Memory effect of CO2-hydrate formation in porous media

Abstract

Understanding the mechanism of synthesis and dissociation of gas hydrate in porous medium is of great significance for selecting appropriate depressurization strategy during the hydrate production. Based on a self-designed nuclear magnetic resonance (NMR) experimental set-up, this study monitored in-real-time the formation process of CO2 hydrate in a sandstone core plug, and investigated the formation mechanism of CO2 hydrate under memory effect. For the same sample with the same initial water saturation, the average formation rate of the secondary synthesis is 9 times higher than that of the primary synthesis, and the final hydrate saturation of the secondary synthesis is 2 times higher than that of the primary synthesis, which indicate the existence of memory effect during multicycle of synthesis processes. To explain the memory effect mechanism, we assume that after a synthesis/decomposition cycle process, some water molecular with normal chain structure transferred into dissociated water having residual cage structure that is favorable for hydrate formation. Moreover, the synthesized CO2 hydrate mainly occupy small pores with a ‘filling model’ during the primary synthesis process, whereas they enter both small and large pores with a ‘coating model’ during the secondary synthesis process. For the pore filling model in the primary synthesis, the earlier formed CO2 hydrate in large pores can prevent the water in small pores participating in the formation of CO2 hydrate, and this blocking effect cannot be found for coating model in the second synthesis.