Effects of Temperature and Pressure on the Phase State of Oils and Asphaltene Solutions Observed Using Dielectric Spectroscopy

Abstract

The dielectric spectra of Caspian Oil were measured at atmospheric pressure in temperature range from 123 to 203 K. Also dielectric responses of Kumkolskaya Oil and asphaltene solutions were obtained in pressure range up to 1 GPa and in temperature interval 250−320 K. The dependences of glass transition temperature (Tg) on pressure for oil samples were obtained for the first time by dielectric spectroscopy, they increase approximately from 125 at atmospheric pressure to over 280 K at 1 GPa. The phase diagrams of oil samples were constructed using Williams−Landell−Ferry equation. For asphaltene solutions, the relaxation times depend on viscosity only prior to the beginning of a crossover region nearby crystallization pressure of solvents. It is shown that the dielectric strength is not proportional to total concentration of asphaltenes in polydisperse solutions. The values of the thermal expansion coefficient of the free volume (αf), the isothermal compressibility of the free volume (βf), and the relative free volume at Tg (fg) were obtained for the examined oils. The received data are in agreement with the data obtained by independent thermophysical methods.