Study On The Microscopic Molecular Dynamics And Macroscopic Multiphysics Field Coupling For Natural Gas Hydrate Exploitation

At present,human society is facing a serious energy and environmental crisis.Natural gas is the key to alleviate the energy and environmental crises due to its green,clean,stable and reliable nature and low carbon emissions.Since a huge amount of natural gas is stored in natural gas hydrate(NGH),it is of great significance to realize large-scale commercial exploitation of natural gas hydrate,and it is necessary to conduct comprehensive and multi-scale research on it.In this paper,the microscopic processes and the macroscopic multi-physical field coupling processes of natural gas hydrate extraction are investigated by molecular dynamic method and computational fluid dynamic method,respectively,to reveal the microscopic and macroscopic mechanisms and laws of natural gas hydrate extraction,and to lay a theoretical foundation and provide some guidance for the large-scale development and utilization of natural gas hydrate.The main work of this paper is as follows:(i)The microscopic decomposition processes of natural gas hydrate under different temperatures,different inhibitors,different electric field strengths,and different electric field directions are studied based on the molecular dynamic method.The results show that higher temperature leads faster hydrate decomposition.When no other external conditions are applied,the decomposed hydrate in the outer layer will form a water film that will adsorb and accumulate on the surface of the undecomposed hydrate,forming a sub-stable structure with it and hindering its further decomposition.The addition of alcohol hydrate inhibitors will promote the decomposition of hydrate.Glycol has a stronger inhibitory effect than methanol because it contains more hydroxyl groups.In addition,alcohol inhibitors can prevent decomposing water from forming a surface water film that hinders further decomposition of the hydrate.An electric field with intensity higher than 0.1 V/A can accelerate the dissociation of hydrate.The higher the electric field intensity,the stronger the ability to accelerate the decomposition of hydrate.Under the action of electric field,water molecules will be gradually stripped from the hydrate surface,which leads to its decomposition,and the water and methane produced by hydrate decomposition will show a clear demarcation.The electric field perpendicular to the gas-solid interface has the strongest promotion effect on hydrate decomposition.(ii)The gas replacement processes in natural gas hydrate under the action of different temperatures,different pressures,different temperature oscillations,and different flue gas components are studied and evaluated based on the molecular dynamic method.The results show that increasing the temperature can increase the amount of escaped methane,and increase the amount of captured carbon dioxide in the short term.However,it is not conducive to the longterm stable storage of carbon dioxide and the maintenance of hydrate cage structure.The effect of pressure is less pronounced than that of temperature.Temperature oscillation can optimize the carbon dioxide replacement process in several aspects,including promoting methane escape,facilitating carbon dioxide sequestration,and better maintaining the hydrate cage structure.The key to optimize the carbon dioxide replacement process using temperature oscillation is to adequately coordinate the deep diffusion of carbon dioxide molecules at high temperatures and the rapid reconstruction of carbon dioxide hydrate at low temperatures.The addition of nitrogen and nitrous oxide to carbon dioxide promotes the escape of methane,but exacerbates the destruction of the hydrate cage structure,while the opposite effect is observed for sulfur dioxide and hydrogen sulfide.The addition of any flue gas component inhibits the capture of carbon dioxide,and the inhibition effect is particularly pronounced for sulfur dioxide and hydrogen sulfide.(iii)A multi-physics field coupling mathematical model of natural gas hydrate extraction by depressurization is established,validated,and solved based on the finite element method.The evolution characteristics of saturation,pressure,temperature,and velocity in the process of hydrate decomposition by depressurization are obtained and analyzed.Parameter sensitivity studies are conducted on the boundary pressure,hydration number,initial absolute permeability,and pore size distribution of porous media.Their influences on the gas production characteristics and the evolution characteristics of saturation,pressure,temperature,and velocity during the decomposition of natural gas hydrate are obtained.