Search

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]