The vapor extraction (VAPEX) process, as a non-thermal process, may be suitable for the recovery of heavy oil and bitumen. In this process, the injected solvent diffuses into the heavy oil/bitumen, reduces its viscosity, and drains it to the producing well. The VAPEX process is more acceptable than other processes due to its environmental friendliness, low capital and operating costs, and suitability for thin reservoirs.
Most of the efforts in the modeling of the VAPEX process have concentrated on the application of fluid flow equations to the solvent and the diluted oil inside each grid block used in the simulation of VAPEX. This is adequate when very fine grid blocks are chosen to simulate the process where the boundary layer (transition zone) occurs over a number of grid blocks. Fine grid blocks, however, require a large amount of simulation time, which is not applicable for field-scale simulation even with today's computing power. To deal with this problem, we introduce a new approach that is based on the application of the fluid flow equations to three phases: solvent, diluted oil, and heavy oil/bitumen. With this approach, it becomes possible to have mobile solvent, mobile live oil, and immobile or slow moving heavy oil/bitumen inside a grid block. The main feature of the proposed model is its ability to capture the boundary layer within a grid block, making very fine grid blocks unnecessary in the simulation of the VAPEX process. In addition, this approach can be applied to modeling viscous fingering inside grid blocks.
Heavy oil and bitumen reserves as an alternative to conventional oil reservoirs have attracted increasing attention in recent years. The magnitude of heavy oil and bitumen resources is about six trillion barrels of oil, which are about six times the total conventional oil reserves[1]. There are different techniques for recovery of these huge resources but one of the techniques which is attractive in thin reservoirs, low permeability carbonate reservoirs, and the reservoirs underlain by aqcuifers and/or gas cap is the vapor extraction (VAPEX) process.
The VAPEX process is a solvent analogue of Steam- Assisted Gravity Drainage (SAGD) in which a steam chamber is replaced by a hydrocarbon vapor chamber. In this process, which was developed by Butler and Mokrys in 1991 [2,3], a vaporized solvent is injected through an upper horizontal injection well into a heavy oil reservoir. The solvent-diluted oil, which is the heavy oil with its reduced viscosity, is then drained downward by gravity to a lower horizontal production well. Therefore, the production rates are directly related to the viscosity reduction, which in turn depends on the amount of solvent dissolved in the crude. The important benefit of this process is the potential to upgrade the oil in-situ[4] resulting from asphaltenes deposition.
The two important mechanisms in VAPEX [5,6] are (1) mixing of fluids and (2) asphaltene precipitation. The mixing of heavy oil and solvent depends on the dispersion and capillary forces.
