The Study Of The Influence Of Guest Molecule Occupancy Rate On The Stability And Physical Properties Of Natural Gas Hydrate

There are a large number of natural gas hydrates around the world.Reasonable exploitation and utilization of natural gas hydrates is expected to solve the current energy shortage problems.However,the current cost of mining hydrates is very high and it is difficult to reach commercial levels.It is known that in the past few decades,although people have gained important understandings of its properties,many aspects of hydrates need to be further studied.People have obtained natural gas hydrate samples through drilling sampling and experimental synthesis,and conducted a lot of research on the stability,occupancy and physical properties of natural gas hydrates so far.Due to the limitation of experimental technologies,it still exists dilemmas regarding to accurately detecting the specific influence of the cage occupancy rate on the properties of natural gas hydrates.In this paper,theoretical calculations were used to reveal the influence of the occupancy of gas molecules in the hydrate cages concerning its stability and physical properties,which can theoretically explain the experimental results,guide the design of experimental schemes,and provide relevant theoretical support for the exploitation and utilization of natural gas hydrates.In this paper,we used first-principles calculations to study how the absence of methane in the two kinds of cages of type I methane hydrates affect its structural stability,electronic density of states,and optical properties.We found that the perfect crystal structure of type I methane hydrates is the most stable one among the three cases.When the hydration number is 6.57,the structure of a small cage lack of methane is more stable than a structure lack of methane in a large cage.And lacking methane in the large cage can easily cause structural deformation,which will lead to changes in the band gap and optical-related characteristic.The results showed that the optical response of the hydrates is in the ultraviolet region.The contribution of electrons to the dielectric constant is negligible,and the dielectric constant of natural gas hydrates is mainly derived from the contribution of the deflection and transport of hydrogen atoms（protons）.Molecular dynamics simulation was used to study the effects of temperature,occupancy and hydration numbers on the stability and physical properties of type I methane hydrates.It was found that the decomposition temperature of hydrates increases with the decrease of hydration number and the increase of cage occupancy rate.When the hydration number is the same,the decomposition temperature of combustible ice lacking methane in the large cage is lower than that in the small cage.The calculation consequences show that when the large cage’s methane occupancy rate is less than 80%,the volume of combustible ice will suddenly decrease during the decomposition process,and there will be a temporary collapse phenomenon.Nevertheless,other occupancy states have no such phenomenon.Both the elastic constant and elastic modulus of hydrates increase with decreasing temperature increasing occupancy rate of cages.Under the same hydration number,the large cage lacking of methane has a greater impact on the C₁₁,C₁₂ and bulk modulus of the hydrates than the small one.The small cage without methane has a more significant impact on C₄₄,shear modulus and Young’s modulus.The changes of the occupancy rate of the two kinds of cages have slight effect on the Poisson’s ratio of the hydrates.The longitudinal and transverse wave speeds of hydrates both increase with the increasing cage occupancy and the decrease of temperature.The absence of methane in small cages will give rise to a greater impact on the sound velocity of the hydrates.In summary,the research results help to reveal the influence of the occupancy rate of different cages on the stability and physical properties of type I methane hydrates.It will provide data support and theoretical reference for the actual storage and transportation,and exploitation of natural gas hydrates.