Page 241 - Enhanced Oil Recovery in Shale and Tight Reservoirs

224 Enhanced Oil Recovery in Shale and Tight Reservoirs

theeffectsofwettabilityandIFTreductiononend-pointrelativepermeabilities
and exponents are based on their effects on residual saturations.
First, a simple interpolation technique is used to consider the wettability
effect only (no IFT or capillary number effect) on a residual saturation:
WA
S WA S r;b1 S WA S WA
rj
r;b1
r;b2
¼ (9.24)
cos q cosq b1 cosq b2 cosq b1
In the above equation, the superscript “WA” refers to the parameter
value when only wettability alteration is considered. The subscripts b1 and
b2 represent two wetting cases that correspond to the two wetting angles
q b1 and q b2 . Then the effect of IFT or capillary number (trapping number)
on the residual saturation is considered using Eq. (9.14).
The above two procedures generate one conventional (without effect of
wettability) capillary desaturation curve (CDC) for one wetting angle, and the
effect of wettability will be reflected by a series of CDC curves with each
CDC corresponding to one wetting angle. Practically, two CDC curves for
each phase are defined; one corresponds to the strongly water-wetting case,
and the other the strongly oil-wetting case.
Once the residual saturations are defined in terms of the effects of wetta-
bility alteration and IFT, the end-point relative permeabilities are estimated
directly from the residual saturations, following Eq. (9.15);and the exponents
are estimated similarly.

9.4.4 CMG-STARS model
The above mathematical models may not readily be available to a practical
engineer or it may not be convenient for the engineer to code the models.
Instead, a commercial simulator is more convenient. In this section, a
CMG-STARS model is presented.
In the CMG-STARS (Computer Modeling Group, 2016) model, it is
assumed that the degree of wettability alteration is scaled by the amount
of surfactant adsorbed. The surfactant adsorption isotherm is described by
the Langmuir-type isotherm:

ax sw
G s ¼ (9.25)
1 þ bx sw
where G s is the adsorbed surfactant, a and b are adsorbing constants for
Langmuir-type isotherm obtained by matching adsorption experiments, and
x sw is the surfactant mole fraction in the aqueous surfactant solution. The
above equation assumes that the adsorbed surfactant is much less than the