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 CO 2 injection rate and memory water on guest exchange and hydrate reformation behavior in CH 4 hydrate-bearing sediment, using a one-dimensional (1-D) reactor to optimize depressurization-assisted replacement. Increasing the CO 2 injection rate facilitated the rapid sweeping of CH 4 into the pore space of the hydrate-bearing sediment, inducing immediate hydrate reformation and preventing CH 4 re-enclathration in the middle region of the 1-D reactor. Despite dissociating 50 % of the initial CH 4 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 CO 2 propagation in the hydrate-bearing sediment resulted in a higher CO 2 concentration in the vapor phase at the point of hydrate reformation, in turn enhancing CO 2 storage efficiency in the newly formed hydrates. These experimental results highlight the importance of CO 2 injection rates and time intervals in determining the hydrate reformation behaviors of hydrate-bearing sediments and optimizing the efficiency of depressurization-assisted replacement. [Display omitted] • The influence of CO 2 injection rate and memory water on depressurization-assisted replacement was investigated. • Increasing the CO 2 injection rate reduced CH 4 re-enclathration and thereby improved replacement efficiency. • High CO 2 injection rates led to the rapid recovery of hydrate saturation, which hindered mass transfer. • Introducing time intervals between depressurization and replacement reduced the residual memory effect of water. • Longer time intervals delayed hydrate reformation, thereby enhancing replacement efficiency. [ABSTRACT FROM AUTHOR]