Page 421 - Petrophysics 2E

WATER-OIL-ROCK INTERFACIAL ACTMTY 389

eo
AGED 5 HRS

9 ,,,,,, , 1 , , 1 1 ~
1
0
300 10 20 30 40
WATER INJECTED, PORE VOLUME
Figure 6.12. Effect of aging of water-oil-rock system on oil recovey eflciency.

amount of water injected, less oil is recovered after water breakthrough
as the system becomes more oil-wet.
Tweheyo et al. observed that water-wet systems exhibit greater
oil recovery at water breakthrough with very little production
thereafter [83]. Progressively more oil-wet systems, however, have a long
period of significant production after water breakthrough. In addition,
neutral-wet (or 50%/50% wettability) systems yield the largest amount
of ultimate recovery.
EFFECT OF BRINE SALINITY ON OIL RECOVERY

Increased oil recovery has occurred in some cases when the injection
brine salinity was substantially decreased. Tang and Morrow concluded
from experimental data that several conditions are necessary [84] :

The reservoir should be a mixed wettability system where residual
oil remains immobile in large oil-wet pores and the connate water
principally occupies the smaller, water-wet pores.
The rock contains potentially mobile particles of clay and other
minerals adhering to the walls of the pores.
The particle size distribution is less than the pore size distribution.
Hence, when the particles are dislodged they can be transported
through the rock by injection brine without damage to porosity and
permeability.

In the mixed wettability system, surface-active compounds in the
oil will tend to migrate to the oil-rock interface and coat the exposed