Countercurrent Extraction of Heavy Oil and Bitumen
Authors: Das S.K. , Butler R.M. 
Published: December 2007

Abstract
The Vapex process represents a unique application of the horizontal well for the recovery of heavy oil and bitumen. In this process low molecular weight hydrocarbon solvent vapour is injected into the reservoir using horizontal well and the solvent diluted oil due to its low viscosity drains by gravity to another horizontal well. The rate of transfer of the solvent molecules in the crude directly reflects on the extraction rate. The higher area of contact between solvent vapour and the crude yields higher rate of mass transfer of the solvent. By using horizontal wells the area of contact for mass transfer is increased enormously. It was observed that in the initial phase, when the solvent vapour rises above the injector forming the extraction chamber, due to the countercurrent nature of the extraction the production rate is about 2–3 times higher than the subsequent gravity drainage rate. In this paper the possibility of enhancing the extraction rate by combining the effect of extended solvent-oil contact and the countercurrent mode of extraction throughout the project life is investigated.
In a reservoir underlain by aquifer the injected steam may preferentially pass through the water zone and condense causing a great loss of energy. In the Vapex process however, the high injectivity in this zone may be utilized to spread the solvent vapour underneath the reservoir extending the solvent-oil contact under the entire pattern simulating a planar well. Both injector and the producer should be placed near the water oil contact, separated by a distance that depends on the oil viscosity and reservoir parameters. The rising solvent vapour contacts oil countercurrently and the diluted oil is carried to the production well. With the remarkable advancement in the technology of drilling multi laterals it is possible to spread the solvent vapour effectively. A mathematical model for this extraction process is presented along with the experimental results that show 4-10 times higher extraction rate in this upward leaching process than in the usual spreading chamber.

Introduction
The Vapex process is emerging as an alternative to the thermal processes for the recovery of the huge resources of heavy and extra heavy oil worldwide. The concept is analogous to that of the Steam Assisted Gravity Drainage (SAGD) process, which has gained tremendous popularity in the industry for its usefulness in producing high viscosity heavy oil and bitumen. Unlike steam in SAGD, in the Vapex process vaporized hydrocarbon solvents are injected into the reservoir through a horizontal well. The injected solvent vapour dissolves in the high viscosity oil at the interface and diffuses through it. In this process the viscous oil gets diluted and drains to the horizontal production well by gravity. Initially the injected solvent vapour rises countercurrently with the draining diluted oil and a rising vapour chamber is created. Once the chamber reaches the top of the reservoir, it spreads sideways until the pattern boundary is encountered. The process may still be continued with a falling interface period until the rate becomes prohibitively low. The Vapex process is highly energy efficient, environmentally friendly, causes in-situ upgrading by deasphalting of the crude and requires lower capital investment compared to steam processes. The applicability of the Vapex process may even surpass the steam processes in thin reservoirs, reservoirs underlain by aquifer, offshore operations etc.
The principal use of horizontal wells is to expose a larger area of the reservoir to the well bore thereby increasing the productivity and to reduce the drawdown. The extended contact of the horizontal well with the reservoir is very useful in case of thermal processes e.g, SAGD or solvent extraction processes like Vapex. In SAGD heat is transported by conduction through the sand matrix containing oil and water. Use of horizontal well increases the area of heat conduction thereby increasing the heat transfer rate. For example with a 0.5 km long well pair in a 30 m thick reservoir with a fully developed steam chamber width at the top, say, 20 m, the area of heat conduction will be greater than 3.6 × 104 m2 compared to 755 m2 for a vertical well with an inverted cone shaped steam chamber of 20 m radius at the top.
As mentioned earlier, the Vapex process involves the diffusion of solvent into the bitumen or heavy oil. Production rates are directly related to the viscosity reduction which in turn depends on the amount of solvent dissolved in the crude. Since the molecular diffusivity of solvent in bitumen is orders of magnitude lower than the thermal diffusivity of the reservoir matrix, it is generally expected that production rates will be much lower in this solvent process than those in a steam process.
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