Steam assisted gravity drainage (SAGD) is the main commercial technology used for in-situ recovery of bitumen in Canada. Solvent-assisted SAGD processes are being pursued to take advantage of heat transfer (steam) and mass transfer (solvent) mechanisms to reduce the bitumen viscosity, swell the bitumen and enhance the mobility of bitumen in an energy efficient way. In the steam-chamber boundary, multiple fluids comprising of solvent, steam and bitumen come in contact with each other, and may exhibit multiphase equilibria depending on the location in the boundary and bulk composition in the pores. These multiphase equilibria can significantly affect the drainage behavior of fluids inside the steam-chamber boundary. Therefore, it is of critical importance to study the phase behavior of these fluids at steam-chamber boundary to obtain a more insightful understanding of the mechanisms of solvent-assisted SAGD process.
In this paper, the phase behavior of solvent-steam-Athabasca bitumen is analyzed with a Peng-Robison equation of state (PR EOS) model at the boundary of steam chamber under typical operating conditions in a SAGD process. The Athabasca bitumen is characterized with six pseudo-components as such characterization of bitumen provides satisfactory matches for experimental gas solubility data in the literature (Kariznovi et al., 2009). The EOS model has been validated with the recently published experimental data on phase behaviour of solvent-steam-bitumen system (where toluene is used as solvent) (Amani et al., 2014). The PR EOS model is able to provide a good representation of the phase behaviour of the water-Athabasca bitumen system and toluene-water-Athabasca bitumen system at temperatures below 317°C. The PR EOS model is used to perform the phase behaviour simulations at a constant pressure of 2.6 MPa and temperatures from 60°C to 250°C. Multiphase equilibria calculations with the PR EOS model indicate that the possible phase equilibria that can occur in the steam-chamber boundary include liquid-liquid-liquid (L1/L2/L3), liquid-liquid-vapor (L1/L3/V), liquid-liquid (L1/L3) and liquid-vapor (L3/V) equilibria. Significant water solubility in bitumen-rich liquid phase can exist at high temperatures, which is consistent with experimental observation in the literature, while a relatively large solvent solubility in bitumen-rich liquid phase is found to exist at low temperatures. The constant flash calculations for a given feed at different temperatures show that: the solvent commences to vaporize drastically from liquid phase and does not play any significant role in viscosity reduction of bitumen when the system switches from L1/L3 to L3/V equilibrium, or from L1/L3/V to L3/V equilibrium. Similarly, the swelling factor of the bitumen-rich liquid phase keeps increasing as a function of temperature until the L1/L3/V- L3/V boundary or L1/L3-L3/V boundary is reached. Viscosity reduction and swelling effect of the bitumen-rich liquid phase L3 due to heating from steam and dissolution of toluene and water can significantly enhance the mobility of bitumen, thus resulting in highly reduced residual bitumen saturation within the SAGD steam-chamber boundary.