Treatment Of High Nitrogen-Containing Natural Gas In Tahe Oilfield

Enhancing oil recovery with nitrogen injection has achieved a good result in Tahe oilfield.However,the nitrogen content of the natural gas in the No.1 combined station of Tahe oilfield increases correspondingly owing to the nitrogen injection.With the maximum nitrogen content of 33.3%,the high heat value of the natural gas is 26.4 MJ/m3 which is far below Class ? natural gas standard and the production of downstream user is affected.Additionally,with the expansion scale of nitrogen injection,the nitrogen content of the natural gas is expected to reach more than 40%.Therefore,it is necessary to propose an economical and effective treatment process for the high nitrogen-containing natural gas in Tahe oilfield.With the advantages of large throughput and good nitrogen removal effect,the cryogenic nitrogen removal process requires a large amount of cold energy and equipment investment and have a relative high energy consumption in operation,which makes it not economical that applying the cryogenic nitrogen removal process directly for treating the high nitrogen-containing natural gas in the Tahe oilfield.With the no need for the LNG product and combining the advatanges of cryogenic nitrogen removal process,three treatment processes for high nitrogen-containing natural gas based on the recovery for cold energy are proposed,the nitrogen removal standard of which is to make the heat value of natural gas meet the class? requirment.Addionally,these treatment processes are analysed from the aspects of LNG separation unit,refrigeration unit,excrgy loss and adaptability.The following contents are included:(1)Peng-Robinson equation is selected as the calculating method for the physical properties of nitrogen-containing natural gas and the analysis of the effect of the pressure,temperature and nitrogen mole fraction on the enthalpy,entropy and exergy of nitrogen-containing natural gas during the process of liquef-action,separation and gasification is conducted.The results show that the existence of the nitrogen can improve the energy grade and exergy efficiency and speed up the rate of cold energy of the natural gas in low temperature.Meanwhile,the nitrogen has more obvious effect on the enthalpy,entropy and exergy of nitrogen-containing natural gas at low pressure.(2)The fitting analysis of the nitrogen volume of the nitrogen injection wells in Tahe oilfield shows that the average nirtrogen content of the feed gas that flows into the No.1 combined station of Tahe oilfield is 21.5%and the maximum is 33.3%amd will reach more than 40%with the expansion scale of nitrogen injection.In order to predict the natural gas heat value under different nitrogen content and relative density,the high heat value chart of the nitrogen-containing natural gas is obtained and it shows that when the molar fraction of the nirtrogen in the natural gas reaches 20%,the heat value is lower than the standard of class? standard.Therefore,three treatment process,respectively combined with single mixed refrigeration(SMR),the propane refrigeration and the expansion refrigeration for high nitrogen-containing natural gas are proposed.(3)The Pareto Frontier in the process of LNG separation is defined and the research conditions and product indexes are given.The model of LNG separatio unit with one stage,two stages and three stages are established and the Matlab optimization tool,which is based on the genetic algorithm(GA),is used to search the Pareto Frontier of the methane loss about the nitrogen content of the product gas.Additionally,the methane loss under diffirent separation pressure and stages are compared on the condition of the high nitrogen-containing natural gas in Tahe oilfield.The results show that with the separation of 500kPa and the methane loss being lower than 5%,natural gas with nitrogen mole fraction less than 33%can be treated by one stage separation process,and that less than 40%can be treated by two stages separation process and that less than 42%can be treated by three stages separation process.The pressure of LNG separation unit for high nitrogen-containing natural gas in Tahe oilfield is supposed to be 500kPa and the two separation stages process is supposed to be used.(4)The HYSYS models of the three treatment processes for high nitrogen-containing natural gas are established and used to analyse the energy consumption,the equipment heat load,the utilizaiton of cold energy,the heat transfering performance of the cold boxes and the investment.The results show that the expansion refrigeration process has the lowest energy comsumption and investment with the total energy consumption of 170.9×104kJ/h,the energy comsumption for per feed gas of 180.7kJ/m3 and the energy comsumption for per product gas of257.5kJ/m3.(5)The exergy calculation model for expansion refrigeration treatment process is established and the exergy loss and the exergy efficiency of every equipment in the process are analysed.Addtionally,a two-stage pressurization scheme for inlet of feed gas is proposed in order to improve the exergy efficiency furtherly.The results show that with the boost ratio of the first stage compressor of 1.9 and that of the second stage compressor of 1.6,the pressurization unit has a highest exergy efficiency and the lowest exergy loss.Compared with the initial process,the exergy loss of the improved process is reduced by 6.3%and the exergy efficiency of that is improved to 97.1%.(6)The adaptability analysis show that the expansion refrigeration treatment process for for high nitrogen-containing natural gas has good adaptability to the fluctuation of nitrogen molar fraction in 25%?60%,the fluctuation of temperature in 30??40? and pressure higher than 1590kPa.Additionally,compared with other commonly used treatment process for nitrogen-containing natural gas,the process proposed has the lowest specific energy consumption under the condition that the requirement fo the molar fraction of nitrogen in the product gas is 5%.