Hydrate Morphological Evolution and Its Relation with High Permeability Characteristics in Hydrate-Bearing Sediments

Abstract

Permeability is a key parameter that reflects the flow capacity of the hydrate-bearing sediments. Permeability decreases with increasing hydrate saturation (Sh), as predicted by conventional permeability reduction models. Our previous research proposed that hydrate-bearing sediments exhibit high permeability characteristics; however, the contributing factors were not systematically investigated until now. This study proposed two reasons for the high permeability characteristics from the natural and artificial aspects. We applied both experiments and numerical simulations: 10 wt % tetra-n-butyl ammonium bromide (TBAB) and 40 wt % Tetrahydrofuran (THF) hydrates were formed into dispersed and bulk hydrates (Sh = 46%) to mimic the original and after-geotime-scale hydrates. Subsequently, pure water was injected to observe changes in hydrate morphology and permeability. The result showed that the flow channel size increased over time from 7.90 to 13.54 μm due to hydrate Ostwald ripening and sintering from the natural aspects, resulting in high permeability characteristics. Hydrate dissociation was also occurring due to the injection of pure water from the artificial aspects, resulting in high permeability characteristics. Natural aspects are more dominant than artificial aspects, which can be controlled manually by the injected volume. Such results can also be used to interpret relevant parameter changes in CH4- and CO2-hydrate-bearing sediments, where geotime-scale morphological changes can lead to preferential pathways increasing the leakage risks.