Memory effect of gas hydrate: Influencing factors of hydrate reformation and dissociation behaviors☆

Memory effect of gas hydrate is a double-edged sword in hydrate-based application and natural gas hydrates exploitation. In this work, in order to acquire a comprehensive understanding of memory effect, we conduct a series of experiments on hydrate reformation and dissociation under different grain filling patterns and thermal history conditions. Experimental results reveal that the memory effect can not only shorten the induction time of hydrate nucleation but also significantly reduce the hydrate formation rate by enhancing the homogeneous distribution of gas hydrate in pores. The homogeneous hydrate distribution under memory effect has been further investigated and evaluated by the hydrate heterogeneity degree and dead-end porosity for the first time. More importantly, the decrease in heterogeneity degree and dead-end porosity driven by memory effect shows significant effects on hydrate dissociation behaviors. On the one hand, the improved homogeneous distribution of gas hydrate under memory effect impairs the heat transfer from the environment to hydrate-bearing sediments, thereby reducing the hydrate dissociation rate. On the other hand, the decreased dead-end porosity can lead to the expansion of fluid flow channels in hydrate-bearing sediments, thus increasing the hydrate dissociation rate. These findings are significant for efficient and secure gas production in field tests since the violent gas/water flow in reservoirs would lead to the rapid hydrate reformation during gas production from hydrate-bearing reservoirs.