Research On Change Rule Of Carbon Dioxide Physical Properties Near The Critical Point

Supercritical carbon dioxide?SC-CO₂?is a special fluid with unique properties,like relatively high density,low viscosity and high diffusivity.SC-CO₂ has drawn more and more attention from the oil and gas industry.Laboratory studies and field practice indicate that SC-CO₂ is a good candidate in well drilling,fracturing,etc.However,drastic changes of properties near the critical point make it hard to predict and control the phase behavior of SC-CO₂.It is necessary to study the microscopic mechanism of phase behavior near the critical point,then the behavior of macroscopic properties can be obtained to provide basis for field applications.In this thesis,the molecular aggregation behavior near the critical point is investigated through molecular dynamics simulation.The macroscopic physical properties?i.e.density and viscosity?of carbon dioxide are obtained around the critical point.Experiments were conducted to verify the results of the simulation work with the“Measurement System of Carbon Dioxide Properties”.This work indicates that carbon dioxide exhibits the following unique features.The density and viscosity decrease as pressure increases at a given temperature.On the contrary,the density and viscosity decrease as temperature increases at a given pressure.The changing trend of the macroscopic properties is the same with that of the temperature and pressure.The aggregation level of carbon dioxide molecules under supercritical conditions is similar to that under liquid phase condition?within the same order of magnitude?.This phenomenon makes the density of carbon dioxide under supercritical conditions is close to that of liquid carbon dioxide.However,if the temperature is high,the change in the aggregation level of carbon dioxide molecules becomes more moderate.This scenario results in the insignificant change in density as pressure increases.The change of density and viscosity becomes more moderate,and the boundary between the supercritical phase and the gas phase becomes more unconspicuous.The area near the critical point can be divided into three distinct regions according to the changing behavior of macroscopic physical properties:the linear-growth region where the density and viscosity grow linearly,the rapid-growth region where a rapid growth is presented in the density and viscosity,and the slow-growth where the properties increase slowly.Based on the molecular simulation and experiments,a method is proposed to divide the area near the critical point?the critical region?according to the characteristics of aggregation of carbon dioxide molecules.The critical region of carbon dioxide is:the pressure ranges from 5.88 MPa to 15.5 MPa,and the temperature range from 297.07 K to 297.07+?P-5.88?×7.75?K?.