Key factors controlling the kinetics of secondary hydrate formation in the porous media

Abstract

Secondary hydrate formation (SHF) usually occurs in hydrate formation and decomposition experiments, and its manifestations behave differently in promoting gas consumption and water-hydrate conversion. In this work, in order to investigate the kinetic processes and controlling factors of the SHF in the porous media, we conduct a series of formation and dissociation experiments in a three-dimensional cubic hydrate simulator (CHS). Hydrate dissociation is accomplished by the depressurization method after the formation experiments. The experimental results reveal that a rapid growth rate of equal magnitude in the SHF stage follows a relatively slow hydrate growth in the process of hydrate formation. The occurrence of the SHF is accompanied by a sudden pressure drop and temperature rise, and a heterogeneous temperature distribution shows significant effects on hydrate formation behaviors in the vessel. It has been verified that the SHF is controlled by hydrate thermodynamic stability at a subcooling degree of −0.8 °C during the hydrate formation. The SHF is initiated by the collapse and crack of the local thin hydrate film, which results in an accelerated triggering of gas diffusion for further hydrate formation. In the dissociation experiments, the slight temperature increase caused by the SHF can promote the hydrate dissociation rate. The subcooling degree in the hydrate formation can also be suitable for predicting SHF in the dissociation experiment. It is found that a rapid depressurization rate can effectively shorten the duration of SHF. These insights can be beneficial for the energy transition and natural gas hydrate extraction in the future.