Page 232 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 215

Water
0.4
n Oil
o
i 0.3
t Microemulsion
c
a
r water
f 0.3
,
n oil
o
i 0.2
t
a microemulsion
r
u
t 0.2
a
s
l
a 0.1
u
d
i
s 0.1
e
R
0.0
0.000001 0.00001 0.0001 0.001 0.01 0.1
Capillary number
Figure 9.1 Capillary desaturation curves for water, oil, and microemulsion phases.
it is 0.00,001. This minimum capillary number is called critical capillary
number (N C ) c . When the capillary number is greater than 0.01, even though
it is increased, the residual oil saturation does not decrease significantly any
more. This capillary number is called the maximum capillary number
(N C ) max . Between (N C ) c and (N C ) max , the change of residual oil saturation
may be described by this equation:
1
ðN C Þ
ðN C Þ
S or ¼ S or max þ S or C S ðN C Þ max (9.2)
or
1 þ T p N C
where T p is the parameter used to fitting the laboratory measurements. The
subscript o means oil phase; it can be substituted by w for water, me for
microemulsion phase, or p for any phase p in general.
The CDC for oil in this figure shows when the capillary number is
increased to 0.0001, the residual oil saturation is decreased from 0.3 to
0.2. In other words, to decrease residual oil saturation by 0.1, the capillary
7 4
number needs to be increased by 1000 times from 10 to 10 .
According to the definition, the capillary number can be increased by one
of three methods: increasing v or m, or decreasing s, if the contact angle is not
changed. It is easy to understand that it is not practical to increase v or m by
1000 times in a real reservoir. However, it is well known that it is feasible to
reduce the oil-water interfacial tension 1000 times by adding surfactants.
Similarly, by adding surfactants and reducing interfacial tensions, residual
water and residual microemulsion saturations can also be decreased.