Assessment of the applicability of binary mixture hypothesis to modeling the phase behavior of heavy oil/bitumen-solvent systems for EOR applications

Abstract

Conventional production methods in heavy oil reservoirs are not efficient primarily due to the high viscosity of the oil. Several methods such as VAPEX and ES-SAGD have been developed to overcome production challenges from these reservoirs, which mainly rely on oil viscosity reduction. Solvent injection dilutes the oil and produces a complex asymmetric mixture. It is difficult to study the phase behavior of oil due to the diversity of its components, and it gets more complicated with the addition of solvent. One of the assumptions to simplify the problem is considering heavy oil as a single pseudo component and investigating the thermophysical properties of binary mixtures with diluents. This study collected and evaluated more than 1000 experimental data on various types of heavy oil/bitumen and several hydrocarbon and non-hydrocarbon solvents. The data were divided into two groups, below and above the critical temperature of the solvent, according to experimental temperatures. Modeling of mixture phase behavior as a binary mixture was performed using the advanced Peng-Robinson equation of state (APR-EoS). EoS tuning to match the experimental data was performed by optimizing the binary interaction coefficients (kij). The results showed that the binary mixture assumption could be used as a practical approach to construct pressure-composition phase diagrams. Optimized values of kij were proposed as two empirical correlations according to the dimensionless ratio Tco/Tcs (oil critical temperature to solvent critical temperature). The model constructed using these correlations showed an average deviation of about 16% compared to experimental data. It is necessary to note that the vapor-liquid-liquid three-phase regions may occur in a heavy oil/bitumen and solvent mixture. But according to the Gibbs phase rule, with two components and three phases in the system, the degree of freedom equals one. So, investigating the effects of pressure change on three-phase regions is not possible using this model.