MRI investigation of hydrate pore habits and dynamic seepage characteristics in natural gas hydrates sand matrix

Abstract

A clear understanding of methane (CH4) hydrate pore habits and gas seepage behaviors in hydrate bearing sediments is critical for the exploitation of natural gas hydrate (NGH) reservoir. In this paper, we presented the experimental data of CH4 hydrate formation and decomposition in sand packs with the aid of magnetic resonance imaging (MRI) technique. The results showed that in homogeneous sand packs CH4 hydrates preferentially formed in the places where the water saturation was lower than the others, whereas in heterogeneous sand packs, the small grain size parts were the preferred locations for hydrate nucleation and the final hydrate saturation was much higher than the other parts. In this study, the CH4 hydrate decomposition process was found to consist of three stages, the initiation stage, the rapid decomposition stage and the post-decomposition stage. The relationship between gas permeability and hydrate saturation was obtained by analyzing the MRI scanned images during percolation experiments. The dynamic gas permeability indicated that the hydrate occurrence mode shifted from grain-coating and pore-filling co-existing to grain-coating predominant with the decrease of hydrate saturation. Moreover, the flow obstruction caused by the presence of hydrate barrier and sand migration was much severer in NGH reservoir.