Abstract

Steam assisted gravity drainage (SAGD) is the only economical and applicable method which the majority of companies are using to extract bitumen from the Alberta oil sands. However, there are some deficiencies in this method, including a high rate of heat loss to overburden and underburden formations in narrow (less than 15 m) oil bearing strata, a high operational cost for producing and injecting steam, and the treatment of the contaminated produced water. These disadvantages have researchers searching for other methods, such as solvent injection.

Different solvents can be considered for injection into a bitumen reservoir, such as ethane and carbon dioxide (CO2). Using CO2 as the solvent has two major advantages: achievement of miscibility with the bitumen, making it mobile by reducing its viscosity, and storage of CO2 underground. The latter is the goal of many researchers addressing the challenges of climate change.

In large-scale compositional simulations, equilibrium constants for different components in a solvent/bitumen mixture are a prerequisite. When there is no initial guess or empirical correlation for K values, the only way to obtain a true equilibrium constant is through phase stability and phase splitting calculations. Since experiments have acknowledged the formation of a second liquid phase in certain ranges of pressures and temperatures when mixing Alberta bitumen samples with solvents, fast and robust four-phase stability and splitting calculations are necessary. The four phases are the liquid diluted bitumen phase, the liquid solvent-rich phase, the solvent gas phase and the water (aqueous) phase.

The Peng-Robinson equation of state and its derivatives have been used for compressibility calculations in this study. Also, a combination of successive substitution and the Newton method has been used for phase splitting calculations for robustness.

The results have been compared to those of the available PVT simulation software (WinProp, Computer Modelling Group) and also to the experimental and modeling results of Mehrotra and Svrcek (1984). These results and comparisons indicate that the second liquid (solvent-rich) and water phases remain almost unchanged with changing pressures and temperatures; however, the components of the first liquid (diluted bitumen) and gas phases do change. This supports the idea of considering water as a phase and not as a component, which is the assumption in most compositional simulations.

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