  Brine fluids have recently been of high interest to enhanced oil recovery.
Mechanisms for such improvement were widely proposed, including wettability
alteration and reduction in the oil-brine interfacial tension (IFT), although
their synergistic contributions were vaguely clarified. Crude oil displacement
by spontaneous imbibition was conducted in the current research with focus on
the effects of brine valency and concentration. Monovalent (NaCl) and divalent
(CaCl2) brines at elevated concentrations were examined. Imbibition results
showed that NaCl brine at suitable concentration (100 mM) displaced greater oil
than too-low or too-high concentrations, and these monovalent brines displaced
more effective than those of divalent CaCl2 due to an oil-wetting as a result
of divalent ion bridging phenomenon. This echoes crucial influences of both
brine valency and concentration. The findings revealed that in a given
imbibition system a required IFT is a wettability-dependent, water-wet system
needs high IFT to enhance a driving capillarity while oil-wet system prefers
lower IFT to weaken a resisting capillary force. Brine formula directly
attributed to wettability and IFT, NaCl brines secure water-wetting with high
IFT while CaCl2 brines reduced IFT more effectively with an assured
oil-wetting. Low-salinity enhanced oil recovery mechanism was thus found to be
contributed from capillary effect, which was an interplay between the
interfacial tension and wettability. Paring these two parameters by formulating
imbibing brine to anticipate high oil recovery is crucial and of challenge.
