Heat transfer and water migration rules during formation/dissociation of methane hydrate under temperature fields with gradient

摘要

To investigate formation and dissociation behaviors of hydrate under non uniform temperature fields and mechanisms behind, methane hydrate was formed and then dissociated within a porous medium in a reactor with fixed positive and reverse temperature gradients. Results show that owing to the water migration under temperate gradients, hydrate concentrates inside the regions at lower temperature. Under temperature gradients, hydrate distribution heterogeneity appears inside the medium during formations. During dissociation under reverse gradient, the upper half part of medium is at higher temperature. Hence, this part is easy to dehydrate with flow up from bottom to top of gas released from dissociation, resulting in emergence of significant medium deformation phenomenon. Under these non uniform temperature fields, hydrate dissociation is primarily controlled by heat transfer. Hydrate distribution heterogeneity directly relates to the water migration under temperature gradients during formation, and the medium deformations along with water discharge during dissociation. Owing to these replicated water migration and dehydration processes under reverse gradient, final hydrate saturation is gradually enhanced accompanying with the increase in formation/dissociation cycle times; on the contrary, is lowered under the positive one. These findings may provide some implications for future methane storage at industrial scale with hydrate technology, and the practical exploitations of natural gas hydrates in nature.