Page 262 - Enhanced Oil Recovery in Shale and Tight Reservoirs

244 Enhanced Oil Recovery in Shale and Tight Reservoirs

Figure 9.23 z-potential for shale cores at different surfactant concentrations.
These data indicate the anionic surfactants were better than the nonionic
surfactants in terms of wettability alteration, as the higher the z-potential,
the more water-wet the cores are. It is confirmed by direct angle measure-
ments after 48 h of soaking in the surfactant solutions shown in Fig. 9.21
and by visual oil shown in Fig. 9.24 coming out of the cores by spontaneous
imbibition of different surfactant solutions for 48 h. However, the mecha-
nism of wettability alteration by forming ion-pairs for anionic surfactants
took some time, while the mechanism by surfactant adsorption made the
contact angle decrease immediately. For example, the time-dependent
wettability alteration for AOE and AES anionic surfactants are shown in
Fig. 9.25. However, the effectiveness of a surfactant to alter wettability

Figure 9.24 Oil seen when the shale cores were soaked in different surfactant concen-
trations for 48 h.