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Sahimi et al.
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To control the mobility, many different methods have been used, such as
water-alternating-gas (WAG), gas-soluble viscofiers, gelled and cross-linked poly-
mers, and surfactants and foams (Smith, 1988). We describe one of these, namely,
the WAG process.
8.10.3 Miscible Water-Alternative-Gas Process

This method is the only gas-flood mobility control technique which is used regularly
in field applications. Its chief virtues are familiarity to the petroleum engineers, as
it consists of the familiar water flooding, alternated with gas flooding; simplicity,
having timing and the ratio of water to gas as its only design parameters, and its low
cost. It is used mostly to maintain pressure while the solvent supply is interrupted, or
to stretch out injection costs by substituting water for the more expensive solvent.
Miscible WAG injection has been implemented successfully in a number of fields
around the world (Christensen et al. 1998a,b). In principle, it combines the benefits
of miscible gas injection and water flooding by injecting the two fluids either simul-
taneously or alternatively. Miscible gas injection has excellent microscopic sweep
efficiency, but poor macroscopic sweep efficiency due to fingering and gravity over-
ride. Furthermore, it is expensive to implement. The optimal ratio for simultaneous
WAG injection in a relatively homogeneous reservoir can be estimated by matching
the advance rates of the water-oil and solvent-oil displacement fronts. Water flooding,
on the other hand, is cheaper and less vulnerable to gravity segregation and frontal
instabilities. However, the residual oil saturations after water flooding are relatively
high.
Johns et al. (2003) studied optimization of the WAG processes for enriched gas
floods, particularly as a primary recovery method. Al-Shuraiqi et al. (2003) carried
out laboratory investigations of first-contact miscible WAG to study the effects of
WAG ratio and flow rate on recovery efficiency. Their experiments (using bead-
packs) indicated that, (1) recovery from WAG may vary with rate, even when gravity
is neglected, and (2) water-solvent and water-oil relative permeabilities may not be
the same if they are first-contact miscible. The influence of the rate on WAG recovery
appearstobeduetotherate-dependencyofwater-oilandwater-solventpermeabilities.

8.10.4 Relative Permeabilities

Enhanced oil recovery processes that are carried out under near miscible conditions
require relative permeability data to calculate the flow behavior of the low interfa-
cial tension (IFT) fluids. In systems where the displacing and displaced fluids are
miscible and the flow velocity is low enough that the process is controlled by diffu-
◦
sion, the fractional flow versus saturation curve is a 45 line, and is referred to as
miscible relative permeability function. The term near-miscible relative permeability
functions is used to denote the curves found in the region between the immiscible
limit and miscible limits. If the IFT is finite and there is no mass transfer, the relative
permeabilities of the phases are, as is well-known, more complicated.