Page 165 - Primer on Enhanced Oil Recovery

Chemical EOR 155

address all this, an alternate liquid injection with high and low interfacial tension is
used. For example, water and alkaline solution (see Fig. 12.10) alterations are used.
In this case, just water is first injected and has high absorbent properties, and then
slug of liquid with improved washing properties is injected. Water, characterized by
a high interfacial tension value, is sucked into the capillaries of small diameter and
displaces oil from them into large pores. As a result of the subsequent injection of
an alkaline solution having improved washing properties easily moves oil towards
production well. This leads to increase in oil recovery from both low and high per-
meable interlayers. However, this situation is possible only in the case when the
flow of water in the capillaries of small diameter was greater or at least equal to the
rate of flow of water in the capillaries of large diameter.
Let us estimate the ratio of the flow rates of water in the capillaries of small and
large diameter. The rate of movement of water in the capillaries of small diameter
can be determined from the following expression:

Rσ
v k 5 (12.6)
4μρL

where: v k liquid speed in small capillary: R capillary radius; μ kinematical
viscosity; ρ liquid density; L capillary lengths.
Liquid speed in the pores of big diameter can be defined from the following
equation:

Q
v f 5 (12.7)
Fm
where: v f liquid speed flow in big pores; F cross-section area of the oil con-
taining formation; Q injected water volume; m porosity.
In order for oil from small-diameter capillaries to be displaced into large-
diameter capillaries, the front of capillary impregnation must be ahead or at least
keep up with the front of the water flow in large pores, i.e.

(12.8)
v k $ v f
In the oil formations we can expect the following spread of values:
3
3
2
R 5 10 28 4 10 26 m; σ 5 10 23 4 10 21 N/m; μ 5 2U10 27 2 2U10 26 m /s; ρ 5 10 kg/m ;
2
L 5 10 4 5U10 m; Q=F 5 0 4 10 25 m/s; m 5 0:2 2 0:35:
Use of those values in the above equations produces the following values for the
liquid speed:
25
v k 5 2:5U10 212 2 1:25U10 25 and v f 5 0 2 5U10 м=с.
Consequently, the range of change in the flow rate in large pores completely
overlaps the range of change in the rate of flow of water in small-diameter capillar-
ies. This shows that by adjusting the rate of water injection for all possible condi-
tions it is possible to fulfill the condition of 12.8.