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[Display omitted] • An approach of warm brine injection to promote CH 4 /CO 2 replacement is proposed. • Warm brine injection provides a 2.0 times enhancement of the cumulative gas yield. • A potential risk of hydrate reformation is alleviated by increasing salinity. • Gas production is insensitive to injected heat as the significant heat loss. • Brine injection scheme should be optimized based on energy harvest and CO 2 storage. CH 4 /CO 2 replacement for natural gas hydrates (NGHs) exploitation is a promising method for CO 2 geological sequestration and energy recovery simultaneously. However, the puzzles of low replacement efficiency and slow reaction rate caused by the mass transfer obstacle of gas exchange are fatal bottlenecks for field application of gas replacement method. Therefore, we propose a method that uses warm brine injection during CH 4 /CO 2 replacement process to break the barrier of CO 2 diffusion and enhance CH 4 recovery as well as CO 2 storage. Benefiting from the synergistic influence of salt effect and thermal stimulation as well as water flow erosion, warm brine injection provides three dimensionally connected channels in hydrate for subsequent mass transfer, improving CH 4 /CO 2 replacement in the deep layer of hydrate. CO 2 sequestration efficiency of the newly proposed method reaches 76.46%, and the maximum amounts of CH 4 recovery is nearly treble than that achieved by single CO 2 replacement method. Notably, a potential risk of secondary hydrate formation could occur upon the pressure surge and fluid migration attached to warm brine injection, which could be effectively alleviated by increasing salinity to destabilize hydrate lattices by reducing water activity. The gas production is insensitive to the injected heat as the significant heat loss during the transportation of injected brine in pipelines and thermal diffusion through boundaries. The introduction of free water increases the complexity of the reactions in hydrate reservoirs, and the formulation of brine injection regimes should be synergistically optimized based on energy harvest, energy efficiency and CO 2 sequestration. This work extends the previous knowledge on CH 4 /CO 2 replacement and shows important practical significance for future field studies of NGHs exploitation and CO 2 geological sequestration. [ABSTRACT FROM AUTHOR]

[Display omitted] • CH 4 /CO 2 replacement behaviors in natural marine sediments are investigated. • CO 2 diffusion is weakened by clayey sediment during replacement process. • NGH reservoir with moderate water content has excellent potential for replacement. • Optimized P-T conditions are crucial to improve CH 4 recovery from hydrates. The use of CH 4 /CO 2 replacement from hydrate bearing sediments for CH 4 recovery and CO 2 storage is an alternative option to mitigate energy shortage and global warming. Fine marine sediments are highly attractive for abundant gas hydrate reserves and tremendous CO 2 sequestration potential. However, the CH 4 /CO 2 replacement regularity previously obtained from coarse sands may not be suitable for fine marine sediments because of their distinct differences in physical properties. In this study, fine natural marine sediments obtained from the South China Sea were used as porous media to investigate the CH 4 /CO 2 replacement characteristics. The results indicated that the pressure and the temperature were the main controlling factors affecting the replacement efficiency. The content of initial methane hydrate and water in reservoir had a more significant effect on CO 2 storage than CH 4 recovery. The gas exchange kinetics in hydrates presented in fine marine sediments were significantly different from those in coarse grained sediments. CH 4 /CO 2 replacement in fine marine sediment seemed to be inhibited by weak CO 2 diffusion as peculiarities of the sediment including fine grain size, clay swelling and high proportion of bound water. According to the observed experimental results, the pressure and temperature conditions should be comprehensively optimized to enhance the mass transfer effect and improve economic benefits. This work provided greater insights into future marine NGH exploitation and contributed to carbon sequestration with applying CH 4 /CO 2 replacement method. [ABSTRACT FROM AUTHOR]

Pressure oscillation controlled CH 4 /CO 2 replacement in methane hydrates to enhance CH 4 recovery and CO 2 storage. [Display omitted] • The oscillated pressure control is proposed to enhance CH 4 /CO 2 replacement. • Inner replacement cause CH 4 hydrate decomposition and mixed hydrate regeneration. • Optimized depressurization is crucial to improve CH 4 production and CO 2 storage. The recovery of CH 4 from natural gas hydrates via CH 4 /CO 2 replacement is a highly attractive method for achieving the recovery of CH 4 and storing CO 2. However, although extensive simulation and experimental studies have been conducted, applying CH 4 /CO 2 replacement is currently hampered by a lack of in-depth understanding about the replacement mechanism and the insufficient replacement efficiency and rate achieved. Therefore, to enhance CH 4 recovery and CO 2 storage, we propose a method that uses pressure oscillation during CH 4 /CO 2 replacement. The effects of depressurization pressure, depressurization time, and depressurization frequencies were analyzed in this study. The characteristics of pressure and temperature were analyzed during the depressurization process, and the decomposition and regeneration of hydrates relating to the oscillation pressure were observed on a macro level. The results show that this newly proposed combined method effectively improves CH 4 /CO 2 replacement, and the maximum amounts of CH 4 recovered and the CO 2 storage efficiency are nearly double than those achieved without pressure oscillation. CH 4 /CO 2 replacement is promoted by the pressure oscillation mechanism by breaking the balance of the hydrate layer, which results in the dissociation of CH 4 hydrates and finally forms a CO 2 /CH 4 mixed hydrate layer with free water. The results of this study provide a valuable method that can be used to enhance CH 4 recovery and CO 2 sequestration. [ABSTRACT FROM AUTHOR]