In nature, permafrost regions and deep ocean sediments contain a large amount of gas hydrate. As a basic property of the sediments, the particle size of the porous media is a critical factor affecting hydrate production. In this study, methane hydrate formed and dissociated in the sediments with different particle sizes, including the particle sizes of 14–20 mesh, 35–60 mesh, 80–120 mesh, and 400–500 mesh. The experimental results showed that two stages were included during the hydrate formation process. In the first stage, the hydrate was mainly formed in the upper of the sediments, which hindered the further contact of gas/water and resulted in the decrease of the hydrate formation rate in the second stage. As the particle size of the porous media decreased, the induction time for the hydrate nucleation decreased and the hydrate formation rate increased. In the porous media with 400–500 mesh, the hydrate started forming while the gas was injected into the hydrate simulator. It was found that the hydrate formation rate in the sediments was limited by the mass transport rate of gas and water. In the constant pressure stage (CPS) of the hydrate dissociation, the maximum value of the hydrate dissociation rate was obtained in the porous media with 35–60 mesh. It was found for the first time that the change characteristics of the average hydrate dissociation rate with the medium particle size of the porous media were similar to those of the effective thermal conductivity with the medium particle size of the porous media. This demonstrated that the heat transfer rate of the sediments determined the hydrate dissociation rate, and the influences of the capillary force and the hydrate distribution on the hydrate dissociation were minor. The experimental results also suggested that the coarse-dominated sediments are more advantageous for gas hydrate production.

Most resources of natural gas hydrate (NGH) exist in marine sediments where salts and sea mud are involved. It is of great importance to investigate the effects of salts and sea mud on NGH formation kinetics. In this study, the mixture of silica sand and montmorillonite was used to mimic sea mud. The effects of the NaCl concentration of pore water and montmorillonite content on methane hydrate formation were studied. A low NaCl concentration of 0.2 mol/L and a low montmorillonite content range of 10–25 wt% is beneficial to reduce the induction time of hydrate formation. The high NaCl concentration and high content of montmorillonite will significantly increase the induction time. The average induction time for the experiments with the NaCl concentrations of 0, 0.2, 0.6, and 1.2 mol/L is 20.99, 8.11, 15.74, and 30.88 h, respectively. In the pure silica sand, the NaCl concentration of 0.2 mol/L can improve the final water conversion. In the experiments with pure water, the water conversion increases with the increase of the montmorillonite content due to the improvement of the dispersion of montmorillonite to water. The water conversion of the experiments in pure water with the montmorillonite contents of 0, 10, 25 and 40 wt% is 12.14% (±1.06%), 24.68% (±1.49%), 29.59% (±2.30%), and 32.57% (±1.64%), respectively. In the case of both montmorillonite and NaCl existing, there is a complicated change in the water conversion. In general, the increase of the NaCl concentration enhances the inhibition of hydrate formation and reduces the final water conversion, which is the key factor affecting the final water conversion. The average water conversion of the experiments under the NaCl concentrations of 0, 0.2, 0.6 and 1.2 mol/L is 24.74, 15.14, 8.85, and 5.74%, respectively.