Page 234 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 217

numbers. For the ease of description and understanding, we describe water-
oil two phase flow.
From the above description, it can be understood that as the capillary
number is increased, residual oil saturation is decreased, the end-point water
relative permeability at any N C , k e;N C is increased following this equation:
rw
ðN C Þ N C h i
e ðN C Þ ðS or Þ c ðS or Þ e ðN C Þ e ðN C Þ
k e;N C ¼ k rw c þ k rw max k rw c
rw
ðS or Þ ðN C Þ c ðS or Þ ðN C Þ max
(9.5)
Similarly, for the exponent of water n N C at any N C , we have
w
ðN C Þ N C h i
ðS or Þ c ðS or Þ
n N C ¼ n ðN C Þ c þ ðn w Þ ðN C Þ max ðn w Þ ðN C Þ c (9.6)
w
w
ðS or Þ ðN C Þ c ðS or Þ ðN C Þ max
In the above equations, if the subscripts w and o are exchanged for their
positions, the parameters for the oil phase can be written similarly. Further-
more, three-phase relative permeabilities can be written similarly as well.
For detailed formulation, see Chapter 7 in Sheng (2011).
In summary, when surfactants are added, the oil-water interfacial tension is
reduced greatly, the residual oil saturation is decreased, and the oil recovery is
improved.

9.3 Mechanisms of wettability alteration on oil
recovery
In an oil-water-rock system, the water-oil capillary pressure p cwo is
defined as

2s wo cosq w
p cwo ¼ p o p w ¼ (9.7)
r
where p o and p w are the oil phase pressure and water phase pressure,
respectively, r is the pore radius, s wo is the water-oil interfacial tension, and
q w is the water contact angle. When the rock is water-wet, q w is less than

90 , and the oil phase pressure p o is higher than the water phase pressure p w
so that oil will flow out of the pores, meanwhile water will imbibe into the
pores, resulting in countercurrent flow of oil and water. The countercurrent
flow will drive oil out of the rock system. This is the fundamental principle
to use wettability to produce oil.

When the rock is oil-wet, q w is greater than 90 , and the oil phase pressure
p o is lower than the water phase pressure p w . The water cannot imbibe into the