The impact of mineral compositions on hydrate morphology evolution and phase transition hysteresis in natural clayey silts

Natural gas hydrate is a clean and high-efficient energy resource, and more than 90% of its reserves are contained in fine-grained (typically clayey silts) sediment. In this work, for the first time, the micro-scale imaging is performed to explore hydrate phase transition, morphology evolution and fundamental characteristics (mineral compositions, pore structures and seepage capacity) in clayey silt sediments. The results indicate that clayey silts formation properties strongly depend on dominant minerals component in the sediments. The clay-rich clayey silt possesses more microcapillary interstice with smaller permeability than that in quartz-rich sediment. Hydrates generally occur as microfracture-filling (veins) and grain-displacing (nodulus) in sand-rich clayey silt. While they occur in the form of fracture-filling (vein) and foraminifera-filling in clay-rich clayey silt sediments. Biological fossils (especially foraminifera) provides potential space for hydrate aggregates. But hydrate formed inside it depends on the structures of fossils, the mineral components and pore structure of surrounding matrix. The hysteresis between hydrate formation and decomposition found to be more significant in clay-rich clayey silt sediment than quartz-rich sediment. And this hysteresis inside foraminifera is much more serious compared with that in matrix. In addition, dispersed pore-filling hydrates forms during decomposition, which has adverse effect on continuous gas production.