Study On The Thermodynamics Of CO₂-Hydrocarbon-Formation Water System By Using The Association Theory

Among the global oil and gas resources,high CO2-content natural gas reservoirs are widely distributed.In the development process of the high CO2-content natural gas reservoirs,the phase equilibrium of the CO2-hydrocarbon-formation water system is complicated with the decline of the formation pressure,and considering the influence of H2O/H2O and CO2/H2O intermolecular hydrogen bonds,the thermodynamic behavior of natural gas with high CO2-content in formation water is difficult to be described accurately according to the theory and method.Therefore,the thermodynamic theory of those system is significant theoretical basis for safe and efficient development of high CO2 natural gas reservoirs.Based on the combination of experiment and theory,the solubility of natural gas with different CH4 content and the data of saturated water content in the gas phase are tested.The experimental data are verified by the PR-HV state equation,and the CO2/CH4-pure water/brine mixture system is established on the basis of the association fluid theory.The CPA-SRK-WS thermodynamic model has be established to calculate the phase equilibrium of CO2-hydrocarbon-formation water system.The research results are of practical significance to reveal the dissolution mechanism of CO2 underground effective storage and the water production mechanism of the high CO2 gas well,and to enrich and develop the phase state of the CO2-oil water system.The main conclusions of this paper are as follows:(1)Comparative analysis of the thermodynamic model of CO2-hydrocarbon formation water mixture system is described.It mainly includes the conventional cubic equation of state(PR/SRK)combined with different mixing rules(HV/WS),and describes the process of calculating the gas-liquid equilibrium of the conventional cubic Eos and considering association Eos in detail.(2)Balanced-Liquid-Sampling-Method and Flash-Method were used to measure the gas solubility and water content in gas mixture solution system,under the experimental conditions(temperature of 65℃～133 ℃,pressure 10MPa～60MPa,CH4 content 20～60%)measured in 108 experimental data;and the PR-HV(NRTL)model was validated against the experimental data,and the following conclusion is:gas solubility increases with the increasing of pressure;the saturated water content decreased with the increase of pressure under the same temperature.When the temperature is greater,the saturated water content changes greatly with the changes of pressure.(3)Based on association theory model,the improved model(iCPA-PR and iCPA-SRK)were proposed.On the other side,using CPA-SRK combination with WS mixing rule and NRTL activity coefficient model,a new thermodynamic model are innovatively established for calculating the phase behavior of CO2-H2O,CH4-H2O and CO2-CH4-H2O system.The binary interaction parameters(τij、τji、α、kij、kji)of that model were obtained by experimental data.Through the comparison of the 720 groups of the experimental data with the other six thermodynamic models are found that the thermodynamic model for associating the forecasting effect is better than that of the conventional cubic equation of state;and the CPA-SRK-WS model has the highest prediction accuracy(AARD=4.02%)in a wide range conditions(T=273.15～573.15K,P=150MPa).In cubic Eos,the results of PR-HV and PRSV-WS model are the better,and the result of UMR-PR model is the worst.(4)For the CO2/CH4-formation water system,the CPA-SRK-WS can be popularized to calculate the thermodynamic equilibrium problem of the aqueous solution system of the CO2/CH4-formation.A comparative analysis of PR-HV model,Duan2003 model,Duan2006 model and SRK-HV model in the prediction of xCO2/xCH4 in AB(NaCl)and AB2(CaCl2)system.And the conclusion as follows:in the prediction xCO2 of CO2-H2O-CaCl2 system in the experimental condition(T=303～523K、P≤50MPa、m=1.01、2.28、3.90 mol·kg-1),the CPA-SRK-WS model performs the best results with all AARD lower than 6%,the average relative error of AARD is 3.81%,which predicts far below the AARD of PR-HV,SRK-HV model(11.78%and 14.76%);in the prediction xCO2of CO2-H2O-NaCl system in condition of T=303K～473K,P=0.63MPa～40MPa,m=0.173～6mo1·kg-1,CPA-SRK-WS also displays the highest prediction accuracy(AARD=4.15%)among the Duan2003 model,the PR-HV and SRK-HV models;in the prediction xCH4 of CH4-H2O-NaCl system in the temperature(372～513K),pressure(13.5～156.8MPa),concentration(0.06-5.05 mol·kg-1),the CPA-SRK-WS model get the best prediction results comparison with other models(AARD=6.63%),Duan2006 model takes second place,SRK-HV has the worst error;in prediction xCH4 of CH4-H2O-CaCl2 system,the average AARD of CPA-SRK-WS model is 2.67%,much lower than the PR-HV and SRK-HV model(6.97%and 10.91%).