Experimental study of methane hydrate formation in aqueous foam stabilized by surfactants

• Methane hydrate formation from aqueous foam under high pressure. • Modified phase-change number for hydrate formation. • Deformation of the foam structure behind the hydrate formation front. • Generation of polycrystalline conical conglomerates (hydrate needles). • Methane hydrate foam melting. This work presents experimental studies of methane hydrate formation from aqueous foam stabilized by surfactants. The experiments were carried out in a high-pressure optical chamber. It is shown that the process of hydrate formation begins near the chamber walls. The front of hydrate formation propagates through the entire volume of a sample at a rate of ∼ 1 mm/s which leads to the hydrate crust formation on the surface of foam cells, while a liquid layer of surfactant solution remains between the hydrate films. The paper discusses the phase transition criterion, which quantifies the fraction of water that converts into hydrate during the adiabatic process. The following phenomena accompanying hydrate formation in foam are described: (1) deformation of the foam structure behind the hydrate formation front due to a decrease in the gas pressure; (2) subsidence of foam associated with capillary suction of solution through the inter-hydrate media; (3) generation of polycrystalline conical conglomerates (hydrate needles) at the foam – solution boundary at the moment, when the front of hydrate formation reaches this boundary. It is noted that these formations have an elongated shape and are oriented along the normal to the surface of the hydrate growth front. Upon melting, the needles retain a transparent hydrate framework, but lose their tightness. As a result, gas is displaced by liquid from their inner volume into foam cells from which these formations have grown.