Deep-Sea Field Test of the CH₄ Hydrate to CO₂ Hydrate Spontaneous Conversion Hypothesis

Abstract

We have carried out a small-scale deep-sea field test of the hypothesis that CH4 gas can be spontaneously produced from CH4 hydrate by injection of a CO2/N2 gas mixture, thereby inducing release of the encaged molecules with sequestration of the injected gas. Pressure cell studies have shown that, under some pressure and temperature conditions, this gas mixture can induce formation of a solid N2/CO2 hydrate with no associated liquid water production. We transported a cylinder of pure CH4 hydrate, contained within a pressure vessel, to the sea floor at 690 m depth off shore Monterey, CA, using the remotely operated vehicle (ROV) Ventana. Upon opening the pressure vessel with the vehicle robotic arm, we emplaced the hydrate specimen on a metal stand and covered this with a glass cylinder full of a 25% CO2/75% N2 gas mixture, thereby fully displacing the surrounding seawater (T = 4.92 °C). We observed complete and rapid dissociation of the CH4 hydrate with release of liquid water and creation of a mixed gas phase. This gas composition will undergo transition over time because of the high solubility of CO2 in the displaced water phase. We show that the experimental outcome is critically controlled by the injected gas/hydrate/water ratio.