F. Schoeggl

Case Study: Modeling the Phase Behavior of Solvent Diluted Bitumen

The design of solvent-based and solvent assisted heavy oil recovery processes requires accurate predictions of phase behavior as straightforward as saturation pressures and as potentially complex as vapour-liquid-liquid equilibria and asphaltene precipitation. In this case study, saturation pressures of dead and live bitumen were measured in a Jefri PVT cell at different concentrations of a multi-component solvent at temperatures from 20 to 180°C. Saturation pressures and the onset of asphaltene precipitation were also measured for the bitumen diluted with n-pentane. The onset of precipitation was determined by titrating the bitumen with pentane and periodically circulating the mixture past a high pressure microscope. The data were modeled with the Advanced Peng-Robinson equation of state (APR EoS). The maltene fraction of the bitumen was characterized into pseudo-components based on extrapolated distillation data. The asphaltenes were characterized based on a Gamma distribution of the molecular weights of selfassociated asphaltenes. The APR EoS was tuned to match the saturation pressures by adjusting the binary interaction parameter between the solvent and the pseudo-components via a correlation based on critical temperatures. Rather than adjusting the interaction parameters for each pair of components, only the exponent in the correlation was adjusted. The role of mixing rules in correctly predicting the onset and amount of asphaltene precipitation is discussed. Introduction In Western Canada, thermal recovery methods such as cyclic steam stimulation and steam assisted gravity drainage are the methods of choice to recover heavy oil and bitumen with viscosities exceeding 10,000 mPa.s. These methods require significant volumes of natural gas and water to generate steam: approximately 34 m3 of natural gas and 0.2 m3 of groundwater (assuming 90 to 95% recycle) to produce one barrel of bitumen (1). Solvent based and solvent assisted recovery methods are a potential alternative to reduce or replace steam usage. However, potential solvents, such as light n-alkanes , are expensive relative to heavy oil and the success of process depends on how much solvent can be recovered. Predicting the performance of solvent-based and solvent-assisted processes (including both oil and solvent recovery) is challenging because the introduction of a solvent can lead to complex phase behavior. For any given heavy oil and solvent mixture, it may be necessary to predict the phase boundaries, amounts and compositions for liquid-liquid (LL), vapour-liquid (VL), vapour-liquid-liquid (VLL), and asphaltene precipitation regions.