Page 231 - Enhanced Oil Recovery in Shale and Tight Reservoirs

214 Enhanced Oil Recovery in Shale and Tight Reservoirs

include the EOR mechanisms of wettability alteration and IFT reduction,
formulation of these two functions, IFT reduction versus wettability alter-
ation, surfactants used to alter wettability, determination of wettability, and
conversion of wetting angles.

9.2 Mechanisms of interfacial tension (IFT) reduction

To understand mechanisms of interfacial tension reduction, first review
the concept of capillary number. The dimensionless capillary number, N C ,is
defined by the ratio of the viscous to capillary force:

F v vm
N C ¼ ¼ (9.1)
F c scosq
where F v and F c are viscous and capillary forces, respectively, m is the displacing
fluid viscosity, v is the pore flow velocity of the displacing fluid, s is the
interfacial tension (IFT) between the displacing and displaced phases, and q is
the contacting angle defined by the displacing fluid. A set of consistent units
are used so that the dimensionless group is dimensionless. For example, v is in
m/s, m in mPa$s, s in mN/m or dyne/cm.
Let us first use the above equation to calculate the waterflooding capil-
lary number in a conventional reservoir. A typical injection velocity may
6
be 1 ft/day (3.528 10 m/s), the water viscosity is close to 1 mPa$s,
the oil-water interfacial tension is assumed to be 30 mN/m, and the
contact angle is assumed zero. Then the corresponding capillary number is
6
um 3:528 10 m=s ð1 mPa$sÞ 7
N C ¼ ¼ z10
s ð30 mN=mÞ
It has been established that as the capillary number is increased, the residual
oil saturationis decreased, thus the oil recovery will beimproved. The relation-
ship between the capillary number and the residual oil saturation is the capillary
desaturation curve (CDC), as shown in the solid curve and square points in
Fig. 9.1. It is also known that as the capillary number is increased, the residual
water saturation and residual microemulsion saturation are also decreased.
The CDCs for them are also presented in this figure. In the figure, the discrete
data points are experimental data and the smooth curves are fitted curves.
The presented CDC curves show that when the capillary number is less
than 0.00,001, even though it is increased from 0.000,001 to 0.00,001, the
residual oil saturations are barely decreased. It means that for a residual satu-
ration to be decreased, a minimum capillary number is required; in this case