# New Insights into the self preservation phenomenon of methane hydrates

Belkacem Samar, Saphir Venet, Arnaud Desmedt, Daniel Broseta

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Belkacem Samar, Saphir Venet, Arnaud Desmedt, Daniel Broseta. New Insights into the self preservation phenomenon of methane hydrates. ECGH 2024 - European Conference On Gas Hydrates, Jun 2024, Trieste, Italy. hal-05042932

HAL Id: hal-05042932

https://hal.science/hal-05042932v1

Submitted on 22 Apr 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# New Insights into the self-preservation phenomenon of methane hydrates

Belkacem Samar<fnref n="1" />, Saphir Venet<fnref n="1" />, Arnaud Desmedt<fnref n="2" />,<fnref n="3" />, Daniel Broseta<fnref n="1" />

Within a certain range of temperature below 0°C, gas hydrates have the unique property of metastably preserving themselves when pressure is lower than the equilibrium value, say, a few bar or even at atmospheric pressure, i.e. far outside their stability zone. This phenomenon, referred to as “anomalous preservation” or “self-preservation”, is characterized by an extremely slow rate of dissociation of gas hydrates which can be preserved for weeks (Stern et al. 2001). The studies of this phenomenon have been carried out mostly with methane hydrates in macro-reactors, and a tentative explanation for such a stability is that the hydrates are covered with an ice layer (or “ice shield”) that serves as a diffusive barrier and thus prevents the guest molecules to escape through the ice. Even though ice seems to play a role on the phenomenon of self-preservation, the ice-shield theory has been facing a lot of skepticism as it is considered that the sole presence of an ice layer is not sufficient to contain the guest molecules and withstand the pressure induced by the liberation of the gas initially enclathrated.

In this work, we monitor, by means of a novel experimental methodology that enables us the observation by optical microscopy of hydrates in thin glass capillary cells (High pressure Optical Cells – HPOC), the dissociation of methane hydrates into methane-saturated supercooled water. To our surprise, anomalous preservation of methane hydrates in the absence of ice is also possible. The melting of a porous, surfactant-promoted hydrate induces the formation of a thin water film whose thickness increases slowly over time. Micro-Raman spectroscopy experiments confirm the presence of a dissolved methane concentration gradient within this film which increases over time until a methane bubble nucleates and grows near the hydrate – the water film is then expelled far from the skeleton as a lamella, and the process of water film generation and thickening starts again. This process can be explained by considering the methane’s diffusive transport from the hydrate skeleton to the surface of the water/methane meniscus, which becomes too slow for large enough water films: the concentration of dissolved methane then exceeds the supersaturation limit of water and a gas bubble nucleates and grows as shown in the figure below

<figure><img src="image_2.png" /><figcaption>1st period: Hydrates dissociate into supercooled water supersaturated with methane</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>2nd period: The thickness of the supercooled water film doesn't permits methane to diffuse through it quicker than hydrates to dissociate.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Final period: Concentration of methane in supercooled water reach a critical value that triggers nucleation of gas bubble.</figcaption></figure>

Self-preservation of methane hydrates at -5°C and ambient pressure, where the average dissociation rate is two orders of magnitude lower than normal.

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Candidature to the best PhD poster presentation

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Candidature to the best Young Researcher oral presentation (PhD student or young researcher – PhD defended < 5 years)

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I want to deposit (end of June, I will be contacted by reviewvers) a full document for reviewing and publication

<footnote n="1">¹ Laboratoire des Fluides Complexes et leurs Réservoirs (LFCR), UMR 5150 CNRS-Université de Pau et des Pays de L'Adour, 64000 Pau, France</footnote>

<footnote n="2">² Institut des Sciences Moléculaires, UMR 5255 CNRS-Université de Bordeaux, Talence, France</footnote>

<footnote n="3">³ Laboratoire Léon Brillouin, UMR12 CEA-CNRS, Gif-sur-Yvette, France</footnote>