Studies On Density Models Of H₂O-CO₂-Salt(Li⁺,Na⁺,K⁺,Mg²⁺,Ca²⁺,Cl^-,SO₄^2-) System

Recently, the global climate became warmer and warmer because of emissions of the greenhouse gases like CO2. For this reason, scientists proposed an effective and feasible technology for carbon capture and storage. Deep saline aquifer is one of the best places, because it has wide distribution and large area, and can sequestrate CO2 for a long time. To this technique, the density of H2O-CO2-salt system is a very important physicochemical parameter, so it is very necessary to establish a density model for H2O-CO2-salt system under CO2 geological storage conditions(typically less than 200 ℃ and 500 bar).Firstly, based on the density model of chloride-water system(Mao and Duan, 2008), we established a density model of binary sulfate-water systems(Li2SO4-H2 O, Na2SO4-H2 O, K2SO4-H2 O, Mg SO4-H2O) with the Pitzer electrolyte theory. Corresponding model parameters were obtained by the least squares method. Compared with reliable experimental data for the Li2SO4-H2 O, Na2SO4-H2 O, K2SO4-H2 O, Mg SO4-H2 O systems, the average density error was 0.046%, 0.036%, 0.051%, 0.038%, respectively. Calculated apparent molar volumes at infinite dilution from the density model are within the scope of the values given by the literatures, which further validates the reliability of the density model.Then, according to the mixing rule of Young and Smith(1954), a predictive density model of mixed water-salt systems(H2O, Li+, Na+, K+, Mg2+, Ca2+, Cl-, SO42-) was proposed. Compared to the density of experimental data of multi-component water-salt systems, the average density error is usually less than 0.1%. Meanwhile, in terms of the IAPSW97 equation(Wagner et al., 2000) of H2 O, a density model of pure CO2 fluid explicit in Gibbs free energy was developed, with a valid T-P range of 216.592~623.15 K and 0~1000 bar.Finally, according to the method of Duan et al.(2008), the parameters of liquid density model of H2O-CO2 system was refitted. The density calculated by this model is more precise with an average density error of 0.083%. Using an approximate method(CO2 apparent molar volume in the water-salt system is equal to that in water), the H2O-CO2 density model was extended to multi-component H2O-CO2-Li Cl/KCl/Mg Cl2/Ca Cl2/Li2SO4/Na2SO4/K2SO4/ Mg SO4 systems. Compared with the experimental density data of H2O-CO2-Na Cl system, the average density error is about 0.179%. Due to the lack of experimental data of other multi-component H2O-CO2-salt systems, future experimental work can be focused on these systems to further verify the validity of these models.