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This paper reports a laboratory mimic study that focused on the extraction of methane (C[H.sub.4]) from hydrates coupled with sequestration of carbon dioxide (C[O.sub.2]) as hydrates, by taking advantage of preferential thermodynamic stability of hydrates of C[O.sub.2] over C[H.sub.4]. Five hydrate formation-decomposition runs focused on C[H.sub.4]-C[O.sub.2] exchange, two baselines and three with host sediments, were performed in a 200 mL high-pressure Jerguson cell fitted with two glass windows that allowed visualization of the time-resolved hydrate phenomenon. The baseline pure hydrates formed from artificial seawater (75 mL) under 6400-6600 kPa C[H.sub.4] or 2800-3200 kPa C[O.sub.2] (hydrate forming regime), when the bath temperature was maintained within 4-6°C and the gas/liquid volumetric ratio was ~17:1 in the water-excess systems. The data show that the induction time for hydrate appearance was largest at 96 h with C[H.sub.4], while with C[O.sub.2] the time shortened by a factor of four. However, when the secondary gas (C[O.sub.2] or C[H.sub.4]) was injected into the system containing preformed hydrates, the entering gas formed the hydrate phase instantly (within minutes) and no lag was observed. In a system containing host Ottawa sand (104 g) and artificial seawater (38 mL), the induction period reduced to 24 h. In runs with multiple charges, the extent of hydrate formation reached 44% of the theoretical value in the water-excess system, whereas the value maximized at 23% in the gas-excess system. The C[O.sub.2] hydrate formation in a system that already contained C[H.sub.4] hydrates was facile and they remained stable, whereas C[H.sub.4] hydrate formation in a system consisting of C[O.sub.2] hydrates as hosts were initially stable, but C[H.sub.4] gas in hydrates quickly exchanged with free C[O.sub.2] gas to form more stable C[O.sub.2] hydrates. In all five runs, even though the system was depressurized, left for over a week at room temperature, and flushed with nitrogen gas in between runs, hydrates exhibited the 'memory effect', irrespective of the gas used, a result in contradiction with that reported previously in the literature. The facile C[H.sub.4]-C[O.sub.2] exchange observed under temperature and pressure conditions that mimic naturally occurring C[H.sub.4] hydrates show promise to develop a commercial carbon sequestration system. Key words: sediment hosted hydrates, gas exchange in hydrates, methane hydrate, carbon dioxide hydrate, carbon sequestration. Cet article presente les resultats d'une etude visant a reproduire en laboratoire les conditions d'extraction du methane (C[H.sub.4]) a partir d'hydrates et, simultanement, la sequestration du dioxyde de carbone (C[O.sub.2]) sous forme d'hydrates, en tirant parti de la plus grande stabilite thermodynamique des hydrates de C[O.sub.2] par rapport aux hydrates de C[H.sub.4]. Nous avons mene cinq essais de formation-decomposition d'hydrates centres sur l'echange entre le C[H.sub.4] et le C[O.sub.2], dont deux essais de reference et trois, dans des sediments contenant des hydrates. Les essais ont ete realises dans une chambre Jerguson a haute pression de 200 ml munie de deux fenetres de verre permettant l'observation du phenomene de transformation des hydrates en fonction du temps. Les hydrates purs de reference se sont formes a partir de l'eau de mer artificielle (75 ml) sous une pression de C[H.sub.4] de 6400 a 6600 kPa ou de C[O.sub.2] de 2800 a 3200 kPa (conditions de formation d'hydrates) lorsque la temperature du bain etait maintenue entre 4°C et 6°C et que le rapport volumetrique gaz/liquide etait d'environ 1,7: 1 dans les systemes ou l'eau etait en exces. Les donnees revelent que la periode d'induction requise pour l'apparition de l'hydrate etait la plus longue, soit 96 h, dans le cas du C[H.sub.4], tandis qu'elle etait quatre fois plus courte dans le cas du C[O.sub.2]. Toutefois, lorsque le gaz secondaire (le C[O.sub.2] ou le C[H.sub.4]) etait injecte dans le systeme contenant des hydrates deja formes, le gaz entrant s'est tout de suite transforme en hydrate (en quelques minutes); aucun delai n'a ete observe. Dans un systeme contenant le substrat dans du sable d'Ottawa (104 g) et de l'eau de mer artificielle (38 ml), la periode d'induction etait reduite a 24 h. Dans les essais a charges multiples, l'hydrate s'est forme dans une proportion atteignant 44 % de la valeur theorique en systeme ou l'eau etait en exces, tandis que la proportion a atteint une valeur maximale de 23 % en systeme ou le gaz etait en exces. La formation d'hydrates de C[O.sub.2] dans un systeme qui contient deja des hydrates de C[H.sub.4] etait aisee, et les hydrates formes sont demeures stables. A l'inverse, les hydrates de C[H.sub.4] formes dans le systeme compose prealablement d'hydrates de C[O.sub.2] etaient initialement stables, mais ont rapidement vu leur C[H.sub.4] gazeux s'echanger contre le C[O.sub.2] gazeux libre pour former des hydrates de C[O.sub.2] plus stables. Dans les cinq essais, meme dans le cas d'un systeme etait depressurise, laisse a temperature ambiante pendant plus d'une semaine et purge avec de l'azote gazeux entre les essais, les hydrates ont manifeste l'<>, quel que soit le gaz employe, un resultat qui est en contradiction avec les resultats publies auparavant. L'echange aise entre le C[H.sub.4] et le C[O.sub.2] observe en conditions de pression et de temperature imitant les conditions naturelles de formation des hydrates de C[H.sub.4] semble un moyen prometteur de realiser un systeme commercial de sequestration du carbone. [Traduit par la Redaction] Mots-cles: hydrates d'origine sedimentaire, echanges gazeux dans les hydrates, hydrate de methane, hydrate de dioxyde de carbone, sequestration du carbone., Introduction With the world population expected to reach over 10 billion by 2050, (1) the world is facing an ever increasing need for new energy sources. Increasing greenhouse gas levels [...]

Sequestration of carbon dioxide (CO2) in the form of its hydrates in natural methane (CH4) hydrate reservoirs, via CO2/CH4 exchange, is an attractive pathway that also yields valuable CH4 gas as product. In this paper, we describe a macroscale experiment to form CO2 and CH4-CO2 hydrates, under seafloor-mimic conditions, in a vessel fitted with glass windows that provides visualization of hydrates throughout formation and dissociation processes. Time resolved pressure and temperature data as well as images of hydrates are presented. Quantitative gas conversions with pure CO2, calculated from gas chromatographic measurements yielded values that range from 23 - 59% that correspond to the extent of formed hydrates. In CH4-rich CH4-CO2 mixed gas systems, CH4 hydrates were found to form preferentially. [ABSTRACT FROM AUTHOR]