The Impact Of Reservoir Properties And Operating Conditions On Mixing Of CO₂ Cushion And Natural Gas In Storage Reservoirs

Natural gas is a kind of national strategic resource,so underground gas storage is significant for the guarantee of national economic development and national energy security.Natural gas storage and transportation are special,in different seasons and periods,there will be gas peak and low peak.Underground natural gas storage is a process which effectively balances a variable demand market with a nearly constant supply of energy provided by the pipeline system.Cushion gas in storage reservoirs provide the necessary pressure for the withdrawal of working gas and it makes up for the largest part(30%?70%)of investments in underground storage reservoirs.Usually,apart of the cushion gas is replaced by an inert gas such as CO2 in order to reduce the investment costs.COZ is the most important greenhouse gas,the most effective way to deal with the CO2 is the geological storage,buried location of the main selection of depleted oil and gas reservoirs,deep brine reservoir,coal mining,and the deep sea.Depleted gas reservoir is the most ideal and most economical COZ storage place.Due to this replacement threr might be problems caused by the mixing of inert cushion and natural gas,which is a cause of concern as it affects the quality of the natural gas.In this study,the inert gas mixing problem is investigated in storage reservoirs by using a reservoir simulator based on the equation of state.The degree of mixing between the two dissimilar gases was found by the amount of inert cushion that is produced along with the withdrawn gas.Reservoir parameters like the volume of inert,porosity,permeability,pressure and temperature were varied to study their effect on mixing.The main results are as follows:(1)Outlines the domestic and foreign research development status quo of underground gas storage,this paper discusses the CO2 as the cushion gas.(2)This paper discusses the advantages of carbon dioxide as the cushion gas,and discusses the advantages of using the gas reservoir to establish underground gas storage,and the operating mechanism of carbon dioxide as cushion gas in gas storage is discussed.(3)The feasibility of carbon dioxide used as the cushion gas in the gas reservoir is studied.Compared with natural gas,CO2 has the characteristics of high condensability,high viscosity and high density which are benefited for cushion gas.High condensability can maintain the pressure of reservoir;high viscosity can avoid the mixing with working gas;High density can arouse gravity segregation phenomena.(4)Several experiments have been carried out to demonstrate the feasibility of carbon dioxide as the cushion gas of gas storage.(5)This paper researches the sensitivity of the mixing between CO2 and natural gas by numerical simulation,finds out the main factors influencing the mixing,proves the great influence of dynamic injection-production on mixing.It's shown that it's better to inject cushion gas from the side of reservoirs.(6)A case study validates the feasibility and effectiveness of the proposed method.The results showed that CO2 Cushion gas in storage reservoirs provide the necessary pressure for the withdrawal of working gas and it makes up for the largest part of investments in underground storage reservoirs.