Page 414 - Enhanced Oil Recovery in Shale and Tight Reservoirs

384 Enhanced Oil Recovery in Shale and Tight Reservoirs

100
Tight rock
Ra o-R
80
Flowback%
60

40

20

0
water non-ionic surf anionic surf
Figure 12.42 Permeability recovery ratios and flow back efficiencies for tight oil-wet
cores (k in the 10th md) of different fluids.

the fluids flow out (back) through the end B. In addition, the water close to
the end B would not stick to the core as the core was oil-wet. As a result,
both the permeability ratio and flow back efficiency in the water case
were the highest.
In the anionic surfactant case, the invaded zone close to the end B was
changed to water-wet, at least partially water-wet. The oil from the end
A would have resistance to imbibe into this zone. And the water would pref-
erentially stay inside the core owing to the capillary end effect. The resulting
permeability ratio and flow back efficiency would be lowest.
In the nonionic surfactant case, although the invaded zone remained oil-wet,
the IFT was reduced so that the capillary drive force was lower than that in
the water case. Thus, the permeability ratio and flow back efficiency were
lower than those in the water case.
Fig. 12.43 shows the permeability recovery ratios (R) and flow back ef-
ficiencies for the conventional cores. The performance was similar to that in
the tight cores, except that the differences among the cases of different so-
lutions were lower. In the conventional cores, the low IFT and high flow
velocity made the capillary numbers in the order of magnitudes of
5 3
10 e10 (Tangirala and Sheng, 2019b) which made the capillary desatu-
ration work. Thus, the performance in the nonionic and anionic surfactant
solution cases was improved, and their differences with those in the water
cases became smaller.
However, field data suggest that the IFT reduction decreases the draw-
down required to initiate and sustain the flowback; the load recovery from
over 200 wells in Cotton Valley, Greater Green River, Piceance, San Juan,