Knowledge on the kinetics of gas hydrate dissociation in microporous sediments is very important for developing safe and efficient approaches to gas recovery from natural gas hydrate (NGH) deposits. Herein, molecular dynamics (MD) simulations are used to study the dissociation kinetics in microporous sediments. The hydrate phase occupies a confined sandy nanopore formed by two hydroxylated silica surfaces with a buffering water layer between the hydrate and silica phase, meanwhile, this system is in contact with the bulk phase outside the pore. The hydrates in this sediment system dissociate layer-by-layer in a shrinking core manner. The released methane molecules aggregate and eventually evolve into nanobubbles, most of which are spherical cap-shaped on the hydroxylated silica surfaces. At high initial temperatures, a faster decomposition of the hydrate phase is observed, however, fewer methane molecules migrate to the bulk phase from the pore phase. These phenomena may occur because more methane molecules are released from the hydrate phase and facilitate the formation of nanobubbles with large heat injection; these nanobubbles can stably adsorb on the surface of silica and capture the surrounding methane molecules, thereby decreasing the number of methane molecules in the water phase. In addition, the injection speed of heat flow should be significantly increased at high dissociation temperatures when using the thermal stimulation method to extract gas from hydrates in tight sediments. This study provides molecular level insight into the kinetic mechanism of hydrate dissociation and theoretical guidance for gas production by thermal injection from sediments with low permeabilities.

A sound knowledge of thermodynamic properties of sII hydrates is of great importance to understand the stability of sII gas hydrates in petroleum pipelines and in natural settings. Here, we report direct molecular dynamics (MD) simulations of the thermal expansion coefficient, the compressibility, and the specific heat capacity of C₃H₈, or tetrahydrofuran (THF), in mixtures of CH₄ or CO₂, in sII hydrates under a wide, relevant range of pressure and temperature conditions. The simulations were started with guest molecules positioned at the cage center of the hydrate. Annealing simulations were additionally performed for hydrates with THF. For the isobaric thermal expansion coefficient, an effective correction method was used to modify the lattice parameters, and the corrected lattice parameters were subsequently used to obtain thermal expansion coefficients in good agreement with experimental measurements. The simulations indicated that the isothermal expansion coefficient and the specific heat capacity of C₃H₈-pure hydrates were comparable but slightly larger than those of THF-pure hydrates, which could form Bjerrum defects. The considerable variation in the compressibility between the two appeared to be due to crystallographic defects. However, when a second guest molecule occupied the small cages of the THF hydrate, the deviation was smaller, because the subtle guest-guest interactions can offset an unfavorable configuration of unstable THF hydrates, caused by local defects in free energy. Unlike the methane molecule, the carbon dioxide molecule, when filling the small cage, can increase the expansion coefficient and compressibility as well as decrease the heat capacity of the binary hydrate, similar to the case of sI hydrates. The calculated bulk modulus for C₃H₈-pure and binary hydrates with CH₄ or CO₂ molecule varied between 8.7 and 10.6 GPa at 287.15K between 10 and 100 MPa. The results for the specific heat capacities varied from 3155 to 3750.0 J kg^-1 K^-1 for C₃H₈-pure and binary hydrates with CH₄ or CO₂ at 287.15K. These results are the first of this kind reported so far. The simulations show that the thermodynamic properties of hydrates largely depend on the enclathrated compounds. This provides a much-needed atomistic characterization of the sII hydrate properties and gives an essential input for large-scale discoveries of hydrates and processing as a potential energy source.