Physical Properties of Nanoconfined Methane Hydrate: Structure, Thermoelasticity, and Thermal Conductivity

Large amounts of methane hydrate are trapped in natural porous media, such as marine sediments and permafrost. In this context, understanding the physical and physicochemical properties of confined methane hydrate, sometimes down to the nanoscale, is crucial for environmental and energy applications. Here, a molecular simulation strategy is employed to assess some important properties of nanoconfined methane hydrate: density, structural order parameters, thermal expansion, compressibility, and thermal conductivity. Confinement is found to affect only slightly the microscopic structure of methane hydrate close to the pore surface, with a structural ordering more pronounced than for its bulk counterpart. On the other hand, under a typical temperature and pressure range relevant to real conditions, confinement decreases the thermal expansion of methane hydrate, while it increases or decreases the isothermal compressibility depending on pressure. As for the thermal conductivity, which is determined from the anisotropic heat-flux vector using the Green-Kubo formalism, confinement increases the thermal conductivity in the tangential and normal directions with respect to the pore surface. The thermal conductivity components decomposed into acoustic and optical modes and are compared to their bulk counterpart.