Vapor extraction (Vapex) has been widely researched in the past two decades and is believed to be a promising technique for the recovery of heavy oil especially for thin reservoirs where thermal methods, such as steam-assisted gravity drainage (SAGD) are not economical. Experimental results of Vapex by Dunn et al.showed that the bitumen drainage rate, when CO2 was used as a solvent, was unexpectedly higher than what was theoretically predicted. Moreover, the drainage rate with CO2 was much higher than that when ethane was used. This cannot be explained by the currently proposed mechanisms for Vapex.
Different from hydrocarbon gases, dissolving of CO2 in oil results in an increase in the density of the oil phase. If CO2 is used as a solvent in Vapex process, the density increase in oil phase near the CO2-oil contact region can induce a natural convection, which, in turn, enhances the mass transfer of CO2 in the oil phase. This paper examined the likelihood of the onset of natural convection in CO2-oil contact region of CO2-Vapex process using the convective instability theory. A mathematical model to calculate the transient Rayleigh number (the criteria for the onset of natural convection) within the diluted mobile oil layer in CO2-Vapex process was developed. Convective instability analysis based on the conditions of the CO2-Vapex experiments reported by Dunn et al. was conducted using the model presented in this paper. The calculation results showed that the natural convection within the mobile oil layer of CO2-oil contact region could occur. This natural convection can explain why the drainage rate in CO2-Vapex was surprisingly higher than that in ethane-Vapex as found by Dunn et al. .
As the light oil production is declining, more attentions have been paid to the heavy oil and bitumen reservoirs. However, due to the extra-high viscosity of these oils, productions by the conventional primary recovery techniques are extremely limited. For some cases the primary productions cannot even be implemented. Processes based on viscosity reduction both by thermal steam injection and non-thermal solvent dilution have been widely investigated since 1980's. Currently, steam assisted gravity drainage (SAGD) [2–4] is believed as an effective thermal method and has been successfully applied in fields for recovery of heavy oil and bitumen . However, steam process may not be applicable for some thin reservoirs where excessive heat losses to the overburden and the underburden make it economically unfeasible. For such cases, vapor extraction (Vapex), a nonthermal solvent gas injection method, was probably one of the most promising techniques for the recovery of extra-viscous heavy oils and bitumen [5, 6].
In Vapex process, the vaporized solvent is injected into the heavy oil reservoirs through a horizontal injection well. The injected solvent diffuses into the heavy oil, resulting in a considerable reduction in viscosity due to the dilution and deasphalting effects. The diluted oil will drain along the interface between solvent chamber and the immobile heavy oil to the production well by gravity.