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Vapor extraction, also called Vapex, is the process of recovering heavy oil and bitumen from the reservoir utilizing vaporized solvents. Vaporized solvents are injected mostly through horizontal wells. After injection, the solvent vapors dissolve into the high viscosity oil at the gas/oil interface and diffuse into the interior of the oil zone, which reduces the oil viscosity and promotes the gravity drainage of oil towards a production well, often located a short distance below the injection well.
The ultimate objective of the process is to produce oil by reducing its viscosity in order to increase the gravity drainage rate. Vapor extraction is considered advantageous in some reservoirs, which are not amenable to thermal recovery methods. This advantage comes from the absence of issues like heat loss to overburden rock in thermal recovery methods. However, the oil production in this process is slow compared to thermal recovery, due to its reliance on mass transfer by solvent diffusion which is much slower than heat transfer by conduction and convection.
- The process does not require expensive surface facilities such as water treatment facility and steam generators needed in steam injection processes.
- The Vapex process has lower greenhouse gas emission (80% lower than steam injection processes) and offers the additional scope for carbon dioxide sequestration when CO2 is used as a component in the injected solvent. Hence this process is environmentally friendly.
- The Vapex process is technically applicable to a wider range of heavy oil and bitumen reservoirs. It can be applied to reservoirs that carry high amounts of water in pores, thin pay zones, low thermal conductivities of the formations, and reservoirs with bottom water layers.
- A known limitation of vapor extraction is its slower rate of mass transfer compared to heat transfer. Because of that, the recovery takes longer time to start and then results in lower rates of oil production.
Factors affecting process
- Viscosity of oil: which depends on its composition, temperature, pressure and concentration of gases
- Diffusion coefficient of the solvent gas: which controls how efficient the absorption and mixing process of the solvent is to reduce the oil viscosity.
- Dispersion of solvent gas: which can enhance the mixing of solvent with the oil above what can be achieved by diffusion.
- Deasphalting of heavy oil: which reduces the viscosity of the oil. However, the precipitated asphaltene can cause severe permeability damage, which can negate the advantage of reduced viscosity.
- Solvent selection & injection: as Upreti  mentioned in his review paper; the solvent selection is affected by many factors, including equilibrium pressure, molecular weight, density difference, solubility, diffusivity, and reservoir temperature and pressure.
- Geology aspects are also important: because they will help to define how the solvent and the oil will mix in the reservoir. Factors to know in this respect are: permeability, the ratio of vertical and horizontal permeability, the height of the reservoir, and the dip angle among others.
- S.R. Upreti at al. (2007) Vapor Extraction of Heavy Oil and Bitumen: Review. Energy & Fuels, 21, 1562-1574
- Butler R. M., Mokrys I. J. (1991) A New Process (VAPEX) For Recovering Heavy Oils Using Hot Water and Hydrocarbon Vapor. JCPT, Vol.30, No.1, 97-106
- Haghighat, P and B.B. Maini (2010) Role of Asphaltene Precipitation in Vapex Process. JCPT, Vol. 49, No. 3, 14-21