Study On Density Of CO₂ Saline Solution And CO₂ Decane Solution Under Geological Sequestration Conditions

At present, global climate change caused by large emissions of greenhouse gases is a serious threat to the living environment of human being. Carbon Capture and Storage (CCS) technology can reduce atmospheric concentrations of CO2 immediately and effectively. Among all the geological sequestration technologies, the deep saline formations were considered the most promising sequestration sites because they provide the largest capacity for CO2 geological sequestration; Injection of CO2 into depleted oil reservoirs can not only enhance the oil recovery but also sequestrate partial CO2 within the subsurface reserviors.In order to develop reliable CO2 geological sequestration technology, it is necessary to know the knowledge physical and chemical properties of CO2 based formation fluids. The density change of formation fluids due to CO2 dissolution affect the diffusion and moving of CO2 and then affect the safety and efficacy of CO2 sequestration. In this study, we measured the density of CO2 saline solution under geological sequestration conditions systematically using the Magnetic Suspension Balance experimental system. We also used decane as simulated crude oil and measured the CO2 decane solution under sequestration conditions using the Magnetic Suspension Balance experimental system. We developed density modal of CO2 saline solution and CO2 decane solution suitable to sequestration conditions by correlating the experimental density data. Besides, the volumetric behavior of CO2 saline solution and CO2 decane solution were studied based on the experimental density data.The research result of CO2 saline solution shows that:under experimental conditions, the density of CO2 saline solution increase with pressure linearly and the increasing rates are all the same in all cases; the density of CO2 saline solution increases with temperature; the density of CO2 brine solution increased with increasing CO2 mass fraction almost linearly and the slope of the CO2 brine solution density vs CO2 mass fraction curves is closely related to temperature; it decreases from 0.222 to 0.185g·cm-3 as temperature increases from 313 to 353 K. The density data of CO2-free brine and CO2 saline solution have been fitted to equations. Comparing with the experimental results, the equations can accurately reproduce the density data where the maximum deviations are 0.004% and 0.03%, respectively. The excess molar volume of CO2 saline solution is negative in all cases. It is less negative with increasing pressure and more negative with increasing temperature. The apparent molar volume of CO2 in saline solution increases with increasing temperature and CO2 concentration. The research result of CO2 decane solution shows that:under experimental conditions, the density of CO2 decane solution increase with pressure and decrease with temperature almost linearly. The density of CO2 decane solution shows a crossover with CO2 mass fraction and the crossover pressure increases with temperature. We developed a density modal for CO2 decane solution based on the experimental density data. The predicted values are in good agreement with experimental data. The excess molar volume of CO2 decane solution is negative in all cases. It becomes less negative with increasing pressure and more negative with temperature. When temperature and pressure are constant, the excess molar volume shows a trend of first decrease and then increase with CO2 concentration. The apparent molar volume of CO2 in decane solution increases with increasing temperature and pressure. And the relationship between it and CO2 concentration is not obvious.In summary, the density and volumetric properties of CO2 saline solution and CO2 decane solution were studied using Magnetic Suspension Balance experimental system, which provides theoretical guidance and database supports for CO2 deep saline sequestration and CO2 oil reservoir sequestration.