Molecular Simulation Study On Micro-mechanism Of Gas Hydrate Exploitation

Natural gas hydrate is an important alternative clean new energy source for oil and gas,and has a wide application prospect.Shallow burial,unconsolidated and low permeability of argillaceous silt hydrate in the South China Sea lead to complex phase change-seepage mechanism and low production capacity.At present,the process of phase transformation decomposition and fluid flow mechanism of hydrate in micro-pore of argillaceous silt reservoir are still unclear,which has become one of the key scientific problems restricting the effective exploitation of hydrate in China.In this paper,molecular dynamics simulation is used as the main means to study the micromechanism of gas hydrate decomposition process under micro-pore conditions and the flow characteristics of two-phase fluids in the pore,so as to elaborate the micro-mechanism of gas hydrate exploitation.Firstly,molecular dynamics simulation of hydrate decomposition process was carried out to verify the reliability of the hydrate crystal model and force field parameters.Secondly,the molecular model of hydrate under the pore of hydroxyl silica was established to study the molecular dynamics mechanism of hydrate decomposition process and the gas-water distribution in the pore.Finally,the non-equilibrium molecular dynamics of pore gas-water system was carried out.The two-phase flow process in micro-pore is simulated and analyzed,and the flow parameters are characterized.The results show that when the number of hydrogen bond breaks in the hydrate cage structure increases to a certain extent,methane gas begins to escape,and the number of hydrogen bond breaks will continue to increase until the hydrate completely decomposes.When the hydrogen bond is broken in large quantities,the hydrogen bond potential energy increases gradually,the gas-water interaction energy increases,and the hydrate decomposition rate increases accordingly.When the hydrate phase changes,two non-uniformly distributed water molecules adsorbed on the surface of hydroxyl silica are formed,and there are strong adsorption sites of water molecules on the surface of silica.Methane gas exists in the center of the pore in the form of bubbles.With the increase of the pore size,it gathers as a continuous phase fluid.The main driving force of gas-liquid gathering is the interfacial tension between gas and water.With the increase of system pressure,the adsorption of water molecule on the wall is enhanced,the water molecule in the adsorption layer increases,and the water molecule in bulk phase decreases.With the increase of temperature,the adsorption of water molecule on the wall is weakened,and part of water molecule is desorbed to become bulk phase fluid.When a certain driving force is applied to the system,water molecules still exist adsorption phenomenon in the flow state,but the flow of water molecules affects the adsorption stability of the adsorption layer and reduces the density of water molecules in the adsorption layer,which is related to the slip phenomenon of water molecules on the pore surface.With the increase of driving acceleration,the apparent viscosity of two-phase fluid decreases.With the increase of pore size,the slip length of fluid tends to decrease,and the apparent viscosity of two-phase fluid decreases.