Study On The Phase Equilibrium Prediction Model Of High CO₂ Natural Gas And Water System

More and more high CO2 natural gas fields have been discovered at home and abroad.However,the development of high CO2 natural gas reserves still lacks more mature theories and technologies,and the solubility of CO2 in formation water is far greater than other hydrocarbon components in natural gas.Knowing the dissolution between high CO2 natural gas and formation water is an important basis for the high-yield and reasonable exploitation of such gas reservoirs.Therefore,this paper proposes a research topic for balancing the high CO2 natural gas and water systems.This study provides theoretical and practical significance for fluid migration,accumulation and efficient and safe development of reservoirs with high CO2 content in natural gas fields.For this practical problem,this paper uses a combination of experimental and theoretical research methods.The phase equilibrium experiment and prediction model study of different CO2 content natural gas PVT phase state experiment,different CO2 content natural gas and formation water mutual solution were carried out.The equilibrium prediction model and experimental data of high CO2 gas and water system were compared and analyzed.The main achievements of this paper are as follows:(1)The volume coefficient,relative volume and compression coefficient decrease with the increase of pressure.The deviation factor decreases and then increases with the increase of pressure.The density and viscosity increase with the increase of pressure;For natural gas with different CO2 content without stratum water and formation water:the deviation factor of the non-stratum water is larger than that of the water-bearing stratum.The relative volume,compression coefficient,volume coefficient,density,viscosity and expansion coefficient are very small.(2)Within the scope of this study(temperature 141?,145?,148?;pressure 3-53.01 MPa),as the pressure of the system decreases,the gaseous water content in natural gas increases continuously,and the increase range under high pressure conditions is obviously lower than that under low pressure conditions;Under the same pressure,with the increase of temperature,the gaseous water content in the natural gas rises;For the calculation of the theoretical method:the calibrated Campbell plate method and the Wecht chart method in the plate method have the highest accuracy,and the accuracy of the acid gas,the salinity and the relative density in the empirical formula is higher.(3)Natural gas samples with CO2 content of 9.16%,21.97%and 53.88%are used.The solubility of high CO2 natural gas in formation water is between 0.12-20.19m3/m3,With the decrease of pressure in the system,the solubility of natural gas in formation water continuously decreases,and the higher the CO2 content,the greater the reduction;and the molar composition of dissolved natural gas in formation water is measured.(4)Considering that water molecules belong to strongly polar molecules,water molecules themselves or water molecules may associate with other molecules.Based on the SAFT equation of state,a phase equilibrium prediction model for high CO2 natural gas and water systems is established.The SAFT equation of state takes into account the short-range repulsive force and long-range dispersion force between molecules,the aggregation and association of chemical bonds,and the solvation among different molecules,which is mainly composed of the hard sphere contribution term,the chain contribution term,the dispersion force contribution term,and the association contribution term.(5)Developed a program for predicting the phase balance of high CO2 natural gas and water systems,and verified that the model calculation data and experimental data can be well fitted.For the gas water content in natural gas,the calculated value of the model is slightly larger than the experimental value;for the solubility of natural gas in the formation water,the calculated value of the model is slightly smaller than the experimental value.It shows that the prediction model in this paper can better describe the dissolution of high CO2 natural and formation water.