Experimental Characterization of Memory Effect, Anomalous Self-Preservation and Ice-Hydrate Competition, during Methane-Hydrates Formation and Dissociation in a Lab-Scale Apparatus

Abstract

This study explores the process of methane hydrate formation and dissociation in a small-scale confined environment and in the presence of a porous sediment. The research is focused on answering the shortage of information about the intrinsic properties of the hydrate formation and dissociation processes, such as memory effect and anomalous self-preservation, in a lab-scale apparatus. Experiments were carried out consecutively and with the same gas–water mixture. The temperature reached during dissociation was high enough to ensure the complete dissolution of water cages. At the same time, it was sufficiently low to keep the system able to retain the memory of the previous formation of hydrates. Different well-known phenomena were observed and described; memory effect, anomalous self-preservation and competition between ice and hydrates were shown in detail. Experiments confirmed that the memory effect improves the process mainly during the initial nucleation phase, while it does not provide significant changes in the following massive growth phase. Finally, experiments proved that the formation process can be divided in two different steps: the initial intense growth, due to the small difference in local equilibrium conditions, and the subsequent asymptotic growth, which continues until the process is completed.

Definition of the induction time for CO2 and CH4 hydrate via evaluation of the heat released during the process and the gas consumption rate.

“…The induction time was found to be longer for carbon dioxide hydrates than for methane hydrates. The results obtained confirmed what asserted in literature [24,25]. Such difference must be considered when evaluating the efficiency of replacement processes, because it is responsible for the occurrence of a timelimited phase during which the modified (from the external) pressure and temperature conditions will cause the dissociation of methane hydrates in absence of replacement, thus causing the release of gaseous methane in the surrounding environment and the dissociation of water cages, with possible deformation of soils in correspondence of the reservoir.…”

Gas Hydrates as High-Efficiency Storage System: Perspectives and Potentialities

“…The molecule of methane has a smaller size than the one of carbon dioxide and is capable of fitting both the types of cavities present in sI. Conversely, the molecule of carbon dioxide prefers the largest 5 12 6 2 cages and rarely enters the 5 12 cages [45]. Properly, for this reason, the CO 2 /CH 4 exchange efficiency during replacement is far from 100% and is estimated to range between 64% and 75% [46,47].…”

Hydrate Formation with the Memory Effect Using Classical Nucleation Theory

“…A phenomenon recognized by many researchers as the water "memory effect", however, may cause natural gas hydrates to form easier after being dissociated or melted down [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. In these studies, the memory effect is observed as a shorter induction time during a constant subcooling or as a smaller subcooling phase during a linear cooling ramp.…”