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236 Enhanced Oil Recovery in Shale and Tight Reservoirs

anionic surfactants. An additional benefit of combining anionic and cationic
surfactants was the reduction of adsorption of anionic surfactant on the posi-
tively charged carbonate rock surface. However, it should be aware that a
mixture of opposite charge surfactants may tend to precipitate.
Although Chen and Mohanty (2015) presented the property data of each
surfactant, the property data of the mixed surfactant solutions were not pro-
vided. It could be possible that the properties of the mixed surfactant solutions
were very different from their counterparts of each individual surfactant. To
understand or confirm the real mechanisms or synergy of those surfactants, the
properties of the mixed surfactant solutions should have been measured, so
that it became more clear what properties had made the mixed surfactant
solutions more effective in improving spontaneous oil recovery.
To have such synergy for incremental oil recovery from spontaneous
imbibition, at least two properties of a mixed surfactant solution must be
met: (1) wettability alteration to water-wet, (2) the interfacial tension may
be intermediately high (it cannot be ultralow or too high). These properties
will make sure that the surfactant solution can enter the porous medium to
displace oil out, and the solution also has the favorable flow properties like
improved relative permeability.

9.6 Specific surfactant EOR mechanisms related to
shale and tight formations
Some surfactant EOR mechanisms specifically related to wettability
alteration are reviewed in this section.

9.6.1 Bilayer mechanism by anionic surfactants
The mechanism of bilayer formation is shown schematically in Fig. 9.14.
The negative EO-surfactant adsorbs onto the positive surface of the chalk
through hydrophobic interactions with the adsorbed crude oil components
to form a monolayer as Chen and Mohanty (2015) called it. The water-
soluble head-group of the surfactant, the EO-group, and the anionic sulfo-
nate group may decrease the contact angle below 90 by forming a small

water zone between the organic coated surface and the oil. The formation
of the bilayer must not be regarded as a permanent wettability alteration of
the chalk. In fact, it will probably be fully reversible due to the weak
hydrophobic bond between the surfactant and the hydrophobic surface
(Standnes and Austad, 2000).