Influence of CO2 injection rate and memory water on depressurization-assisted replacement in natural gas hydrates and the implications for effective CO2 sequestration and CH4 exploitation

This study was conducted to investigate the influence of CO2 injection rate and memory water on guest exchange and hydrate reformation behavior in CH4 hydrate-bearing sediment, using a one-dimensional (1-D) reactor to optimize depressurization-assisted replacement. Increasing the CO2 injection rate facilitated the rapid sweeping of CH4 into the pore space of the hydrate-bearing sediment, inducing immediate hydrate reformation and preventing CH4 re-enclathration in the middle region of the 1-D reactor. Despite dissociating 50 % of the initial CH4 hydrate for depressurization-assisted replacement, the replacement efficiency converged at around 70 % due to drastically reformed gas hydrates hindering mass transfer. Introducing time intervals between depressurization and replacement to reduce the residual memory effect of water led to delayed hydrate reformation, altering the longitudinal distribution of replacement efficiency in the 1-D reactor. An increase in replacement efficiency toward the outlet was seen, suggesting that improved CO2 propagation in the hydrate-bearing sediment resulted in a higher CO2 concentration in the vapor phase at the point of hydrate reformation, in turn enhancing CO2 storage efficiency in the newly formed hydrates. These experimental results highlight the importance of CO2 injection rates and time intervals in determining the hydrate reformation behaviors of hydrate-bearing sediments and optimizing the efficiency of depressurization-assisted replacement.