The Vapour Extraction (VAPEX) process is a promising technique directed towards heavy oil reservoirs that are typically thin and underlain with water, and cannot be exploited economically or technically by conventional thermal recovery methods. The VAPEX technique was developed by Butler and Mokrys in the 1990s as an alternative to Steam-Assisted Gravity Drainage.
This process is mechanistically complex and some questions regarding its expected performance are still pending. A numerical model can play a critical role in addressing important questions about the process. Specifically, a numerical model can predict the performance of the process, especially the occurrence and effect of asphaltenes precipitation during the ‘upgrading’ process.
This research proposes an alternative approach to simulate numerically the asphaltene precipitation effect of the VAPEX process. The model was constructed using a commercial thermal reservoir simulator. It was then validated using published experimental data. The effect of relative permeability curves, reaction frequency factor, selection of reactant, apparent dispersion coefficient, and operating parameters on performance were investigated. In addition, the model was used to design a physical experiment. The operating conditions of the experiment were optimized to represent the main mechanisms of the VAPEX process.
The results of the study indicate that the numerical model can reproduce the process with acceptable accuracy. Moreover, despite the significant viscosity reduction, it wasfound that there was no significant evidence to demonstrate blockage of fluid flow through the porous medium due to asphaltene precipitation. Further experiments would be required to confirm these findings.
The vapour extraction (VAPEX) process1 (Figure 1) has drawn the increasing attention of the heavy oil/bitumen industry since it was developed by Butler and Mokrys in 1991. In this process, the same well configuration, as well as the same counter-current drainage concept as the popular steam-assisted gravity drainage (SAGD)2 process (Figure 2), are used. However, solvents, such as butane and propane3,4 are injected, rather than steam, at or near their dew point. The mobility of the oil is improved through the mass transfer (diffusion and dispersion) effect between the solvent vapour chamber and the oil.
The VAPEX process is a non-thermal recovery method. It is more energy efficient and environmentally friendly than thermal processes. It is expected to improve recovery in problematic reservoirs, such as thin reservoirs and reservoirs with bottom water3,5, where thermal recovery methods are not economically or technically feasible.
Another benefit of the VAPEX process is the potential to upgrade the oil in-situ3 resulting from asphaltenes deposition. However, the initiation and effect of asphaltenes precipitation on fluid flow during the VAPEX process has not been resolved. Asphaltenes deposition was observed in almost all the physical experiments that Butler and his colleagues performed3–9. However, other researchers10–13 observed less or no asphaltenes deposition in their experiments.
It is also plausible that asphaltenes precipitation during the VAPEX process might plug the pore throats under reservoir conditions. Investigations on scaled physical models with significant permeability suggest that such is not the case. However, the performance of reservoirs with a range of the reservoir permeabilities is of concern.