Methane replacement into hydrate reservoirs with carbon dioxide: Main limiting factors and influence of the gaseous phase composition, over hydrates, on the process

Replacement processes are probably the most attractive methodologies for methane recovery from natural gas hydrate reservoirs. The injection of replacing guest species enhances the recovery of methane and, at the same time, strengthens the hosting sediments and allows to permanently store carbon dioxide is solid structures. Unfortunately, the process is still far from the industrial maturity and its field application is limited. This study deals with a critical discussion about the main influencing factors of replacement; in detail: heat and mass transfer limitations; extension of the thermodynamic region available for replacement; maximum ideal efficiency achievable and role of composition of the gas phase present in the replacement region. This latter aspect is then experimentally investigated and its effect on the quantities of, respectively, methane recovered and carbon di oxide stored, together with the whole amount of hydrate structures (preserved or re-formed) is quantified in detail. Within the selected range of CO2/CH4 compositions for the replacing phase (CO2 content higher than 65 vol%), the percentage of methane recovered gradually diminished: it passed from 84.14%, related to the initial gas composition equal to 67.55–32.41 vol%, to 21.54%, corresponding to 91.83–8.17 vol% CO2/CH4. The maximum quantity of CO2 stored into hydrates via replacement (thus not through formation of new hydrate structures) was reached at 67.55–32.41 vol% CO2/CH4 and was equal to 70.40%, proving that the presence of methane in the replacing gaseous phase plays a key role during replacement processes.