Page 427 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fracturing fluid flow back 395

oil-wetness at the ends of early-phase (A), intermediate-phase (B), and late-
phase (C) for a tight model (500 nD). An optimum condition for high k ro
appears to be low IFT with a wide range of wettability in the early phase,
oil-wetness with a wide range of IFT in the intermediate and late phases.
Probably the oil recovery is more related to the late-phase behavior. The
oil-wetness is an optimum condition for the tight model. This condition
corresponds to a condition to favor negative p c according to Table 12.3.
Apparently, this example indicates that a favorable k r condition is good for
a low-permeability water-wet formation; a favorable p c condition is good
for a tight water-wet formation.

12.9 Solutions to deal with flow back

According to the fundamental flow theory (the concept of relative
permeability), higher flow back will enhance oil and gas recovery, because
more water blocking is removed. Sometimes, a high oil or gas production
rate is observed when the flow back is low. That may be because trapped
water near fractures dissipates deep into the formation so that the blocking
near fractures is mitigated. In this situation, there may exist a complexity of
fracture networks to facilitate quick water dissipation. If a low oil or gas rate
is observed even though the flow back is high, the opposite situation may
exist where there is not a complexity of fracture networks. In the following
sections, several solutions are proposed or practiced to dealing with flow
back.

12.9.1 Avoid using trapping fluids
Flow back is to remove phase trapping. Ideally, use of trapping fluids should
be avoided. For example, use of water should be avoided in strong water-
wet formations. The problem is many shale and tight formations exhibit
mixed wettability, either oil or water can spontaneously imbibe into forma-
tions. More importantly, which fluid should be used depends on many fac-
tors in addition to the trapping issue, like economic and environmental
concerns. This solution, in most of cases, is not practical.

12.9.2 Early high drawdown
As the aqueous fluid is more significantly accumulated near fracture faces
similar to the capillary pressure end effect in core flood experiments, it causes
the blockage to oil and gas flow into the fractures. High pressure drawdown