Study On Measurement And Modeling Of CO₂-Brine Solution Density Under Geological Sequestration Conditions

The CO2 concentration in atmosphere has been increasing in recent years,due to the extreme dependence on fossil fuel,which leads to global warming and some natural disasters.CO2 sequestration into brine reservoir,as one of the promising CO2 geo-sequestration methods,is widely studied.Density of CO2-brine solution is one of key parameters to affect the brine mitigation underground,which further influence the safety of CO2 geo-sequestration.Hence,in this work,some measurements of the CO2-brine solution density were done,and the influence of temperature,pressure,NaCl concentration and CO2 concentration were analyzed.In addition,the density models were built to predict the CO2-brine solution density.The study provides high-precision CO2-brine solution density data support for theoretical model research and the evaluation of risk analysis and site selection on the CO2 geo-sequestration in brine underground.The density of CO2-H2O-NaCl was measured under the condition of typical geo-sequestration,with different temperature(60??140?),different pressure(10?18 MPa),different CO2 mass fraction(0%.1%.2%.3%)and different NaCl concentration(1?2?3?4 mol/kg)on the basis of the density measurement of CO2-brine solution using high-precision Magnetic Suspense Balance experiment system.When NaCl concentration was in the range of 1?3 mol/kg,the density of CO2-brine solution linearly increases with pressure and slopes close to equal,decreases with temperature,and linearly increases with increasing CO2 mass fraction and increasing NaCl concentration.What's more important,the phenomenon of equal density temperature was found in this study.On the condition of 4 mol/kg NaCl concentration,when temperature is above 120?,the density of CO2-H2O-NaCl decreases with the increase of CO2 mass fraction.Therefore,on the geo-condition of high temperature and salinity,there is possibility that brine density decrease after the dissolution of CO2,which is harmful for CO2 sequestration.Based on the experiment data,the empirical density model and density difference model were built,and the models have good accuracy when applying to prediect the density of brine from Tianjin Dagang reservoir,brine from Tianjin Tanggu reservoir,The volume features of apparent molar volume of CO2-H2O-NaCl were also analyzed,and further,the models of apparent molar volume and equal density temperature for CO2-H2O-NaCl were established.Then the equal density temperature of CO2-H2O-NaCl was calculated using the equal density temperature model,and the calculation result fitted the experiment data well as NaCl concentration of 4 mol/kg.Meanwhile,we predicted the equal density temperature of pure water,the brine from Tianjin Dagang reservoir and brine from Tianjin Tanggu reservoir,and prodive support for the evaluation of the temperature standard of the sequestration site selection.ePC-SAFIT model was unproved and applied to study the density of CO2-H2O-NaCl solution.The molecular parameter of H2O and the binary interaction coefficients of CO2-H2O and H2O-NaCl were optimized,and the relationships between these parameters and tengerature were built.The improved parameters were applied to improve ePC-SAFT model,and the calculated results fitted very well to the experimental data.In addition,introducing the NaCl concentration transformation method,the inproved ePC-SAFT model was applied to predict the density of experiment brine of H2O-NaCl and actual underground brine from Tianjin Dagang reservoir and Tianjin Tanggu reservoir with CO2 dissolving in.The simulated results agreed well with the e3q)erimental results.In addition,the density prediction of the CO2-Weyburn brine using this model was also accurate.Thus,the improved ePC-SAFT model has good precision and can provide support for the CO2 geo-sequestration into underground brine.