# New Insights into the self-preservation phenomenon of methane hydrates

Belkacem Samar, Saphir Venet, Daniel Broseta, Arnaud Desmedt

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Belkacem Samar, Saphir Venet, Daniel Broseta, Arnaud Desmedt. New Insights into the self-preservation phenomenon of methane hydrates. Journées Hydrate de Gaz 2023, Groupement De Recherche (GDR) Hydrates du CNRS, Oct 2023, Le Croisic, France. hal-05042879

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https://hal.science/hal-05042879v1

Submitted on 22 Apr 2025

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# New Insights into the self-preservation phenomenon of methane hydrates

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

Within a certain range of temperature below the ice freezing point, gas hydrates have the unique property of metastably preserving themselves even when pressure is low, say, a few bar or event 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 are 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”) preventing the guest molecules to escape through the ice. However, controversies are still hanging over this so-called ice-shield model as it seems that metastable (supercooled) water rather than ice coexist with the methane hydrate. We monitor in this work, through the usage 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 very slow dissociation of methane hydrates into methane-saturated supercooled water. The melting of the surfactant-promoted hydrate porous skeleton 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 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 superstation 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.

<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>