Permeability analysis of gas hydrate-bearing sand/clay mixed sediments using effective stress laws

For natural gas hydrate reservoirs in the South China Sea, particle types of sediments and compaction of overlying rocks significantly affect the variation of porosity and permeability in sediment, leading to changes in gas production efficiency in natural gas hydrate reservoirs. In this paper, the permeability experiments ranged from 0 MPa to 5 MPa effective stress have been performed in a cylindrical reaction vessel with various conditions of hydrate saturations. The relationships between permeability and porosity and effective stress in methane hydrate bearing sediments made of quartz and quartz-montmorillonite mixture were investigated, accounting for hydrate saturation and particle types. The experiment results indicated that in quartz and quartz-montmorillonite mixture without methane hydrate, both permeability and porosity decrease gradually with the increase of effective stress. When the effective stress exceeds 2 MPa, the decrease of porosity and permeability of pure quartz-montmorillonite mixture is significantly higher than that of pure quartz. The permeability of quartz decreases with the increase of hydrate saturation, while permeability of quartz-montmorillonite mixture is opposite due to the loss of water and shrinkage of clay. In addition, the porosity and permeability of the two hydrate bearing sediments decrease with the increase of effective stress. The porosity of the two types of methane hydrate samples changes with the effective stress in accordance with the law of exponential function. In addition, the relationship between permeability and effective stress in quartz and quartz-montmorillonite mixture sample is appropriate for the exponential law and power law, respectively. These results provide guidance for studying the effect of effective stress on porosity and permeability for different particle types sediment in the process of natural gas hydrate production in South China Sea.