Page 380 - Enhanced Oil Recovery in Shale and Tight Reservoirs

352 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 12.11 Schematic of electric double layer.

cannot pass through the diffusion layer, in the case of shale with such pores
that the gap between the solid surfaces is bigger than the electric double
layer. Then the electric double layers will overlap each other. As a result,
charged particles cannot pass through the pores; but neutrally charged water
molecules can flow through the pore center.
The conditions under which osmotic pressures can be observed are rare.
A large difference in solute concentration or the existence of clay-rich strata
may not be sufficient to generate an osmotic pressure. There must be a very
low-permeability barrier between two strata which have very different so-
lute concentrations. Such barrier must be effective in large scale. If there
are transmissive fractures between the two strata or units, osmotic pressure
may not build up because the fractures behave like short-circuits for solvent
(water). Also, for a geologic stratum or unit to have osmotic pressure build
up, it must be hydraulically isolated, so that there is no leakage. In a real shale
formation, the pore sizes have a wide range. Thus the shale cannot serve as
an ideal semipermeable layer. Some solutes in the solvent can pass through,