Page 138 - Primer on Enhanced Oil Recovery

128 Primer on Enhanced Oil Recovery

sweep efficiency of the formation, is associated with a small viscosity of gas com-
pared to water, which produces high mobility ratio.
In order to eliminate the mentioned negative results, combined methods were
developed, including the injection of gas and water, so named water alternating gas
(WAG) technology. It is believed that in 1951, the gas injection technology was
used in conjunction with the contour-boundary flooding to intensify the process of
reservoir pressure recovery and increase oil recovery in the Starogroznenskoye field
(Russia). The method included sequential with gas injection of water in volumes
3
0.5 1.0 m into the gas injection wells. This significantly reduced gas breakthrough
into production wells occurrences. It was also found that the periodic injection of
water (alternating slugs) not only eliminates gas breakthroughs, but also contributes
to an increase in oil recovery from the formation in this pilot site. For the first time
in the West, WAG technology was implemented in 1957 in Canada at the North
Pembina field.
Later, other variants of WAG were developed and optimized. There are technol-
ogies such as: sequential (sequence) water and gas injection, joint (simultaneous)
injection of water and gas, as well as foam WAG. The later relies on injection of
mixture when a foaming surfactant is added to the injected water.

11.1 WAG methods

The essence of the water alternating gas (WAG) process is the simultaneous or sep-
arate injection of water and gas into the oil reservoir.
The gas pumped into the reservoir, which is a non-wetting phase, moves into
large pores and into top layers of the formation under the action of gravitational
forces. The water on the contrary, under the influence of capillary forces occupies
small pores of the hydrophilic reservoir and generally concentrates at the lower
strata. Taking into account the uniqueness of water and gas, and in order to achieve
a greater effect from the overall injection process, it is advisable to perform alter-
nate, cyclic injection. With this technology, it is necessary that the optimum ratio
of the injected volumes of water and gas be proportional to the ratio of the volumes
of small and large reservoir pores. The use of water gas mixtures in this sequence
will give the maximum positive effect as a result of combining the properties of the
injected water and gas.
Enhanced oil recovery during alternate water and gas injection is also occurs due
to the fact that the phase permeability of the wetting phase depends only on the sat-
uration of the formation with water. The increase in oil displacement from the res-
ervoir in the presence of free gas occurs by the value of the maximum gas
saturation and is 10 15%, at which the gas is stationary.
There are various WAG implementations which are used and differ in the
method of supplying the working agents to the reservoir and in they are applied
by taking into the account the mixability of the injected gas in the reservoir
oil.