Natural gas hydrates represent a valid opportunity to counteract two of the most serious issues that are affecting humanity this century: climate change and the need for new energy sources, due to the fast and constant increase in the population worldwide. The energy that might be produced with methane contained in hydrates is greater than any amount of energy producible with known conventional energy sources; being widespread in all oceans, they would greatly reduce problems and conflicts associated with the monopoly of energy sources. The possibility of extracting methane and simultaneously performing the permanent storage of carbon dioxide makes hydrate an almost carbon-neutral energy source. The main topic of scientific research is to improve the recovery of technologies and guest species replacement strategies in order to make the use of gas hydrates economically advantageous. In the present paper, an experimental study on how salt can alter the formation process of both methane and carbon dioxide hydrate was carried out. The pressure–temperature conditions existing between the two respective equilibrium curves are directly proportional to the effectiveness of the replacement process and thus its feasibility. Eighteen formation tests were realized at three different salinity values: 0, 30 and 37 g/L. Results show that, as the salinity degree increases, the space between CO₂ and CH₄ formation curves grows. A further aspect highlighted by the tests is how the carbon dioxide formation process tends to assume a very similar trend in all experiments, while curves obtained during methane tests show a similar trend but with some significant differences. Moreover, this tendency became more pronounced with the increase in the salinity degree.

Natural gas hydrates represent a valid opportunity in terms of energy supplying, carbon dioxide permanent storage and climate change contrast. Research is more and more involved in performing CO₂ replacement competitive strategies. In this context, the inhibitor effect of sodium chloride on hydrate formation and stability needs to be investigated in depth. The present work analyses how NaCl intervenes on CO₂ hydrate formation, comparing results with the same typology of tests carried out with methane, in order to highlight the influence that salt produced on hydrate equilibrium conditions and possibilities which arise from here for improving the replacement process efficiency. Sodium chloride influence was then tested on five CO₂/CH₄ replacement tests, carried out via depressurization. In relation with the same typology of tests, realised in pure demineralised water and available elsewhere in literature, three main differences were found. Before the replacement phase, CH₄ hydrate formation was particularly contained; moles of methane involved were in the range 0.059–0.103 mol. On the contrary, carbon dioxide moles entrapped into water cages were 0.085–0.206 mol or a significantly higher quantity. That may be justified by the greater presence of space and free water due to the lower CH₄ hydrate formation, which led to a more massive new hydrate structure formation. Moreover, only a small part of methane moles remained entrapped into hydrates after the replacement phase (in the range of 0.023–0.042 mol), proving that, in presence of sodium chloride, CO₂/CH₄ exchange interested the greater part of hydrates. Thus, the possibility to conclude that sodium chloride presence during the CO₂ replacement process provided positive and encouraging results in terms of methane recovery, carbon dioxide permanent storage and, consequently, replacement process efficiency.

Natural gas hydrates are the largest reservoir of natural gas worldwide. This paper proposes and analyzes the CH₄-CO₂ replacement in the hydrate phase and pure methane collection through the use of membrane-based separation. The investigation uses a 1 L lab reactor, in which the CH₄ hydrates are formed in a quartz sand matrix partially saturated with water. CH₄ is subsequently dissociated with a CO₂ stream supplied within the sediment inside the reactor. An energy and environmental analysis was carried out to prove the sustainability of the process. Results show that the process energy consumption constitutes 4.75% of the energy stored in the recovered methane. The carbon footprint of the CH₄-CO₂ exchange process is calculated as a balance of the CO₂ produced in the process and the CO₂ stored in system. Results provide an estimated negative value, equal to 0.004 moles sequestrated, thus proving the environmental benefit of the exchange process.