Experimental and theoretical study on water solubility of carbon dioxide in oil and gas displacement

The emission of greenhouse gas carbon dioxide causes the global temperature to rise year by year. A possible method for CO2 storage and emission reduction is to use it to displace oil and gas. Solubility of CO2 in water is key to replace oil and gas. The present study investigates the effects of the temperature, pressure, and addition of salt and methane on the water solubility of CO2 by high temperature and high pressure cell combined with chromatographic analysis. The results show that the phase state of CO2 in water with temperature and pressure greatly affects its solubility. Besides, its solubility in the supercritical and liquid state is significantly greater than that in the gaseous state. The solubility of CO2 gas in brine and water increases with an increase in pressure; however, this increasing tendency gradually decreases. At the same time, the addition of CH4 gas can boost the dissolution of CO2 gas, while the addition of NaCl can significantly reduce its solubility; as the salt content in the water increases, the solubility of CO2 gradually decreases. Given the influence of saturated vapor pressure of water at vapor-liquid equilibrium, a thermodynamic constitutive equation was established to calculate the water solubility of CO2. The results showed that the prediction error of CO2 solubility in water is within 8%, that of pure CO2 is less than 4%. A comparison of these results with those predicted using the artificial neural network model showed that the thermodynamic model is more suitable for predicting water solubility required for replacing oil and gas by carbon dioxide.