Experimental Modeling of Methane Hydrate Formation and Decomposition in Wet Heavy Clays in Arctic Regions

Round  1

Reviewer 1 Report

This paper presents the effect of wet fine clay on hydrate formation and dissociation behaviour. It is really a nice summary of the experimental work in this domain. There is  very limited research work available in this direction. Thus it is good to understand these clays-hydrate system. 

Minor comment:

-As this paper summarises the literature data, The title should be modified accordingly to reflect the review of the literature in wet fine clay-hydrate system. 

- If possible a table can be included to summarise the effect of clay in different systems along with experimental conditions and the main conclusions.

Author Response

Thank you for the review!

My answers:

As this paper summarises the literature data, The title should be modified accordingly to reflect the review of the literature in wet fine clay-hydrate system.

The paper summarizes  original experimental data obtained by the author in different time and presents their analysis. There is no general review of all known literature on gas hydrate formation and decomposition in wet clayish soils. Such review is a separate task.

If possible a table can be included to summarise the effect of clay in different systems along with experimental conditions and the main conclusions.

To compose such a table (or classification) much more different experiments with wet hydrate-containing clays have to be done. The paper represents just first attempt to show the effect of frozen and thawed clay composition and inner conditions on hydrate formation and decomposition. And different experimental approaches to hydrate formation in wet gas-impermeable clays.

Reviewer 2 Report

This paper presents some interesting experiments about methane hydrate formation and dissociation and the roles of clay. But many places are not clearly written and I suggest revisions before accepted.

1.     I suggest improving the language by some native speakers.

2.     Line 22, I suggest using “gas” rather than “gas resources” because the low-concentration methane hydrate in marine muds may not become resource.

3.     Line 30-31, please explain the upper limit of hydrate stability zone there before concluding that these hydrates are metastable.

4.     Line 52, unfrozen water content should decrease rather than increase with clay content.

5.     Line 58, what is “forming hydrate decomposition”?

6.     Lines 71-73, how do you know the pressure decrease from A to B is due to hydrate formation rather than ice melting?

7.     Have you measured the amount of methane you injected into the cell and released from the cell? And are they equal or not? This might be a more direct way to demonstrate if there is hydrate or not at the end of the experiments.

8.     Lines 86-87, what is “that value”?

9.     Line 88, please describe in detail how you rebuilt the hydrate formation condition. And I think you need a figure to describe the pressure, temperature condition during this experiment, too.

10.  Line 96, is the volumetric water content compared to the total volume including the water and clay? How does the 3-99% volumetric content compared with the water content in your experiments which are mass fraction?

11.  In Figure 2, you present the subcooling with water content. But you measured them from hydrate formation experiments. Hydrate formation is well known to have long induction time. That is why such curves are usually measured from hydrate dissociation rather than formation experiment.

12.  You talked about the different roles clay play in hydrate formation with different water contents, please explain the reason behind their corresponding roles?

Author Response

1. I suggest improving the language by some native speakers.

Made, where possible.

2. Line 22, I suggest using “gas” rather than “gas resources” because the low-concentration methane hydrate in marine muds may not become resource.

Agree. Corrected

3. Line 30-31, please explain the upper limit of hydrate stability zone there before concluding that these hydrates are metastable.

Agree. Corrected.

4. Line 52, unfrozen water content should decrease rather than increase with clay content.

Unfrozen water content at the same subzero temperature and the same water content of a sample   is growing with the growth of fine mineral particles content in the sample (with clay particles content increase). This is proven by many experimental results (see, for example,  Principles of Geocryology. Volume 1. Physico-chemical bases of geocryology.(Editor E.D.Ershov). Lanzhou University Press Lanzhou, 2015, China, ISBN 978-7-311-04791-7

5. Line 58, what is “forming hydrate decomposition”?

Agree. Grammar mistake. Corrected to “formed hydrates decomposition”

6. Lines 71-73, how do you know the pressure decrease from A to B is due to hydrate formation rather than ice melting?

Frankly speaking, there was no ice melting as we imagine usually. That was ice decomposition caused by two forces: hydrate formation and bonded water formation. Initial ice content of the sample (12% mass) was too low to produce 0,5 MPa pressure reduction due to water specific volume change. Moreover, specific water volume of hydrate lattice is more than specific  volume of liquid water, so, mechanically, hydrate formation has to increase pressure (if exclude gas incorporation into hydrate lattice).

7. Have you measured the amount of methane you injected into the cell and released from the cell? And are they equal or not? This might be a more direct way to demonstrate if there is hydrate or not at the end of the experiments.

Unfortunately, the cell was not equipped by gas metering system. But intensity of pressure reduction (AB) corresponded to experiments with hydrate formation in fine-grained ice without sediments.

8. Lines 86-87, what is “that value”?

Corrected. “(water content of upper boundary of plasticity)” is added.

9. Line 88, please describe in detail how you rebuilt the hydrate formation condition. And I think you need a figure to describe the pressure, temperature condition during this experiment, too.

The initial pressure was established as in the first experiment, but its strong decrease during hydrate formation forced to add gas to the cell manually few times, until it stabilized for a while at initial 7.0 MPa. So,P/T conditions at the beginning and at the end were the same. Some explanations are added to the text.

10. Line 96, is the volumetric water content compared to the total volume including the water and clay? How does the 3-99% volumetric content compared with the water content in your experiments which are mass fraction?

Yes, volumetric water content is water volume compared to the total volume including the water and dry clay. We did not convert volumetric % to mass.

11. In Figure 2, you present the subcooling with water content. But you measured them from hydrate formation experiments. Hydrate formation is well known to have long induction time. That is why such curves are usually measured from hydrate dissociation rather than formation experiment.

Hydrate formation in this case means equilibrium points. It is often case. As usually, they were measured during dissociation.

12. You talked about the different roles clay play in hydrate formation with different water contents, please explain the reason behind their corresponding roles?

This is subject of a big study. Now we can only suppose the mechanism of different roles of clays. We assume, that at low water content clay is hydrate inhibitor due to bonded water layer growth on mineral surfaces (see experiment 1), At great water content mineral surfaces having enough water to compose layers of strongly and weakly – bonded water molecules, create some – pre-hydrate lattices in upper water molecule layers, which are favorable for gas molecules adsorption. But this is only assumption. Also much is dependent on salt ions content and their composition.

Reviewer 3 Report

This paper presents the effect of clay on hydrate formation and decomposition. Experiment result is interesting. However, results and interpretations are mixed and written, making it difficult to read. I think description of results and interpretation should be separated. In addition, there are some doubts about the description of the background.

(1)   P1L30 The existence of metastable hydrate should be cross-checked by the method such as Raman spectroscopy.

(2)   P1L31 The definition of meta stable hydrate is ambiguous. Please indicate the definition more clearly. For example, temperature pressure condition where metastable hydrate exists.

(3)   Please show the porosity and saturation of the MH-sand sample.

(4)   P2L63 Please describe the summary of sample cell.

(5)   P2L73 Is there evidence that all free water reacts?

(6)   P3L88 Please describe the specific numerical value of equilibrium value.

(7)   P3L93 Is there evidence strongly bonded water does not form hydrate?

(8)   The information is needed for the volume of vessel and sample core for experiment of Figure3.

(9)   For experiment figure 3, comparative target such as experiment without mud is necessary.

Author Response

(1) P1L30 The existence of metastable hydrate should be cross-checked by the method such as Raman spectroscopy.

This is good idea. But having no Raman spectrometer metastable hydrate existence was confirmed by liberated gas volumes measurements and pressure changes when thawing of different sediments (for example, see articles of E.M.Chuvilin)

(2) P1L31 The definition of meta stable hydrate is ambiguous. Please indicate the definition more clearly. For example, temperature pressure condition where metastable hydrate exists.

Metastable hydrates, or self-preserved hydrates, or relic (in nature) hydrates – that are hydrates being once formed at normal hydrate-formation P/T conditions and then, due to pressure drop at subzero temperatures, covered by isolating ice film formed from water liberated  as the result of upper hydrate layer decomosition (see for example, Yakushev V.S. Gas Hydrates in Cryolithozone. 1989. Soviet Geology and Geophysics, N11.  Yakushev V.S. and Istomin V.A., 1992. Gas-Hydrates Self-Preservation Effect. In: Physics and Chemistry of Ice (edited by N.Maeno and T.Hondoh), Hokkaido University Press, Sapporo, p.136-140.). They can exist at temperatures below 0oC, when ice formation is possible.

(3) Please show the porosity and saturation of the MH-sand sample.

There is no data in the article about experiments with sand samples. Experiments with sands were made many times by different researchers (see, for example Wright, J.F., Chuvillin, E.M., Dallimore, S.R., Yakushev, V.S. and Nixon, F.M.1998. Methane hydrate formation and dissociation in fine sands at temperatures near 0oC. Proceedings of the 7th International Conference on Permafrost, Yellowknife, NWT, Canada, 23-28 June, 1998 p.1147-1153. or

Hachikubo A., Takeya S., Chuvilin E., Istomin V. Preservation phenomena of methane hydrate in pore spaces. PCCP: Physical Chemistry Chemical Physics. 2011. Т. 13. № 39. С. 17449-17452.)

(4) P2L63 Please describe the summary of sample cell.

There were applied three different experimental cells and main difference is that in the first experiment there was completely static cell, in second case – static cell with gas flow injection regulation in the third – cell with magnetic stirrer. Detail description of all three cells is represented in articles from references list:

Yakushev V.S. Composition, structure and properties of frozen hydrate-saturated deposits. Ph.D Thesis, Lomonosov Moscow State University, 1991, 132 p. (in Russian), and

Yakushev V.S. Natural gas and gas hydrates in permafrost. Moscow, VNIIGAZ, 2009, 192 p. (in Russian)

Uchida, T., Takeya, S., Chuvilin, E., Ohmura, R., Nagao, J., Yakushev, V., Istomin, V., Minagawa, H., Ebinuma, T., Narita, H. Decomposition of Methane Hydrates in Sand, Sandstone, Clays and Glass Beads.J. Geoph.Res.,2004. 109, B05206.

Yakushev V.S., Semenov A.P., Medvedev V.I., Bogoyavlensky V.I., Bogoyavlensky I.V. Experimental Modeling of Methane Release From Intrapermafrost Relic Gas Hydrates When Sediment Temperature Change. Cold Regions Science and Technology. 2018. v. 149,p. 46-50

(5) P2L73 Is there evidence that all free water reacts?

Back pressure growth in the cell is the evidence, that there is no more ice and free water inside the sample and only hydrate and clay particles hold water. But continuation of bonded water layer formation on clay particles surfaces decomposes formed hydrates. This is the reason of back gas pressure growth. There is no opportunity for free water existence at hydrate-formation conditions in such system.

(6) P3L88 Please describe the specific numerical value of equilibrium value.

Added to the text.

(7) P3L93 Is there evidence strongly bonded water does not form hydrate?

Experiment 1 has shown that at conditions, close to equilibrium of hydrate formation, strongly bonded water decomposes hydrates. Experiment 2 has shown, that some part of bonded water is involved into hydrate formation according to the degree of supercooling. This is like unfrozen water content in frozen sediments: the less is temperature, the less unfrozen water content (i.e. some part of bonded water transforms to ice).

(8) The information is needed for the volume of vessel and sample core for experiment of Figure3.

All details of experimental plant and sample mass are in the paper 13 from reference list.

(9) For experiment figure 3, comparative target such as experiment without mud is necessary.

Experiment without mud is methane hydrate formation and decomposition in contact with deoinized water in the cell with magnetic stirrer. There is a lot of such experiments worldwide. At subzero temperature methane hydrate-ice agglomerate after pressure drop and self-preservation is decomposed at temperature very close to 0oC (temperature of isolating ice melting).

Round  2

Reviewer 3 Report

The author described that metastable hydrate exists in the natural permafrost region. This is an impactful claim. I have read Yakushev and Chuvilin, 2000, cold regions science and technology. However, I consider the evidence for that existence are insufficient. 

Therefore, I can not agree on the description of metastable hydrates in the background section.

Author Response

Thanks.