Page 139 - Fundamentals of Enhanced Oil and Gas Recovery

Miscible Gas Injection Processes 127

variations, which in turn affect flow parameters such as capillary pressure and relative
permeability [110,111].
Availability and composition of injection gas. The availability of gas, in terms of quan-
tity and composition, plays a vital role. Usually, the gas produced with oil from a res-
ervoir is reinjected during the WAG process.
WAG ratio. The WAG ratio is highly significant in WAG process design [112].A
WAG ratio of 1:1 is normally used in field applications.
Heterogeneous permeability. The vast majority of reservoirs have nonuniform pore
size distribution with varying degrees of interconnectivity, giving rise to heteroge-
neous permeability. Sometimes, the heterogeneity can be segregated in the form of
layers, constituting homogeneous layers in the reservoir [113].
Injection pattern. Well spacing is critical in WAG process design [7,104]. The five-
spot injection pattern is very popular, as it can provide better control on frontal
displacement.
The other affecting parameters are capillary pressure, relative permeability, and
wettability.

4.10.2 WAG Ratio Optimization
The WAG ratio has a key role in determining the shape of the oil production perfor-
mance and the CO 2 utilization curves. As the WAG ratio increases, the peak oil pro-
duction rate decreases, the time to reach the peak is delayed, and CO 2 utilization
decreases [114]. For different conditions of WAG process, a reservoir simulation is
implemented, in which its results are available in the work of Ettehadtavakkol et al.
[115]. Time dependency of oil production rate and CO 2 utilization and also the trend
of oil production rate which generally rises to a peak and then follows an
exponential-like decline can be observed in this figure [115].
Christensen et al. [44] studied the use of WAG in different formations with vary-
ing injecting gases and drive mechanisms. In fact, several projects faced either
channeling problems or reduced injectivity, whereas optimal flow allocation has the
potential to vanish these two primary problems and increase the recovery through
WAG project.

4.11 ESTIMATING RECOVERY

The theoretical carbon dioxide sequestration capacity may be calculated using
the reservoir data on the basis of reservoir rock volume, porosity, and oil saturation
[116,117]. It should be noted that in reservoirs flooded using water, the available