Numerical Simulation Study On Cushion Gas Mechanism And Parameter Optimization In Aquifer Gas Storage

As an important part of underground gas storage,cushion gas plays an indispensable role in the construction and operation of aquifer gas storage.Selecting appropriate cushion gas can save the construction and operation cost of gas storage and improve the production degree of natural gas,but the mixing of cushion gas and working gas will also affect the quality of produced gas to a certain extent.Therefore,this paper selects CO₂ from multi-angle comprehensive,study on mechanism analysis of cushion gas,distribution law,geological risk assessment and the reasonable matching optimization of cushion gas amount,so as to form a set of reasonable optimization schemes for the types and parameters of cushion gas in aquifer gas storage,and provide basic theoretical basis for the construction of aquifer gas storage.Mainly includes the following research content:（1）By establishing the mechanism model of CO₂ cushion gas,numerical simulation research on the mechanism of CO₂ cushion gas is carried out from four aspects of recovery degree,formation pressure,edge water intrusion and gas production,and the advantages of CO₂ as cushion gas are clarified.（2）Based on the mechanism model of CO₂ cushion gas,the numerical simulation research on the distribution law,diffusion law and change law of air-water interface of cushion gas is carried out,and the influence of different factors on the distribution,diffusion and change of air-water interface of cushion gas in the construction and operation of gas storage is clarified.（3）Based on the geological model of the actual block,the numerical simulation model of the gas reservoir is established to clarify the influence of uncertain aquifer parameters,reservoir heterogeneity and gas diffusion parameters on the distribution cushion gas and gas mixing.（4）Based on the numerical simulation model of actual gas reservoirs,the reasonable matching optimization of cushion gas and working gas is carried out.The influence of storage construction and working system parameters on the proportion of CO₂ in the produced gas and the degree of natural gas recovery is analyzed,and the operation parameters of aquifer gas storage are optimized.The research results show that CO₂ as cushion gas can not only maintain reservoir formation pressure,improve natural gas recovery,but also inhibit edge water intrusion.During gas injection,the gas preferentially diffuses around,and then diffuses to the lower reservoir,and the plane diffusion efficiency is better than the longitudinal diffusion efficiency.During gas injection,the injected gas diffuses to the surrounding,and then diffuses to the lower reservoir.The plane diffusion efficiency is better than the longitudinal diffusion efficiency.At the end of the reservoir construction,the cushion gas is mainly distributed in the cushion gas is mainly distributed in the pure cushion gas area and gas mixture area at the outer and lower of the gas phase region.The greater the reservoir permeability,the more easily the cushion gas migrates and diffuses to deeper and farther locations.The greater the gas injection rate,the wider the cushion gas distribution.The longer shut-in time,the gas diffusion recovers to the upper reservoir.The larger the diffusion coefficient,the wider the range of gas migration and diffusion,and the higher the proportion of CO₂ in the produced gas.The greater the comprehensive water compression coefficient,the smaller the risk of gas storage.Low permeability zone has the greatest influence in working gas zone and the least influence in edge water zone.Under the simulation conditions in this paper,when the cushion gas ratio is 30%,it can not only meet the quality requirements of the produced gas,but also maintain low formation pressure and high recovery degree.Injecting cushion gas from the middle of the reservoir,injecting working gas from the upper part of the reservoir,and producing gas from the upper part of the reservoir,the proportion of CO₂ in the gas produced by the injection-production well pattern is less than 1.5%.When the shut-in time is 4 months,the proportion of CO₂ in the produced gas is low and the degree of natural gas recovery is high.In the periodic injection-production process of gas storage,the greater the number of injection-production cycles is,the smaller the proportion of CO₂ in the produced gas is,and the lower the degree of natural gas recovery is.