Peer review of: Contactless probing of polycrystalline methane hydrate at pore scale suggests weaker tensile prop (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
In this paper, the authors reported the growth of thin films of the hydrate at 15MPa in thin glass 
capillaries, and they showed that the thermal-induced stress results in the failure of their prepared 
hydrate samples (hydrate shell). However, there are several important issues the authors needed 
to address further before publication in Nature Communications. 
(a) My first important concern: How to evaluate the feasibility and availability of the experimental 
method in obtaining the failure properties of hydrate shell in this manuscript? In other words, the 
authors should show the converged results. How much will the thermal-induced lattice expansion 
of hydrate shell and the thin glass capillary affect the present results? It should be noted that the 
small expansion of the thin glass may produce large internal stress on the hydrate shell. This 
internal stress may bring about important effect on the structure of the hydrate shell. The authors 
should give the change in the radius of the glass capillary as the temperature increases. How much 
will the structure of the hydrate shell be affected by the temperature change? 
(b) My second important concern: As shown in the paper (Page 19), the grain size is a key 
parameter which will influence the mechanical properties of polycrystalline materials. Besides grain 
size, grain morphology texture, hydrate sample purity, and hydrate phase structure (sI, sII, and 
sH) can also play an important role in the mechanical strength of polycrystalline hydrates. The 
information is unclear. As shown in the paper and supplementary videos, the grain size of their 
prepared samples fluctuates greatly. How to determine these impacts on the present results? 
(c) My third important concern: In their experiments, the contacting force between their grown 
hydrate shell and the thin glass capillary seems to be larger than those of hydrate shell itself. As 
pointed in the paper in Page 5, a thin water layer is found between micron-thin hydrate shell and 
the sediment particles, how to understand the contacting interface structure between hydrate shell 
and the thin glass capillary in this paper? 
There are also other issues as bellows: 
(1) The writing of this paper is not good at all. There are a lot of mistakes in English writing. And 
the presentation of this paper is also not good, many parts of this paper need to be rewritten. 
(2) In Page 1, “… and tensile strength as functions of the parameters controlling grain size…”, the 
controlled parameter may be the thickness of hydrate shell, not grain size. The thickness of 
hydrate shell and the grain size are two different parameters. This is absolutely misleading. Thus, 
using the shell thickness as a proxy for grain size is not correct. 
(3) In Page 6, “…by creating a greater pressure on its gas than on its water side…”, it makes 
readers very confused because this description is contradictory with the content in Page 1 (…A 
novel method of thermally induced stress generation enables contactless characterization of the 
tensile). 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
The manuscript presents an elegant experimental approach to study a complex and difficult 
material. The following comments are triggered by observations reported in the manuscript (some 
of them unprecedented – e.g., Figure 2). The emphasis is on potential pitfalls and alternative 
interpretations rather than the many lessons learned. 
 
 
KEY QUESTIONS/ISSUES 
 
• Neat experimental study leading to surprising measurements. The limited discussion in the text 
hides inherent experimental complexities and associated uncertainties in indirect measurements 
(some discussed in supplementary material). In this context, I wonder about the distinction 
between “what we think we measure, what we want to measure and what we measure”. The 
discussion section should address experimental challenges and uncertainty propagation before any 
in-depth physical interpretation (e.g., Hall-Petch effect). This review attempts to identify some 
potential issues. 
•All tests are run at high degree of supercooling, i.e., below water freezing. Therefore, the 
coexistence of ice-and-hydrate and experimental implications must be carefully addressed. 
•What is the meaning of tensile strength in a material that is pressure sensitive? In other words, 
what is the difference between fluid pressure and solid stress at the atomic scale and their 
implications on hydrate stability? Could the tensile failure of hydrate mass result from local 
hydrate dissociation during tensile loading due to the effective decrease in internal stress? Such a 
possible interpretation was raised in Jung and Santamarina (2011). If this were the case, the high 
supercooling conditions used in this study could explain -at least in part- the higher tensile 
strengths reported by the authors. 
 
 
OTHER COMMENTS 
 
Note: references to our own work are added to several comments, NOT to promote our work (in 
fact, the authors have already recognized our efforts generously), but to provide clarity and 
support to my comments (furthermore, our papers cite articles of potential interest to the 
authors). 
 
• Page 1: requires careful editing 
 
• Page 2: 
o Strength: hydrate effects dilation in short-term tests (the effect depends on pore habit – see 
various DEM studies, such as Brugada et al 2010 – Granular matter; Jung et al., 2012 JGR). 
o Hydrate creep may be more relevant than strength to long-term stability (Refer to page 21). 
 
•Page 5&6 
o Why run the system at such a low temperature T=-23C? Clearly, the induction time will be 
shorter, but it drives hydrate formation unrealistically fast after nucleation (e.g., consider crystal 
growth, the local effects of exothermic heat, diffusion from the initially saturated water phase, 
etc). 
o Ice: the authors do not report ice formation. As freezing point depression can be disregarded for 
this pore size, I wonder whether I am missing something critical. Line 5 on pg 6 does not clarify 
the matter. Induction times may be long under quiescent conditions, but active hydrate formation 
should be enough to trigger ice formation - or, in fact, it may be the other way around (Dai et al. 
2014, Fluid Phase Equilibria 378, 107–112). Any evidence of ice content? Raman data? 
o Annealing by “slightly raising the temperature”: how much? As you started at T=-23C, I assume 
you are still much lower than T=0C? If so, why not anneal at T>0C to make sure there is no ice? (I 
can see implications on volume contraction, but the alternative is concerning). 
o Could the authors discuss hydrate wettability on borosilicate (data from the literature may help). 
Consequences of rapid transformation? Evidence of tip adherence? Implications on results. 
o Did the authors observe moisture migration and condensation ahead of the meniscus, 
particularly during the first stage at 20C (it will probably diminish at -23C - see Sun et al., 2018. 
Time-dependent pore filing. Water Resources Research)? This will have important implications on 
the halo formation and does not require water film effects and transport. The movie may support 
this mechanism: as soon as the seal breaks during tension, vapor transport/condensation could 
sustain hydrate formation ahead of the initial halo. 
o Young hydrate is porous and permeable (also page 9 - Evidence and references in Liang and 
Santamarina JGR 2018; also observed in other crystallizations, e.g., Dai et al 2016 – Acta 
Geotechnica) 
o End of page 6: Ostwald ripening (ok: listed on page 13) 
•Page 7 
o “The most obvious is to increase the pressure of the gas”: why would this work for hydrate 
resting on stiff water? expansion of the tube? (note: both longitudinal and transverse)? 
o Increase temperature: water contraction and simultaneous glass expansion/extension (ok: 
addressed on page 8) 
o Failure appears to happen away from the cap. Why? Tension decreases away from the cap due 
to hydrate-wall interaction, doesn’t it? 
 
• Page 8 
o The measurement of displacements (described in S1): it appears to be a featured-centered 
digital image correlation – correct?. Please mention in the manuscript. Critical to all analyses 
 
• Page 9 
o “Crystal growth is not significantly influenced by the applied stress”: explain 
 
• Page 10 
o Supercoooling temperatures: once again, address issues related to the apparent absence of ice. 
 
• Page 15 
o Water flow without freezing or forming hydrate: it implies that the rate of transport is faster than 
rate of transformation - Damkohler (also observed in Jung and Santamarina, 2012, J. Crystal 
Growth 345 61–68). Related discussion? 
 
• Page 16 
o Mostly summary of previous parts (clearer and better written). Avoid duplication. 
 
• Page 17 
o Indeed, hydrate growth reported in this study is particularly relevant to excess-gas / water-
limited conditions (e.g., at the base of the BSR, next to faults and gas-release features such as 
pipes) 
 
• Page 21 
o Considering geological time scales: hydrate creep will be critical on the long-term behavior of 
hydrate bearing systems 
 
 
EDITORIAL 
 
• Abstract and some figure captions: require careful editing. 
• The text could be tighter (it could afford a 40% shortening without loss of information)