Page 302 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 275

r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2s wo cosq os
t o;SI
V o;SI r
d o;SI ¼ ¼ s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
V o;SI þ V o;FI
2s wo cosq os 2s wo cosq os
t o;SI þ DF o þ t o;FI
r r
1
¼ s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi:

DF o t o;FI
1 þ 1 þ
p co t o;SI
(9.61)
In the above equations, p cw and p co are the water-oil capillary pressure,
when respective water or oil imbibes; F w and F o are applied water and
oil potentials, respectively.
For the Amott-Harvey method, the wettability is determined by the
difference of d w,SI and d w,SI .
In shale and tight cores, the spontaneous imbibition times and forced
imbibition times are very long. It is not practical to use the Amott-Harvey
method.
Recently, Siddiqui et al. (2018) raised three critical questions about
wettability:
1. Which contact angle measurements represent in-situ reservoir
wettability, air/water/rock, air/oil/rock, or oil/water/rock?
2. If oil/water/rock contact angle measurements represent actual reservoir
wettability, can it really be reliable if it does not behave similarly in
imbibition tests?
3. The electrostatic and chemical forces can significantly contribute to
water and oil imbibition volumes in shales, and they are also very
active in controlling contact angles on the rock surface. Why does the
imbibition volume measurement lead to different wettability than that
reached by contact angle measurements?
The above question 1 is answered by Conclusions 1 and 2 earlier in this
chapter. Put it simply, the wettability cannot be determined by contact
angles in a gas-liquid-solid system, and it must be determined in a liquid-
liquid-solid system.
The above question 2 is answered by Conclusion 3: the water and oil
volumes by spontaneous imbibition in two same dry cores cannot be used
to determine the wettability.