Ionic liquids (ILs)
can play a vital role in clay swelling inhibition
during hydraulic fracturing. Previous studies highlighted the effect
of side-chain length attached to the cationic core and different anions
possessing almost the same chemical properties on inhibition performance.
However, polyatomic anions have the potential to superiorly inhibit
swelling compared to monoatomic anions. In this study, three ILs,
namely, 1-butyl-3-methylimidazolium chloride (BMIMCl), 1-butyl-3-methylimidazolium
tetrafluoroborate (BMIMBF4), and 1-butyl-3-methylimidazolium
hexafluorophosphate (BMIMPF6), were utilized to assess
the effects of polyatomic anions on clay swelling inhibition. These
performances were compared with the performances of a conventional
inhibitor, potassium chloride (KCl). X-ray diffraction (XRD) testing
was applied to check the mineral components present in the bentonite
clay sample studied in this research. Clay swelling inhibition performance
and rheological properties of these ILs were evaluated by the bentonite
plate soaking test, linear swelling test, and rheological test. The
swelling inhibition mechanisms were investigated through ζ-potential
measurement, Fourier transform infrared (FT-IR) spectroscopy, and
contact angle measurement. Moreover, COSMO-RS computer simulation was conducted to
explain the inhibition mechanisms theoretically. The results demonstrated
that BMIMPF6 showed superior inhibition performance and
reduced the swelling by 21.55%, while only 9.26% reduction was attained
by potassium chloride (KCl). The adsorption ability on the bentonite
surface through electrostatic attraction, higher activity coefficient,
and less electronegativity of PF6
– anion
played a vital role in attaining such superior inhibition performance
by BMIMPF6.