Effect of thermal formation/dissociation cycles on the kinetics of formation and pore-scale distribution of methane hydrates in porous media: a magnetic resonance imaging study

A magnetic resonance imaging study was conducted to explore the kinetics and spatial characteristics of the thermally induced methane hydrate formation in both synthetic and natural sediment samples. Low-resolution images were taken from the sediment samples during the hydrate formation and dissociation stages of three consecutive thermal cycles and the induction time, hydrate formation rate and duration, spatial distribution of water, and saturation of all co-existing phases were determined in order to understand the effect of the first cycle of the formation/dissociation on the subsequent cycles. The results demonstrate that the induction and hydrate formation times of the second and third thermal cycles decrease due to the memory effect, enhanced dissolution of methane in the aqueous phase and the redistribution of water associated with the first cycle of the hydrate formation and dissociation. Moreover, the hydrate formation proceeds with a fairly smooth and fast trend in the subsequent cycles primarily due to the multiple nucleation events, in contrast with the traditionally believed “fits and starts” manner which was observed for the first cycle. The thermal cycles for the natural sediment sample were compared with those for the synthetic sediment sample in terms of the induction time, hydrate formation behaviour and duration, and spatial distribution to understand how the sediment particle type and size distribution could influence the cyclic hydrate formation/dissociation. High-resolution images were also taken from the samples and used to infer the spatial distribution of methane hydrates, gas and water in pore space after completion of the hydrate formation stage of each thermal cycle, by applying an innovative image analysis approach.