Effect of methane hydrate dissociation and reformation on the permeability of clayey sediments

Natural gas hydrates in the South China Sea generally exists in clayey sediments, whose permeability is significantly affected by the swelling-shrinkage characteristics of clay that are caused by hydrate dissociation and reformation, and these characteristics influence the efficiency of gas production. In this study, montmorillonite with the most significant swelling characteristics and kaolinite with weak swelling characteristics in South China Sea sediments were investigated experimentally, respectively. The gas permeability was experimentally investigated for varying hydrate saturations during hydrate dissociation and reformation. The results showed the following: The clayey samples after hydrate dissociation had lower permeability than those without dissociation due to the clay particle swelling and the existence of bound water, but the swelling and hydrophilicity of kaolin are lower than for montmorillonite, which is confirmed by observation of SEM photography. Furtherly, the hydrate reformation improved the gas permeability of clayey samples to a certain extent except for those kaolin samples with initial ice saturations of 5% and 10% because of the increased pore space due to dehydration shrinkage of clay particle and the formation of typical aggregate structures, which is in contrast to the results for sand. Furthermore, the damage value of permeability after hydrate dissociation and the ratio of the permeability after reformation of hydrate to the initial permeability in both the montmorillonite and kaolin were obtained in this study.