Probing the mechanism of salts destroying the cage structure of methane hydrate by molecular dynamics simulation

Abstract

Much attention has been generated in the exploitation of natural gas hydrate by injecting brine due to its low cost and eco-friendliness. However, it is not clear how inorganic salts destroy the cage structure of methane hydrate. This work focuses on the mechanism of inorganic salts destroying the cage structure of methane hydrate by molecular simulation. And the influence of cation type on methane hydrate decomposition is studied in detail. The results show that inorganic salts can effectively reduce the temperature required for methane hydrate decomposition. And inorganic salts can weaken the interaction between water molecules by reducing the dipole moment of water molecules in methane hydrate, then the cage structure of methane hydrate is destroyed. Methane hydrate both can completely decompose without and with CaCl2 solution at 307 K and 20 MPa, but the decrease rate of the dipole moment autocorrelation function (DACF) of water molecules in the system with CaCl2 solution is 6.10 times than that without CaCl2 solution, and the decomposition rate of methane hydrate increased by 5.8 times because of adding CaCl2 solution. In addition, the increasing charge and radius of cations result in greater methane hydrate decomposition which is caused by the faster destruction speed of cage structure, the faster rate of methane molecules escaping from cage structure and the faster diffusion velocity of cations are demonstrated. This study provides a theoretical basis for the efficient exploitation of methane hydrates by brine injection.