Page 172 - Fundamentals of Enhanced Oil and Gas Recovery

160 Forough Ameli et al.

As it is clear, heavy oil is a mixture of cycloparaffins, paraffins, and aromatic com-
pounds. Catalytic reactions continue by B-scission and intermediates by chain reac-
tions [46]. The latter include three stages, namely, initiation, propagation, and
termination. The first step consists of carbenium ions formation which occurs during
protonation of catalysts acid sites. There may be other routes that lead to producing
carbenium ions including (1) paraffins hydride abstraction, (2) olefins protonation,
(3) protolytic cracking [46]. Carbenium ions are produced by hydride ion transfer
from carbocation and oil molecules in a chain reaction. It is possible to generate
stable secondary or tertiary carbenium ions by alkyl or hydride shift [47]. The gener-
ated carbeniums are unstable and are converted to smaller hydrocarbons by cracking,
isomerization, ring opening, alkylation, etc. Through the last stage, namely, termina-
tion, protons are separated from carbenium ions and change to lighter hydrocarbons,
hydrogen, and tricoordinated carbenium ions [46].

5.2.6 Steam/Solvent-Based Hybrid Processes

Vapor extraction (VAPEX) consists of injecting a solvent into heavy oil reservoir to
reduce the oil viscosity. The solvent is injected within the upper well and is produced
from the lower one by gravity drainage mechanism. The first studies on this issue was
performed by Mokrys and Butler [49] using a similar solvent to SAGD process. In this
study, toluene was applied for extraction of two oils, namely, Suncor Coker and
Athabasca. However, the initial idea of proposing this technique was introduced by
Allen [50] as he changed “huff-and-puff” process by changing the solvents of butane
and propane. Moreover, liquid solvents accompanied by a noncondensable gas were
applied for injection through the reservoir [51]. Pure gas and a mixture of gases were
also applied while the injection pressure is less than the vapor pressure to increase the
recovery of heavy crude oils. As CH 4 and CO 2 were inexpensive and available, they
were selected for heavy oil recovery by Dunn et al. [52]. The drawback of using these
solvents was low production rates which caused this process never be introduced in
field scale. The idea of horizontal wells leads to revival of solvent injection after 10
years of interruption. In this period, some lab-scale efforts were focused on porous
media and nonporous media models [53]. Moreover, the VAPEX technology was
upgraded by introducing hybrid VAPEX and warm VAPEX in which the solvent was
heated. This causes heat transfer to the VAPEX interface and in situ condensation
occurs in heavy oil. In Hybrid or wet VAPEX, steam is injected into solvent. In order
to increase the rate of viscosity decrease, these techniques combine the effects of heat
and mass transfer to optimize the rate of production. Another technique represented
by Farouq Ali and Snyder [54] and Awang and Farouq Ali [55], involved application
of solvents in hot miscible displacement. The effect of high temperature on this pro-
cess is represented by Butler and Jiang [56] and Karmaker and Maini [57]. In Butler