Modelling of Mass Transfer Boundary Layer Instability in the CO-Vapex Process

Abstract

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. CO 2-based Vapex is an attractive option from both economical and environmental perspectives. In CO 2-based Vapex, unlike other hydrocarbon solvents, the dissolution of CO 2 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 CO 2-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 CO 2 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 CO 2 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 (Ra c), convection does not happen.

A Review on the Application of Enhanced Oil/Gas Recovery through CO2 Sequestration

“…Its dissolution and mixing leads to density increase followed by density-driven natural convection phenomena. There are several published studies which reported that this phenomenon has a significant enhancement in hydrocarbon recovery and sequestration potential [24][25][26][27].…”