A Numerical Simulation Study Of Underground Gas Storage In Depleted Gas Reservoirs With N₂ As Cushion Gas

In the total investment of underground gas storage,the investment of cushion gas accounts for a large part.If cheap inert gas is used as cushion gas,the high investment can be saved.But inert gas and natural gas will mix in underground gas storage,which affects the quality of natural gas production.In this paper,the operation process of underground gas storage with N2 as cushion gas has been studied and the results are as follows:Feasibility study of N2 as cushion gas in the underground gas storage is studied from three aspects:the thermodynanic properties,the source and the cost.In the themodynamic properits part,it is found that there are density difference and viscosity difference between N2 and CH4 under the same reservoir condition.And the density ratio between N2 and CH4 varies in the range of 1.4-1.7 with the variation of temperature and pressure.When the pressure is greater than 12 MPa,the viscosity difference between N2 and CH4 increases with the increase of pressure.Both the density difference and viscosity difference between N2 and CH4 means that the mixing of N2 and CH4 can be limited.And the greater density of N2 means a better compressibility,which provide larger power in the period of production,and improve the recovery capacity.In the study of source and cost,it is obtained that the industrial source of NZ is mainly air with three motheds and the price of N2 is much lower than CH4.The price difference is in the range of 2800 yuan per ton to 7000 yuan per ton.Using N2 instead of natural gas as cushion gas in underground gas storage can save much investment.In the study of recovery gas purity,a single well mechanism model for vertical well gas production is established to study the effects of several reservoir parameters and design parameters on the purity of recovered gas,when N2 is used as cushion gas in a gas reservoir reconstructed from porous sandstone lithologic gas reservoir.The results of numerical simulation obtain that the proportion of cushion gas,N2,formation pressure and injection-production period have greater influence.When the design parameters of underground gas storage are unchanged,the best cushion gas ratio is 50%,which can satisfy the requirement of producting degree and ensure that the concentration of N2 in recovered gas is below the upper limit.When the design parameters of underground gas storage are unchanged,the optimum proportion of N2 is 15%.Under this condition,the maximum molar fraction of N2 in recovered gas is about 5%in the first five injection-production cycles.After that,with the increase of N2 proportion,the increase of N2 molar fraction in recovered gas is more and more faster.Under this condition,the molar fraction of N2 in recovered gas is about 0.3%higher when the reservoir pressure increases by 2 MPa during N2 injection.In the study of evaluation,an evaluation model is set up by the data of numerical simulation.The main function of the evaluation model is to judge whether the parametes is conducive to the construction of underground gas storage with NZ as cushion gas and determine its superiority and inferiority.The accuracy rate of the evaluation model is 96.26%,and the effect is good.The study of this paper can provide theoretical guidance and reference for the application of underground gas storage building in depleted gas reservoirs with N2 as cushion gas.