A novel cationic
gemini surfactant was prepared by chlorination
and quaternization in this paper. The target product was successfully
synthesized, which was confirmed by infrared (IR) spectrometer and
NMR analysis. Different methods including zeta potential determination,
scanning electron microscope, IR, sessile drop method and spontaneous
imbibition were used to investigate the mechanism of the wettability
alteration of the oil-wet surface by the synthesized gemini surfactant
(GABEO) in this work. Results show that the gemini surfactant has
excellent surface activity (32.5 mN/m). Ion pair formation is responsible
for the mechanisms of wettability alteration of the oil-wet sandstone
surface by GABEO, dodecyltrimethylammonium bromide (DTAB), and didodecyltrimethylammonium
bromide (GDTAB). Owing to better wettability alteration ability of
the synthesized gemini surfactant, there are fewer asphaltene particles
on the solid surface treated with GABEO. Compared with DTAB and the
traditional gemini surfactant GDTAB, a lower balanced contact angle
was obtained for GABEO due to the stronger ion pair desorption capacity;
that is, GABEO has a stronger ability to change the wettability of
the oil-wet sandstone surface. In addition, owing to the strongest
ability to change the surface wettability, the core has the largest
ultimate imbibition recovery among the three systems when GABEO is
used.
Various
experimental methods including atomic force microscopy,
scanning electron microscopy, zeta potential measurement, and contact
angle measurement were used to analyze the mechanisms of wettability
alteration of an oil-wet sandstone surface by cationic/nonionic surfactant
mixtures in this work. Due to the synergies between cationic surfactants
and nonionic surfactants, head groups of CTAB and TX-100 interact
with each other, making the cmc of the cationic/nonionic surfactant
mixtures lower compared to the single surfactant CTAB or TX-100. Ion
pairs are produced by the carboxylic substances and the aggregates
formed by CTAB and TX-100, which are irreversibly desorbed from the
quartz surface and are solubilized into the mixed micelles formed
by the CTAB/TX-100 mixture. The CTAB molecules are preadsorbed on
the oil-wet sandstone surface by electrostatic attraction, acting
as anchor particles, and the aggregates are formed by TX-100 and CTAB
through hydrophobic interaction, thereby increasing the adsorption
amount of CTAB on the oil-wet sandstone surface. The ability to form
ion pairs for CTAB and the carboxylic substances in the presence of
TX-100 and the solubilization ability of the mixed micelles are all
enhanced, making the desorption capacity of ion pairs stronger. Thus,
the CTAB/TX-100 mixture is more effective than the single surfactant
CTAB in altering wettability of the oil-wet sandstone surface toward
a more water-wet condition.
Different analytical methods were utilized to investigate the mechanisms for wettability alteration of oil‐wet sandstone surfaces induced by different surfactants and the effect of reservoir wettability on oil recovery. The cationic surfactant cetyltrimethylammonium bromide (CTAB) is more effective than the nonionic surfactant octylphenol ethoxylate (TX‐100) and the anionic surfactant sodium laureth sulfate (POE(1)) in altering the wettability of oil‐wet sandstone surfaces. The cationic surfactant CTAB was able to desorb negatively charged carboxylates of crude oil from the solid surface in an irreversible way by the formation of ion pairs. For the nonionic surfactant TX‐100 and the anionic surfactant POE(1), the wettability of oil‐wet sandstone surfaces is changed by the adsorption of surfactants on the solid surface. The different surfactants were added into water to vary the core surface wettability, while maintaining a constant interfacial tension. The more water‐wet core showed a higher oil recovery by spontaneous imbibition. The neutral wetting micromodel showed the highest oil recovery by waterflooding and the oil‐wet model showed the maximum residual oil saturation among all the models.
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