Experimental investigation into the dissociation of methane hydrate near ice-freezing point induced by depressurization and the concomitant metastable phases

Abstract

Methane hydrate (MH) has been viewed as an important potential energy resource, drawing attention of its efficient exploitation. An intriguing phenomenon that gas hydrates dissociate in a slow rate and can preserve for long periods of time out of the stable region was considered as a performance of hydrates metastability. A mechanism that supercooled water can lead to the retarded dissociation of hydrates was proposed by researchers, which will affect the gas production efficiency and sediment safety during the exploitation of MH. In this work, experiments on MH dissociation and production of methane gas from porous MH sediment near ice-freezing point induced by depressurization were performed to obtain the evidence of the metastability of the gas hydrate system. A series of experiments at different production pressures, depressurization rates and initial water saturations were carried out to study the effects of supercooled water on gas production performance. The results show that the existence of supercooled water below the freezing point of water would retard the dissociation of MH, by which the gas production rate was slowed down. The duration of this metastable state of the system was dependent on the induction time of the ice formation during the MH dissociation process. The formation of ice would promote gas production from the sediment, while the formation was observed as a stochastic process at 2.0 MPa and 2.2 MPa. The method employed should be adjustable and amendable to investigate the metastability of the gas - liquid - gas hydrate system and the related effects on gas production during depressurization-induced MH dissociation process below ice-freezing point in pilot scale.