Page 381 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fracturing fluid flow back 353

but the rest cannot. We use the membrane efficiency to describe the effec-
tiveness which is defined as the actual pressure increase across the membrane
divided by the theoretical osmotic pressure. The literature information
shows that the membrane efficiency is low (less than 5% according to Neuzil
and Provost’s (2009) review of public experimental data). When Fakchar-
oenphol et al.’s (2014) history matched experimental data using the
TOUGHREACT simulator, they used 5% membrane efficiency. It is
possible that theoretical pressures could exceed 30 MPa at a porosity of
0.1 and 10 MPa at a porosity of 0.2 (Neuzil and Provost, 2009). However,
the average osmotic pressure measured in laboratory and in situ is 0.128 MPa
at the porosity of 0.206 from the published data summarized by Neuzil and
Provost (2009). The possible reasons to cause the discrepancy between the
theoretical estimation and actual osmotic pressure could be one or more
of the following.
(1) Due to the wide range of pore sizes, formation acts as a nonideal
semipermeable membrane which only restricts passage of some of the
solutes in the solvent (Fakcharoenphol et al. 2014). Ghanbari and
Dehghanpour (2015) observed that significant permeability parallel to
the laminations acts as a preferential pathway for the imbibing water
than the semipermeable clay layers, therefore, reducing the favorable
conditions for the development of osmotic pressure.
(2) Some of assumptions in the osmotic theory may not hold in reality
(Neuzil and Provost, 2009).
(3) The conditions for the osmosis to occur are difficult to meet (Neuzil and
Provost, 2009).
(4) Formations that are effective membranes in tests are ineffective at large
scale (Neuzil and Provost, 2009).
However, anomalously high pressures were observed in shales of Triassic
Dunbarton Basin, Eastern United States (Marine, 1974; Marine and Fritz,
1981), and in an argillite in the eastern Paris Basin, France (Gueutin et al.,
2007). There could be many reasons, such as tectonic deformation, compac-
tion, diagenesis and heating, to cause anomalously high pressure behavior. It
seems difficult to explain those anomalously high pressures except by the
concept of osmotic pressure.

12.3.4.3 Implications of osmotic phenomenon
The osmotic pressure will drive lower-salinity water like fracturing fluid into
higher-salinity shale, displacing oil and gas out of shale formation. Fakchar-
oenphol et al. (2014) simulated the osmotic effect between a fracture and a