In development of early generations of thermal compositional simulators several assumptions are used based on characteristic of dead oil and steam mixture. K-value approach is used for phase splitting and equilibrium ratios are only function of temperature and pressure. Phase properties such as density, enthalpy and internal energy are calculated from correlations and ideal solution assumption. Excess properties such as excess enthalpy and density and mutual solubility of water in oil phase and vice versa are neglected in such models.
The latter assumptions may work properly for simple fluid mixtures with pure steam injection but could produce false results in more complicated processes such as hybrid processes with more intermediate components. In hybrid processes, where a hydrocarbon solvent is added to the steam, equilibrium ratios change with the variation of composition and neglecting this effect may lead to thermodynamically inconsistent or wrong results. Solubility of water in oil phase increases with temperature and it could become significantly high in some cases.
The purpose of this study was to develop a 3-D, fully implicit, equation of state (EOS) based thermal compositional simulator capable of modeling hybrid and thermal process of heavy oil recovery. By using an equation of state we aim to correctly model the thermodynamic and compositional effect on the phase behaviour. Water is allowed to be soluble in all phases and mutual solubility of oil and water is taken into account in our proposed simulator and its effect on the oil recovery can be investigated. Thermal expansion, fluid compressibility, solvent extraction, and steam distillation are calculated by our thermodynamic model. Steam properties are calculated from EOS or steam tables.
Different features of current simulator were validated against the analytical models and commercial simulators. Then several synthetic mixtures from published papers were selected and used in thermal and hybrid processes. The field production and injection rates from the current simulator are compared with the K-Value approach simulators. Detailed grid to grid level comparisons between EOS and K-Value approaches were also performed to investigate the effect of solvent additives on the equilibrium ratios, mutual solubility of oleic and aqueous phase, and phase splitting.