Clayey-silty sandy media have been widely discovered in naturally occurring hydrate-bearing sediments in the South China Sea. However, the phase change behavior of CH 4 hydrate (MH) and the resulting pore structure change and the migration of fluid in clayey-silty sediments remain less known and warrant investigation. In this study, we examine the pore-scale behavior of MH formation and dissociation in clayey-silty sediments and the associated fluid migration by low-field nuclear magnetic resonance (NMR). Based on T 2 spectra measurement, MH starts to grow in small pores (pore size <1 μm) first and in large pores (pore size >10 μm) subsequently. The presence of clay, i.e., Na-MMT, practically retards the overall growth kinetics of MH evidenced by the low H 2 O conversion (<10%) to MH in clayassociated small pores. During depressurization, MH starts to dissociate in sand-associated large pores first. Free water migrates to clay-associated small pores and partially converts to clay-bound water. MRI visualization depicts the heterogeneous spatial distribution of both MH and the residual water in the process. The experimental results provide possible explanations on the spatial heterogeneity of MH in clay-silty sediments in nature and on the multiphase fluid migration during energy recovery from MH reservoirs.

Efficient gas enrichment approaches are of great importance for the storage and transportation of clean energy and the sequestration of carbon dioxide. Of special interest is the regulated gas hydrate-based method; however, its operation requires adequate additives to overcome the lowstorage capacity issue. Thus, this method is not economically feasible or environmentally friendly. In this work, a novel recyclable hydrate promoter of copolystyrene-sodium styrenesulfonate@Fe 3 O 4 (PNS) nanoparticles with an integrated core−shell structure was synthesized through emulsion polymerization. This was found to effectively reduce the induction time of methane hydrate formation by one-third compared with the widely used sodium dodecyl sulfate (SDS); the corresponding gas storage capacity was also comparable, up to 155 v/v. In addition, the PNS nanoparticles showed a good performance in foam inhibition upon hydrate decomposition, which frequently occurred with the use of SDS and other surfactant-based promoters. In particular, the new promoters contributed to a more than 30% increase in CO 2 storage capacity, coacting with the fine sediments that mimic a marine environment. This provided further possibilities of sequestering CO 2 in the form a gas hydrate under the seafloor. The underlying mechanism was proposed to involve anchored surfactants on the surface and tiny channels between the nanoparticles that lead to rapid hydrate nucleation and controlled growth. The results showed that the integrated magnetically recovering nanoparticles developed in this study could improve the efficiency of gas storage by forming gas hydrates; the excellent recycling performance paved the way for solving the economic and environmental problems encountered in additive usage.

Owing to the heterogeneity of methane hydrate reservoirs in the South China Sea, hydrate and silty clay particles mostly exist in the form of interbedded reservoirs. Permeability is a decisive factor for the efficiency of interbedded methane hydrate deposits. Therefore, using montmorillonite as the simulated sediment material, permeability measurements of interbedded sediments with different methane hydrate distributions and effective stresses were carried out for the first time in this study. By comparing with homogeneous sediments, the permeability evolution of interbedded sediments under different hydrate distributions and effective stresses is explored, and the effect of methane hydrate dissociation on the permeability of interbedded sediments is analyzed. The results show that the permeability of interbedded sediments with different hydrate saturation levels has little difference and is almost the same as that of pure soil sediments. With the increase of effective stress, the porosity of methane hydrate interbedded sediments decreases and permeability damage is caused, but the final permeability damage is less obvious than that of homogeneous hydrate sediments. In addition, the gas slip effect exists in methane hydrate interbedded sediments. Moreover, the dissociation of methane hydrate causes clay swelling, which leads to the decrease of interbedded permeability, but the degree of permeability damage is lower than that of homogeneous sediments. The results of this study provide a theoretical basis for the development and utilization of interbedded methane hydrate sediments in the South China Sea.

The effective stress and types of particles significantly affect the permeability of hydrate sediments. In this study, effects of hydrate saturation, hydrate decomposition, effective stress, and particle type on gas permeability and sample deformation of montmorillonite samples and quartz-montmorillonite mixtures were investigated. The results showed that the gas phase permeability increased with hydrate formation. In addition, the decline of sample porosity is the fundamental factor determining the permeability failure rate under effective stress. During methane hydrate decomposition by depressurization, the compression coefficient of samples increases with decreasing pore pressure, which means that decomposing natural gas hydrate makes experimental samples more sensitive to stress. Furthermore, the addition of sand particles significantly affects the compressibility of sediments. In addition, because of the decline of sample porosity and the swelling of montmorillonite, the results of gas permeability show a downward trend when the pore pressure decreases during the decomposition of methane hydrate. Gas permeability of the sample ranges from a few millidarcies to several hundred millidarcies. Furthermore, the content of clay and the particle size in the sample are the key factors in determining the permeability damage rate when the stress increases during decomposition of methane hydrate. Therefore, the addition of quartz sand can reduce damage of gas permeability of sediments caused by effective stress and hydrate decomposition.

Permeability is a key parameter to characterize fluid flow in hydrate-bearing sediments. Figuring out dynamic permeability evolution is of great importance for the effective development of hydrate-bearing deposits. In this paper, a grain-coating hydrate-bearing regular pore network model with complex pore throat cross-sections is first constructed. Afterward, the dynamic permeability evolution regularity is calculated. After the validation, the effects of initial aspect ratio, coordination number, and pore throat cross-sections on dynamic permeability evolution are investigated. The results show that hydrate narrows the effective flow space, which results in the exponential decrease of dynamic permeability with the increased hydrate saturation. The larger initial aspect ratio aggravates the heterogeneity of the pore network, resulting in a faster permeability decline rate. However, hydrate weakens the effect of initial aspect ratio on dynamic permeability evolution since the physical hydrate thickness in large pore bodies and throats is larger. The high initial coordination number reduces the dynamic permeability decline rate with the increased hydrate saturation since the higher coordination number increases the topology of the network, while hydrate compresses or blocks the effective pore throat space. Pore throat cross-sections have nothing to do with dynamic permeability evolution, but they dramatically influence the absolute permeability values. This study provides a novel insight into dynamic permeability evolution in hydratebearing sediments.