Author Response

Reply:

We gratefully appreciate for your valuable comments and thank you for your significant reminding. We have shortened the introduction and to make it more concise seen in the revised manuscript.

Reviewer 2 Report

The authors have made a good attempt in characterizing pore pressure upon association of gas hydrate. Some comments are:

1. Please define p', Δp' and Δu, or remove symbols from the abstract.
2. Eq 1: what do you mean by "the excess pore pressure change Δu"?
3. Fig 1: make figure caption self-descriptive of the figure.
4.  Fig 3b is not consistent with Fig 3a and 3c, please check your calculation. Use smaller increment of axial strain in code. Similarly check calculation of Figs 3d, e & f.
5.  Similarly, check other Figs.

I am unable to follow the behaviours of the pore pressure coefficients at different hydrate saturation. It looks so chaotic. Please check the calculations carefully.

There are so many sentences without proper meaning, read carefully and correct them.

All the best

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Overview and general recommendation:
Understanding the mechanical response of gas hydrate-bearing sediments (GHBSs) under stress (i.e.,  analyzing their strength and deformation characteristics), is essential to assure production efficiency as well as the  mechanical stability of the reservoir and of other infrastructures associated during exploitation. This manuscript presents a correlation between the pressure coefficient A and the mechanical response of GHBSs subjected to triaxial shear under undrained conditions. Even though this is a new way of presenting\evaluating the mechanical response of GHBSs and the conclusions presented here match previous observations of the effects of hydrate saturation and effective confining stress on the overall mechanical response of GHBSs, I struggle to see the novelty of this manuscript.
In particular, one of the main issues related to the mechanical behaviour of GHBss during hydrate exploitation, which is mentioned in the manuscript, is that the main volumetric changes occur due to hydrate phase change and the depressurization path required for unstabilizing hydrates. However, this study is limited to examining the mechanical characteristics of GHBSs at constant hydrate saturation, ignoring the effect of Sh changes, or the stress path. Thereby, and because extensive studies have already characterized the mechanical behavior of GHBSs under similar conditions, I recommend reconsidering the paper publication after major revision. The items that should be addressed in revision are listed below:

General comments:
I think the effort of this paper should be directed towards examining the volumetric response of GHBSs during dissociation. If not, the authors should clearly specify why do they do not account for volumetric changes associated to hydrate dissociation and also highlight what is the novelty of this work and how that could be used/applied in future reserach.

The authors should justify why they use the experimental data selected and why this is relevant/representative (including testing conditions) for potential in-situ exploitation scenarios. In addition, the potential effects on the sediment mechanical behavior of the procedure used in the experiments to form hydrates and the different gas compositions (CO2-CH4) used should be discussed.

Along the manuscript (including the abstract) the authors use acronyms that have not been described before (e.g., Ap’ in Line 27). I recommend including a table summarizing all the nomenclature used to avoid this type of issues.

It is worth noting previous notable numerical and experimental efforts at studying the geo-mechanical properties o of MHBS  (e.g., https://www.sciencedirect.com/science/article/abs/pii/S1875510020306090?via%3Dihub)

The coefficient A requires to be derived from experimental data. How is then this parameter useful when geomechanicals tests are not available? How does this compete with numerical models developed to predict the mechanical response of GHBSs? ( An international code comparison study on coupled thermal, hydrologic and geomechanical processes of natural gas hydrate-bearing sediments - ScienceDirect)

I would highly recommend showing the q vs axial strain plot for all the geo-mechanical experiments analyzed to better correlate the plots presented (peak, post-peak ... behaviors).

Captions of Figures 2 to 6 can be massively shortened using the following structure: Effect of hydrate saturation on ? value of sand sediments at (a) ?0′=1MPa, (b) ?0′=2MPa ….

Specific comments:

I would suggest including “under triaxial shear” at the end of the current title to define the scope of the work presented.

Line 35: The exploitation of methane as an energy source is not clean!, you could say, as much, “cleaner”, but is important to remember that is still an energy source based on hydrocarbon burning.

Line 39: “exploitation stability”. Note that there is a clear difference between the mechanical stability of the reservoir and the stability of the exploitation process, which includes allowing production efficiency without further complications like clogging, thermic limitations, etc... The authors should specify that you are refering to the mechanical stability of the reservoir during the exploitation process.

Lines 43-44: Excess pore-pressure can also be related to the dissociation of hydrates, the overall response of the reservoir is linked to a fully-coupled thermo-hydro-geochemical-mechanical process.

Line 45: You should mention that the volumetric response before dissociation depends on the hosting sediment type as well as maybe the habit of the hydrate crystal.

Line 64: I would recommend to initiate this paragraph with a detailed explanation of coefficient A and the meaning of their values and implications to assess the mechanical behavior of GHBSs.

Line 147: What do you mean by isolated?

Section 3: Specify within the text the equations used to estimate A, Af, and A’ and graphically indicate Af on the plots presented.

Lines 161-162: “in Figure 2 (c), the curve of hydrate-bearing specimen exceeds that of hydrate-free only at small strain stage.” Please explain why this behavior is different here.

Line 169: Couldn’t it just be that both sediments haven’t reached yet the critical state? It seems that after 15% of deformation both curves tend to the same value.

Line 206:  The distinct behavior of the blue line observed before the post-peak required further explanation.

Figure 6: What is the difference between MSc and M?

Reviewer 2 Report

I found the  article interesting of quality: the theme discussed is well defined and the following discussion is concretely supported by data provided in the manuscript. I believe that the article is suitable for publication in this Journal. I would only suggest to revise some minor elements. In Section 2, the meaning of coefficient A and B must be expressed immediately below Equation 1. The first coefficienct is only mentioned in the previous section, while the second appears for the first time in the equation. More in general, all parameters used during the analytical discussion, must be specified clearly and in advance. 

There are two Section 3, put it together.

The first sentence used in Section 3.1 is exaggeratedly long and difficult to folow; it should be modified. In the same way, the first sentence in Section 3.2 is very similar to the previous one I mentioned and shoud be revised accordingly.