Page 420 - Enhanced Oil Recovery in Shale and Tight Reservoirs

390 Enhanced Oil Recovery in Shale and Tight Reservoirs

because the surfactant solutions reduced IFT and increased capillary number.
These expectations are consistent with the experimental results presented in
the preceding paragraph. Note that surfactant cannot change the chip wetta-
bility from oil-wet to water-wet in this case owing to the chip material. If a
surfactant can change the wettability to water-wet, then the capillary pressure
becomes a resistance, but the oil relative permeability may be increased during
the flow back. The net effect of these two factors determines the oil flow rate
for surfactant solution compared with the oil rate for water.

12.8 Effect of surfactants on flow back
In preceding sections, the roles of surfactants are discussed when they
are related to shut in, invasion depth, and initial rock wettability. This sec-
tion is to further discuss the roles in flow back. Surfactants have basically two
functions: interfacial tension (IFT) reduction and wettability alteration.
Generally, when IFT is reduced, the capillary number is increased, residual
saturations are reduced, and relative permeabilities are increased. At oil-wet
conditions, oil relative permeability is depressed; similarly, at water-wet
conditions, water relative permeability is depressed. From these general
observations, surfactant solutions should have high IFT and oil-wetness
alteration to mitigate water blockage from favorable p c point of view; but
they should have low IFT and water-wetness alteration to increase k ro and
depress k rw from k r point of view. More detailed ideal functions of surfactants
are summarized in Table 12.3. Obviously, the requirements to favor p c and k r

Table 12.3 Favorable conditions for invasion, flow back, and oil recovery.
Higher oil
Less invasion More flow back recovery
Initial
Favorable wettability IFT Wettability IFT Wettability IFT Wettability
p c for SI WW Low OW High OW High OW
k rw for WW High WW Low OW
displacement
k ro for WW Low WW
displacement
p c for SI OW High OW High OW High OW
k rw for OW High WW Low OW
displacement
k ro for OW Low WW
displacement