Phase behavior of C3H8–CO2–heavy oil systems in the presence of aqueous phase under reservoir conditions

Abstract

Phase behaviors including phase boundaries, volumes, and compositions of reservoir fluids during solvent(s)-assisted heavy oil recovery processes have been experimentally and theoretically determined in the presence of an aqueous phase. More specifically, a water-rich aqueous phase (A), an oil-rich liquid phase (L), and a solvents-rich vapor phase (V) can coexist under certain reservoir conditions. The phase boundary between AL and ALV on a pressure–temperature phase diagram, i.e., three-phase bubblepoint pressure, has been measured for C3H8–CO2–water–heavy oil systems at temperature ranging from 298.15K to 383.15K. The phase volumes of A+L and V at thermodynamic equilibrium state have been determined at temperatures of 321.55K and 344.95K, respectively. Moreover, the fluids in both L and V phases are sampled in an isobaric manner to perform the compositional analyses by using gas chromatography (GC) method. Meanwhile, two heavy oil samples are theoretically characterized as the multiple pseudocomponents. The Peng–Robinson equation of state (PR EOS) together with two recently developed alpha functions for water and non-water component(s) is applied as the primary thermodynamic model. A previously developed binary interaction parameter (BIP) correlation for CO2–water pair is combined with the van der Waals’ mixing rule to improve the phase behavior prediction for water-contained system. A volume translation method proposed by Peneloux et al. (1982) is then incorporated to correct the calculated phase volume. Without tuning any parameters, the developed water-associated and water-free mathematical models are found to be able to accurately reproduce the measured phase boundaries in the presence and absence of the aqueous phase, respectively. The three-phase bubblepoint pressure is found to be reduced in the presence of water. More accurate prediction can be achieved by considering the effect of aqueous phase. Finally, the GC analyses and flash calculations demonstrate that CO2 is more easily to be vaporized than alkane solvent (i.e., C3H8) when phase splitting occurs for a C3H8–CO2–water–heavy oil system.