Page 92 - Fundamentals of Enhanced Oil and Gas Recovery

80 Ramin Moghadasi et al.

As it can be seen, the mobility ratio increases through A to F, and as a result, flow
instability occurs. At high mobility ratios, gas will break through via narrow fingers at
very low pore volume injected. This results in a low sweep efficiency, which means
that low oil recovery would be achieved. Favorably, a stable displacement would occur
as long as M ,1, and an unstable fingering displacement would occur for M .1. The
other plot for representing such instability is shown in Fig. 3.10 [12,48].
Based on the abovementioned discussions, a modification to M toward smaller
values would stabilize the flow condition. When CO 2 is injected into the reservoir, it
dissolves in oil and reduces the oil viscosity. Therefore mobility ratio is modified to a
smaller value. However, CO 2 mobility is still high compared with that of the oil, and
fingering potential could be very high, especially when permeability variations in a
stratum are considerable.
Indeed, viscosity reduction is an effective mechanism accounting for increasing oil
recovery during immiscible CO 2 injection; nonetheless, the degree of effectiveness is
highly dependent on oil properties and rock characteristics. Viscosity effects are totally
more profound on heavy oils than light ones. At low viscosity values, this is water
flood, which is technically superior to immiscible CO 2 flood. This is due to the more
favorable mobility control through the water flooding process. However, CO 2 immis-
cible flooding recovers significantly more oil than inert gas drive at low viscosities.
This observation could be attributed to the better mobility ratio due to viscosity
reduction and greater swelling of the oil, thus leaving less residual oil in place. At
higher viscosities (i.e., 70 1000 mPa s), CO 2 injection appears to be superior than
the other methods. This is because of higher viscosity reduction and swelling associ-
ated with CO 2 injection. Typically, natural gas-saturated oils have viscosities in the
range of 0.7 to 700 mPa s, and carbonated oils have viscosities between 0.3 and
30 mPa.s. This serves less mobility ratio during CO 2 injection, which is more

Producer Producer

Displaced fluid Displaced fluid

Displacing Fingering
fluid Displacing
fluid
M > 1 M < 1
Injector Injector
Figure 3.10 Effect of mobility ratio on flow stability for both values less than and more than
unity [49].