Effect of decomposition water content of natural gas hydrate on permeability and gas production of clay sediments based on numerical simulation

The solid phase swelling of the clay sediment is related to water from gas hydrate dissociation. Therefore, the relationships among decomposed water content, porosity and effective gas permeability in clay sediment are derived in this paper. A formula for predicting the evolution of effective gas permeability in sediments during clay particle expansion caused by decomposed water content is proposed, which is in good agreement with experimental data. The model of gas production from clay sediments by depressurization is established for the first time, which is verified by gas production rate obtained by gas production experiment. The results show that the porosity and gas phase permeability of clay sediments decrease with the increase of decomposed water content. Then, with the decrease of hydrate saturation, the effect of clay expansion caused by decomposed water content on porosity and gas phase permeability decreases. The amount of decomposed water needed for montmorillonite to expand is less than that of illite. After the gas production rate drops suddenly at 3000s, it could be considered that clay swelling affected by decomposed water basically ends.