Page 141 - Primer on Enhanced Oil Recovery

Water altering gas injection 131

Figure 11.6 Foam-assisted WAG.
f. Foam-assisted WAG (see Fig. 11.6), is carried out by adding a foaming surfactant to the
injected water. This provides the best possible reservoir coverage (see Fig. 10.6). At the
same time, the gravitational separation of working agents and the influence of rock het-
erogeneity are significantly reduced.
Depending on just gas flooding or different WAG methods sweep efficiency in
the oil containing strata changes as shown in Fig. 11.7. Single (no-water) gas injec-
tion leads to fast development of fingering and fast breakthrough. There is also
high gravity segregation. Both of those unwanted processes lead to significantly
reduced sweep efficiency.
Adding water helps to reduce viscous fingering but still has problems with the
vertical gravity segregation. Adding a foam reduces vertical gravity segregation and
helps to reduce heterogeneities influence on the sweep propagation through an oil
strata.
There are further possible different implementations of WAG depending on the
miscibility of gas and strata crude (see Fig. 11.8):
a. WAG in the presence of gas and oil miscibility with pressure above the minimum misci-
bility pressure. In this case the viscosity of the oil decreases and its mobility increases.
Gas dissolves in oil and there is no well-defined front between gas and oil;
b. WAG in the absence of gas and oil miscibility. The process is carried out at a pressure
below the minimum miscibility pressure. In this oil as a phase is not affected by the gas
and there is a border between the oil and gas phases. The process continues until gas
breaks into the production well.

11.2 WAG applicability criteria

As a result of alternating water and gas injection, the sweep efficiency increases by
the more uniform interaction with the heterogeneous zones. There is a certain