Study On Density Of CO₂Pure Water Solution Oand CO₂Brine Solution Under Sequestration Conditions

With the rapid development of social economy and large-scale use of fossil fuels, the sharp increase of CO2and other greenhouse gas emissions on a global scale lead to the greenhouse effect more and more serious. In order to effectively reduce the content of CO2in the atmosphere and alleviate global warming, carbon capture and Storage (CCS) is considered to be one of the most effective ways. Among all CO2sequestration technology, the saline formation sequestration is considered to be one of the most promising methods of storage due to the great potential and a long time for buried. In order to evaluate the safety of CO2saline formation sequestration, should understand the migration situation after CO2dissolved in underground brine solution, and the solution density is an important basic physical properties. In this paper, we measured systematically the density of CO2-pure water solution and CO2-brine solution from Dagang Oilfield reservoir. According to thermodynamic theory, we obtained density model of CO2pure water/brine solution based on our experimental data.When temperature and pressure are1～140℃and10-18MPa, respectively, the research results about CO2-pure water solution density show that:the density of CO2-pure water solution relations with pressure is linear increase, with temperature is parabola decrease and with CO2concentration is approximately linear increase. According to the relationship between the density of the solution and CO2concentration, we obtained the equal density temperature (Te) of this solution system which is255.4℃. The solution density will not change with CO2concentration change when the temperature is equal to the Te. The solution density will decrease with increasing CO2concentration when the temperature is higher than the Te. Therefore, it is very important to ensure that storage temperature is lower than the inherent Te when actual CO2geological sequestration. In order to improve the safety of CO2sequestration, high geothermal gradient saline formation is unsuited for CO2sequestration.When temperature and pressure are40-90℃and10-18MPa, respectively, the research results about the density of CO2-brine solution from Dagang Oilfield reservoir show that:the density of CO2-brine solution from Dagang Oilfield reservoir relations with pressure is linear increase, with temperature is parabola decrease and with CO2concentration is approximately linear increase. These conclusions are consistent with CO2-pure water solution system, but the Te of CO2-brine solution from Dagang Oilfield reservoir is161.5℃, this Te is lower than the CO2-pure water solution system. According to thermodynamic basic theory, a semi-empirical density model of CO2pure water/brine solution is obtained. And coming up with an idea that apparent molar volume of dissolved CO2as a function of temperature, CO2mass fraction and total dissolved solids (TDS) in brine solution. By comparing the experimental data of other researchers, the accuracy of our model have certain to improve, the maximum deviation (MD) and absolute average deviation (AAD) between our model and other experimental data are0.26%and0.07%, respectively. Therefore, this semi-empirical density model can accurately predict density change of CO2dissolved in brine solution, this provides data and theoretical support for CO2geological and deep-sea sequestration.