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Being a promising potential source for natural gas, methane hydrate (MH) is attracting increasing interest due to its great amount and diverse geographic distribution. The formation of MH is significantly influenced by the properties of sediment media such as porosity and permeability. In this study, in order to have a better understanding on the relationship between MH formation behavior and sediment properties, as well as to synthesize representative hydrate samples, MH was formed in six different sets of mixed-size porous media composed of clay, silt and fine sand, with saline water and circulating methane gas to reflect MH formation with free methane flux in marine sediment. The sediment composition, experimental pressure (15 MPa) and temperature (286.2 K) were chosen based on the SH2 drilling site in the Shenhu area in South China Sea. A two-stage growing behavior was observed for all systems. The gas consumption and hydrate formation rate exhibited positive relations with the permeability and porosity of the sediments. Furthermore, hydrates were found to be preferably formed in the bottom layer of the sediment, which could be attributed to the drastic drop of permeability at the early stage of hydrate formation. Lastly, the hydrate formation rate constant was calculated based on the intrinsic kinetic model and found to be a good reflection of the mass transfer properties of different porous media. [ABSTRACT FROM AUTHOR]

[Display omitted] • Phase equilibrium for CH 4 -DIOX hydrates in DIOX concentrations 0.15 mol%–5.56 mol%. • Low dosage DIOX induces more CH 4 in sII hydrate large cage but with inferior gas uptake. • The kinetics and morphology of mixed CH 4 + DIOX hydrate formation were observed. • An evaluation of DIOX promoting effect based on thermodynamics and kinetics. Hydrate-based CH 4 storage is a promising technology for safe and economically-viable natural gas storage and transportation. Solid CH 4 hydrate possesses advantages in both energy density and storage stability. The quest for an effective and economic-friendly thermodynamic promoter for CH 4 hydrate formation is imminent with the aim of (a) reducing CH 4 hydrate formation P - T condition and (b) increasing both the CH 4 gas uptake rate and final CH 4 storage capacity. In this study, a non-carcinogenic thermodynamic promoter 1,3-dioxolane (DIOX) was systematically investigated to quantify the effect of DIOX concentrations (0.15–5.56 mol%) on the process of mixed CH 4 + DIOX hydrate formation. A series of thermodynamic, kinetic, Raman spectroscopy measurement, and morphology observation experiments were conducted to elucidate the mechanism of the promotion effect of DIOX at concentrations below the stoichiometric concentration (5.56 mol%). DIOX with various concentrations shifts the thermodynamic phase equilibrium of binary hydrate to the region of moderate temperature and pressure. Raman spectra provided evidence of more CH 4 molecules filling in the large cages of sII hydrate with the decrease of DIOX concentration. The ratio of CH 4 in large cages to CH 4 in small cages of sII hydrate increases from 0.05 to 0.38 with the decrease in DIOX concentration. The gas uptake rate of CH 4 declined with the decrease in DIOX concentration. We also observed that DIOX concentration is an important factor affecting the morphologies during hydrate formation. These findings provide fundamental evidence for improving the CH 4 storage capacity and environmental friendliness in the application of DIOX in hydrate-based CH 4 storage. [ABSTRACT FROM AUTHOR]