Page 138 - Fundamentals of Enhanced Oil and Gas Recovery

126 Pouria Behnoudfar et al.

Mobility of gas can be controlled by water injection. The cyclic nature of the
WAG process leads to an increase in water saturation during the water injection half
cycle and a decrease of water saturation during the gas injection half cycle. The pro-
cesses of imbibition and drainage taken by inducing cycles cause the residual oil satu-
ration to be usually lower than that of waterflooding and similar to those of gas
flooding [106]. The oil recovery factor can be described by two factors that are the
macroscopic sweep efficiency and the microscopic sweep efficiency. Furthermore, the
macroscopic sweep efficiency is defined by the horizontal and the vertical sweep effi-
ciencies. This can be formulated as follows [106]:

R f 5 E v E h E m (4.32)
k rg =μ
M 5 g (4.33)
k ro =μ
o
vμ L

R v 5 o (4.34)
g kgΔρ h
where E v , E h , E m , k r , μ, Δρ, g, L, and h stand for vertical sweep efficiency, horizontal
sweep efficiency, microscopic sweep efficiency, relative permeability, viscosity of fluid,
density difference, constant of gravity, the length of porous media, and net-pay thick-
ness, respectively. The subscripts o and g are in correspondence with oil and gas.
Literature on the WAG process typically discusses two major management para-
meters that affect the economics of a WAG project. These operational aspects are the
half-cycle slug sizes and the WAG ratio. The two major problems faced are early
breakthrough and injectivity losses. It is therefore proposed that the third parameter to
be studied is the operation of the smart wells. The two most common distinctions in
the classification of the WAG process are miscible WAG injection and immiscible
WAG injection. Miscible WAG injection occurs when the reservoir is above the
MMP, and it is immiscible when injection pressure is below the MMP value. When
the initial reservoir pressure is just above the MMP, it often moves in and out of mis-
cibility condition in part of the reservoir or all of it [101].

4.10.1 Factors Influencing Water-Alternating-Gas
Considerable parameters in the design of the WAG process are as follows
[7,16,107 109]:
Fluid properties and rock fluid interaction. The fluid behavior within the reservoir is a
key parameter which still requires more knowledge and development for better
understanding. This phenomenon becomes even more complex when the prevailing
conditions within the reservoir change as a result of undergoing processes. Variations
in rock fluid interaction with changing conditions in a reservoir result in wettability