Variations in terms of CO2 capture and CH4 recovery during replacement processes in gas hydrate reservoirs, associated to the “memory effect”

The replacement of methane molecules, contained within natural hydrates reservoirs, with a theoretical equal number of carbon dioxide molecules, represents a promising opportunity for the near future. However, the maximum theoretical efficiency was proved to be approximately equal to 75% and, in the real applications, it is drastically lower. This article focused the attention on the possible reasons of such difference and found in the memory effect one of them. Firstly, the formation process of pure CH4 and CO2 hydrates was performed and thermodynamically characterized; then, the replacement process was realized on a system excluding memory and on a system including it. In the first situation, the theoretical efficiency was reached, while, in presence of memory effect, the results were drastically less effective, both in terms of methane recovery and in terms of carbon dioxide storage. When the system did not retain memory of previous formation processes, the quantity of CO2 captured ranged from 73.97 to 75.05 vol% and the quantity of methane released was approximately equal to the one of CO2 captured. Conversely, in presence of memory effect, the percentage of CO2 captured dropped to 26 vol% and the quantity of methane release was still lower (24.97–26.03 vol%).