An experimental comparison between methane and carbon dioxide hydrates formation: characterization of common aspects and differences between the two multiphase processes

The injection of CO2 in gas hydrate reservoirs has been intensively studied for its role in methane recovery and carbon dioxide storage. This study investigated the formation of methane and carbon dioxide hydrate in a smallscale experimental apparatus, which was well-suited for simulating natural gas hydrate reservoirs and intervening with replacement strategies. Four tests were conducted, equally divided between the two guest compounds under consideration. The process was considered as a first-order chemical kinetic equation for the time dependence and the hydrate formation rate was calculated according to it. In addition, studies were classified on the basis of the quantity of hydrates generated and the gas inputs with considering the temperature trend and total time length. As expected, CO2 hydrate formation showed better kinetics during the first phase of tests, proving a more intense nucleation and rapid growth of crystals. After a certain time period, CH4 hydrate formation became more prominent, mainly due to the appearance of secondary nucleation sites, which were not observed in the presence of CO2. The results shown in this work explain why the CO2 replacement process usually of interest for only the most superficial layers; moreover, the explanation is not limited to the formation of a superficial CO2 hydrate shell, which prevents the flow of gaseous CO2 into deep layers. The results prove that the different kinetics of the process, depending of the guest compound, makes replacement extremely effective in the first period, while lowers it drastically on the second phase, due to the higher production of secondary nucleation sites in the presence of methane.