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Depressurization is considered to be the most promising method for exploitation of natural gas hydrate. To analyze the characteristics of hydrate dissociation during depressurization, methane hydrate (MH) dissociation was performed at different depressurizing ranges and rates, and the hydrate dissociation process was directly observed using magnetic resonance imaging (MRI). The experimental results indicate that with increased depressurizing rate from 0.01 MPa/min to 0.1 MPa/min, the average dissociation rate increases for a given depressurizing range. Meanwhile, with an increase in depressurizing range from 0.3 MPa to 1.1 MPa, the average dissociation rate increases for a given depressurizing rate. Moreover, the hydrate dissociation process can be divided into two main stages: hydrate saturation remains constant with little fluctuation for several minutes after back-pressure decreases, and then the hydrate dissociates continuously until dissociation completes. In addition, excessively high depressurizing range and rate result in hydrate reformation and ice generation, which slow the rate of hydrate dissociation. Furthermore, it was also determined that MH reformation and ice generation always occur at the higher depressurizing range and rate due to insufficient heat transfer. [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]

Several trial productions have been conducted on marine natural gas hydrate reservoirs, demonstrating its enormous potential as an alternative energy source. Nonetheless, sustained and efficient production still remains challenging, with crucial issues on low gas production efficiency and blockages. The large amount of hydrate decomposition and resulting heat absorption can lead to prolonged subzero conditions close to the well; this can readily induce ice formation and blockage. This study aims to investigate the issue of ice blockages caused by insufficient energy supply during the recovery process. A systematic analysis of the energy consumption and ice blockage quantification at each stage of the traditional multi-stage direct depressurization and constant speed depressurization methods is conducted. A hybrid optimized scheme is proposed, which enhances energy utilization efficiency by appropriately utilizing reservoir sensible heat during the initial stage of production. Additionally, a multi-stage constant speed depressurization method is introduced in the later stages to mitigate production fluctuations and reduce blockage duration. Results showed a 69.5 % increase in energy efficiency and an average 67.1 % decline in gas blockage duration with comparable gas productivity. The proposed method could be a viable solution to mitigate challenges of icing blockages and fluctuating production during trial tests of marine hydrate reservoirs, achieving improved energy efficiency and sustainable production. • Ice blockage in hydrate reservoirs reduced production efficiency during direct and constant-speed depressurization methods. • Proposed a hybrid method to address gas production fluctuations caused by ice blockage through energy consumption analysis. • Multi-stage analysis revealed 70% energy efficiency improvement and 67% reduced ice blockage duration with the hybrid method. [ABSTRACT FROM AUTHOR]

• The micro-nano bubbles shorten the induction period of methane hydrate nucleation. • The nucleation rate of methane hydrate increased by 4.73 times. • Micro-nano bubbles of guest molecules with milder phase equilibrium conditions have more advantages in promoting methane hydrate nucleation. • The concentration of micro-nano bubbles affects the memory effect of hydrate, and the memory effect disappears when the concentration of micro bubbles decreases by 73.5%. Hydrate technology has significant application potential in the fields of phase-change cold storage, seawater desalination, gas separation, etc. The key factors influencing the development of hydrate technology are the reduction in the induction time and the promotion of nucleation and growth of hydrates. Micro-nano bubbles (MNBs) can significantly increase the nucleation rate of hydrates owing to their advantages of increasing the gas–liquid interface, enhancing mass transfer, and providing sites for nucleation. In this study, a visualization experimental system for in situ hydrate formation promoted by MNBs was developed. The induction time and the nucleation rate of methane hydrate under the action of MNBs, and the influence mechanism of different guest molecule bulk MNBs on the induction time of methane hydrate were studied. Furthermore, the effects of hydrate formation with injection of bulk MNBs and the reformation with dissociation solution were compared, and the mechanism of bulk MNBs on the memory effect of hydrates was analyzed. The results show that under the action of bulk MNBs (size 80–240 nm), the average induction time of hydrate nucleation is reduced by 80.9%, and the average nucleation rate is increased by 4.73 times. At the same time, by comparing the bulk MNBs of different guest molecules (CH 4 , N 2 , CO 2 , R134a), it was found that R134a has relatively mild phase equilibrium conditions and is easier to nucleate, which has an enhancement effect on the nucleation of methane hydrate. Finally, compared with the concentration changes of bulk MNBs in the dissociated solution, it was found that bulk MNBs play an important role in the hydrate memory effect. When the concentration of bulk MNBs in the dissociated solution decreases by 73.5%, the memory effect disappears, which quantitatively proves the mechanisms of bulk MNBs on memory effects. [ABSTRACT FROM AUTHOR]