Page 130 - Fundamentals of Enhanced Oil and Gas Recovery

118 Pouria Behnoudfar et al.

The VAPEX process was introduced by Mokrys and Butler [69] and Butler and
Mokrys [70]. In this process, a pure hydrocarbon vapor or a vaporized hydrocarbon
mixture diffuse and dissolve in heavy oil leading to a reduction in oil viscosity and an
increase in the oil mobility. This method is mostly used in cases in which solvent-
assisted gravity drainage (SAGD) process is not applicable. In other words, VAPEX is
utilized in cases where SAGD leads to excessive heat losses and extra costs. These
reservoirs are as follows [68]:
• Thin reservoirs
• Low permeability carbonate reservoirs with high heat capacity per unit volume of
contained oil
• Reservoirs underlain by aquifers and/or gas cap
Solvents used frequently include [71]
• Ethane
• Propane
• Butane
However, it should be noticed that pure solvents are applicable for only limited
conditions. In the most reservoirs, pressure and temperature do not allow to inject a
pure solvent in its saturated vapor (dew point) condition [72].

4.5.1.1 The Solvent Requirement for the Vapor Extractions Process
Amount of the used solvent is a critical parameter which affects the costs of operation.
The injected solvent undergoes different phenomena including dissolution into heavy
oil and then stripping from heavy oil in order to create a recycling process.
It is shown that one barrel of propane is required for recovery of one barrel of oil
in a condition in which there is no stripper and recycle loop; however, a drastic
decrease as much as 74% of the injected propane as solvent can be recovered when
recycling is employed [73].
Various studies have been done to model solvent material balance in a reservoir
stimulated by the VAPEX recovery process. The method used by Butler et al. [74],
which was based on specific volumes of the solvent and heavy oil, failed as it was not
able to approximate the specific volumes of the two solvents correctly. Afterward,
Mokrys [72] proposed an equation based on Gibbs theory [75] for calculating the vol-
ume of solvent mixture required for recovery. The mathematical form of this model
can be states as follows [75]:

ig ig
M ð T PÞ 5 M ðT p i Þ (4.23)
i i
where
X
M 5 x i M i (4.24)
i