Study On The Preservation Of Natural Gas Hydrate For Gas Storage At Low Pressure And Unsteady State

Natural gas is one of the cleanest fossil energy sources and plays an increasingly important role in China's energy structure.Natural gas storage is an important part of natural gas utilization.Clathrate hydrates is an ideal host for natural gas,by entrapping gas molecules in cage-like structures composed of hydrogen-bonded water molecules,natural gas can be stored in solid form with high density.However,there is still a contradiction between high gas storage capacity and low pressure storage in the preservation of natural gas hydrate above ice point.In order to alleviate this contradiction,this paper proposes a surface treatment method for coating natural gas hydrate with cyclopentane hydrate shell.After verifying the feasibility of this method,the stability of natural gas hydrate storage at low pressure above the ice point is studied.Finally,on the basis of experiments,the mechanism of low-pressure preservation of hydrates is analyzed and described with a diffusion model.This study first proposed a method to achieve low-pressure and unsteady state preservation of natural gas hydrates above ice point through hydrate shell coating.Use methane to simulate natural gas to formed hydrate in a 300 ppm sodium dodecyl sulfate(SDS)solution.The formed hydrate and cyclopentane are mechanically extruded to form a cyclopentane hydrate shell.It was confirmed by laser Raman that the treated outer surface hydrate shell was cyclopentane-methane hydrate,and the inner surface was methane hydrate,indicating that this method is feasible in operation.Methane and methane/propane(98:2 mol%)are used to simulate natural gas to intensify the formation of hydrates in ice powder and SDS solution respectively.After surface treatment,the formed hydrates are stored at low pressure above the freezing point,and all treated hydrate have the characteristics of high stability and high storage capacity.Among them,the methane hydrate formed in ice powder has the highest stability,after 8 hours of storage at 275.0 K and atmospheric pressure,more than 99.7 mol%of methane is still stored in the hydrate;the methane hydrate formed in the SDS solution has the highest gas storage,the storage capacity of the gas is 152.9 V/V on average.Considering the conditions of hydrate formation rate and gas storage capacity,the methane hydrate formed in SDS solution stored at low pressure above the freezing point has the most application prospects.The mechanism of this preservation method to achieve low-pressure preservation of natural gas hydrates above the ice point is that the cyclopentane hydrate shell hinders the diffusion of internal natural gas,and the diffusion model and experimental data can be better fitted.The simulation shows that the diffusion coefficient of the cyclopentane hydrate shell formed in deionized water is the smallest,which is on the order of 10^-14 m²/s,and the existence of SDS will increase the diffusion coefficient of gas in the cyclopentane hydrate shell.The method used in this study was first used for natural gas hydrate preservation.It is believed that with more research,the applicability and gas storage capacity of this method can be improved.