Simulation On The Micro-mechanism Of Natural Gas Hydrate Dissociation

Natural gas hydrates are widely distributed in the deep sea and permafrost.They are favored by all countries due to their large reserves,high calorific value of combustion,and low pollution.They are regarded as one of the potential new sources of energy in the 21 st century.At present,the microscopic mechanism study of methane hydrate dissociation and its production process is not perfect yet,which restricts the research progress of gas hydrate to a great extent.In this paper,the molecular dynamics method was used to simulate the microscopic process of the decomposition of sI methane hydrate on the Materials Studio.And the variation of crystal structure at the microscopic level during the hydrate mining was summarized.The effects of cage-specific occupancy and adjacent phase state on the decomposition of methane hydrate in different crystal structures were also studied.This work provides theoretical support for the development of natural gas hydrate production technology.The main results are shown as follows:(1)The accuracy of the CVFF force field used in molecular dynamic simulation was verified by high-precision quantitative calculation methods.It was found that CVFF can reasonably describe the intermolecular interactions in methane hydrate system.(2)The decomposition behavior of methane hydrate in single-crystal and three-phase systems was studied by molecular dynamics method.It was found that the decomposition process of hydrate in the single crystal system mainly includes the deformation of water cages until it is ruptured and the escape of methane molecules,then a methane bubble formed because of the aggregation of methane molecules.Due to the existence of phase interface in the threephase system,the hydrate decomposition process is divided into two stages: the fast diffusion of methane molecules in partial cages and the layer by layer breakdown of closed cages from the two interfaces towards the center.(3)The influence of different adjacent phase states on the decomposition rate of methane hydrate was studied.The presence of liquid or gas phases adjacent to the hydrate has an effect on the rate of hydrate dissociation.At the beginning of dissociation process,hydrate layers in contact with liquid phase dissociated faster than layers adjacent to the gas phase.As the dissociation continues,the thickness of water film near the hydrate-liquid interface became larger than the hydrate-gas interface giving more resistance to the hydrate dissociation.Dissociation rate of hydrate layers adjacent to gas phase gradually exceeds the dissociation rate of layers adjacent to the liquid phase.The difficulty of methane diffusion in the hydrate-liquid side also brings about change in dissociation rate.(4)The effect of cage-specific occupancy is considered in this simulation with energy evolution used to describe the dissociation rate.Hydrates with similar occupancy differ in dissociation behavior depending on the type of empty cages(small or large).Hydrate system with empty small cages show higher stability than system with larger empty cages.It is because the stability decrease of large cages loss of guest molecule is found to be higher than small cages from the calculation of stabilization energy.Moreover,with the temperature increases,the effect of cage-specific occupancy on hydrate dissociation rate decreased.