Page 242 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 225

surfactant in the solution. In building a model, the user directly inputs a table
of adsorption G s versus surfactant concentration x sw . The upper boundary
U
L
(G ) and lower boundary (G ) are set and they correspond to the adsorbed
s
s
surfactant at the completely water-wetness and the completely oil-wetness.
At any intermediate wetness, k r and p c are interpolated:
L !
G L G s G s G U G L
k r ¼ k s þ k s k s (9.26)
r U L r r
G G s
s
L !
p c ¼ p G L s þ G s G s p G U s p G L s (9.27)
c U L c c
G G
s s
Water-oil IFT is required to define the capillary number that is used to
calculate residual saturations and to scale capillary pressure. One of the
simplest ways is to input IFT versus surfactant concentration.
To include the effect of IFT in CMG-STARS, the IFT (s) is calculated
from Huh’s (1979) equation:

C H
(9.28)
2
s ¼
V om
V sm
where V om and V sm are the volumes of oil and surfactant, respectively, in the
microemulsion phase, C H is an empirical constant obtained by fitting
experimental data of s versus V om /V sm . The volumes of oil and surfactant
need to be converted to the liquid-liquid K-values required in CMG-
STARS.
The K-value for a component c is defined as

mole fraction of component c in Phase A
K AB ¼ (9.29)
c
mole fraction of component c in Phase B
If the microemulsion is type I (oil-in-water microemulsion), the micro-
emulsion phase is actually the water phase solubilized with oil and surfactant.

c W
K WO ¼ o ¼ c W (9.30)
o O o
c o
c O
w
OW
K w ¼ ¼ 0 (9.31)
c W
w