Page 137 - Fundamentals of Enhanced Oil and Gas Recovery

Miscible Gas Injection Processes 125

yields
H 2 CO 3 1 C 2 S 2 H ! amorphous silica gel 1 CaCO 3
yields
H 2 CO 3 1 Ca OHÞ ! CaCO 3 1 2H 2 O
ð
2
yields
H 2 CO 3 1 CaCO 3 ! Ca HCO 3 Þ
ð
2
In the foregoing reactions, calcium silica hydrate, C S H, compounds are major
components in Portland cements, whereas free lime, Ca(OH) 2 , constitutes about 20%
of the cement composition in set Portland cements. Because CO 2 corrosion of cement
is thermodynamically favored and cannot be entirely prevented, various solutions have
been developed to limit CO 2 attack on the cement sheath. Most of these approaches
involve substituting materials such as fly ash, silica fume, or other nonaffected filler or
other cementation materials for a portion of the Portland cement. The water ratio of
the cement slurry is designed to be low to reduce the permeability of the set cement.
The permeability of the set cement may be further lowered through the addition of
materials such as latex (styrene butadiene) to the compound [99].

4.10 WATER-ALTERNATING-GAS PROCESS

WAG injection is a tertiary oil recovery process. First, it was introduced in the
1950s and its popularity has grown since then. WAG is a combination of the two sec-
ondary recovery processes of waterflooding and gas injection, and its original aim for
the ideal system of oil recovery is to enhance the macroscopic and microscopic sweep
efficiency simultaneously [100].
It is classified based on the types of fluid involved and the manner in which they
are injected to different groups. In general, it can be divided into miscible and immis-
cible displacement processes [7,16,102]. In the miscible WAG process, the injected gas
is miscible with the reservoir oil under the prevailing conditions. In the immiscible
WAG process, the injected gas is not miscible with the reservoir oil and it displaces
the oil while maintaining its gaseous phase, with a front between the two phases.
Further classification of WAG process is given below, depending on the injection
technique used in the process [103,104].
Further classification of WAG process depends on the injection method used in
the process. A WAG process can be implemented as hybrid WAG injection, in which
injection of a large volume of gas is done at the first step, and then small volumes of
water and gas are injected maintaining a WAG ratio of 1:1. Moreover, simultaneous
water and gas injection is also one of the popular approaches [103,105].