Carbon dioxide-induced liberation of methane from laboratory-formed methane hydrates

This paper reports a laboratory mimic study that focused on the extraction of methane (CH₄) from hydrates coupled with sequestration of carbon dioxide (CO₂) as hydrates, by taking advantage of preferential thermodynamic stability of hydrates of CO₂ over CH₄. Five hydrate formation-decomposition runs focused on CH₄-CO₂ 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 CH₄ or 2800-3200 kPa CO₂ (hydrate forming regime), when the bath temperature was maintained within 4-6 °C and the gas/liquid volumetric ratio was ∼1.7:1 in the water-excess systems. The data show that the induction time for hydrate appearance was largest at 96 h with CH₄, while with CO₂ the time shortened by a factor of four. However, when the secondary gas (CO₂ or CH₄) 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 CO₂ hydrate formation in a system that already contained CH₄ hydrates was facile and they remained stable, whereas CH₄ hydrate formation in a system consisting of CO₂ hydrates as hosts were initially stable, but CH₄ gas in hydrates quickly exchanged with free CO₂ gas to form more stable CO₂ 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 CH₄-CO₂ exchange observed under temperature and pressure conditions that mimic naturally occurring CH₄ hydrates show promise to develop a commercial carbon sequestration system.