Phase Behavior Modelling of Oils in Terms of SARA Fractions

Abstract

Abstract One of the key steps towards improving the predictability of air-injection-based processes relies on the development of accurate phase behavior models of the oil. Historically, for in-situ combustion (ISC) in heavy oils and bitumens, phase behavior was often ignored, as the physical aspects of the process (e.g. distillation) were not considered to be as significant as the oxidation reactions. However, this step is important for several reasons. First, the compositional model should reflect the phase behaviour of the original fluids. Second, reaction rates are dependent on the concentration of the reactants, which in turn are affected by the volatility of the components. This is particularly important for lighter oils (but not unimportant for heavier oils) where the phase equilibrium between the liquid and vapour can have a significant impact on the flammability range for vapour phase combustion at given temperature and pressure conditions. Finally, for the case of lighter oils, a good phase behaviour model is required to capture the compositional effects of the resulting flue-gas drive. This study presents a practical workflow to develop a phase behavior model in terms of SARA fractions (saturates, aromatics, resins and asphaltenes), which is aligned with the reaction modelling approach used in most kinetic models. The methodology requires conventional oil characterization (i.e. based on distillation cuts) and conventional phase behavior experiments (e.g. differential liberation), as well as oil characterization in terms of SARA fractions. The first step of the method consists of splitting of the heaviest oil fraction (i.e. plus fraction), followed by the lumping of all single-carbon-number components, in such a way that the new oil characterization honours the SARA data available, such as composition, and physical properties of each fraction (e.g. molecular weight). In addition, the gas components (e.g. Methane) would be treated as additional components as necessary. The second step is to tune an equation of state (EoS), in terms of the SARA-based model, to match the relevant laboratory experiments. Finally, the tuned EoS would be used to export the equilibrium constants (K-value tables) to the thermal numerical simulator. Different examples on the application of the phase behavior modelling workflow are presented and discussed in detail, for heavy and light oils. This work opens up opportunities to model the ISC process for any oil (i.e. light or heavy) by utilizing the currently available kinetic models, which in turn is an important step towards improving the predictability of ISC processes using reservoir simulation.