Visual study on methane hydrate formation and depressurization-induced methane hydrate dissociation processes in a micro-packed bed reactor

Natural gas hydrate has become one of the most promising green alterative energies. The safe and efficient exploitation of natural gas hydrate is of importance. Depressurization method has been considered as one of the most feasible hydrate exploitation approaches. The methane hydrate dissociation by depressurization has been widely investigated by using a variety of reactors or simulators. However, the micro-packed bed reactor has been rarely used for the hydrate-related studies. The packing materials in the micro-packed bed reactor are useful for simulating the porous condition of sediments. The optical transparency of the microreactor can provide the direct visualization of phase behavior, morphology, interfacial phenomenon and fluid migration in pore spaces. Thus, a high-pressure transparent micro-packed bed reactor recently developed was used to investigate the methane hydrate formation and depressurization-induced methane hydrate dissociation behaviors in porous media in this study. The hydrate formation results identify the different patterns of methane hydrate formation caused by the various methane bubble dispersion conditions, such as size and distribution in pores. The depressurizationinduced methane hydrate dissociation behavior was investigated by continuously extracting fluids after the methane hydrate formation in the micro-packed bed reactor. The hydrate reformation phenomenon was observed during the early stage of depressurization process. This was triggered by the methane escaping out of the collapsed hydrate shell. The impact of fluid extraction rate of depressurization was studied. The results indicate higher fluid extraction rate leading to more severe hydrate shell collapsing and significant gas migration, resulting in more significant methane hydrate reformation. The hydrate dissociation process and dramatic fluid migration phenomena in the pore spaces were also visualized during the later stage of depressurization.