Page 378 - Enhanced Oil Recovery in Shale and Tight Reservoirs

350 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 12.9 Comparison of the overall water saturation during flow back inside the
core between the methanol and water cases (Liang et al., 2017a).

12.3.3 Fluid entrapment
During the drawdown, the pore pressure is decreased. Then the net confining
stress is increased, resulting in closure of some pores or fractures. Then fluid
can be entrapped in the formation (Bertoncello et al., 2014; Ezulike et al.,
2016). And incomplete water drainage can occur due to adverse mobility
ratio and gravity segregation (Parmar et al., 2013; 2014). In an oil-wet forma-
tion, water can be trapped by snap-off (Bertoncello et al., 2014).

12.3.4 Osmosis

In this section, osmosis in shale and its mechanism are introduced, and the
implication in shale formation is discussed.

12.3.4.1 Osmosis in shale
Osmosis is the spontaneous movement of solvent molecules (e.g., water)
through a semipermeable membrane from a region of lower solute concen-
tration into a region of higher solute concentration. It tends to make the
solute concentrations on the two sides equal. A semipermeable membrane
is permeable to the solvent, but not the solute. This process can be