Page 308 - Fundamentals of Enhanced Oil and Gas Recovery

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Figure 10.2 (A) Water fingering due to unfavorable mobility ratio and (B) favorable mobility ratio
as a result of increasing water viscosity by polymer injection [64].

where M is the mobility ratio, k w and k o are water relative permeability in the water-
flood zone and oil relative permeability in oil-saturated zone, respectively. μ and μ
o w
are the respective oil and water viscosities. The difference between the oil and water
phases’ mobilities is a factor leading to poor volumetric sweep efficiency [17]. The
favorable condition in which a uniform oil displacement will occur is mobility ratios
less than 1, while much greater values than unity are not favorable and will result in
water fingering. Polymers such as xanthan gum can be used in waterfloods to increase
the waterflood viscosity, which consequently decrease the mobility ratio and increase
the volumetric sweep efficiency (see Fig. 10.2).
The other factor that can adversely affect the volumetric sweep efficiency is
the differences between the permeability of different zones of the formation [40,65].
In general, most of the oil reservoirs are composed of different layers of rocks with
different permeabilities thus, permeability variation is often the most important
factor that control the sweep efficiency and consequently the overall recovery factor
[17]. Employing biomass to plug the highly permeable channels and correct the
permeability profile can be a suitable solution, which will be explained in details in
Section 10.6.
The other influential factor would be the density difference between the displacing
(water) and displaced (oil) fluids. Large differences may lead to gravitational segrega-
tion through which underriding or overriding of the displaced fluid would take place.
As the result, the fluid would be bypassed at the bottom or top of the reservoir, which
reduces the macroscopic efficiency [60].