Vapex (vapor extraction) is a promising technique for the recovery of heavy oil and bitumen reservoirs, especially for cases where steam-assisted gravity drainage and other thermal recovery methods are not economical. In the Vapex process, a solvent is injected into the reservoir to reduce the oil viscosity and mobilize it towards the production well. CO2-based Vapex is an attractive option from both economical and environmental perspectives. In CO2-based Vapex, unlike other hydrocarbon solvents, the dissolution of CO2 in oil can result in a density increase of the diluted oil. As a consequence, the diluted oil has a higher density than the immobile oil beneath and a gravitationally unstable diffusive boundary layer is induced, which may lead to natural convection. In this paper, a mathematical model for the diffusive boundary layer in the CO2-oil contact region is developed; and, the possibility of convective mixing is examined using linear stability analysis, based on the amplification of the initial perturbations. It is found that in most experimental cases, depending on the Rayleigh number of the porous medium, convective mixing occurs, which results in higher dissolution of CO2 in oil and thus a higher oil production rate than what is expected from theoretical analysis. This would explain the unexpected higher oil production rate of some experiments in Vapex when CO2 was used as a solvent. In field-scale operations, the results are different. In field cases, since it is almost impossible for the Rayleigh number to exceed the critical Rayleigh number (Rac), convection does not happen.
The world's total reserve of heavy oil and bitumen is about six trillion barrels, which is about six times the amount of the conventional resources[1]. A major part of these resources is in Canada, Venezuela and the United States. Most of these reserves are at such depths that open-pit mining cannot be used economically, and in-situ methods have to be used to reduce the viscosity of the oil in-place and mobilize it. Either thermal methods or non-thermal methods can be used to recover these reserves. The viscosity of oil is a strong function of temperature and decreases sharply with increasing temperature. Currently steam-assisted gravity drainage (SAGD), a thermal method, is a popular method for the recovery of heavy oil and bitumen and has been successfully applied in several fields. Despite the success of this process for some reservoirs, there are many reservoirs that SAGD cannot be applied due to excess heat loss, which makes it uneconomical to operate. In thin reservoirs, the need for steam increases and the steam-to-oil ratio (SOR) is prohibitively high. Many oil and bitumen reservoirs have a bottom aquifer, and heat loss to the water can make the process infeasible[2]. There are also reservoir conditions where SAGD may not be applied, such as when water saturation is high, or porosity is low. In cases where SAGD cannot be applied, Vapex is the most promising technique for the recovery of these resources.
