In both SAGD and VAPEX, recovery agents are introduced into the reservoir and these displace oil to horizontal wells located at the base. In each process it is desirable to inject at a pressure equal to or slightly below the pressure of the surrounding reservoir, so as to prevent flow of fluids to outside the recovery pattern.

The pressure in the recovery pattern is reduced by allowing production to draw vapour almost to the production well. The recovery agent that is produced is liquid (water condensate or dissolved solvent). This agent is separated and recycled.

The recycled quantity is about 2–6 tonnes per tonne (t/t) of produced oil for SAGD and 0.3–0.5 t/t for VAPEX. Recycle is much simpler for VAPEX.

The pore volume that contained the produced oil becomes filled with recovery agent either vaporized (steam or solvent vapour) or liquid (water or dissolved solvent). The net requirement of recovery agent is equal to the make-up supplied to the system to maintain operation.

For VAPEX, it is this quantity of solvent that determines the net cumulative solvent-to-oil ratio (NCSOR) before blowdown. Subsequently, during blowdown, a considerable fraction of this net solvent is recovered.

At one extreme the NCSOR is the weight of the solvent vapour that has the same volume as the produced oil. This is usually very small, e.g. 0.02 tonnes of propane per tonne of produced oil. At the other extreme it is the weight of the corresponding volume of liquid solvent and this is much larger, e.g. about 0.5 tonnes of propane per tonne of oil.

In practice the solvent in the reservoir is partly vaporized and partly dissolved in the draining liquid and, as a result, the NCSOR lies between the two extremes. In this paper, this situation is analyzed and experimental results from scaled-model studies are presented. These results show that, with propane as solvent, NCSOR values of about 0.1 to 0.2 may be expected in practice. This range of values corresponds to about 20–30% of the net solvent make-up remaining in the form of vapour.

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