Solvent-based processes for recovery of heavy oil and bitumen have potential application to a variety of reservoir situations. Potential processes range from SAGD to VAPEX, with a range of hybrid processes in between. Over 50 lab-scale and 80 field scale simulations were run to determine optimum operating points for various hybrid processes. The results showed that steam-butane simulations yielded two "sweet spots? where the cost objective function was lower than that for SAGD. Economic analysis was done based on a set of field scale simulations. This analysis showed that a hybrid solvent process for an Athabasca reservoir was an alternative to SAGD. The analysis may be extended to other reservoir types as needed.
SAGD is the main commercial technology used for in-situ recovery of Athabasca bitumen. Due to the increasing costs for energy (natural gas) and the increasing restrictions on fresh water usage, VAPEX (Ref. 1) has been proposed.
The VAPEX process may be augmented by adding heat. Heating will reduce the oil viscosity sufficiently to produce a large increase in oil rate. The heat will serve to speed the diffusion of solvent into the oil. The heat will also serve to initiate communication between the injector and the producer.
Heat may be injected by using vaporized solvent or steam. Because of the low latent heat capacity of solvent, it is expedient to heat the solvent by co-injection of steam. The result is a Hybrid Solvent process (Figure 1). This process may be operated at any set of steam and solvent rates between pure SAGD and pure VAPEX. Detailed experimental, modelling and economic studies were done to determine an optimum point or points for this process.
Numerical 2D field-scale simulations were used to compare VAPEX, SAGD and Hybrid solvent processes for an Athabasca bitumen reservoir. The comparisons considered propane, n-butane and n-pentane as solvents, and considered effects of steam rate, solvent rate, pressure and steam sub-cool setting of the production well. The results are displayed in more detail in the following figures.
The numerical simulations were based on experiments done at Alberta Research Council to model the Steam-Solvent Hybrid process. Figure 2 shows a photo of the experimental apparatus. Figure 3 shows a diagram of the experimental model. The scaling criteria used for ARC lab model experiments on thermal processes are the Pujol and Boberg scaling criteria (Ref. 2). This set of scaling criteria matches the ratios of gravity, viscous forces, conductive and convective heat transfer, and diffusion, at the expense of incorrectly scaling pressure drop vs. capillary forces, and dispersion vs. diffusion. This scaling method is acceptable for SAGD, where thermal conduction is the rate-controlling step. The scaling will be less certain for VAPEX and hybrid solvent processes, where diffusion and dispersion play major roles in controlling process rates. Heat transfer, diffusion and dispersion are all important in hybrid solvent processes. These values must be determined experimentally. Experimental values of diffusion as a function of temperature are not yet available.
