Modeling Phase Behavior of Heavy Oil/Bitumen and Solvent Binary Mixtures

Conventional production methods in heavy oil reservoirs are not efficient due to high viscosity of oil. Therefore, several new 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 diversity of its components, and it gets more complicated by addition of solvent. One of the assumptions to simplify the problem is considering heavy oil as a single pseudo component and to investigate thermophysical properties of binary mixtures with diluents/mixtures. In this study, more than 1000 experimental data of various types of heavy oil/bitumen and several hydrocarbon and non-hydrocarbon solvents were collected and evaluated. The data were divided into two groups, below and above the critical temperature of the solvent, according to experimental temperatures. Also, modeling mixture phase behavior as a binary mixture was done using the advanced Peng-Robinson equation of state (APR-EoS). EoS tuning to match the experimental data was done by optimizing the binary interaction coefficient (kij). The results showed that binary mixture assumption could be used as a practical approach to construct P-X diagrams. Optimized kij was proposed according to dimensionless ratio Tco/Tcs (oil critical temperature to solvent critical temperature) for each solvent. The model constructed using suggested kij shows an average deviation of about 13% compared to experimental data. It is necessary to note that vapor-liquid-liquid three phase region 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. Therefore, investigating impacts of pressure change on three phase region is not possible using this model.