Page 164 - Primer on Enhanced Oil Recovery

154 Primer on Enhanced Oil Recovery

Figure 12.10 Alkaline flooding application criteria.

Indeed, let us consider the process of alkaline solution filtration, with low inter-
facial tension in a heterogeneous oil reservoir. In this case we assume that the
porosity has sets of big and small pores. During water flooding the water is predom-
inantly adsorbed into capillaries of small diameter under the action of capillary
pressure. According to the Laplace equation the capillary pressure is associated
with interfacial tension:

2σcosθ
P k 5 (12.5)
r
where P k is the capillary pressure, in MPa; σ is the interfacial surface tension, in
N/m; Θ is wetting angle, in degrees; r is the radius of the capillary, in meters.
As can be seen from formula 12.5, a decrease in σ by the use of alkalies can sig-
nificantly weaken the absorbing capacity of water in small-diameter capillaries
under the action of capillary pressure, which can lead to a decrease in oil recovery
of low-permeable interlayers. In highly permeable layers, in which the effect of
capillary absorption can be neglected, the alkaline solution, due to the low interfa-
cial tension and emulsification of oil, will increase oil recovery.
It is possible to increase the oil recovery from small-diameter capillaries with
the use of water characterized by an increased value of σ. Freshwater, sea water,
chlorine-calcium type formation water, connate water solutions have rather high
interfacial tension values. However, in the high-permeable layers, the washing prop-
erties of such water are much worse than those of an alkaline solution. In order to