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The replacement of CH 4 by CO 2 in methane hydrates is a promising method for simultaneously achieving CO 2 storage and CH 4 recovery for global warming mitigation and energy production, respectively. However, gas replacement is restricted to the slow diffusion-limited transport of CO 2 caused by the formation of a mixed hydrate layer, and little attention has been paid to the storage of CO 2 . Therefore, this study proposed a combination of CH 4 /CO 2 replacement and thermal stimulation to enhance CH 4 recovery and CO 2 storage. The effects of the methane hydrate saturation level, replacement zone, and freezing point on the replacement were analyzed. The CH 4 replacement percentage and energy efficiency were obtained and compared using the replacement and combined methods. The results suggested that the combined method effectively improved CH 4 recovery, with the CH 4 replacement percentage exhibiting an upper limit of 64.63%. Moreover, In CH 4 /CO 2 replacement, the total number of moles of CO 2 stored is unequal to CH 4 recovered, because the replacement is sensitive to the free water in the pores of the hydrate sediments. In addition, the CO 2 storage efficiency was first discussed. The results proved that the CH 4 /CO 2 replacement has obvious advantages in CO 2 storage, and a maximum CO 2 storage efficiency of 96.73% was achieved by combined method. [ABSTRACT FROM AUTHOR]

• Capillary pressures in different directions are presented in this study. • Hydrate formation makes the skewness of capillary pressure finer slanting degrees. • The heterogeneity in hydrate formation determines capillarity in anisotropy. • The stronger capillary heterogeneity occurs in the vertical direction. The random nucleation and growth of hydrates in the pore space contribute to the heterogeneity of hydrate sediments, which results in anisotropic fluid flow. An exact forecast for capillary pressure in different directions, which governs the gas/water distribution and the volume of residual water saturation in porous sediments, is of significant importance for hydrate exploitation. In this study, the index properties of the artificial hydrate sediment were characterized through X-ray computed tomography (CT) visualization scanning. The spatial structure feature of hydrate sediment was captured via a topological pore network, which was established from CT imaging. Then, the evolution of capillary pressure in anisotropy with the formation of hydrates was simulated and discussed. The results indicated that with hydrate formation, the capillary pressure curves became steeper, representing skewness with finer slanting degrees and poor sorting. Moreover, the differences in capillary pressure with the same hydrate saturation between different directions was enlarged. Stronger capillary heterogeneity in the vertical direction represents greater heterogeneity, according to the Leverett J-Function. [ABSTRACT FROM AUTHOR]

• Organic exclusions with hydrate growth were firstly simulated and in-situ observed. • Progressive organic exclusion caused substance migration and concentration gradient. • Higher driving formation force generated the precipitation of excluded organics. Inorganic substances and most organic compounds are typically excluded from crystalline hydrate structures. In this study, the phenomenon and mechanism of organic exclusions during hydrate formation were investigated using MD simulations and in-situ observations with an MRI experimental apparatus. The results showed that the organic macromolecules could not be trapped in the hydrate structure and formed a concentration gradient of dissolved organic substances from the growth front of the CO 2 hydrate into the aqueous solution. This study demonstrates the hydrate-based technology would be a promising method for organic wastewater treatment. [ABSTRACT FROM AUTHOR]

Natural gas hydrate is recognized as an ideal substitute for traditional energy resources. Gas production from hydrate reservoirs is accompanied by the phase transition of solid hydrate, which inevitably results in changes to the microstructure of hydrate sediments. Therefore, the interaction mechanism between the microstructure and permeation characteristics of hydrate sediments must be investigated, especially permeability anisotropy. In this study, we assess the variations in permeability anisotropy during krypton hydrate formation using a pore network model and X-ray computed tomography. The results showed that absolute permeability in the vertical (k v) and horizontal (k h) directions both decreased throughout the formation process, but that the decline in k v was steeper, thus resulting in a sharp increase in the degree of permeability anisotropy (k h /k v). The relative permeability to gas was always higher in the horizontal direction than of that in the vertical direction, whereas the relative permeability to water was the opposite. Moreover, the difference in the relative permeability to water and gas between the two directions increased with the increasing hydrate saturation (S h). When extrapolating to the field-scale for hydrate field trials, we consider that horizontal wells provide a better option for gas production, especially for hydrate reservoirs with a higher hydrate saturation. • Prediction of permeability anisotropy during hydrate formation is presented. • Hydrate formation increases the degree of permeability anisotropy (k h /k v). • Difference in gas/water flow between two directions increases with high S h. • Horizontal wells are found to be a better option for gas hydrate production. [ABSTRACT FROM AUTHOR]

Kinetic enhancement for capturing and storing harmful gases into hydrates simultaneously to achieve low energy penalties and environmental mitigations. • Kinetic surfactants accelerate gas captures in pore spaces. • Oscillated pressure controls are first proposed to enhance CO 2 gas captures. • Enhanced gas capture processes change microstructural features. • The hydrate technology is a safely long-term storages of harmful gases. The use of hydrate-based technology for gas capture and storage is highly attractive for environmental mitigation, as it entails low energy penalties and provides gas storage density maximization and long-term storage stability. Although this method has been investigated in extensive researches, its development is restricted by the obscure underlying gas capture micro-mechanisms, elusive micro-structures of stored forms, and insufficient hydrate film growth rates. In this study, the Magnetic Resonance Imaging technique was employed to analyze the hydrate growth micro-processes for greenhouse gas (imitated by CO 2 , CH 4 , and various fractions of CO 2 -CH 4 mixed gases) and volatile organic compound (simulated by C 2 H 4 and C 2 H 2 gases) capture and storage. The hydrate film growth was enhanced with the addition of 288 ppm sodium dodecyl sulfate (SDS), which significantly improved the hydrate growth in the cases of hydrocarbon gases, but not CO 2 gas due to the competing adsorption of bicarbonate and dodecyl sulfate ions. With SDS, hydrocarbon gas hydrates grew via the patchy model at 65–105 mm/s, and 65–95% liquid water was converted into hydrates for gas capture and storage. However, only about 1.4% water was converted into CO 2 hydrates with SDS, at 10.4 mm/s. Thus, a multi-pressure control mechanism for secondary hydrate growth was developed to promote CO 2 capture and storage, based on a large amount of dissolved CO 2 gas compared to the other investigated gases. The enhanced CO 2 capture has important implications for the optimized harmful gas sequestration, due to preferentially patchy hydrate morphologies and associated impacts on permeability. [ABSTRACT FROM AUTHOR]